Chapter 16

Chemical Laboratory Microwave Safety

H. M. "Skip" Kingston, Peter J. Walter Department of Chemistry and Biochemistry Duquesne University, Pittsburgh PA 15282-1503

W. Gary Engelhart Milestone MLS, 7289 Garden Road #219, Riviera Beach, FL 33404

Patrick J. Parsons Wadsworth Center, New York State Department of Health, P.O. Box 509, Albany, NY 12201-0509, and Department of Environmental Health and Toxicology, School of Public Health, State University of New York at Albany, Albany, NY 12201-0509


The unsafe conditions and practices that may occur in the chemical laboratory related to operation of microwave systems are evaluated. A detailed discussion of relevant equipment standards, safety-code requirements, and general safety guidelines relating to laboratory microwave systems is presented.Laboratory safety considerations are extensively discussed including chemical hazards that have been identified in the literature. Case studies are presented for specific types of accidents that have previously occurred to assist in preventing repetition of similar incidents. A mechanism for periodic updating of safety information, including the capability for microwave system users to input their own contributions and for others to access this information, is provided.

Abstract 1
Introduction 4
Safety Standards and Regulations 5
Microwave Exposure 5
Table 16.1 Microwave Energy Depth of Penetration for Human Tissues (13) page 482 6
Figure 16.1 Exposure Limits for Microwave Radiation from ANSI, ACGIH, and IRPA Standards (16) 6
Table 16.2 Microwave Energy in Comparison with other Electromagnetic Energy7
Table 16.3 Chemical Bonds with Related Energies7
The Function of Microwave Digestion Systems 9
Codes and standards Relevant to Microwave Digestion Systems10
Figure 16.2 Parameters for Distinguishing between High- and- low-Pressure Reactions (reproduced from ref (19))11
Pressure-Vessel Design and Safety Codes 14
Pressure Relief Devices 15
Figure 16.3 Microwave Vessel with External Pressure Relief Adjustable Valve External to the Vessel and System 16
Figure 16.4 Microwave Vessel with Non-reclosing Pressure-Relief Mechanism 17
Figure 16.5 Diagram of MDR Vessel in Normal and Venting Modes 18
Figure 16.6 The ultraCLAVETM Microwave Autoclave for High -Pressure Chemical Reactions19
Safe usage of Laboratory Microwave Instrumentation 20
General Safety Issues20
Figure 16.7 Vessel with Internal Physical Wear21
Figure 16.8 Vessel with External Physical Wear21
Good Laboratory Practices and Common Sense 22
Open vessel microwave safety22
Muffle furnace safety23
Intermixing Equipment of Different Manufacturers 24
Modification of Microwave Equipment 25
Chemical/Acid Safety26
Sparking and Metallic Samples27
Use of Flammable Solvents or Production of Flammables 27
Scale-up of Dissolution Procedures 29
Labware for the Microwave Environment 30
Table 16.4 Thermal and Microwave Characteristics of Laboratory Container Materials 31
Microwave Safety Illustrative Case Histories 31
Equipment Failure 32
Case #1 - Chemical Attack of a Safety Interlock 32
Case #2 - Muffle Furnace Meltdown 32
Highly Reactive Samples and Acid Mixtures 33
Case #3 - Hypergolic Mixture33
Case #4 - Ammonium Perchlorate34
Case #5 - Reactive Sample Component34
Case #6 - Perchloric Acid and Easily Oxidizable Organic Matter35
Figure 16.9 Exothermic Run-Away Reaction During the Digestion of 0.25 g SRM 1515 Apple Leaves with 10 g Nitric and 0.5 g Hydrofluoric acids 36
Case #7 - Hydrazine Hydrolysis 36
Case #8 - Large Organic Sample in a Closed Vessel by EPA Method 3015 37
Microwave Heating of Exposed Solid Sample Material 37
Case #9 - Microwave Absorbing Viscous Matrix 37
Improper Assembly/Usage of Vessels 38
Case #10 - Exposure of Flammable Solvents to a Microwave System 38
Case #11 - Inappropriate Equipment Use 38
Chemically Dissimilar, Unknown Samples 39
Case #12 - Digestion of Unknown Samples 39
Figure 16.10 Picture of laboratory after vessel explosion39
High-Boiling-Point Acids/Reagents 40
Case #13 - Melting of Polymer Vessels by Sulfuric Acid 40
Figure 16.11 Vessel Damage Contained Within a Multi-mode Microwave Cavity 41
Case #14 - Melted Vessel 41
Vessel Failures: In-depth Case Studies 41
Case #15 - Digestion of Lyophilized Human Placental Tissues 42
Table 16.5. Heating Program for 500 mg Placental Tissue and 10 mL Nitric Acid 42
Figure 16.12 Result of a Catastrophic Failure of a Lined Digestion Vessel 43
Table 16.6 Heating and Pressure Program for 200 mg Placental Tissue and 10 mL Nitric Acid 43
Figure 16.13 Lined Digestion Vessel Temperature and Pressure Profiles44
Case #16 - Digestion of a Fuel Oil44
Table 16.7 Heating Program for 500 mg Fuel Oil and 12 mL Nitric Acid 45
Figure 16.14 Catastrophic Failure of a Vessel Due to an Uncontrolled Exothermic Reaction 46
Design and Installation of a Safety Shield 46
Figure 16.15 Microwave Door Safety Shield 46
Outcome 46
Case #17 - Non-intuitive Nature of Microwave Interactions47
Conclusion 49
Appendix A: Hazard Monitors For The Detection Of Microwave Leakage51
Appendix B: Safety Terminology 52
Appendix C: General Safety References 55
Acknowledgments 56
References 57


The majority of unsafe conditions or practices that can arise during use of laboratory microwave systems are avoidable. As described throughout this text, many diverse chemical procedures are now performed in either atmospheric-pressureor closed pressurized-vessel microwave systems. Microwave techniques introduce unique safety considerations that are not encountered by the analyst in other non-microwave methods. Differences in operating conditions between traditional laboratory practices and microwave-implemented methods should be examined before applying microwave energy to heat reagents or samples.

Accepted procedures and good laboratory practices pertaining to microwaves are scattered throughout the literature, as are recommended safety suggestions for using reagents to decompose samples, synthesize compounds, and extract analytes. Complete coverage of this literature and the range of possible sample types is beyond the scope of this chapter; however, general guidelines and some specific examples of unsafe conditions previously reported are provided. Microwave-enhanced chemistry is not exempt from traditional safety considerations; sources on chemical laboratory safety should be consulted for documentation of explosive mixtures, toxic chemical behavior, and reagent-handling precautions. Examples of literature sources that can be consulted are; for specific mineral acids decomposition reactions (1-4) and Chapter 2 of this book, for flash points, autoignition points and flammability hazards of solvents used in synthesis, and extraction (5, 6). These hazards are temperature-dependent and specific conditions are documented to prevent these situations. Lists of incompatible chemical combinations are available (7-9), and general chemical and laboratory hazard lists have been compiled to assist in safety evaluations (10-12).

Specific standards that apply to microwave laboratory equipment are evaluated extensively as they relate to the apparatuses used in microwave enhanced chemistry. Previous instances of serious, extreme, and explosive reactions that have occurred using laboratory microwave equipment are included to prevent repetition of these known hazards. Other safety concerns unique to microwave systems, such as the direct effects of microwave energy and special design and performance characteristics are presented to prevent inappropriate equipment configurations and usage.

Microwave interactions mechanisms producing heat were described in detail in Chapter 1. Microwave energy is absorbed not only by polar solutions (e.g.; mineral acids, organic solvents, reactants, and aqueous mixtures) producing heat and accelerating chemical reactions, it is also absorbed by some sample molecules, container materials, and surfaces of an apparatus that may not be intended to heat during a reaction. Microwave energy at 2,450 MHz may also be absorbed by mammalian tissue. All laboratory microwave equipment is designed to shield the analyst and prevent such exposure.

Safety Standards and Regulations

Microwave Exposure
There are standards, limits, and ranges of tolerance established for microwave radiation exposure in most of the industrial world. The United States, Russian Republics, Germany, Belgium, Denmark, France, Italy, United Kingdom, Poland, the former Czechoslovakia, Canada, Australia, Sweden, EEC, military and governmental organizations in these countries as well as international organizations have all established safety standards (13, 14). An underlying reason for the large number of exposure standards is the manner in which they are defined, e.g., by electromagnetic energy frequency, duration of exposure, body mass, and time/periodicity of exposure.

Studies into the biological effects of microwave radiation exposure have been extensively detailed (~1,000 references) in several reviews dealing with scientific, industrial, and medical applications (13-15). Overall, the effects on human tissue are thermal in nature, and relate to overheating of exposed tissue. The underlying protective principle of several standards is derived from data on the amount of energy necessary to raise human skin/tissue temperatures to biologically significant levels. Exposure to energy such as sunlight is basically a surface phenomenon; however, microwave energy penetrates the skin into subcutaneous tissue and therefore also raises the temperature level of tissue and blood (13-15).

Table 16.1 provides an indication of microwave half-power penetration depth in a dielectric material such as tissue and the energy associated with particular frequencies. Energy variations between frequencies is the major reason why there is not a single standard for exposure to microwave energy (different frequencies of microwave energy penetrate to different depths and result in different amounts of energy being absorbed).

Table 16.1 Microwave Energy Depth of Penetration for Human Tissues (13) page 482

Frequency (GHz) Penetration Depth (cm) Energy (µJ cm-1)
0.915 3.03 17.3
2.450 2.05 20.6
3.0 1.97 20.9
30.0 0.078 143.3
100.0 0.032 376.4
300.0 0.023 579.1

©"Laboratory Microwave Safety", Kingston, H.M.; Walter, P.J.;
Engelhart, W.G.; and Parsons, P.J., Chapter 16 In Microwave-
Enhanced Chemistry: Fundamentals, Sample Preparation, and
Kingston, H.M. and S.J. Haswell, Eds.; American
Chemical Society: Washington, D.C., 1997: 697-745.

A diagram (Figure 16.1) has been prepared to illustrate a single unified set of exposure criteria based on the American National Standards Institute (ANSI), American Conference of Governmental Industrial Hygienists (ACGIH), and the International Radiation Protection Association (IRPA) standards (16). As can be seen, the standard is frequency/wavelength-specific, including the three most commonly used laboratory (and commercial) microwave frequencies of 2450, 915 and 27 MHz.

Figure 16.1 Exposure Limits for Microwave Radiation from ANSI, ACGIH, and IRPA Standards (16)

Like ionizing-radiation regulations, the standards for microwave exposure have developed over a period of time with a series of criteria, each taking into account additional limiting factors. A comprehensive history of the exposure limits for scientific, industrial, and medical applications was compiled in 1992 (13) and provides insight into development of standards in many countries.

The potential of microwave energy to cause ionizing radiation effects can be evaluated by examining its energy in comparison with other forms of electromagnetic radiation and common chemical bond energies. Compare a list of nominal energies in electron volts (eV) for the major forms of electromagnetic energy, Table 16.2, and selected common organic bond energies in Table 16.3 It is apparent that microwave radiation does not have sufficient energy to be classified as ionizing radiation. Microwave energy is two orders of magnitude below the energy necessary to disrupt bonds of common organic molecules. This is not to say that there are not other biological effects, or significant interactions of electromagnetic radiation that have been and that are still under investigation, but it does dispel the notion of classical bond ionization (13).

Table 16.2 Microwave Energy In Comparison with Other Electromagnetic Energy

Radiation Type Typical Frequency (MHz) Quantum Energy (eV)
Gamma Ray 3.0 x 1014 1.24 x 106
X-Ray 3.0 x 1013 1.24 x 105
Ultraviolet 1.0 x 109 4.1
Visible 6.0 x 108 2.5
Infrared 3.0 x 106 0.012
Microwave 2450 0.0016
Radio 1 4 x 10-9

©"Laboratory Microwave Safety", Kingston, H.M.; Walter, P.J.;
Engelhart, W.G.; and Parsons, P.J., Chapter 16 In Microwave-
Enhanced Chemistry: Fundamentals, Sample Preparation, and
Kingston, H.M. and S.J. Haswell, Eds.; American
Chemical Society: Washington, D.C., 1997: 697-745.

Table 16.3 Chemical Bonds with Related Energies

Chemical Bond Type Chemical Bond Energy (eV)
H-OH 5.2
H-CH3 4.5
H-NHCH3 4.0
H3C-CH3 3.8
PhCH2-COOH 2.4
Hydrogen bond (water) 0.21

©"Laboratory Microwave Safety", Kingston, H.M.; Walter, P.J.;
Engelhart, W.G.; and Parsons, P.J., Chapter 16 In Microwave-
Enhanced Chemistry: Fundamentals, Sample Preparation, and
Kingston, H.M. and S.J. Haswell, Eds.; American
Chemical Society: Washington, D.C., 1997: 697-745.

Currently, in the United States, microwave energy exposure from laboratory and domestic equipment at 2450 MHz is limited to 5 mW cm-2 at a distance of 5 cm from any surface of a product, or from an insulated wire inserted through any hole into an energy-containing space. These performance standards are incorporated in the Radiation Control for Health and Safety Act, a Federal Law enacted in 1968. These regulations were promulgated in 1970, became effective in 1971, confirmed in 1974, and are administered by the Center for Devices and Radiological Health (CDRH) of the US Food and Drug Administration (FDA). The regulations are contained in Title 21 of the Code of Federal Regulations (CFR), Part 1030.10. While there is no microwave leakage limit for other microwave heating products, such as laboratory and scientific units, manufacturers of these products are subject to other FDA regulations (21 CFR 1002-1004) such as: reports of design and quality control, including radiation safety measures; reports of accidental radiation exposures to users, service, or production personnel; and recall of any product that is found defective (i.e., presents a risk of radiation injury to any person). Such reports should be addressed to FDA (FDA, Center for Devices and Radiological Health, 5600 Fishers Lane (HFZ-312, Rockville, MD 20857).

Manufacturers subject to the FDA regulations include original manufacturers, importers, and persons who re-manufacture products for distribution to others. Re-manufacturing includes adapting a product for a new intended use, such as converting household cooking ovens for laboratory use, and reselling them, but it does not include user modification of a product once purchased. Twenty-nine U.S. states have their own regulations and some have adopted the latest ANSI guideline of 5 mW cm-2 maximum exposure at a frequency of 2450 MHz. Reviewers of these exposure standards seem to be satisfied that the ANSI C95 committee recommendations on standards are adequate and consistent with current understanding of the biological effects of microwave energy fields (13).

Additionally, in the United States, scientific, industrial, and medical products are covered under regulations by the Occupational Safety and Health Administration (OSHA). They require that the maximum exposure to radiofrequency (RF) energy for an operator in a safe work place is <10 mW cm-2 averaged over a 6 minute period (established in 1970 in 29 CFR 1910.97). OSHA regulations also require that, if microwave equipment is modified or the integrity of a safety device is violated, the product must be demonstrated to be safe by measuring the microwave radiation exposure potential. Many references to other international standards are available and should be consulted if your geographical location places the use of microwave under other jurisdictions (13).

Most laboratory microwave equipment surpasses (lower than) the protective requirements in emission standards. As long as damage, wear, or misuse have not lessened the effectiveness of the instrument, all the exposure limits of various national/international standards are met or exceeded.

The Function of Microwave Digestion Systems

Microwave digestion systems are not analytical instruments. Functionally, they are chemical reaction systems for acid decomposition of samples, producing solutions suitable for introduction into common analytical instrumentation.

Most chemical reaction systems are built for conducting a specific, well defined, reaction, or for the study of a group of reactions, such as catalytic hydrogenations. Reaction conditions and operating parameters are specified in detail so that appropriate engineering decisions can be made regarding system design and safety.

Microwave digestion systems differ from other types of chemical reaction systems in two important ways:

1. Microwave reaction and digestion systems are general-purpose systems. Reactants and reaction conditions are not specified and are unknown to the manufacturer in many cases.

2. Heat transfer in conventional chemical-reaction systems with external heating jackets is indirect via conduction and convection. The temperature of reactants rises slowly and internal cooling coils can be used to remove heat from exothermic reactions and to moderate reaction rates. Heat transfer in microwave digestion systems is via direct absorption of microwave energy by reactants inside the pressure vessel. Energy transfer is instantaneous and the temperature of reactants rises rapidly. Threshold activation temperatures for exothermic reactions are attained more quickly in microwave-heated systems. A temperature control device may stop further microwave heating when a setpoint value is reached, but there is no effective means of cooling and removing heat from exothermic reactions inside microwave digestion vessels.

Microwave equipment specified by manufacturer's and the literature rarely list pressures in the International System of Units (SI) units of Pascals (Pa). Instead pressures are commonly listed in atm, bar, or psi. The following pressure unit conversions may be helpful (5):

1 atm = 1.01325 x 105 Pa 1 atm = 1.01325 bar
1 bar = 1 x 105 Pa 1 atm = 14.69595 psi
1 psi = 6894.76 Pa

Pressures reported throughout this chapter are in the units specified by the equipment, standard, or case study.

Codes and Standards Relevant to Microwave Digestion Systems

A number of existing safety codes and standards are relevant to microwave digestion systems and vessels (17-19). The National Fire Protection Association (NFPA) has issued a standard: NFPA 45 - Fire Protection for Laboratories Using Chemicals, containing criteria for classifying laboratory hazards. Equipment design, construction, sizing and safety measures required to deal with each class of hazard are defined in this standard.

If any of the five conditions defined in Section 2.3 "Laboratory Work Area and Laboratory Unit Explosion Hazard Classification" exist, the laboratory is considered to contain an explosion hazard.For instance, laboratories operating closed-vessel microwave digestion systems may be considered to have an explosion hazard because two of the five classification criteria may be present: exothermic oxidation reactions and high-pressure reactions (19).

A large percentage of the acid digestion procedures performed in microwave systems involve the decomposition of organic compounds/matrices having endothermic heats of formation with oxidizing acids (the precise definition of an exothermic oxidation reaction) (20). Guidelines for distinguishing between high- and low-pressure reactions are defined in NFPA 45 based upon a reaction vessel's operating pressure and volume. These two parameters are plotted as a curve and reproduced here in Figure 16.2. Paragraph C-4.5.2 of NFPA 45 states: "Reactions that produce pressures above the curve in Figure C-4.5 should be classified as high pressure reactions". According to this criterion, a single microwave digestion vessel with a capacity of 0.1 L, rated for operation above 400 psi (27 atm) would fall in the high-pressure category.

Figure 16.2 Parameters for Distinguishing Between High- and Low-Pressure Reactions (reproduced from ref (19))

Pressure conditions generated during microwave acid digestion are the result of two factors; (1) microwave heating, which raises acid temperature and vapor pressure, and (2) accumulation of gaseous decomposition products (CO2, NOx, SiF4 ) of the reaction inside the vessel. The general reaction mechanisms responsible for formation of non-condensable gaseous decomposition products during microwave acid digestion are:

Increasing sample weight produces more gaseous decomposition products within the fixed volume of the vessel, resulting in higher pressure conditions. An understanding of these reaction mechanisms is important for selection of microwave digestion vessels with appropriate pressure ratings for the type and amount of sample being decomposed, as the following example illustrates.

Pressure Calculation for Microwave Digestion of Glucose

Vessel: 100 mL volume, 10 mL of nitric acid, 90 mL vapor phase, and 30 atm capacity
Liquid and vapor temperatures of 180°C and 150°C, respectively.
Neglecting other chemical reactions, vapor pressure of water, and assuming ideal gas laws

Vapor pressure of nitric acid at 150°C is 20.1 atm (21)
Residual pressure capacity for digestion products is 9.9 atm

Yields 0.63 L of gas

(Xg Glucose/180gmol-1)(14molCO2 and NO)(22.4 Lmol-1) = 0.63L

0.36 g of Glucose will cause this vessel to vent.

This calculation experimentally corresponds well with Figure 3.27 (in chapter 3) in which 0.306 g of motor oil produced a pressure of ~24 atm.

According to NFPA 45 hazard analysis criteria, laboratories operating microwave digestion systems may be considered to contain an explosion hazard. An explosion is defined in NFPA 45 as follows: "(1) a violent bursting, as of a pressurized vessel or (2) an extremely rapid chemical reaction with the associated production of noise, heat, and violent expansion of gases". Container failure is also defined in NFPA 45; "When a container is pressurized beyond its burst strength, it may violently tear asunder (explode)."

NFPA 45 contains three recommendations for protection against explosion hazards arising from reactions conducted above atmospheric pressures that are relevant to microwave digestion systems. The first of these recommendations is; "High-pressure experimental reactions should be conducted behind a substantial fixed barricade that is capable of withstanding the expected lateral forces."

In the case of microwave digestion systems, the cavity, door, and structural frame of the system must serve as the primary protective barricade for operators in the case of vessel explosions. The force exerted upon these system elements from a microwave digestion vessel venting or failing can be calculated and used by engineers to design the system to withstand such an event. The volume of the microwave cavity, digestion vessel volume, relief-device venting pressure, and surface area of the structural component that the resulting force acts upon are the principal calculation factors. An example of the force exerted upon the door of a domestic microwave oven resulting from the venting of a single 30 atm (440 psi) vessel follows.

Calculation of Door Excess Pressure on Vessel Venting

A single 100 mL vessel (at 30 atm) vents into a microwave cavity
Microwave Cavity Volume: 30 L
Surface Area of Microwave Door: 900 cm2
Vessel Volume: 100 mL
Relief Device Venting Pressure: 30 atm = 30 kg cm-2
(30kgcm-2)(0.1 L) = (xkgcm-2)(30L)

Yields a force of 0.1 kg cm-2 onto the walls and door of the cavity
Acting upon a cavity door surface area of 900 cm2

(0.1kgcm-2)(900cm2) = 90kg

The force of 90 kg is exerted on the door!
(refer to case # 12 below for the result of a vessel bursting at 200 psi)

The manufacturers of domestic microwave ovens have not designed the door of their products to be capable of withstanding this magnitude of force. The force generated by the venting or failure of a microwave digestion vessel is capable of removing the door of such a system and turning it into a secondary missile. Case study #12, later in this chapter, describes such an event when a domestic microwave door was used by a laboratory microwave company.

The functions of explosion-resistant shields and barriers defined in NFPA 45 are; "(a) withstand the effects of an explosion, (b) vent overpressures, injurious substances, flames, and heat to a safe location, (c) contain missiles and fragments, and (d) prevent the formation of secondary missiles caused by failure of hood or shield components" (19).

For a microwave digestion system to be an effective protective barrier for laboratory personnel it must be designed to withstand the force resulting from venting or failure of one or more vessels used inside. Systems that cannot confine venting or failing vessels and have cavity doors that can become detached, secondary missiles do not meet the first NFPA 45 recommendation for protective measures against explosion hazards.

The second recommendation in NFPA 45 for explosion protection is; "Reaction vessels should be built of suitable materials of construction and should have an adequate safety factor." A third recommendation states; "All reaction vessels should be provided with a pressure relief, valve, or rupture disc" (19).

Pressure-Vessel Design and Safety Codes

At this time, no design or safety code has been specifically developed for microwave-transparent vessels heated inside multi-mode or single-mode microwave cavities. In the absence of such a code, manufacturers are responsible for establishing prudent design practices for their products.

The American Society of Mechanical Engineers (ASME) has developed a comprehensive body of American National Standards for industrial process equipment. ASME Boiler and Pressure Vessel Code Section VIII - Division 1, defines engineering principles, design calculations, and safety factors for metal pressure vessels. While the ASME Code does not specifically encompass microwave-transparent plastic pressure vessels, some of the design and safety principles it contains represent prudent engineering practices, and are applicable to such vessels.

The ASME Code requires that all pressure vessels, irrespective of size or operating pressure, be equipped with a pressure relief device that prevents pressure from rising more than 10% above the maximum allowable working pressure (MAWP) of the vessel. Two basic types of pressure relief devices are recognized in the code; reclosing and non-reclosing type safety devices. By ASME Code definition "A pressure relief valve is a pressure relief device designed to reclose and prevent further flow of fluid after normal conditions have been restored. A non-reclosing pressure relief device is a pressure relief device designed to remain open after operation." Rupture disks are non-reclosing type pressure relief devices. Both types of pressure relief devices are employed on microwave digestion vessels (22-27).

Pressure Relief Devices
Pressure-relief setting and venting capacity are the critical performance parameters of a pressure-relief device. Formulas and test procedures for calculating and validating these performance parameters are explicitly defined in the ASME Code. The requirements stipulate that, when a vessel is equipped with a single pressure-relief device, it must be set to allow operation up to the vessel's maximum allowable working pressure.

The first microwave digestion vessels were equipped with external valves to protect them from over-pressurization. Figure 16.3 demonstrates the design of this vessel, the pressure-relief valve, and the measurement system. The pressure tube exits the microwave cavity through a wavelength attenuator cutoff to prevent loss of microwave radiation. The valve relief pressure can be set to correspond to the vessel pressure range. Pressure inside the vessel is transmitted to both the external spring-loaded relief valve for safety and the transducer for measurement and recording (28). In the case of over pressurization, the vessel permits a preset release of pressure above the valve limit and reseals, protecting the integrity of the vessel. The gas is expelled external to the unit; however, the content of the vessel may, or may not, be compromised depending on the amount of escaping gas and whether any liquid or analyte vapor is expelled with the gas. This type of mechanism is still used for prototype and specialty vessels, but has been largely replaced with integrated vessel relief devices.

Figure 16.3 Microwave Vessel with External Pressure Relief Adjustable Valve External to the Vessel and System.

A microwave digestion vessel equipped with a non-reclosing type pressure relief device is depicted in Figure 16.4. In this design, the vessel is protected from overpressurization by a fluoropolymer rupture membrane (22). The membrane functions as a rupture disk that bursts to prevent vessel overpressurization and failure. Once the membrane has burst, the vessel depressurizes to atmospheric pressure and remains open. The use of non-reclosing-type pressure-relief devices has certain disadvantages in microwave digestion systems. First, the vessel contents are forcibly expelled and lost for analysis. Second, the cavity exhaust system evacuates non-condensable vapors; however, certain vessel contents may condense or deposit on surrounding vessels and other components in the cavity. If the microwave run is not aborted, condensed liquids and deposited solid materials will absorb microwave energy, heat, and possibly damage other vessels and system components. Acid, or other absorbing reagents, remaining in the open vessel will also continue to heat and gradually outgas until the vessel is dry. When using rupture disks it is advisable to terminate a run on the basis of the nature of the reagents, e.g.; corrosiveness or flammability.

Figure 16.4 Microwave Vessel with Non-reclosing Pressure-Relief Mechanism

The membrane burst pressure (relief setting) is a function of material properties of the membrane, dimensions of the membrane's effective venting area, and geometry of the vent passage behind the membrane. Because the properties and thickness of rupture disk materials (both metals and plastics) vary from lot to lot, each lot of material must be tested to establish its burst pressure rating. Section UG-127, of the ASME Code details requirements for testing and rating rupture disks, accounts for lot-to-lot variability, stating; "Every rupture disk shall have a stamped bursting pressure within a manufacturing design range at a specified temperature, shall be marked with a lot number, and shall be guaranteed to burst within 5% of its stamped bursting pressure at coincident disk temperature".

A microwave digestion vessel equipped with a reclosing pressure relief device is depicted in Figure 16.5. In this vessel design, a torque wrench is used to apply sealing force equivalent to the maximum allowable working pressure. The applied torque compresses a spring in a thrust plate on top of the vessel's fluoropolymer cover. This spring loading arrangement functions as a reclosing type relief-valve mechanism. When pressure inside the vessel exceeds the sealing force applied by the torque wrench, the cover will lift to vent excess pressure and reclose, resealing at the maximum allowable working pressure initially set by the torque wrench. Non-condensable vapors creating the overpressure condition, (e.g., CO2, NOx, etc.), are released into the cavity and evacuated by the exhaust system. This type of mechanism frequently reseals after venting without compromising the sample integrity. This reclosing-type relief-valve design was developed and patented specifically for microwave vessels (29). The pressure relief device setting is mathematically calculated to be equivalent to the MAWP and is applied by the use of a calibrated torque wrench. This design provides rapid and minimal torque strain of the vessel during overpressurization events.

Figure 16.5 Diagram of MDR Vessel in Normal and Venting Modes

The venting capacity of relief devices on microwave digestion vessels is especially important. The instantaneous heating of reactants and rapid attainment of threshold temperatures for activation of exothermic reactions precludes cooling and heat removal for moderating reactions. Accordingly, the rate of pressure rise from acid vapor and accumulation of decomposition gases is significantly faster in microwave systems than in conventional acid digestion bombs. For proper vessel overpressurization protection, the rate of vapor and gas removal must be equal to, or greater than, the rate of vapor and gas generation (30). Section UG-131 of the ASME Code provides theoretical formulae to calculate the venting capacity of non-reclosing relief devices for various media. A safety factor coefficient of 0.62 is then applied to the calculated value to ensure that the device is sized conservatively.

The majority of laboratory microwave digestion systems in use today have multi-mode cavities with doors for introduction of microwave-transparent plastic vessels. The vessels contain pressure and the microwave system functions as a heating and fume evacuation device and protective barrier for laboratory personnel. The extent to which existing safety codes and standards apply to these systems has been documented to assist the chemist in their safe and reliable use.

Recently, a new type of microwave system (Figure 16.6) has been developed that combines microwave heating and high-pressure vessel technology with increased safety (31). This microwave-heated autoclave is designed for conducting chemical reactions at pressures and temperatures up to 200 bar (2,900 psi) and 350°C. The 4.2-L pressure vessel is constructed of forged stainless steel and has been hydrostatically proof tested to 800 bar (11,600 psi) according to the German Technische Überwachnung Verein (TUV). The vessel meets ASME Code design requirements, with a safety factor > 4x the maximum allowable working pressure. All safety relief devices employed on the vessel system comply with TUV and ASME requirements. The vessel interior is protected by a titanium nitride coating for acid and chemical resistance. Continuous (i.e. non-pulsed) microwave energy, at operator selectable settings of 0-1000 watts, is delivered into the vessel through a microwave-transparent port. Samples are not processed in closed vessels; rather the unit applies pressure to the samples in open vessels. This eliminates any unsafe conditions where vessels are at unknown temperatures and pressures.

Figure 16.6 The ultraCLAVE' Microwave Autoclave for High-Pressure Chemical Reactions

A unique class of microwave pressure digestion systems is flow through or stopped-flow reactors. In these systems, pressure is typically built-up within a Teflon tube (the reactor) contained in a microwave cavity. Without warning these tubes, fittings, and other parts are prone to blow-outs. Safe operation advises making the entire tubing inside the microwave out of a single piece (without connections) to minimize connector related failures, see chapter 6. Frequently these Teflon tubes are reinforced against stretching and bulging, that weaken the tubing, with braiding constructed form of polyetherimde. While ruptures still occur in these armored tubes, they are less frequent.

Recently, a new approach to flow through microwave digestion that utilizes an external gas pressurization outside the Teflon tubing was developed (32). The externally applied pressure is computer controlled to balanced with the pressure inside the digestion tubing. Balancing the pressure minimizes the stress on the Teflon tubing and effectively minimizes the primary safety hazard of flow-through microwave digestion systems.

Safe usage of Laboratory Microwave Instrumentation

General Safety Issues

Proper usage of laboratory microwave equipment is the responsibility of laboratory personnel. It is possible to render the safety devices of many instruments and vessels ineffective by carelessness or misuse. It is the responsibility of the analyst to follow good laboratory practices and the manufacturers' instructions when assembling, using, and maintaining the equipment. For example, by placing a microwave system inside a fume hood, where exhausted acid fumes may get circulated around the unit, the designed physical isolation of the electronics from the cavity is defeated. Accelerated corrosion of the electronics, including the safety interlock mechanisms and control circuits can result. Chemical vapors should always be transported away from the unit, or the cavity air swept away to an exhaust hose, fume extraction and/or neutralization system, or hood. Deterioration of the waveguide, door seals, or cavity walls can provide leak paths for the escape of microwave energy as well as degradation of the equipment.

The hazards associated with inappropriate use of microwave equipment cannot be entirely prevented by interlocking devices and other safeguards. However, the risk can be minimized if the analyst continually inspects the system to ensure that the equipment is maintained in safe working order. If any portion of the microwave unit such as a door seal or vessel casing becomes damaged by a catastrophic event such as an acid spill, prolonged wear, or impact, the safety of the equipment should be re-evaluated before it is returned to service.

In addition to compliance with microwave energy leakage standards, safety interlock devices are required to prevent accidental exposure on all commercial and consumer microwave equipment (33). These interlocks protect against initiating or continuing the emission of microwave energy into the cavity if the microwave system door is open or misaligned. Safety devices should never be removed or defeated on any microwave equipment, but especially on laboratory systems. Other components important for safe operation, such as wavelength attenuators in atmospheric-pressure systems,door seals, or waveguides (if the units cover is removed) should be inspected and tested for microwave leakage if corrosion is noted or if a vessel vents and reagents have prolonged contact with non-resistant parts of equipment.

Figures 16.7 and 16.8 show a vessel that has not failed, but is about to do so and should be permanently removed from use. While the crack and nitric acid degradation are obvious in this example, many other stresses, or chemical interactions, are less obvious, and diligence is required on the part of the analyst to maintain a safe working environment. The authors are asked frequently how long a vessel can be used. This depends on how much you are stressing it each time it is used, on how it has been maintained, and on how much residual pressure capability has been designed into that particular model by the manufacturer. Some manufacturer's have vessel designs that are very rugged and last for hundreds of uses; others have designs that are not as robust and do not last if taken to the upper limit of their specifications for more than a dozen uses. Frequently, degradation is obvious but occasionally there are no obvious warning signs of an impending vessel failure. In these cases, secondary safety systems outside the vessel such as doors, exhausts, cavity structure, preventive measurement devices, and active cutoff switches are required to handle catastrophic vessel failures.

Figure 16.7 Vessel with Internal Physical Wear

Figure 16.8 Vessel with External Physical Wear

Good Laboratory Practices and Common Sense

A seemingly innocuous event that has been observed by one of the authors on several occasions is the dropping of a vessel casing from a laboratory bench. In each instance, the analyst retrieved the casing, placed it back among the others on the bench, and failed to inspect it for damage such as cracks or chips. Polyetherimid (in most cases) vessel casements are brittle, owing to their high mechanical strength and therefore must be inspected for damage before every usage. This is a clear example of a responsibility that is requisite of the analyst using the equipment.

Another incident involved the deliberate removal of the mesh screen from the inside of a microwave door to allow the technician to see the reaction in all Teflon(tm) PFA vessels more clearly. The perforated metal screen functions as a microwave barrier that prevents passage of microwave radiation at 2450 MHz. Once removed, the plastic window is transparent to microwave radiation, which is transmitted into the laboratory. While this example is obvious to some, to others who do not understand the proper use of this component, it is not. Education and consultation are always prudent before using any scientific instrument, but especially laboratory microwave systems.

Open vessel microwave safety

The use of a single-mode microwave system operating at atmospheric pressure alleviates some safety concerns. However, this does not mean that you can take safety lightly when dealing with such systems. The presence of hot reagents, open to the lab, still needs great attention.

The use of a fume scrubber is a requirement for any laboratory that is performing microwave digestions/extractions (leaching) at close to the boiling point of the acids used. Not only does a fume scrubber stop aggressive fumes and/or volatile toxic species from entering the laboratory environment; but it also mitigates the effects of vigorous reactions that may occur upon reagent additions to hot solutions. With the microwave waveguide open to the environment this also minimizes the possibility of acid attack on the waveguide and possible feedback of microwave energy to the magnetron or microwave leakage to the environment. Careful cleaning of the scrubber transfer line is essential after every use, as gases can condense in the line and can cause problems in subsequent runs, especially if perchloric acid or other condensable reagents are used.

Problems with the waveguide and magnetron assemblies can also be caused if any artifacts drop down into the waveguide so as to disturb the microwave field pattern. Leakage of microwave radiation is a serious potential problem if a microwave-conductive material is introduced into the microwave vessel. Thus, under no circumstances should mercury thermometers, thermocouples, thermistors, or metal stirrers be inserted into the waveguide. These devices conduct microwave energy from the cavity on their surface and radiate it into the laboratory.

The analyst should make sure that all safety interlocks are always operational, and be very careful when removing a digestion vessel from the cavity, as the vessel itself can be as hot as the solution it contains.

Muffle furnace safety

Microwave muffle furnace systems are unique combinations of new and old technology. These systems are usually constructed of a strong microwave absorbing material with an insulator to create a small muffle furnace that can be quickly heated and cooled within a dedicated uncoated microwave oven cavity. The sample or matrix is not directly heated by microwave energy, but through convection and conduction heating from the muffle furnace. The microwave muffle furnace has not been fundamentally altered from its original concept described previously (34, 35).

The sample is fused or ashed in traditional quartz, porcelain, nickel and/or platinum crucibles that are shielded from microwave radiation by the strong microwave absorbing furnace. The safety considerations are usually similar to classical muffle furnace systems, with a few exceptions. While the furnace heats rapidly it also cools very rapidly, thereby minimizing the hazards of exposing the analyst to high temperature devices. Specific reaction temperature protocols can be developed for individual samples for reaction optimization and safety because of the rapid heating method.

Sulfuric acid is frequently used as an ashing aid especially in the pharmaceutical industry. When performed in a microwave muffle furnace the sulfuric acid fumes have no direct exit from the furnace so they attack and damage the muffle furnace. In these cases, the muffle furnace typically must be replaced in less than 1 year. A manufacturer designed a muffle furnace with a vacuum port from a quartz ceiling of the furnace that connects to acid neutralization system (36). As the sulfuric acid fumes are generated, they are evacuated outside the entire microwave system into a condenser where the sulfuric acid is neutralized in a collection flask. Additionally, the sulfuric acid muffle furnace was designed with a chemically resistant thermocouple measurement system.

Intermixing Equipment of Different Manufacturers

In General, the intermingling of different manufacturers' equipment is inadvisable. Under certain circumstances it may be possible if the analyst has an excellent understanding of the limitations of each piece of equipment and consults each manufacturer prior to use. The divergent pressure conditions produced in vessels with different thermal insulation characteristics was described in Chapter 3 and clearly indicates why using vessels of dissimilar design for the same digestion protocol can (and has) produced disastrous results (see case #17). It is interesting to note that the intention of the analyst was to be as safe as possible by minimizing the pressure developed inside the higher-pressure vessels. Paradoxically, the non-intuitive nature of microwave/thermal interactions caused an uncontrolled situation to occur that was catastrophic.

Modification of Microwave Equipment

It is sometimes necessary to modify microwave equipment to produce special research configurations that permit the direct addition of reagents or the measurement of temperature and pressure inside a microwave cavity. Modifications should be performed only by those trained in high-voltage circuitry and who understand the shielding requirements of microwave equipment. It is highly advisable to consult the manufacturer about proposed modifications, especially those involving the waveguide, mode stirrer, metrology devices, vessels, and electronics.

After any modification of microwave equipment, it is important to ensure that no path for radiation leakage from the microwave cavity has been created. Measurements to detect microwave radiation must always be performed in and around the modified portion of the equipment. A microwave survey meter with good sensitivity is the recommended method of evaluating such alterations for leakage (see Appendix A). The unit should be run at full power with no load. If an appliance-grade microwave oven is being evaluated, a small load (~100 mL of water) should be in the unit at the time of testing to prevent possible damage to the magnetron.

A common modification to microwave equipment is the attachment of a wavelength attenuator port to permit safe introduction of tubing, thermocouple wires, and/or fiberoptics into the microwave cavity. These devices have been used in many configurations. Two types of attenuators tested by the authors are rigid stainless steel tubing (28) and flexible, tin-plated copper braid (37). The common construction features of both attenuators are metallic tubing constructed of conducting metal with the smallest diameter possible to allow a tube, wire, or other device to pass through its' inside diameter. The effectiveness of small-diameter holes as a barrier to microwave energy is the basis of the perforated metallic screens located at air intakes, and grid on the inside of an otherwise transparent microwave door.

A cut-off device is effective only if the diameter is small and the length exceeds the 12.25 cm wavelength of microwave radiation at 2,450 MHz. Dimensions of the devices used by the authors were approximately 0.7 cm inside diameter and between 35 and 70 cm in length. However, many various dimensions of cut-off devices are possible. The attenuator must be grounded by making electrical contact with the microwave cavity wall around the hole and the unit grounding circuitry. Metallic devices passing through the attenuator must also be grounded at the microwave cavity wall to prevent them from acting as an antenna for transport of the electromagnetic energy out of the cavity, totally defeating the wavelength attenuator cutoff (28). Experiments in which thermocouple wires were passed through the attenuator without grounding were conducted and energy fields > 10 mW cm-2 were measured at the end of the attenuator. Six separate attenuator designs were tested and found to prevent microwave radiation from escaping (<0.01 mW cm-2, the limits of the detection equipment).

Chemical/Acid Safety

Acid temperatures that are achieved after 5-30 minutes of heating using traditional methods are attainable within seconds in a microwave cavity, greatly reducing reaction times. The coupling of electromagnetic energy with the molecular dipoles of a sample may aid the digestion, extraction, synthesis and further accelerate the specific reaction. With rapid heating, the evolution of reaction products may occur too quickly and in too large a quantity to be vented or contained. Thus, caution should be used not to speed up a reaction to such an extent that safety of the individual or structural limits of the equipment will be compromised.

A comprehensive review of all unsafe chemical reactions that may occur in microwave chemistry is beyond the scope of this chapter. These uncontrollable reactions are a function of the reagent or combination of reagents reacting with the sample. Reference books are available identifying classes of incompatible chemicals, toxicity of substances, and general laboratory safety guidelines (7, 8). Sources are also available that discuss the reactivity of the decomposition reagents, unsafe reagent combinations, and proper usage/handling precautions (2, 4, 7, 8, 38).

Many of these unsafe combinations of dissolution reagents and samples were addressed in Chapter 2. Many hazardous combinations involve the use of strong oxidizing agents or dehydrating agents in the decomposition of organic matrices.

Sparking and Metallic Samples

Many alloys interact with microwave energy and will heat sufficiently to melt plastic vessels; others accumulate large electrical discharge potentials. One case, involving a large alloy sample melting through a container, has been related to the authors. Several types of ferrous alloys were tested in which spark discharges were quite spectacular. Discrete pieces (> 1 mm) of metallic samples should be avoided, because electrical arcs may occur between individual sample pieces or between these alloys and the microwave cavity's metallic devices including the walls. The formation and intensity of electrical sparks depend on the composition of the alloy and other conditions such as electric field strength. Electrical arcs from inadequately grounded thermocouples constructed of 316 stainless steel have been found to be sufficiently energetic to puncture 1/16 in Teflon(tm) PFA (39, 40). It has also been noted that some concentrated solutions also exhibit sparking. Concentrated sodium hydroxide, copper nitrate, and nitric acid have all been observed to discharge electrically within the solution.

Use of Flammable Solvents or Production of Flammables

Another hazard that can be anticipated to occur as chemical applications for microwave equipment diversify, is the use of flammable solvents in electromagnetic environments. A documented instance of such a potential problem occurred when students heated diethyl ether in an open container (41). Luckily no ignition occurred in this instance. Because sparks are common in microwave systems, flammable or explosive substances mixed with oxygen from the air pose especially hazardous situations.

When vessels containing flammable solvents vent from over-pressurization, or vessel failure, the microwave should be equipped with a sensor to terminate power to the magnetron. Flammable vapors must be cleared from the cavity before restarting the microwave system. Most systems capable of microwave extraction, organic synthesis, or other applications based on flammable solvents are equipped with an organic solvent sensor mounted in the cavity or cavity exhaust system.

The mineral-acid decomposition of metal and alloy samples has a unique hazard that must be considered before traditional methods of hot mineral-acid decomposition are applied to the microwave environment. Metals below hydrogen on the electromotive series readily liberate hydrogen when dissolved in acid. If hydrogen is released from the sample into an open beaker or a vessel sealed in air, a potentially flammable or explosive mixture with oxygen may result. In traditional digestion procedures, an ignition source would not be present; however, in microwave equipment, because metallic particles can interact with the strong electromagnetic field, a spark may be generated from the sample, and ignition of the hydrogen may cause a fire or explosion. To prevent this potentially hazardous situation, closed digestion vessels should be sealed in inert-gas atmospheres to eliminate oxygen. In open-vessel microwave applications, purging the vessel and/or compartment with inert gases will prevent air from coming in contact with the sample. The limits of flammability of hydrogen in air diluted with an inert gas are presented by Lewis et al. (42). In open-vessel digestions, rapid removal of hydrogen may also prevent the formation of a hydrogen-oxygen mixture sufficient for ignition. Unfortunately, the amount of hydrogen in air that supports combustion ranges from 4 to 75% (43) and the flammability range of hydrogen in oxygen is 4-94% (42). The energy of activation necessary to ignite this mixture can be achieved by the weakest spark, and may also be catalytically ignited, making mixtures of hydrogen and oxygen anywhere in this range extremely hazardous.

The elimination of oxygen and air by purging the vessel and cavity with an inert gas such as argon or nitrogen is a prudent precaution id hydrogen generation is a potential hazard. The oxidant is the only component that can be elimination in such an instance.

Scale-up of Dissolution Procedures

Increasing the sample size can involve significant potential safety problems including solubility problems, the necessity for additional reagents, and the development of too much pressure. The safety and chemistry factors involved in the scaling-up of EPA 3052 digestion method were discussed in detail in Chapter 3 and serve as a guide of good general practice. The most important consideration, based on safety, is the production of gas during dissolution.

Digestion of organic matter generates significant quantities of gaseous decomposition products, primarily CO2 and NOx. Always begin acid digestion method development of an organic matrix with a small sample size, less than 0.25 g. Scale-up the sample size after evaluating the final pressure attained during each successive digestion and pressure limitations of the vessels employed. In general, each incremental scale-up step should only be a maximum of 0.2 g.

A simple example of this approach is the digestion of motor oil. A 0.25 g sample was digested with 10 mL nitric acid in a 100 mL vessel. The vessels were heated to 180°C and held for 9.5 minutes; the final pressure inside the vessels was ~21 atm. The experiment was reproduced with only nitric acid and the final pressure was ~6 atm. After subtracting the pressure produced by the nitric acid, 0.25 g oil produced approximately 15 atm (60 atm g-1 of motor oil). Based on this gas production factor, the approximate maximum sample size can be predicted.

Assume that the digestion vessel is capable of 65 atm. Subtracting a minimum of a 20% safety factor (20% of 65 atm = 13 atm) leaves a safe vessel capacity of 52 atm. Subtracting the pressure that will be produced by heating nitric acid leaves 46 atm of pressure. Dividing 46 atm by 60 atm g-1 oil results in a maximum safe digestion of approximately 0.75 g of motor oil. Owing to numerous assumptions in this calculation, this is meant for a first approximation. Further tests with sample sizes between 0.25 g and 0.75 g are necessary to determine the true safe upper limit of sample size for this particular apparatus.

Labware for the Microwave Environment

Microwave sample preparations, including digestion, extractions, organic synthesis, flow reactors, muffle furnaces, and other laboratory operations require many different types of sample vessels and apparatus. The suitability of a vessel material depends upon the specific conditions, use, and reagent for which it is intended. In choosing a container material for open-vessel work, the boiling point of the acid or solvent being used is the maximum temperature to which the material will be exposed. When closed vessel systems are involved, temperature measurement is necessary to prevent the liquid contents of the vessel from exceeding the upper temperature limit of the vessel material, which is normally the MAWP. At this temperature, a polymeric device will continue to function as intended without change for an indefinite period of time. Above this point, the material may degrade, deform, or melt so that it no longer functions appropriately. For example, a high-boiling solution of concentrated sulfuric acid will melt Teflon(tm), but is not a problem when used with quartz or borosilicate glass (see case #13).

After chemical and mechanical stability, the dielectric constant, and microwave energy absorption characteristics of the material are the important physical factors to consider. Many materials are compatible for use in a microwave field because they do not absorb radiation in this frequency range. Among the essentially microwave-transparent materials are all types of fluorocarbons (e.g., Teflon(tm) PFA and TFM), Quartz, and some glasses, which are all excellent as sample containers because of their exceptional chemical and thermal durability. Other common laboratory polymers, such as polyethylene and polypropylene, are also suitable for use in the microwave environment, but at low temperatures and under mild conditions.

When selecting containers for use in a microwave field, the composition of all parts needs to be considered (e.g., handles or screw-on caps, which tend to be made from different materials than the vessel itself). Problems can be avoided by selecting materials that do not absorb microwave radiation, or absorb very little. Listed in Table 16.4 are the dielectric constants and melting points of some common laboratory container materials. Fluorinated polymers are excellent materials for microwave containers because their low dielectric constants make them essentially transparent at 2,450 MHz. Labware fabricated from, or incorporating Bakelite(r), Lucite(r), and other thermoplastic resins may be acceptable for some uses, even though they absorb a small amount of energy.

Table 16.4 Thermal and Microwave Characteristics of Laboratory Container Materials

Microwave Safety Illustrative Case Histories

Microwave sample preparation and chemistry procedures impose a unique set of safety considerations in addition to those of good laboratory practice. This is evidenced by the reports of microwave-related incidents described below. Parties involved in some of these illustrative case histories have requested anonymity. In these cases, we have honored their requests and the source of the information is not cited.

The reports below are only brief descriptions of what happened. In some cases, more in-depth descriptions of these events can be found at the SamplePrep Web site (see Chapter 15). As other instances of equipment malfunctions, runaway chemical reactions, or improper use of microwave equipment are reported, they will be added to the Web site database. Readers are encouraged to contact us with any safety problems that they encounter so that we may quickly report them to the microwave community. The majority of these safety problems and potential hazards are not reported by manufacturers, go unreported by the analyst, or are unavailable. It is our hope that reporting of such problems will lead to their solutions and ultimately to their prevention.

Equipment Failure

Case #1 - Chemical Attack of a Safety Interlock

Commercial microwave appliances for cooking food are not designed to withstand chemical attack from corrosive substances. An incident has been documented in which all the safety interlocking devices were rendered inoperative as the result of chemical attack on the metal safety switches. The result was that when the microwave unit's door was opened, full power microwave energy emission continued for 1 minute directly exposing an operator. This is the first reported case of human injury from the use of a home microwave unit in the laboratory (case I8-138 reported to Microwave/Acoustic Products Section, FDA, 1986). These units have not been designed for use in a chemical environment and interactions of the instrumentation with chemicals must always be considered before equipment is purchased. The use of appliance grade microwave equipment in corrosive environments, especially acid vapors, should be avoided. This precaution is a specific recommendation in all U.S. EPA methods using microwave technology.

Case #2 - Muffle Furnace Meltdown

Microwave muffle furnaces are constructed of an insulating block containing a high temperature-resistant and high microwave absorbing material (such as silicon carbide) that fits inside the microwave cavity. The absorber block has one side that opens-up to allow the introduction or removal of samples. The absorbing block's open side must be re-inserted properly onto the block prior to starting microwave heating. Numerous instances have occurred in which the absorbing block's door and accompanying insulation was not replaced prior to microwave exposure. The heat from the absorbing block radiated in the direction of the microwave door initially heating and eventually melting the door's plastic components. It was not determined whether any microwave exposure to personnel occurred.

To approaches to this problem are available. One company uses a solid stainless steel cavity and door with insulation built into them. This unit does not have the potential of a physical meltdown (36). Another manufacturer's approach was the implementation of an IR sensor that audibly signals the operator if the muffle furnace is improperly installed and overheating is occurring (44) .

Highly Reactive Samples and Acid Mixtures

Microwave acid digestion accelerates the rate of sample decomposition reactions, but it does not alter the fundamental chemistry involved. Microwave heating and microwave acid digestion of certain chemical compounds, mixtures, and types of samples constitutes unreasonable, hazardous misuse of laboratory microwave systems. Explosives, propellants, hypergolic chemical mixtures, and pyrophoric chemicals should never be heated inside a laboratory microwave system. Combinations of reagents that are explosive, or so highly reactive as to be uncontrollable, fall into this category. Several case histories are presented here to underscore this important point.

Case #3 - Hypergolic Mixture

A U.S. government laboratory, seeking to analyze the metal content of a candidate drug, attempted to perform acid microwave digestion on the sample dissolved in 3-4 mL of a propylene glycol transdermal patch delivery solution. The sample was dispensed into two vessels and 10 mL of nitric acid was added. A 10 minute pre-digestion step was performed before the vessels were sealed. Both (200 psi) vessels exploded immediately after placement inside the microwave system, without any microwave heating! The reagent and this highly reactive sample are not controllable and a good example of a classical chemical hazard.
Case #4 - Ammonium Perchlorate

A technician, without extensive chemistry training, called the application staff of a microwave digestion system manufacturer with two questions concerning digestion of samples. Question 1: How much (what %) ammonium perchlorate can be digested in a wastewater sample? (Some wastewater samples would contain > 20% v/v ammonium perchlorate.) Question 2: A "polymer" sample with the acronym GAP flashes on a hot plate at ~105°C; is it possible to digest the sample inside a microwave system?

Ammonium perchlorate is an explosive. GAP is an acronym for Gelled Ammonium Perchlorate, a colloidal form used as a solid rocket propellant. Fortunately, the technician was cautious enough to inquire and receive advice that prevented an otherwise inevitable explosion.

Special safety procedures have been developed for handling of explosives and propellants (8, 45-48). Discussions of what constitutes an explosion are also available (49). Certain finely divided metals are pyrophoric, (e.g., calcium and zirconium), and spent hydrogenation catalysts are problematic and hazardous because of adsorbed hydrogen that may be present (10).

Case #5 - Reactive Sample Component

An explosion was reported during the nitric acid extraction of a PTFE Teflon(tm) filter used in air sampling (50). Upon examination, it was noticed that, in addition to the filter, a plastic ring that held the PTFE filter made of polymethylpentene was decomposed and can react violently in the microwave at elevated temperatures. Subsequently, the laboratory cut the PTFE filter from the polymethylpentene ring and investigated pure PTFE filters to eliminate the problem.

Frequently, there is another sample component that has completely different reaction characteristics from those expected from the major matrix component. All sample components being placed in microwave vessels must be considered during method development.

Case #6 - Perchloric Acid and Easily Oxidizable Organic Matter

An analyst called a research laboratory and asked, if he put perchloric acid directly on coal, and heated it in a microwave oven, whether it would be a complete digestion. He was advised an uncontrolled explosion would likely occur due to the combination of easily oxidizable organic matter and hot concentrated perchloric acid. The analyst, frustrated by the ineffectiveness of other methods of digestion, called and reiterated the query and asked if an explosion was a certainty. This analyst was then directed to appropriate literature and assured of this potential and eventuality. Not wanting to abandon the idea, the analyst called another person in the same laboratory and reiterated the question and receiving the same answer. Fortunately, the analyst did not attempt this reaction and took the advice (51). Perchloric acid is a potentially hazardous reagent that requires a highly experienced analyst for its safe use. Because closed vessel microwave digestions can achieve relatively high reaction temperatures and the oxidizing power of nitric acid increases with increases in reaction temperature, nitric acid is usually sufficient to oxidize most materials, see Chapter 3 for details. Therefore rarely, if ever, is perchloric acid necessary for a microwave digestion. See Chapter 2 for general safe use rules.

There is a demonstrated potential for runaway or out-of-control reactions in a microwave. Where exothermic reactions supply enough energy in a very short time frame, they may raise the temperature above that being induced and controlled by the microwave system and the reaction may reach temperatures that are self-sustaining. These reactions are beyond the equipment's ability to control as there is only the cooling of the ambient air on the vessel walls, leaving no way to reduce unwanted thermal accumulation of exothermic reactions (28). In addition, perchloric acid has been documented to decompose autocatalytically in a microwave field at 245°C with no sample present (52).

Elevated temperatures increase the oxidizing power of acids such as perchloric and nitric. Figure 16.9 shows the temperature and pressure curve for the nitric acid/hydrofluoric acid digestion of apple leaves. Even after microwave emission is stopped the oxidation potential of the digest solution is so high that an exothermic reaction occurs. Similarly, if easily oxidizable organic matter is heated with an acid such as hot concentrated perchloric acid, it will eventually thermally run away and then chemically explode. Many in-depth treatises on perchloric acid have been written and should be consulted (1, 40), as well as Chapter 2 of this text. The authors cannot recommend the use of perchloric acid in a microwave system with the exception of only the most experienced analyst and under well known and tightly controlled conditions.

Figure 16.9 Exothermic Run-Away Reaction During the Digestion of 0.25 g SRM 1515 Apple Leaves with 10 g Nitric and 0.5 g Hydrofluoric acids.

Case #7 - Hydrazine Hydrolysis

A technician at a pharmaceutical company was experimenting with a microwave system to develop a method for hydrolyzing a glycoprotein sample for subsequent chromatographic analysis of the constituent sugar units. The technician selected pure hydrazine as the hydrolyzing medium. In less than 30 seconds after starting the microwave heating program, the cover of the Teflon(tm) PFA vessel blew off.

Hydrazinolysis is a relatively common non-microwave hydrolysis procedure performed on glycoproteins and carbohydrates. However, this procedure is typically conducted at a temperature of 0°C. Hydrazine is a liquid rocket propellant with a flash point of 52°C.

Case #8 - Large Organic Sample in a Closed Vessel by EPA Method 3015

A technician from a chemical plant lab reported the explosion of a 200 psi microwave digestion vessel to the applications staff of the manufacturer. The sample involved was stated to be "effluent wastewater" being digested according to U.S. EPA Method 3015, (e.g., 45 mL of water and 5 mL of nitric acid). A duplicate sample of the wastewater was submitted to the microwave system manufacturer for digestion to discern the problem. Upon analysis, it was determined that the "effluent wastewater" sample contained 55% triethanolamine! This concentration equated to ~25 grams of pure organic, which was roughly 50 times the vessel's organic sample size limitation.

This overpressurization of an organic sample would be expected if the liquid or miscible organic portion of the sample was thought of as ~25 g of solid sample. Method 3015 uses the same equipment and temperature profile as EPA method 3051, which restricts organic sample size to 0.25 g maximum.

Microwave Heating of Exposed Solid Sample Material

Case #9 - Microwave Absorbing Viscous Matrix

A U.S. government laboratory seeking to analyze peanut butter failed to place a sample in the bottom of the microwave digestion vessel. The sample was permitted to adhere to the side wall exposed in air, above the liquid level of nitric acid. During the microwave heating cycle, the peanut butter absorbed microwave energy, charred and burned into the Teflon(tm) PFA vessel wall (the melting point of Teflon(tm) PFA is ~300°C and heat softening occurs above 260°C). Pressurized nitric acid vapor forcibly vented through a small hole that formed in the region of vessel wall damaged by localized heating and charring of the sample.

While Teflon(tm) is one of the most chemically resistant polymers with excellent thermal properties, it can be damaged by localized overheating. This specific scenario can be avoided by placing the sample under the reagent solution and not permitting it to adhere to the wall of the vessel while open to the air.

Improper Assembly/Usage of Vessels

Case #10 - Exposure of Flammable Solvents to a Microwave System

A laboratory technician from a U.S. chemical company reported that a fire had occurred in a new microwave system during its first run. The microwave system had been purchased for this facility to duplicate a solvent extraction procedure previously developed at another U.S. company site. Through discussion, it was learned that the technician had not been trained in operation of the system and had not performed the extraction method before. Vessels containing a hexane/acetone mixture were assembled without rupture membranes installed. Upon microwave heating, solvent vapor escaped from the open rupture membrane port of the vessel and ignited inside the cavity. The resulting fire went unobserved until extensive damage had occurred to the system.

Solvent sensors are available for the cavities of laboratory microwave systems to detect the presence of flammable organic solvent(s). They can also be fitted to exhaust ducts of microwave systems. The detection of significant amounts of flammable solvents can be used to automatically stop microwave heating before the lower explosive limit (LEL) concentration in air can be reached. The system reported here was not equipped with a solvent sensor system.

Case #11 - Inappropriate Equipment Use

A laboratory technician called the manufacturer to report that a microwave digestion vessel had failed (melted) upon heating. Through discussion it was learned that the technician had heated the microwave-transparent vessel inside a convection oven, not a microwave system for which it was designed. The outer Ultem' polyetherimid vessel body was charred and destroyed, without the vessel contents heating. Polyetherimid has a melting point of (~200 °C) and is not meant to be used with convective or conductive heating.

Chemically Dissimilar, Unknown Samples

Case #12 - Digestion of Unknown Samples

An environmental lab experienced an explosion of a 200 psi microwave digestion vessel during microwave digestion of two dissimilar samples of unknown composition. The force generated by this vessel failure was sufficient to remove the door of the microwave system, turning it into a secondary missile, which landed approximately 15 feet across the laboratory, where it struck a lab bench, and fell to the floor. Figure 16.10 is a photograph taken of the laboratory immediately following the vessel explosion. Devices are described in Case #15 to modify microwave equipment with inadequate door latches and hinges, with a safety shield.

Figure 16.10 Picture of laboratory after vessel explosion

In this instance, the user had attempted to conduct a microwave digestion run with two samples. The first sample was 0.43g of an "oily waste" with 5 mL of nitric acid, and the second sample was 45 mL of a "soapy aqueous waste" and 5 mL of nitric acid. The precise chemical composition and content (% organic) of both samples was unknown.

The vessel containing the 0.43g of oily waste was connected to the pressure control system of the microwave unit. The explosion occurred in the vessel containing the 45 mL of soapy aqueous waste, and resulted in a rupture of the vessel bottom. A program consisting of four, 10 minute pressure controlled microwave heating steps, all at a 60% power setting (600 W unit) was run. The vessel failure occurred in step 4 with a pressure control setpoint of 180 psi, that had not yet been achieved by the monitor vessel.

The pressure generated from the decomposition of the oily waste sample in the control vessel, attained the pressure control setpoint and thus caused minimal microwave heating during the first three steps. Microwave heating during the fourth step was apparently sufficient to raise the temperature of the larger (45 mL) volume of the soapy aqueous waste beyond the threshold reaction initiation temperature for decomposition of the organic compound(s) in the sample. It is presumed that the sample contained a mass of organics > 0.5 g and rapid evolution of decomposition gases exacerbated the vessel failure.

Organic molecules with different structures are oxidized with varying efficiencies by nitric acid. Thus, samples containing different organic compounds will decompose at different rates, and to different pressures/temperatures, see Chapters 2 and 3. Accordingly, the sample in a pressure/temperature control vessel must be close in character to the sample in all other vessels in a batch/run.

High-Boiling-Point Acids/Reagents

Case #13 - Melting of Polymer Vessels by Sulfuric Acid

Microwave heating of high-boiling-point acids and reagents, (temperatures > 300°C), such as concentrated sulfuric or phosphoric acids is potentially problematic. These acids are strong couplers of microwave energy and their temperatures will rapidly rise to over 300°C. The boiling points of these acids exceed the melting-point temperatures of polymer materials of microwave vessels and labware. Melting of vessels and release of their contents with continued microwave heating can result in fires and irreparable damage to the microwave system. The problem is most acute in cases where the door to the microwave cavity is also constructed of plastic material. Figure 16.11 shows the damage incurred to vessels from a long duration; (> 120 minutes) microwave heating test with concentrated H2SO4. In this instance,the solid steel door construction safely contained the problem inside the microwave cavity. Although the rotor and vessels were damaged, the unit only suffered surface cosmetic damage. The microwave system utilized a sensor to detect overheating, when this sensor detected a problem the system was shutdown the experiment. Inappropriate or erroneous programmed conditions can lead to microwave system damage or failures.

Figure 16.11 Vessel Damage Contained Within a Multi-mode Microwave Cavity

Case #14 - Melted Vessel

A technician at a U.S. chemical company performed an acid digestion procedure on a refractory ceramic utilizing a 1:1 H2SO4:H3 PO4 mixture in Teflon(tm)-PFA vessels. After heating for 20 minutes, the technician removed a sample vessel. The heat softened, melted the bottom of the vessel, and fell apart into the technician's free hand. The technician was not wearing protective gloves and molten plastic caused second-degree burns.

Temperature safeguard mechanisms do exist for heating high-boiling-point acids and reagents inside laboratory microwave systems, and are currently available on some systems. The external surface temperature of vessels can be monitored using an infrared sensing system (53). An upper temperature limit setpoint is programmed to regulate microwave power delivered to the acids/samples and prevent heating beyond the service temperature of the polymeric vessel components.

Vessel Failures: In-depth Case Studies

Case studies #15 and 16 were reported by the New York State Department of Health's Trace Elements Laboratory and Case study #17 was reported by a fortune 500 company. They exemplify how improvements in microwave instrument design can come from unfortunate incidents. They also demonstrate why study of the nonintuitive nature of microwave heating interactions is required.

Case #15 - Digestion of Lyophilized Human Placental Tissues

The analysis of placental tissue for trace metals presents several problems. Placental tissue can be separated into the three anatomically distinguishable components: membrane, umbilical cord, and placental body. Sample digestion is required prior to instrumental analysis. Previous attempts to achieve sufficiently complete digestion of organic matter using a standard program for biological materials were unsuccessful. The efficiency of the digestion process was estimated by observing background absorbance signals from placental samples. In this case, determination of the elements lead and cadmium were carried out using graphite furnace atomic absorption spectrometry with Zeeman background correction. All tissues were lyophilized to constant weight and 400-600 mg samples placed in Teflon(tm) PFA-lined microwave digestion vessels with 10 mL concentrated HNO3. The Teflon(tm) liner was secured inside the Ultem(tm) polyetherimid outer body and the vessel cap tightened by hand. These vessels have an upper pressure limit of 200 psi. Twelve vessels were placed in the carousel of a 630 W microwave digestion system. The unit was equipped with a pressure controller accessory, which was connected to one of the lined-vessels with a special double port cap. The following eight-step heating program was suggested by the manufacturer's technical support staff (Table 16.5).

Table 16.5. Heating Program for 500 mg Placental Tissue and 10 mL Nitric Acid

Step*Time (min)Pressure setting (psi)
1 10 20
2 10 40
3 10 60
4 10 80
5 10 100
6 10 120
7 10 140
8 60 175

*Each step was performed at 100% power - 630 W

©"Laboratory Microwave Safety", Kingston, H.M.; Walter, P.J.;
Engelhart, W.G.; and Parsons, P.J., Chapter 16 In Microwave-
Enhanced Chemistry: Fundamentals, Sample Preparation, and
Kingston, H.M. and S.J. Haswell, Eds.; American
Chemical Society: Washington, D.C., 1997: 697-745.

After running this heating program with several placenta samples, large background absorption signals were still evident in the furnace, particularly at the cadmium 309 nm line, with cadmium barely detectable. It was decided to explore additional heating at 100% power and 175 psi cutoff for a further 60 minutes after allowing the vessels to cool overnight, and manually venting each one before proceeding. This second 60 minute heating step, 24 hours later, had the effect of considerably reducing the background absorbance and noise; thereby, improving the detection limit for cadmium. It was during the post-24 hour heating stage of a subsequent run, that one of the lined digestion vessels suffered a catastrophic failure. The event caused one of the vessels to fragment into ten pieces, although the cap and rupture membrane remained intact (Figure 16.12). The force of the event caused the oven door to open, scattering debris and hot acid into the laboratory. Fortunately, no one was near the oven when this event took place. The damage to the oven was limited to a broken mode stirrer and a second cracked vessel.

Figure 16.12 Result of a Catastrophic Failure of a Lined Digestion Vessel

As follow-up to this event, several freeze-dried placenta samples were provided to the manufacturer's applications laboratory to investigate the conditions that led to vessel failure. Applications staff ran the samples in a 950 W oven with a fiber-optic temperature control system. Placenta samples were digested in 10 mL HNO3 using lined digestion vessels (200 psi), and another set were digested in 10 mL HNO3 in newly developed high-pressure vessels (600 psi) using the program in Table 16.6.

Table 16.6 Heating and Pressure Program for 200 mg Placental Tissue and 10 mL Nitric Acid

Step Power Pressure setting Run time Time at pressure Temperature Fan speed
(%)*(psi) (min) (min (°C) (%)
1 25 20 10 5 120 100
2 35 40 10 5 200 100
3 35 85 10 5 200 100
4 35 150 10 5 200 100
5 35 200 10 5 200 100

*100% power - 950 W

©"Laboratory Microwave Safety", Kingston, H.M.; Walter, P.J.;
Engelhart, W.G.; and Parsons, P.J., Chapter 16 In Microwave-
Enhanced Chemistry: Fundamentals, Sample Preparation, and
Kingston, H.M. and S.J. Haswell, Eds.; American
Chemical Society: Washington, D.C., 1997: 697-745.

The temperature and pressure of one of the lined digestion vessels were monitored and recorded (Figure 16.13). The data show that a maximum temperature of 194°C was obtained during step 4 at a pressure of 200 psi. One sample was cooled and vented, and heated again at 35% power at a temperature cutoff of 200°C and a pressure cutoff of 150 psi. Both temperature and pressure were recorded. The temperature inside the vessel increased quickly and actually exceeded 200°C at one point at 150 psi. These data suggest that the temperature of placenta sample digests probably exceeded the recommended vessel operating limit (225°C). Since the unit did not have temperature control, it was not possible to control this. Further experiments digesting placenta samples using the high-pressure vessels showed that the maximum pressure obtained was 153 psi, below the stated 200 psi tolerance for lined digestion vessels, and below the final pressure cut-off used in the final step above (175 psi). Thus, it would seem that temperature control is essential for the safe digestion of biological tissue samples such as placenta. Additional aspects of temperature versus pressure control are evaluated in Chapters 2 and 3.

Figure 16.13 Lined Digestion Vessel Temperature and Pressure Profiles

Case #16 - Digestion of a Fuel Oil

Refined oils are routinely digested for subsequent analysis by spectroscopic and wet chemical techniques. Closed-vessel microwave digestion offers a rapid and convenient technique for achieving digestion. In this case, a standard application method from the manufacturer was followed for digesting samples of #6 Fuel Oil. The application note called for Teflon(tm)-PFA vessels rated to withstand internal pressures of 120 psi. A 500 mg #6 Fuel Oil sample was placed in Teflon(tm) PFA-lined microwave digestion vessels with 12 mL concentrated HNO3. Twelve vessels were placed in the carousel of a 900 W microwave digestion system. The unit was equipped with a pressure controller accessory, which was connected to one of the lined vessels with a special double port cap. The twelve step heating program recommended in the application note was followed with power settings reduced by 30% to compensate for the field intensity of the 900 W microwave system (Table 16.7).

Table 16.7 Heating Program for 500 mg Fuel Oil and 12 mL Nitric Acid

Step Power (%)* Time (min) Pressure setting (psi)
1 70 6 170
2 0 2 vent
3 70 2 170
4 0 3 vent
5 70 6 170
6 0 3 vent
7 70 7 170 8 0 4 vent
9 70 8 170
10 0 4 vent
11 70 10 170
12 cool to room temperature
*100% power - 900 W

©"Laboratory Microwave Safety", Kingston, H.M.; Walter, P.J.;
Engelhart, W.G.; and Parsons, P.J., Chapter 16 In Microwave-
Enhanced Chemistry: Fundamentals, Sample Preparation, and
Kingston, H.M. and S.J. Haswell, Eds.; American
Chemical Society: Washington, D.C., 1997: 697-745.

While performing this routine procedure, one of fuel samples underwent an uncontrolled exothermic reaction, causing a catastrophic failure of the inner vessel (Figure 16.14). This failure led to fragmentation of the outer liner, and the force of the event caused the oven door to open. Debris from the fuel oil and vessel were scattered across the laboratory. Again, fortunately there were no personnel in the room at the time.

Figure 16.14 Catastrophic Failure of a Vessel Due to an Uncontrolled Exothermic Reaction

Design and Installation of a Safety Shield

Upon suffering a catastrophic vessel failure in two different laboratories that resulted in opening the oven door with near disastrous consequences, actions were taken that, at the very least, would ensure the safety of technical personnel working with microwave digestion procedures. The most important action was to design and install a safety shield that is physically mounted on the oven, and which swings down in front of the oven during use (Figure 16.15).

Figure 16.15 Microwave Door Safety Shield

The shield was assembled from basic, low-cost workshop materials, and is easily constructed by any competent, well-equipped workshop. The shield is made from 6.4 mm (1/4 inch) polycarbonate supported by 6.4 mm aluminum alloy. A complete schematic of this and other such safety devices can be found on SamplePrep Web (Chapter 15). When the shield is in the lowered position, the distance between the shield and the oven door is 8 cm, with 3.5 cm clearance between the oven-door handle, and a 6.4 mm aluminum-alloy support bar that traverses the shield. This is purposely designed to allow the oven door to open slightly during a vessel failure and relieve any pressure build-up in the oven. A plastic stopper on the support bar ensures that, should the oven door burst open during a failure event, the polycarbonate shield will not break.


The vessel failures, and the decision to install an oven safety shield, were reported to the microwave manufacturer in May, 1993. They responded by offering a protective shield for retrofitting the unit. We encourage microwave customers to report problems to instrument manufacturers to help improve the quality of future analytical instruments. The authors also encourage reporting of safety issues, case studies, and preventative methods to the SamplePrep Web site in the hope that this produces effective solutions to prevent additional occurrences.

In conclusion, this experience underlined the need to install safety shields on unprotected microwave digestion systems, to use temperature control when digesting biological tissue samples, and to publicize known safety problems preventing repetition of these events.

Case #17 - Non-intuitive Nature of Microwave Interactions

An analytical laboratory at a Fortune 500 company was performing fluoride analysis on plant tissue. The decomposition was a strong base hydrolysis using a relatively concentrated sodium hydroxide solution. Approximately 0.25 g samples of plant tissue were placed in the Teflon(tm) liners of seven vessels. Vessels from two different manufacturers were used, one monitoring vessel from the laboratory microwave unit manufacturer, and six microwave vessels from another manufacturer of different design and construction. The system was equipped with pressure feedback control to the monitor vessel. The vessels had drastically different pressure ratings. The unit manufacturer's vessel, used for pressure monitor/control, was rated at 200 psi, while the remaining six vessels had a design rating of 1,200 psi. The microwave program used in the 600 W microwave was 75% power for 2 minutes, followed by 40% power for 30 minutes.

At 10 minutes into the decomposition, a violent explosion occurred, opening the door, sending a vessel across the laboratory, and destroying the microwave unit. After the incident, the safety officer at the company decided that: "Microwave digestion will no longer be performed in analytical chemistry as a result of this near miss." (54).

The stated purpose of the analyst for using two different types of vessels was to add an additional pressure safety factor. The logic being that since the monitored vessel had a much lower pressure (200 psi) capacity than the majority of his sample vessels (1,200 psi) from the second manufacturer, they would have additional pressure capacity. Additional information, obtained from the analyst, indicated that carbonaceous material was found on exploded vessel fragments and inside the microwave unit. Additionally, when asked if sparking was ever heard with this reagent mixture, the analyst reported that indeed he had heard sparking sounds frequently when running this reagent mixture.

Failure analysis in this instance is not simple, nor is it appropriate to use conventional logic without understanding the unique interactions of the microwave and vessel. First, one must refer to the discussion in Chapter 3 dealing with "Temperature and Pressure Relationships in Closed Microwave Vessels". Insulated heavy-walled vessels, such as the 1,200 psi high-pressure vessel used in this example, retain heat and follow conventional temperature/vapor pressure relationships. However, thermal characteristics of the 200 psi pressure-monitoring vessel, in this case are more like the example in Chapter 3, where a large amount of heat is transferred and dissipated from the gas phase through the vessel walls. The pressure in this monitor vessel may be only 12-20 % that of the insulated vessel at similar temperatures. Thus, monitoring this vessel gives a false value for pressure control in the remaining vessels.

This particular high-pressure vessel has only had two reported uncontrolled reactions where it disintegrated in this manner. The other incident was shown to be a chemical explosion. In this case, the finding of large amounts of carbonaceous material covering the destroyed vessel fragments indicate that a fire may have occurred in the vessel. Since it has been reported that highly concentrated sodium hydroxide solutions can "electrically arc" when irradiated, the solution may have sparked the fire. No digestion vessel is designed to handle a chemical fire or explosion. The kinetics of chemical explosions are outside the design and operations range of safety relief devices for over-pressurization protection. The microwave cavity itself must act as the next barrier with a door or secondary shield appropriately constructed to prevent the escape of such fragments/missiles.

The failure to understand the design and performance characteristics and unique interactions of microwave equipment, the analyst created an unsafe situation by attempting to control reaction pressure in an inappropriate manner. This case illustrates the non-intuitive nature of microwave technology that requires study of the subject to enable the analyst to predict important interactions during method development. We strongly encourage analysts to learn the subtleties of this new tool as a prudent measure for efficient and safe operation of microwave instruments.

Epilog: The Fortune 500 company, after analyzing the underlying causes of the explosion, has taken steps to educate its staff and is again using microwave digestion successfully and safely.


Safety of the analyst is the most important consideration in analysis or of the sample preparation process. As the result of direct coupling, microwave absorption by chemical reagents, energy is directly transferred to and concentrated on reactants. The advantages of high temperature for chemical procedures are quickly realized, however control of sample heating and reactions is complicated by this rapid heat transfer mechanism. As in all good laboratory practices, after becoming informed and removing ignorance as an excuse, there is no substitute for common sense. Common sense dictates careful planning of experiments and cautious experimentation when the results are uncertain and the equipment is unfamiliar. Many of the instrumentation mishaps reported to the authors involve misuse of equipment. Other unsafe situations arose from failures of equipment that were no longer serviceable. Still others involved equipment that does not perform as specified or as it was expected, because of a lack of thorough testing.

Occasionally, an unanticipated event will occur that results in equipment damage and loss of the sample. These phenomena should be documented and reported to help colleagues in the scientific community avoid these same difficulties. Because science progresses by virtue of the observations and discoveries of our predecessors and peers, we advance science by making our own observations known. This is the purpose of scientific literature and the SamplePrep Web (Chapter 15). As this collection of case studies disappears from common occurrence, newer uses will produce other safety concerns. We hope we have provided some insight into the current state-of-the-art, and await the new contributions from others to advance the safe use of laboratory microwave systems for chemical analysis and reactions.

Appendix A: Hazard Monitors For The Detection Of Microwave Leakage

ANCHOR CHEMICAL AUSTRALIA PTY. LTD Box 474, P.O. Crow's Nest, N.S.W. 2065 Australia Tel. (02) 439-2144

APPLIED MICROWAVE ENERGY INC. 31127 Via Colinas, Westlake Village, CA 91362 USA Tel. (213) 991 4624 BACH-SIMPSON LTD 1255 Brydges Street, London, Ontario N5W 2C2, Canada Tel. (519) 452-3200

GENERAL MICROWAVE CORPORATION 155 Marine Street, Farmingdale, NY 11735 USA Tel. (516) 694-3600

GERLING LABORATORIES 1628 Kansas Ave., Modesto, CA 95351 USA Tel. (209) 521-6549

HOLADAY INDUSTRIES INC. 14825 Martin Drive, Eden Prarie, MN 55344 USA Tel. (612) 934 4920

MICOR, INC. 3901 Westerly Place, Suite 102, Newport Beach, CA 92660 USA Tel. (714) 476 0616

MICROWAVE HEATING LTD 1A Heron Trading Estate, Luton Beds, England LU3 3BB, UK Tel. (0582) 58474

Milestone MLS 7289 Garden Road, #219, Riviera Beach, FL 33404 USA Tel. (407) 863 6991, Fax (407) 863 7758 (for U.S. and Canada)

Milestone s.r.l. Via Fatabenefratelli 1/5, 24010 Sorisole (Bg), Italy Tel. (39) 35 573857, Fax (39) 35-575498 (International)

NARDA MICROWAVE CORPORATION 435 Moreland Road, Happauge, NY 11788 USA Tel. (516) 231 1700

Appendix B: Safety Terminology

An unplanned event, sometimes but not necessarily injurious or damaging, that interrupts an activity. A chance occurrence arising from unknown causes, carelessness, ignorance, lack of training, etc.

The American Conference of Governmental Industrial Hygienists

A high-temperature luminous electric discharge across a gap.

American Society of Mechanical Engineers

Refers to ASME Section VIII Division 1. Pressure Vessel Code. BLEVE
An acronym for a boiling liquid expanding vapor explosion. These are explosions involving vessels that contain liquids under pressure at temperatures above their boiling points.

Design Pressure
Refers to the pressure value used to determine the minimum wall thickness of a pressure-vessel body . In pressure-vessel codes, the design pressure is always higher than the maximum allowable working pressure (MAWP) or operating pressure. The minimum margin for design pressure is 110% of the MAWP (operating) pressure.

A violent bursting, as of a pressurized vessel. An extremely rapid chemical reaction with the associated production of noise, heat and violent expansion of gases. Reactive explosions are further categorized as deflagrations, detonations, and thermal explosion. Failure
Distortion, breakage, deterioration, or other fault in a structure, component, or system resulting in inoperability or unsatisfactory performance of intended function.

Failure Analysis
A logical, systematic examination of an item, component, or assembly, and its place and function in a system, to identify and analyze the probability, causes, and consequences of potential and real failures.

An acronym for electromagnetic radiation to fuel. The potential for electromagnetic radiation to cause ignition of volatile combustibles such as aircraft fuels.

An acronym for hazards of electromagnetic radiation to personnel. The potential for electromagnetic radiation to produce harmful biological effects in humans.

Hypergolic Mixture
Two chemicals that when mixed create enough heat of reaction to cause ignition, without an external ignition source. (Three examples of chemicals that form hypergolic mixtures with concentrated nitric acid include; phenol, acetone, and triethylamine).

Intrinsically Safe
The term classifying electrical components or equipment, approved for use in specific hazardous atmospheres, which are incapable of releasing sufficient energy to cause ignition under normal or abnormal conditions. (Microwave systems for laboratories are not designed or rated intrinsically safe).

International Radiation Protection Association

An acronym for Maximum Allowable Working Pressure. The maximum permissible operating pressure inside a vessel at its specified operating temperature.

An abbreviation for the National Electric Code, issued by the National Fire Protection Association. This code carries the force of law in all 50 states in the U.S.

National Fire Protection Association.

Pressure-Relief Valve
A pressure-relief device which is designed to reclose and prevent the further flow of fluid after normal conditions have been restored.

Pyrophoric Chemicals
Chemicals that react so rapidly with air and moisture that the ensuing oxidation and/or hydrolysis leads to ignition.

Abbreviation for radiation hazard, term describes hazards of electromagnetic radiation to fuels, electronic hardware, ordinance, and personnel.

Rupture Disk Device
A non-reclosing pressure-relief device actuated by inlet static pressure and designed to function by the bursting of a pressure-containing disk

Indicates a mandatory requirement in codes and regulations

Thermal Explosion
An explosion resulting from an exothermic reaction that generates heat faster than the heat can be dissipated, and thus raising the temperature to a level where the reaction rate is catastrophic.

An abbreviation for Germany's Technische Überwachnung Verein, an authorized inspection agency for industrial safty codes and regulations

Appendix C: General Safety References

A. Steere, N.V., CRC Handbook of Laboratory Safety, Second Edition, CRC Press, Inc., Boca Raton, FL, 1982.

B. Stull, D.R., Fundamentals of Fire and Explosion, AIChE Monograph Series, 10, Vol. 73, 1977.

C. Brown, S.J., Impact, Fragmentation and Blast (Vessels, Pipes, Tubes, Equipment, American Society of Mechanical Engineers, New York, Vol. 82 PVP Series, 1984.

D. Chung, H., and Nicholson, D.W., Advances in Impact, Blast, Ballistics, and Dynamic Analysis of Structures, American Society of Mechanical Engineers, New York, Vol. 106 PVP Series, 1986.

E. Kendall, D.P., High Presseure Technology - Design, Analysis, and Safety of High Pressure Equipment, American Society of Mechanical Engineers, New York, Vol. 110 PVP Series, 1986.

F. Mahn, W. J., Academic Laboratory Chemical Hazards Guidebook, Van Nostrand Reinhold, New York, 1991.

G. Mahn, W.J., Fundamentals of Laboratory Safety - Physical Hazards in the Academic Laboratory, Van Nostrand Reinhold, New York, 1991.


We thank Timothy Cole (Case #15) and John Orsini (Case #16) of the Wadsworth Center of the New York State Department of Health for technical assistance and for reporting these incidents.

Thanks to Dr. Bill Sonnefeld for the photographs of unserviceable vessels, and to the Wadsworth Center's Photographic and Illustrations Unit for the photographs associated with cases 15 and 16.

We also thank the staff of the Wadsworth Center's Instrumentation and Automation Support Unit for designing and building the microwave safety shield.


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