Chapter 2

 

Overview of Microwave Assisted

Sample Preparation

 

Peter J. Walter, Stuart Chalk, H. M. 'Skip' Kingston

Department of Chemistry and Biochemistry

Duquesne University

Pittsburgh, PA 15282-1503



1. HISTORICAL PERSPECTIVES ON THE DEVELOPMENT OF MICROWAVE SAMPLE PREPARATION

2. DOCUMENTATION OF MICROWAVE METHODS

3. CURRENTLY APPROVED INTERNATIONAL STANDARD MICROWAVE METHODS

4. ROBOTIC AUTOMATION OF MICROWAVE PROCEDURES

5. RESOURCES FOR METHOD DEVELOPMENT

6. BIBLIOGRAPHY

7. APPENDICES


Sample preparation involves numerous steps; from sample collection to its presentation as a homogeneous solution for instrumental analysis. Sample preparation can involve drying of the sample, leaching, extraction, or digestion of the matrix, and post-digestion chemistry, analytical separation, or solvent removal, and exchange. The use of microwave technologies have been shown to improve sample preparation while also reducing contamination.

This chapter and the remainder of this book will address the diverse field of microwave sample preparation and microwave chemistry. Chapter 3 will address the use of chemistry for microwave acid dissolutions with an emphasis on environmental applications. Chapter 4 will address high pressure acid digestion and chapter 5 will address atmospheric pressure microwave dissolution. Chapter 6 will address flow through microwave reactors. Chapter 11 will describe the use of microwave energy in solvent extraction. Other chapters will address flow through microwave and uniquely designed microwave sample preparation systems. Chapter 13 will evaluate the use of pressure control in microwave dissolutions. This chapter will focus on reviewing the literature in the field of microwave sample preparation and the chemistry of dissolution.

 

1. Historical Perspectives on the Development of Microwave Sample Preparation

Elemental analysis of nearly every matrix requires the dissolution of the sample prior to instrumental analysis. Despite tremendous improvements and discoveries of new analytical instruments over the past decades, few changes in dissolution methodologies have come forth. For centuries, chemists have used some variation of an open vessel digestion or a Carius tube closed vessel digestion. In 1975, microwaves were first used as a rapid heating source for wet open vessel digestions (1-3). Microwaves were used to heat acid(s) rapidly, in Erlenmeyer flasks, to digest biological matrices reducing conventional sample digestion times from 1 - 2 hours, to 5 - 15 minutes using microwave heating, a net reduction in analysis time. These papers spawned the research and development of a new sample preparation technique.

Early microwave sample preparation researchers used common laboratory glassware and open Teflon vessels to digest matrices at the boiling point of the acid(s) in commercial microwave ovens. In the 1980s, researchers began using specially designed closed vessels for microwave digestions to achieve reaction temperatures above the atmospheric boiling point of the acid(s) in order to increase the reaction rates and decrease reaction times. However, this was accompanied by an increase in reaction pressures, a potential safety concern (4-7). These closed microwave digestions vessels were fabricated from polycarbonate or Teflon and were not specifically designed for microwave (7, 8). The first closed Teflon vessels used in this transitional period were designed for leaching of nuclear waste glass samples (8)

Temperature and pressure monitors were adapted with wavelength attenuators for monitoring the reactions and evaluating the conditions in closed microwave systems, see Figure #1 (4, 5). These modifications to commercial microwave systems became the foundation of the laboratory microwave units of today. In 1987, an IR 100 was award for the development of microwave sample preparation to the National Institutes of Standards and Technology (NIST) and CEM Corporation, lead by Dr. H. M. 'Skip' Kingston of NIST (9). Before temperature and pressure monitors were commercially available, digestion procedures were developed by a trial-and-error approach and evaluated on the basis of the recovery of an element or a suite of elements from a material, frequently a standard reference material. This approach brought about the majority of the digestion procedures and research papers that have illustrated the numerous advantages of microwave sample preparation. They have; however, contributed little to the acid concentration, power, and time optimization of digestions, the understanding of the completeness of the digestion, the understanding of microwave interactions, or the digestion mechanisms. Despite the lack of chemical knowledge that is gained by the trial-and-error approach, it was and still is the primary approach to the development of microwave digestion procedures.

Figure 1. First research microwave sample-preparation system with temperature and pressure monitoring capabilities.

In 1985, the first laboratory multimode cavity microwave unit was introduced. Its primary improvements over 'home' or 'domestic' units were the added safety features. The early units, although built from domestic cavities and doors, isolated and ventilated the cavity to prevent acid fumes from attacking the electronics, see Figure #2. Since the first laboratory microwave unit was introduced, numerous companies have continued to improve every aspect of the unit including homogeneity of the microwave field, ability to control the microwave power, and most importantly improvements in safety.

Figure 2. Typical laboratory cavity-type microwave system.

In 1986, the first completely reengineered laboratory focused microwave system was introduced. Contrary to microwave cavity systems, a single vessel is placed directly in a microwave waveguide, see Figure #3. The vessels are constructed of either Teflon or quartz. The bottom few inches of the vessel are exposed directly to the microwaves, while the upper region of the vessel remains cool. This results in an effective condensing mechanism inherent to the design. While the vessels are open to the atmosphere, the refluxing action minimizes the acid and some volatile elemental losses. The vessel openings were designed to permit automated reagent addition and to restrict contamination from the atmosphere.

Figure 3. Typical laboratory focused type microwave system.

In the mid 1980s, a few researchers began building or modifying temperature and pressure monitoring equipment for use inside a microwave cavity. The primary challenges were to develop probes that were non-perturbing to the microwave field, and to build wavelength attenuator cutoffs for these probes so they could enter the microwave region while preventing microwaves from leaving the microwave cavity. Monitoring temperature and/or pressure during digestions/extractions began the age of controlled digestions, the study of microwave digestion mechanisms, and the development of transferable standard microwave sample preparation methods. These developments spawned an outgrowth of microwave use that is illustrated by the increase in microwave sample preparation publications (see Figure #4). Since articles on microwave sample preparation are published in all areas of chemistry, medicine, geochemistry, etc., the complete collection of many articles are only discovered years later through references.

Figure 4. Growth of microwave sample preparation as seen through the growth of research papers.

The first commercial laboratory microwave unit with pressure feedback control in 1989, and the first commercial laboratory microwave unit with temperature feedback control in 1992 have allowed for a more rigorous design and control of microwave sample preparation procedures. Concurrently, with these developments, the microwave vessel has evolved significantly (see Figure #5). The first generation of microwave closed vessels were an all Teflon® design with low pressure limits, approximately 7 atm. These vessels were prone to venting, by exceeding their limited pressure capabilities. These pressure limits decreased as the vessel aged due to stress from previous dissolutions. The second generation microwave was the jacketed vessel. Typically these vessels were constructed of a Teflon liner and cap with a polymeric case (typically polyetherimide) that increased the pressure limits of the vessels to up to 20 atm. The third generation vessels, also lined, have been completely redesigned and are capable of extremely high pressures in the range of 60 - 110 atm. The evolution of the microwave vessel and the capability to monitor the reaction conditions throughout the digestion have allowed researchers to systematically study the decomposition mechanisms of various matrices. These studies have advanced the 'art' of sample preparation into 'state-of-the-art' sample preparation.

Figure 5. Evolution of closed microwave vessels

In 1995, Milestone Corporation developed a completely unique approach to microwave dissolution, a microwave heated autoclave (10). The ultraCLAVE utilizes a very high pressure (200 bar) autoclave cavity with a maximum temperature of 350°C. The temperature and pressure capabilities far exceeds all current microwave closed vessels, enabling this system to be used for a wider variety of chemical applications.

 

1.1 Reaction Control and Transferring of Microwave Methods

As microwave sample preparation methods have been developed, means of reproducing these methods have also been developed. One of the biggest drawbacks with the use of microwave methods has been their dependence on the microwave power, as this changes significantly from unit to unit (not just manufacturer). There are four approaches used to transfer standard microwave methods: specification of microwave unit power settings, instrument power calibration, and pressure and temperature feedback control. A comparison of the effectiveness of these control mechanisms is shown in Figure #6. The improvement in precision and accuracy matches the chronological development of these techniques.

Figure 6. Reproducibility of microwave digestion temperatures for various control techniques.

Transferring a method using microwave unit power settings is best illustrated by ASTM method D4309-91 (11). This method describes two general ranges of microwave powered units and specifies the partial power setting and the time for acid leaching.

With the wide range in the power delivered to the cavity, the dissolution reaction conditions are never under precise control, leading to a range of maximum reaction temperatures of approximately 20°C, based on the methods reaction temperature profiles. Compounding the variation in the reaction temperature is the variability of the reaction time. According to the specification listed above, the reaction times can be either 30 or 50 minutes. These variations lead to a method that is incapable of accurately reproducing the method's designed reaction conditions of a maximum reaction temperature, for example 165 ± 5°C.

Currently, methods in which calibration and feedback control are used can be documented, transferred, and reproduced. Calibration reproduces the reaction conditions through replication of the microwave field throughout the digestion procedure. Feedback control monitors temperature and/or pressure and makes microwave power adjustments based upon the reaction conditions in order to reproduce the desired reaction profile.

Calibration of a cavity type microwave involves the measurement of the microwave field versus percent power setting of the unit. Since the direct measurement of the microwave field would require expensive specialized equipment, modification of the microwave unit, and specialized training, an indirect technique of measuring the microwave field is used. The strength of the microwave field is measured by determining the amount of microwave energy absorbed by a strongly absorbing substance like water. Water is readily available and is used for power calibration almost uniformly in commercial and laboratory microwave units. Using the following thermodynamic relationship, the microwave field strength can be determined and the applied microwave field defined.

 

 

P is the apparent absorbed power (Watts), K is the conversion factor for calories/second to Watts (4.184 J/cal), Cp is the heat capacity of the microwave absorbing solvent (water), m is the total mass of the microwave absorber in the cavity, T is the change in temperature in the microwave absorber from the irradiation of the microwave energy, and t is the time of microwave exposure.

This equation and the method of calibration have been described in detail in several sources (5, 12, 13). Additionally, a computer program was written to guide the analyst through the collection of calibration data as well as the statistical evaluation of the data (14). This program can be downloaded from the SamplePrep Web WWW site, as described in Chapter 15. A calibrated microwave procedure can be developed and reported as watts of microwave power versus time. By simply calibrating another microwave unit and the determination of the partial power settings that correspond to the watts specified in the procedure, the microwave field can be reproduced.

Calibration, as a means of transferring microwave procedures, is only as good as the ability to reproduce the microwave field precisely, to measure it, and to reproduce the exact loading of the microwave unit. Reproduction of the microwave field is limited by the precision with which one can measure and calibrate a microwave unit, and the ability to reproduce the exact wattage required in the procedure. The power of a microwave unit can be calculated to about ± 10 Watts and most laboratory microwave unit's partial power settings (0 - 100%) correspond to changes of 6 - 12 Watts. Combining these factors, the ability to reproduce the reaction conditions of a calibration controlled method, such as EPA Method 3051, is typically ± 5 - 10°C or more (15, 16), as illustrated in Figure #7. The temperature for a given sample procedure using the same unit can be reproduced to less than ± 5°C (15). Transferring microwave methods through calibration, identical microwave digestion vessels and amounts of reagent(s) can reproduce the reaction conditions. In some cases, the sample and the number of vessels must be the same. The most severe limitation of calibration control is that a separate determination of calibrated powers must be determined if the method is varied; i.e. change in the volume or type of reagents, type of vessel, or in some cases, sample size.

Figure 7. Reproducibility of EPA Method 3051 through calibration.

One of the manufacturers of focused microwave digestion systems performs calibration during assembly. At both 10 and 100% power settings, the magnetron power control circuitry is adjusted so that each unit has the same power output within 5%. This is achieved through the use of a continuous flow calorimeter. A precision flow meter is used to ensure a constant flow rate while a thermistor measures the inlet and outlet temperature. For the focused microwave system, the use of a flow through calibration device forces a change on the power equation. Increasing the flow rate through the chamber decreases the residence time in the magnetic field, and thus the power absorbed decreases resulting in a smaller temperature rise. Therefore, an increase in the flow rate causes a decrease in the temperature at the same power as expressed in the following equation.

 

 

Equation (2.1) can also be used for the design of microwave procedures. Knowing the absorption of microwave energy by an acid or combination of acids, Equation 2.1 can be rearranged to predict either the time required to reach a temperature or the temperature at a given time during a digestion.

 

The absorption of microwave energy by numerous acid(s) was studied and the acids' microwave energy absorption versus mass of absorbing acid plots were reported (5). Using these data, the reaction conditions during the initial stages of digestion can be predicted. The later stages of the digestion cannot be predicted due to the equation's inability to correct for heat loss from the microwave vessels. A more complex evaluation played by heat loss of the vessel in a microwave field will be explained in Chapter 3.

Predicting the time to reach a temperature, Equation 2.4, has been used to design a leaching method without the aid of using temperature feedback control. Leaching soils in concentrated nitric has been demonstrated to be nearly quantitative for soluble salts at 175°C for 5 minutes (5, 17). For a specific combination of sample and reagent quantities, the final reaction temperature of 175°C was calculated to be reached in approximately 1 minute (18). Experimentally, the digestion should be complete if the digestion temperature is maintained for 5 minutes, resulting in a total digestion time of 6 minutes. A digestion time versus determined concentration of mercury supported that a quantitative recovery was accomplished in 6 minutes. Even without a temperature feedback control microwave unit or temperature measurement capability, a sophisticated microwave digestion program can be devised with only calibration, calculations, and careful planning.

The introduction of commercial microwave digestion units in the late 1980s with first pressure and later with temperature feedback control has enabled more fundamental microwave digestion research and improved reproducibility and transferability of microwave procedures. The rate of a digestion reaction is controlled primarily by temperature and only indirectly by pressure (6). Both pressure and temperature feedback control microwave digestion systems are appropriate for specific types of digestions. Digestions that produce little or no gaseous products may be controlled well by pressure. If the digestion of the sample produces a minimal pressure, the primary increase in pressure is due to the acid(s) or solvents. If the amount of the solvent is accurately dispensed into identical vessels, pressure feedback control will reproducibly repeat reaction conditions by reproducing the temperature profile through this secondary and related control. However, when digesting materials that produce significant quantities of gas, controlling the digestion by pressure becomes more difficult. The pressure formed during the digestion is the sum of the pressures from the reagent(s), and the pressure from the gaseous digestion products. The latter depends on the sample composition and quantity. If the precise amount of reagent(s) and sample are not reproduced, then a specific temperature will produce different pressures and therefore different temperatures both within and between runs. An evaluation of potential problems inherent in pressure control is further discussed in chapter 3. For some total digestions, this may not be serious, but if the reaction is a leach or extraction method where the reaction conditions are critical for reproducing the leaching/extraction, this type of control is critical and frequently inappropriate. In these and other cases, the best control is through the real-time adjustment and control of the reaction temperature.

Temperature feedback controls the primary factor affecting reaction rates. Regardless of the amount of sample or reagents, the temperature of the solution during the digestion can be reproduced. Methods, such as EPA Method 3051 that were previously reproducible through calibration to about ± 10°C, are now reproducible to ± 4°C or better. This enables the development of a method for any vessel, and the ability to transfer the method to any microwave unit capable of temperature feedback control using vessels with a minimum pressure capacity. Temperature feedback control allowed for the development of EPA method 3052, in which a wide variety of reagents and variable quantities can be used. In all variations of the method, the temperature can be accurately controlled to the methods specifications. Feedback control of temperature enables improved control and reproducibility of reaction conditions over power control, pressure feedback control, and calibration as well as documenting the digestion.

 

2. Documentation of Microwave Methods

The reproducibility of a microwave method is directly related to the detail provided in its documentation. Far too frequently, microwave methods have not been documented well enough to adequately reproduce the methods in another laboratory, as was illustrated in many of the methods listed in Appendices 1-3. Improved documentation could increase the method's usefulness in analytical laboratories. This section will discuss what is critical to properly document microwave methods.

An analogy of complete documentation of a microwave method can be drawn to the documentation of a hot-plate digestion. The early procedures for hot-plates probably said heat the sample until hot. This description of the procedure is lacking any information about what temperature the dissolution is too be run at or how long the digestion should be performed. Analogously, this is similar to early microwave methods that reported heating the vessel at some power until the dissolution was complete.

Microwave units can be calibrated to produce uniform fields of energy that reproduce standardized reaction conditions. Once calibrated, the individual unit's partial power settings from a calibrated microwave unit can be converted into power in watts. Then the method can be described as a series of power steps, each of which are described as a power in watts applied for a specific time. Despite microwave calibration's improvement in documentation of microwave methods, this method suffers from precise reproduction of the temperature. The temperature for a specific time reaction conditions profile and calibration of a method is specific to the exact amount and quantities of acids and the exact type and number of microwave vessels.

The most advanced description of methods involve the complete documentation of the heating profile. For microwave dissolutions, complete documentation of a method involves the description of the entire heating profile using temperature versus time criteria. This has been accomplished in several standard EPA methods, including 3015, 3051, and 3052.

A completely documented microwave method is independent of the specific microwave unit and vessels. A method can be reproduced using microwave vessel with sufficient temperature and pressure capabilities in combination with any microwave system with temperature feedback control. An example of an appropriately described method is illustrated below:


Buffalo River Sediment

Sample Type: Geological, Environmental

Matrix: Clay, Silt, Organic debris

Standard Method: EPA Method 3052

Sample Size: 0.25 - 1.0 g

Control: Temperature Feedback

Stage 1 of 1:

Acid (s): 9 mL HNO3, 3 mL HF

Program: 700 W 700 W 0 W

 

time 5.5 min 9.5 min 5 min

Temp. to 180°C at 180°C cool

Pressure < 15 atm < 15 atm < 15 atm

 

Unit: MLS 1200mega

Vessel: MDR 600/10

No. of Vessels: 10

Maximum Power Required: 700W

Maximum Temperature Obtained: 180°C

Maximum Expected Pressure: 12 atm

Figure: Typical Reaction of the Digestion of a Soil

NOTE: Programming of equipment and the exact temperature and pressure profiles will depend on the microwave unit and the specific vessels.

 

Standard Reference Material: NIST Buffalo River Sediment SRM 2704

Element Analyzed Certified Technique

µg/g µg/g

Cd 3.5 ± 1.2 3.45 ± 0.22 ICP-MS

Cr 132.9 ± 1.3 135 ± 5 ICP-MS

Cu 98.0 ± 4.2 98.6 ± 5.0 AA

Ni 43.6 ± 3.9 44.1 ± 3.0 ICP-MS, ET-AA

Pb 154.5 ± 9.2 161 ± 17 AA

Zn 441.9 ± 0.8 438 ± 12 AA

 

Comments: This analysis was used in part to write the original EPA Method 3052. It is part of a certification of EPA Method 3051 for leaching of six metals from NIST SRMs 2710 and 2711; high and moderately highly contaminated Montana soils (Report to NIST April 5, 1994, Contract # 50SBNB3C7513)

 

Analyst: Peter J. Walter

Location: Duquesne University



Figure 8. Example of complete documentation of a microwave decomposition method for Buffalo River.

These types of documented methods will be posted on the SamplePrep Web WWW site, as described in chapter 15. An outline of the necessary descripers for documenting a microwave method has been described in the literature (15). The most critical descriptors briefly described below:

The description of the sample type into general classifications such as geological, metallurgical, etc.

Analytes of interest

Amount of sample per vessel (Range)

Number of samples per dissolution (vessels per group)

Vessel type

Dissolution reagents specifying the precise quality of reagent (ACS grade or sub-boiled distilled grade, etc.) and the amount of reagent for both dissolution vessels and reagent blank vessels.

The complete digestion program

For calibration: the calibration procedure must be stated along with the specific power settings and durations of each step

For temperature feedback control: The method should be described as a series of heating to temperature, maintaining a temperature, and cooling steps. As illustrated above, EPA Method 3052 can be described in three stages: heat to 180 ± 5°C in 5.5 minutes, maintain 180 ± 5°C for 9.5 minutes, and cool for at least 5 minutes. The temperature profile is specified, but the tolerances to the temperatures are also stated.

 

This information allows scientists to reproduce methods quickly, accurately, and precisely. Some set of these descripters are used in the documentation of standard methods that are promulgated or suggested by various organizations worldwide. Methods also should be validated for which standard reference materials are available from many international standards organizations for this purpose.

 

3. Currently Approved International Standard Microwave Methods

As microwave sample preparation has evolved, standard microwave procedures have been developed and approved by numerous standard methods organizations. Currently there are 21 methods approved, or in the process of being approved, by the Association of Official Analytical Chemists (AOAC) (19-22), American Society for Testing and Materials (ASTM) (11, 23-25), the United States Environmental Protection Agency (US-EPA) (12, 13, 26-28), Standard Method (29), and French (30) and Chinese (31) national methods for either microwave drying or microwave acid dissolution. These methods are summarized in Table 2-1.

Two microwave drying methods, ASTM methods E1358-90 and D4643-93, have been developed to dry either wood or soil to a constant mass with the use of microwave energy. However, the bulk of the methods are acid dissolution procedures for either total elemental analysis or acid leaching of a matrix. The reaction control of these standard acid dissolution methods range from minimal control using microwave unit power settings and calibration, to moderate control using pressure feedback control, or robust control of the reaction through temperature feedback control. Some methods have been developed and documented with two types of reaction control, such as temperature feedback control and calibration. This allows the methods to be run using calibration for standard microwave equipment or temperature feedback control for more advanced microwave equipment.

ASTM method D4309-91 is an acid leach method for water with the reaction control based on the power rating of the microwave unit. Basic ranges of microwave power ratings are separated into categories of digestion protocols. These power range settings cannot accurately reproduce reaction conditions, resulting in nonreproducible leaching of the sample.

The Chinese method, C303.01T and ASTM method, D5258-92 were both developed for pressure feedback control. Using pressure feedback control, these methods may more accurately reproduce the designed reaction conditions with one limitation. If the digestion produces variable amounts of gaseous byproducts, variable temperatures will result.

In order to improve the digestion reaction control, EPA method 3015 and 3051 and Standard Method 3030K (a reproduction of EPA method 3015) were designed with both calibration and temperature feedback control. The methods were developed using temperature measurement capabilities and both the calibration power settings for a specific microwave vessel as well as the specific temperature reaction profiles were provided. These methods enabled the microwave units at that time, which were primarily unable to perform temperature feedback control, and future microwave units, with temperature feedback control, the ability to use these methods. Since these methods have temperature reaction criteria, these methods can be very accurately and precisely run batch after batch.

The most advanced microwave standard method is EPA method 3052 and is designed to accommodate the variation in the acid chemistry and utilizes temperature feedback control with third generation moderate to high pressure vessels. One should keep in mind that many of these older standard methods were developed with limited instrumental capabilities when laboratory microwave units were little more that converted domestic microwaves systems. Many of these methods have limitations imposed by early low pressure vessels and the lack of temperature feedback control. Newer methods, such as EPA method 3052, were developed with and utilize both third generation microwave vessels and temperature feedback control.

EPA methods 3031 and 3050B and the French standard method V03-100, a Kjeldahl nitrogen method, are examples of traditional methods that are being expanded to allow for microwave heating as an alternative heating source. Other standard methods have been proposed such as EPA Methods 351 and 365. Using atmospheric pressure vessels and temperature feedback control, these methods are capable of improved reaction temperature control that result in improved precision. Improvement in both precision and accuracy for several of the environmental standard methods are described in Chapter 3. As new methods are developed and older methods are reevaluated, more standard hot-plate acid dissolution methods well become available as microwave methods. A complete list of current and developing standard methods will be updated continuously on the SamplePrep Web, see Chapter 15 for details.

4. Robotic Automation of Microwave Procedures

The first fundamental rule of automation of chemical processes is reactions which are not understood or in control cannot be automated successfully. Now that there are approved high volume standard microwave dissolution tests and methods, it makes sense to examine their automation. Two major factors make procedures using closed-vessel, microwave dissolution of acid samples well-suited for intelligent automation. First, the ability to accurately transfer precise amounts of energy to the acid and its ability to control and reproduce reactions by controlling the temperature and mechanisms of the reaction provides reproducible digestion conditions. Second, the rapid heating due to the direct coupling of microwave energy into the solution reduces digestion times from hours to minutes. Automation of microwave dissolution procedures should maintain the accuracy and precision of sample preparation and increase sample throughput.

There have been several efforts to robotically automate microwave sample preparation. The first was developed at Kidd Creak Mines in Canada for the analysis of minerals (32). The second system was developed at the National Institutes of Standards and Technology (15, 16, 33, 34) with cooperation from Zymark Corporation, Hopkinton, MA and CEM Corporation, Matthews, NC. This system was designed primarily to implement EPA microwave methods; however, it had the flexibility to perform nearly any microwave digestion procedure because of its modular programming. Commercial versions of this system is available from Zymark Corporation. Another system was built for the dissolution of Titanium Dioxide by Norris, et. al. (35).

Despite the diversity of samples and the approaches to automation, these systems have common goals and components. All three of these systems were built to increase the throughput of routine dissolution procedures while maintaining or possibly improving the quality over manual manipulations. In order to accomplish these goals, automated components had to be designed to perform many simple manual tasks. Some tasks had to be performed in a completely different manner with a fully automated system.

While the three systems were developed for different purposes, with different equipment the overall robotic layouts are similar. Figure #9 illustrates one robotic table (33). The system uses computer software and hardware to guide the analyst through the logging-in and weighting of each sample, whereas other systems have automated powder sample dispensers (35). Once the samples have been added to the dissolution vessels, the digestion acid(s) are added by automated reagent dispensing stations. The acid dispensing station, shown in Figure #8, is enclosed in a class 100 clean hood to maintain sample integrity by protecting samples from the major source of contamination in trace elemental analysis, the environment. Once the vessel is uncapped, a reagent dispensing arm moves over the vessel and the appropriate amount of acid(s) are dispensed. The vessel is then capped and torqued to the vessel manufacturers' specifications. The NIST system was built as automated modules as described in the Consortium on Automated Analytical Laboratory Systems (CAALS) and was documented in a government report (36-39). This system was developed to use standard method encapsulated for automated transfer and implementation (8, 16, 38).

Figure 9. Fully automated robotic microwave sample-preparation system.

Automation of microwave sample preparation relied on standard laboratory units that were modified for automated door opening, carousel indexing, and remote programming. Recently, a unit was developed with automated control integrated into its basic design (10). As with any dedicated automation instrument, it no longer bares a resemblance to a conventional microwave oven, but is designed around the concept of automated operation.

Because chemistry is much faster and more reliable, it leads itself to automation more readily than modifying traditional sample preparation or chemical reaction apparatus. This makes microwave chemistry an attractive tool for increasing efficiency in the chemical laboratory. Many of the microwave applications in subsequent chapters lend themselves to automation and will be implemented as automated systems in the next few years.

5. Resources for Method Development

5.1 General guide to literature tables

In writing a review of any topic, it is often difficult to incorporate a lot of information, while making it easy to read and making it a usable resource. For this reason, in this overview of microwave sample preparation we have organized the wealth of information into tables of various formats. We hope that this will make the chapter a valuable reference for any scientist needing to develop microwave based procedures.

In deciding how to organize the information relevant to sample dissolution, we looked at the process of developing new methods. Within each part of this process, we have tried to optimize the format for each table to convey the information in a concise yet accessible manner. As this field is still growing at a breakneck pace, we do not want it to be a static entity. We are committed to expanding this resource as the field grows and so we welcome any suggestions that readers have about how we can improve the format, content, and areas of the interest. Not only do we plan to include updated versions of these tables in future editions of this book, but we also will be making them available on the SamplePrep Web WWW site (see Chapter 15). We hope you find the tables useful.

 

5.2 Review papers

Table 2-2 includes brief descriptions of review papers in the area of microwave sample dissolution so that the reader can quickly find a more specific review on their area of interest. Not included in this table are the several chapters in the rest of this book that review current areas of microwave chemical research.

5.3 Microwave drying

Sample drying is not often considered in the development of microwave sample preparation procedures. However, the residual moisture content of samples can play a major part in sampling the matrix reproducibly, as well as the interaction of the microwave energy with the sample. Thus, it is relevant to include references on the use of microwaves for drying as it can and should be included in the preparation procedure for certain matrices. In Table 2-3, we have described the application as it was described in the paper to convey the perspective of that paper to the removal of free water and other free action polar solvents from the matrix. Particular importance should be placed on the drying platform indicated as several authors have commented how important the form of the sample is on the drying characteristics.

5.4 Microwave Ashing

Although the majority of work on microwave sample dissolution concerns wet acid digestions, it is not absolutely necessary to use an aqueous phase while heating with microwaves. Table 2-4 lists papers that address the use of microwave based ashing as an alternative to muffle furnace ashing. This approach may be more productive if the matrix is easily combustible and volatile species are not to be determined.

5.5 Microwave dissolution

This area of research has fueled the growth of microwave technologies because of its superiority over hot plate and block digester wet acid digestion methods. Because of the large amount of work on this in the literature and the limited space in this book, we have chosen to include a subset of the literature. Thus, the following tables contain papers on microwave sample dissolution that have been applied to reference materials to validate the procedure used. Through this approach, we have provided known matrix and analyte data that may be critically evaluated for its relevance to the analyst. This seems to be an appropriate method for grouping the literature for rapid, useful evaluation. The references in this table make up about 50% of the total papers on microwave sample dissolution. The exclusion of the other 50% of papers should not imply that they are "bad science", but they do not provide a basis by which any judgment can be made about the proposed procedure(s). Of the papers cited, we have not distinguished between "good" and "bad" science, rather we have provided relevant information that researchers will find useful in developing microwave methodology.

In the compilation of the tables, only those articles we have been able to obtain in hardcopy have been included. Thus, articles in journals unavailable to us have not been included. We would be delighted to obtain copies of any articles which should be included in the tables, but are not, as well as receiving any comments regarding the accuracy of the information contained in the tables. Please contribute to the authors any articles for publication updating this information.

Comparing table entries for the same reference material (a good example is NIST 1577 - Bovine liver in Appendix 1.1), it can be seen that the procedures used are numerous and varied in both reagents used and microwave conditions. This is largely due to the infancy of the technique with researchers applying anything and everything to find procedures that will provide respectable results on a particular matrix. This has lead to the general uses of reagent outlined in section 5.6, and the realization that performance based methods (the use of temperature and pressure to control reactions) will provide more robust methods for a wider variety of matrices. In adaptation of these procedures for the readers sample matrix, we feel more emphasis should be placed on the reagent compositions reported than the microwave conditions because of the wide disparity in microwave power outputs and scant information on reaction temperatures.

 

5.5.1 General information

The tables on the following pages have been broadly grouped into three categories based on the reference sample matrix types. Appendix 1 includes all biological reference materials, Appendix 2 contains geological and metallurgical reference materials, and Appendix 3 contains other reference materials that could not be categorized within either of the first two tables. Appendices 1 and 2 have been subdivided for easier use.

 

5.5.2 Matrix

The institution that supplied the reference material, the name designation, and the number are included in that order. Within each table, entries are listed in alphabetical order by supplier and sub-listed by reference material number in ascending order. Explanation of the supplier acronyms can be found in Table 2-5 below. No attempt has been made to distinguish between different batches of the same reference material (e.g. 1577a and 1577b are considered an equivalent matrix).


Table 2-5. Sources of reference materials

Acronym

Supplier

AIS Academy of Iron and Steel, Pan Zhi Hua Ministry of Metallurgy, Japan
ANRT Association Nationale de al Recherche Technique, France
AR Alpha Resources Inc., USA
ASCRM Australian Standard Coal Reference Material
BAM Bundensanstalt für Materialforshung und -prüfurg, Germany
BCR Community Bureau of Reference, Commission of the European Community 200 Rue de la Lol, B-1046 Brussels, Belgium
BCS British Chemical Standard, Bureau of Analysed Samples, Middlesborough,UK
BI Behring Institute, PO Box 1140, D-3550 Marburg ,Germany
BRAMMER Not reported
BRL Bio-Rad Laboratories, Munich, Germany
CANMET Canada Centre for Mineral and Energy Technology
CCRMP Canadian Certified Reference Material Project, Energy mines and Resources, Canada
CRPG Centre de Recherches Petrographiques et Geochimiques, France
CSAN Czechoslovakian Analytical Normal
EPA Environmental Protection Agency, Washington DC, USA
GSJ Geological Survey of Japan
IAEA International Atomic Energy Agency, Analytical Quality Control Services Laboratory Seibersdorf, PO Box 100, A-1400 Vienna, Austria
IFM Institutet Far Metallforskoring, Sweden
IGGE Institute of Geophysical and Geochemical Exploration, China
IRSID Not reported
ISS Instituto Superiore di Santa Roma, Italy
IWG International Working Group
KL Kaulson Laboratories, 691 Bloomfield Avenue, West Caldwell NJ, USA
MOE Ontario Ministry of the Environment, Canada
NAGRA Swiss National Cooperative for Radioactive Waste Storage
NIES National Institute for Environmental Studies, Japan Environmental Agency PO Yatabe, Tsukuba Ibaraki 300-21, Japan
NIM National Institute of Metallurgy, South Africa
NIST National Institutes of Standards and Technology, Room B311 Chemistry Building, Office of Standard Reference Materials, Gaithersburg, MD 20899, USA
NRCC National Research Council of Canada, Institute for National Measurement Standards, Ottawa K1A OR6, Canada
RMA Rocky Mountain Arsenal, Program Managers Office, Commerce City, CO 80022, USA
SERONORM Nycomed Diagnostics, Oslo, Norway
SRS Fisher Scientific, Fair Lawn, NJ, USA
USGS United States Geological Survey
XIGMR Xian Institue of Geology and Mineral Resources, Chinese Academey of Geological Sciences, China
ZGI Zentrales Geologisches Institut, Germany


5.5.3 Analytes

Elements determined in the indicated matrix are listed alphabetically by symbol. Other analytes are listed after. Speciation information has been included where possible.

 

5.5.4 Reagents and microwave conditions

In order to give a general sense of the procedure used, the information in these two columns has been organized as follows:-

 

Different procedures within a paper are separated by letter designations, e.g. E., F.

Different steps in a procedure are separated by number designations, e.g. 1., 2.

If, within a step, more than one reagent is used, each reagent is separated by a comma.

Repeat steps within a profile are indicated by (x?) where ? is an numeric.

If the procedure involves criteria other than microwave power, i.e. pressure, this has been included as well.

Other pertinent information from the procedure has been included as necessary.

 

5.5.5 Microwave cavity and reaction vessel descriptions

Considering the variety of modes in which microwave sample preparation can be performed, we have classified methods only by general descriptors (see Table 2-6 and Table 2-7). Information on the material of the digestion vessel has not been included.


Table 2-6. Microwave type designations

 Microwave Type

 Description

 Multimode  Sample exposed to a multimode field (modestirred cavity system)
 Single mode  Sample exposed to a single mode field (focused waveguide system)



Table 2-7. Vessel type designations

 Vessel Type

 Description

 HP closed  High pressure closed vessel (> 80 atm)
 MP closed  Medium pressure closed vessel (>10 atm, < 80 atm)
 LP closed  Low pressure closed vessel (< 10 atm)
 Open  Atmospheric pressure vessel (1 atm)
 Flow through  Flowing stream of reaction mixture passing through a microwave field
 Stopped flow  Flowing stream of reaction mixture that is stopped when it is contained within a microwave field


5.5.6 Detection

Instrumental detection methods are listed. Acronyms for the techniques can be found in Table 2-8.


Table 2-8. Acronyms used for analytical techniques

Acronym

Description

F-AAS Flame atomic absorption spectrometry
ETV-AAS Graphite furnace, electrothermal vaporization/atomization AAS
CV-AAS Cold vapor/mercury vapor atomic absorption spectrometry
HG-AAS Hydride generation atomic absorption spectrometry
FI-F-AAS Flow injection flame atomic absorption spectrometry
SIMAAC Simultaneous atomic absorption
ICP-OES Inductively coupled plasma optical/atomic emission spectrometry
FI-ICP-OES Flow injection inductively coupled plasma optical/atomic emission spectrometry
ETV-ICP Electrothermal vaporization inductively coupled plasma
ICP/MS Inductively coupled plasma mass spectrometry
MS Mass spectrometry
DCP-OES Direct current plasma optical/atomic emission spectrometry
CV-AFS Cold vapor atomic fluorescence spectrometry
CV-FANES Cold vapor furnace atomic non-thermal emission spectrometry
F-OES Flame photometry, flame emission
LC Liquid chromatography
HPLC High performance liquid chromatography
IC Ion chromatography
SPC-IC Solid phase chelation ion chromatography
UV-Vis Ultraviolet visible spectrophotometry
ASV Anodic stripping voltammetry
DP-ASV Differential pulse anodic stripping voltammetry
DPP Differential pulse polarography
NAA Neutron activation analysis
ID Isotope dilution
TXRF Thermal X-ray fluorescence

NOTE: Also used Fluorimetry, Temperature, Pressure, Voltammetry, Total carbon analyzer, Carbon dioxide coulometry, Radiochemical, a-spectrometry


5.6 Dissolution Reagents

As mentioned in the previous section, good dissolution procedures depend on the choice of reagents that are used and the specific temperature profile. These two parameters establish the mechanisms and kinetics of the reaction. This is not only important for complete dissolution, but also reproducible extraction (leaching), analyte solubility, analyte volatility, species stability, and importantly safety.

The process of developing a new microwave digestion procedure involves identifying the samples primary components, understanding the chemistry required to decompose these components, and considering safety factors as applied to a specific digestion and/or equipment.

To propose a digestion procedure, both the reactivity of the common digestion acids and the reactivity of many matrices must be considered. If the primary components are known, procedures from the literature can be consulted to determine guidelines for decomposing the matrix. However, the specific elements of interest and their reactivity with the acids must also be considered. Therefore, consideration of both reference digestion procedures and the reactivity of the acids for the analytes of interest are critical to a successful procedure.

Included here is a review of the reactivity of the primary decomposition reagents. A comprehensive review of the chemistry of each acid and combinations of acids is beyond the scope of this section. Many specific reagent and method dependent mechanisms are described in a discussion of method 3052, in Chapter 3. However, a review of the basic chemistry of several acids and common combinations of acids is discussed. The reactivity of six common dissolution reagents with the elements are presented in terms of their volatility, reactivity, solubility, complexation, stability, and catalytic effects. Additionally, when the information in the literature was available, the specific elemental species or oxidation state was noted. This information is presented in periodic table graphical form as well as in tabular form with comprehensive citations, see Figures 10 - 15 and Tables 2-9 - 2-14. We would be delighted to obtain copies of any articles which should be included in these tables but are not, as well as receive any comments improving the accuracy and completeness of the information contained in these tables.

The most commonly used digestion reagents; nitric acid (Figure 10 and Table 2-9), hydrochloric acid (Figure 11 and Table 2-10), hydrofluoric acid (Figure 12 and Table 2-11), sulfuric acid (Figure 13 and Table 2-12), perchloric acid (Figure 14 and Table 2-13), and hydrogen peroxide (Figure 15 and Table 2-14), are discussed in their interactions with the elements. While these chemical reactivates are the basis for an understanding for the development of a digestion procedure for a matrix, they are not absolute. The analyst will have to evaluate the reactivity of the acids with each specific matrix because every matrix will have unique chemical interactions. Many references for each acid are located in the accompanying tables and figures and related aspects of each reagent have been provided there.

Figure 10. Reactivity of nitric acid with the elements.

Figure 11. Reactivity of hydrochloric acid with the elements.

Figure 12. Reactivity of hydofluoric acid with the element.

Figure 13. Reactivity of sulfuric acid with the elements.

Figure 14. Reactivity of perchloric acid with the elements.

Figure 15. Reactivity of hydrogen peroxide with the elements.

 

 

5.6.1 Nitric Acid (HNO3)

Nitric acid is an oxidizing acid that will dissolve most metals to form soluble metal nitrates. It has poor oxidizing strength below 2 M, but is a powerful oxidizing acid in the concentrated form. Its oxidizing strength can be enhanced by the addition of chlorate, permanganate, hydrogen peroxide, or bromine or by increasing temperature and pressure. Most metals and alloys are oxidized by nitric acid with two categories of exceptions: Gold and Platinum are not oxidized and some metals are passivated when attacked by concentrated nitric acid. These metals can be dissolved by use of a combination of acids or by dilute nitric acid.

Nitric acid is the most common acid for the oxidation of organic matrices. Nitric acid is a more powerful acid when used in combination with a complexing acid such as Hydrochloric acid.

 

5.6.2 Hydrochloric Acid (HCl)

Is a non-oxidizing acid which exhibits weak reducing properties during dissolution. Many metal carbonates, peroxides, and alkali hydroxides are readily dissolved by hydrochloric acid. Some metals, including Au, Cd, Fe, and Sn, can be dissolved by hydrochloric acid, but dissolution is accelerated by the addition of another acid. Most metals form soluble metal chlorides with several notable exceptions: AgCl, HgCl, and TiCl which are insoluble and PbCl2 which is only slightly soluble. Hydrochloric acid's complexing nature allows for complete dissolution of numerous metals, such as Fe(II) and Fe(III) complexing to form [FeCl4]-2 and [FeCl4]-, respectively (40, 41).

Hydrochloric acid is often used in combination with other acids for dissolution. It is frequently combined with nitric acid. If the mixture of hydrochloric acid to nitric acid is a ratio of 3 : 1, this is called aqua regia.

 

5.6.3 Hydrofluoric Acid (HF)

Hydrofluoric acid is a non-oxidizing acid whose reactivity is based on its strong complexing nature. It is most commonly used in inorganic analysis because it is one of the few acids that can dissolve silicates. Hydrofluoric acid's strong complexation capabilities prevents the formation of sparingly soluble products of several metals, increasing the solubility and stability of those elements. Dissolution with hydrofluoric acid produces primarily soluble fluorides, with the exception of insoluble or sparingly soluble fluorides of the alkaline earth, lanthanide, and actinide elements. To improve dissolutions, hydrofluoric acid is routinely combined with another acid, such as nitric acid. Insoluble fluorides may frequently be resolubilized by removing the hydrofluoric acid after digestion.

 

5.6.4 Sulfuric Acid (H2SO4)

Dilute sulfuric acid does not exhibit any oxidizing properties, but the concentrated acid is capable of oxidizing many substances (41). Concentrated sulfuric acid (98.7%) has a boiling point of 339°C, which is greater than the working ranges of all Teflons. Therefore, careful microwave sample preparation method development must be implemented through the measurement of the reaction's temperature to prevent exceeding the thermally stable temperature of Teflon vessels. Quartz vessels are the material of choice for sulfuric acid dissolutions. Sulfuric acid can corrode the surfaces of Teflon during prolonged evaporations (42).

Sulfuric acid is commonly used with other acids and reagents. One of the most common combinations is with perchloric acid or hydrogen peroxide. Sulfuric acid will act as a dehydrating agent that will dramatically increase the oxidizing power of perchloric acid, but this mixture may react to violently with organic matrices in closed vessels or if heated rapidly.

 

5.6.5 Perchloric Acid (HClO4)

Dilute aqueous Perchloric acid, either warm or cold, is not an oxidizing acid. Concentrated Perchloric acid (60 - 72%) is not an oxidizing acid cold, but becomes a powerful oxidizing acid when warm. Therefore, the oxidizing power of perchloric acid is proportional to its concentration and temperature. Warm perchloric acid will readily decompose organic matter, sometimes violently. Due to its extremely rapid reactivity with organic matrices (sometimes explosive), perchloric acid is generally mixed with nitric acid. This combination of acids allows for a controllable digestion of organics; the nitric acid will attack the easily oxidizable matter at lower temperature while diluting the perchloric acid. However, as the temperature rises, the perchloric acid will completely digest matter undigested by the nitric acid. Perchloric acid has been an acid of choice for the destruction of organics using traditional heating systems because it has been shown to decompose nearly any organic matrix, and nearly all perchlorate salts are soluble. However, dry perchlorate salts of many metals are explosive!

Because of the explosive potential of perchloric acid with organic matrices and the fact that microwave sample preparation uses an extremely rapid heating source, the use of perchloric acid should be considered a potential safety hazard. Perchloric acid has been shown to decompose at 245°C in a microwave closed vessel developing dangerous amounts of gaseous byproducts and tremendous excess pressure (5). General use of perchloric acid in closed or atmospheric pressure microwave systems is not recommended due to safety concerns. If a digestion requires the use of perchloric acid then the following rules should be always obeyed:

1: Read all pertinent information about safe handling of perchloric acid. An excellent source of information about perchloric acid is "Perchloric Acid and Perchlorates" by A. A. Schilt (43).

2: Always start with dilute perchloric acid. Dilute perchloric acid will slowly react with organic matter preventing possible explosive reactions.

3: Always use another acid to digest the easily oxidizable matter before perchloric acid can begin reacting with the matter. Similarly to rule #2, dilute the perchloric acid with an acid like nitric acid. Then slowly heat the sample, to enable the nitric acid to destroy as much of the organic matter as possible. Slowly continue heating until the perchloric acid has reached a temperature at which it can oxidize the remainder of the organic matter.

4: The safest method of following rules #2 and #3 are to perform a two stage digestion. In the first stage of the digestion use only nitric acid and digest the sample as completely as possible. Once the digestion vessels are cool, open the vessels and add a minimal quantity of perchloric acid. Then heat the digestion vessels for a second time. This will allow the perchloric acid to only decompose a small quantity of the toughest organic matter.

5: Follow established safety rules regarding the disposal of perchlorates and avoid the uncontrolled collection of perchlorate residues on apparatus.

 

5.6.6 Hydrogen Peroxide(H2O2)

Typically concentrations of about 30% hydrogen peroxide are used in digestions, but more recently 50% concentrations are available. Hydrogen peroxide alone can react explosively with many organics, especially in the more concentrated form. Hydrogen peroxide is usually combined with an acid because its oxidizing power increases as the acidity increases. The combination of hydrogen peroxide and sulfuric acid forms monoperoxosulphuric acid (H2SO5), a very strong oxidizing reagent (41). Because of its oxidizing power, hydrogen peroxide is frequently added after the primary acid has completed a pre-digestion of the matrix. The hydrogen peroxide can complete the digestion and the potential safety hazards previously described are minimized. In this regard, hydrogen peroxide is used similarly to perchloric acid. Using these acids after the primary digestion of organic matter is completed is one way to avoid potentially violent reactions.

 

5.6.7 Physical Properties of the Common Dissolution Reagents

Some general parameters for common dissolution reagents are given in Table 2-15. Note that the dielectric constant, an important measure of the absorptivity of microwave radiation, is listed for the pure substances at specific temperatures. The values for these reagents as they are used in the laboratory will be slightly different due to the addition of water.

 

5.6.8 Guidelines for using the chemical reactivity tables and figures

The following tables and figures contain the important chemical reactivates of the acids with the elements. They address the volatility of the elements, the stability either as stable soluble complexes or precipitates, the reactivity of the pure elements with the acids, and catalytic effects on the acids. Each of these categories will be address separately.

In the compilation of the information, it became apparent there is a considerable controversy in the literature about several of these categories. Unfortunately, the majority of the primary sources lack the citations to the original literature eliminating the ability to resolve the literature's discrepancies. As a result, some of the information in these tables and figures may have less confirmation than the authors would like, but each individual piece of information has been referenced to enable the reader further investigation. It is our intention to keep updating these tables and figures with new information. These updates will be included in any future updates of this book and relevant WWW pages on the SamplePrep Web site that is discussed in Chapter 15. We would be delighted to obtain any comments regarding the accuracy of the information contained in the tables and figures or the submission of information for inclusion into the tables.

 

5.6.9 Volatility of an Elemental Species

Volatility of elemental species is the most controversial of all the information in these tables. While one source may state that a significant percentage of an element is volatilized under specific conditions, another source may state that it is not volatile under the same conditions. This problem lead to the inclusion of the data in tabular format that allows for the complete referencing of all papers that state a particular point of view.

At the heart of the controversy is the experimental designs some of the works used to evaluate elemental volatility. While not identifying the original works, several papers addressed the same controversy and stated that numerous works were published with the conclusion that certain elements were volatile. However, the reviewers felt that the loss of some elements may have been due to interactions with the vessel, precipitation, or experimental error. Therefore, it would be wise to use these tables as a general guide and always evaluate the volatility of the analytes of interest in ones own apparatus, reagent chemistry, and specific sample.

Volatile species can be classified as either oxides, oxyhalides, halide, elemental, or hydrides. The primary volatile oxides are OsO4, RuO4, and Re2O7 which are volatile in nearly any oxidizing solution. The volatile oxyhalide and halide species are generally volatile in a heated solution either with the halide acid present or a sufficient amount of the halide present in the sample. The most volatile of all elements is mercury, it is volatile as the element or as numerous other forms, and commonly classified as an intrinsically volatile element. Under reducing conditions, numerous elements can be reduced to hydrides that are very volatile, such as AsH3, SbH3, etc. The reagent reactivity tables list the elements, oxidation state, and volatile species with the appropriate citation(s). The tables are not intended to suggest that these oxidation states and species forms are the only volatile forms; they are simply a compilation of know information about these elements. A compilation of boiling points for many potentially volatile species is tabulated in Table 2-16.

 

5.6.10 Reactions of the Elements with a Reagent

These listings are, in general, based on the reactivity of the pure element. While this reflects the reactivity of the element with the acid under a specific condition, it may or may not be applicable to an impure matrix that contains the element of interest. For example, while an acid may not react with a pure element, it may react with its alloy. Nitric acid frequently forms oxide coating with many pure metals preventing further digestion. This can easily be avoided by reacting the metals with dilute acid or by the inclusion of a complexing agent.

 

5.6.11 Solubility of the Element

The solubility of the element is critical for the development of digestion procedures, but several additional factors may play a very important role in the solubility in a specific matrix. While a digestion method may not involve hydrochloric acid, it is quite common for samples to contain chlorides that can be solubilized during digestion and potentially precipitate the element of interest. Solubilities are not always as simple as the solubility product, for example, calcium is precipitated as the sulfate, as predicted by the solubility product, but when it precipitates it will frequently cause the co-precipitation of several other elements that are not predicted to precipitate based solely on their solubility product. Sometimes, in order to completely decompose the matrix, the element(s) of interest are precipitated, such as the digestion of a silicate containing rare earth elements with hydrofluoric acid. Despite the precipitation of the rare earth elements during the digestion, these elements can be easily resolubilized with a post-digestion reaction and removal of the fluoride. The addition of a complexing agent such as chloride or fluoride can greatly improve the digestion of the matrix as well as the stabilization of the elements after digestion.

 

5.6.12 Stability (complexation or precipitation)

The stability of the elements is crucial for accurate determination. Once an element is digested, it may either be stable in solution as the hydrated metal ion, complexed with a complexing ligand, or precipitated from solution. To further complicate these issues, the concentration of reagents and the elements will play a very important role in the stability of the elements.

Some reagents have little or no complexation capabilities, these include perchloric acid and hydrogen peroxide. Both hydrofluoric and hydrochloric acids are very good complexing agents that can stabilize many elements in solution. For example, a number of elements will hydrolyze in an acid solution to form a precipitate, but the fluoride ion will complex a number of these elements. Nitric acid will passivate numerous metals through the formation of an oxide coating that is insoluble by nitric acid alone. In several cases, the use of dilute nitric acid alone will digest these metals, but hydrochloric acid is often added to enhance the dissolution. The hydrochloric acid will attack the oxide and complex the metal with the chloride, thus preventing the formation of an oxide coating. Silver will precipitate with hydrochloric acid to form silver chloride, but if the chloride ion concentration is increased to greater than 3M, the silver will re-dissolve as a stable chloride complex.

 

5.6.13 Catalytic Effects

The reactivity of both perchloric acid and hydrogen peroxide are known to increase with the addition of specific catalysts. These catalyst(s), especially in the case of perchloric acid, may increase the reactivity of the reagent to a point where it may be potentially explosive.

Catalytic effects are encountered with the mixing of acids. It is common to digest an organic material with perchloric acid and a secondary acid such as nitric or sulfuric acids. The mixture of acids is designed to allow either the nitric or sulfuric acids to primarily decompose the organic matrix at low temperatures before the temperature is sufficient for the perchloric acid to begin oxidizing the remainder of the matrix (approximately > 160°C) (41). This works well with nitric acid, but sulfuric acid is dangerous. The sulfuric acid dehydrates the starting perchloric acid (~72%) to > 85% which is a strong oxidizer even at low temperatures (41, 44). This mixture is considered explosive and should never be used with organic material, due to the postulated formation of Cl2O7 (41). Perchloric acid is potentially explosive with organics especially hydroxyl compounds and fatty materials (45). Perchloric acid has been implicated in explosions with Bismuth, its alloys, and Sb2O3 when treated with concentrated hot perchloric acid.

Hydrogen peroxide is commonly used as an oxidant that is added to an acid such as sulfuric or nitric. The combination of sulfuric acid and hydrogen peroxide has been postulated to produce permonosulfuric acid in situ, a reagent that is known to oxygenate many kinds of organic molecules (45).

Figures 2-9 to 2-14 are meant to be used individually when a single reagent is being used or collectively with several acids. By referring to these figures when developing a sample preparation acid digestion, many of the relevant interactions should be apparent.



Created and maintained by Ipek Erzi

Revised April 14,1998.

Last Modified Wednesday, May 20 1998
Copyright Duquesne University, 1997

Copyright 1997, SamplePrep WebTM