Overview of Microwave Assisted Sample Preparation

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

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