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A N A LY T I C A L C H E M I S T R Y / J A N U A R Y 1 , 2 0 0 1
Microwave-Enhanced
Robert C. Richter Dirk Link H. M. “Skip” Kingston Duquesne University
Analytical chemists no longer have to accept the technological mismatch between sample preparation and instrumentation.
he art of sample preparation had its beginnings in ancient Egypt and Greece. Early alchemists devel-
T
oped fusion methods for controlling the purity of
gold and silver. The discovery of mineral acids in the 14th century greatly improved the speed and
accuracy of these sample preparations and was the basis for the industrial method of separating gold
and silver in the early 15th century (1). In fact, author Isaac Asimov hailed the discovery of mineral acids as the most important chemical advance since the production of iron from iron ore 3000 years earlier (2). Many of the sample-preparation methods currently in use were actually developed during the 19th century. In the early 1800s, Berzelius developed test tubes, separatory funnels, and platinum crucibles. In 1834, Henry and Zeise developed methods for the gravimetric determination of sulfur NEVERNE COVINGTON
as sulfate in organic samples. Their method called
J A N U A R Y 1 , 2 0 0 1 / A N A LY T I C A L C H E M I S T R Y
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Outer vessel (top) 45° C
taneously detecting three-quarters of the periodic table in a matter of minutes. Such instruments require samples that are homogeneous at the molecular level in the liquid phase. However, while there have been many advances in analytical instrumentation, techniques for transforming solid samples into homogeneous solutions have not advanced with the same fervor. Many chemists continue to use 150-year-old mineral acid, beaker dissolution, and Soxhlet extraction methods, which can take hours or days to complete and are susceptible to biases, including the skill of the analyst and contamination of the sample. Microwave-enhanced chemistry (MEC) is a fast, efficient, and reproducible sample-preparation method. The use of clean chemistry techniques in combination with MEC sample preparation has advanced the levels of sample analysis into the subpicogram range. Many fundamental mechanistic differences separate microwave heating from other heating methods. Solutions are heated so efficiently that reaction timescales are dramatically reduced, often from days to minutes, and the level of reaction and process control offered by microwave heating is better than any other heating method. As a result, microwave methods are highly amenable to standardization and automation.
MEC sample-preparation development
180° C Solution
70° C Outer vessel (bottom)
FIGURE 1. Reflux conditions inside a microwave-closed vessel. The microwave-closed vessel’s liner and outer casing remain relatively cool during the heating process, because they are microwave-transparent and have only a small insulating capacity. The cooler the vessel walls, the more efficient they will be at removing water molecules from the vapor phase. The increased condensation rate results in lower internal pressures at higher temperatures.
for the sample to be digested with fuming nitric acid or aqua regia and fused with potassium hydroxide or potassium nitrate. In 1860, Carius devised a new method for the quick and efficient determination of sulfur and halogens. Barium chloride or silver nitrate was mixed with the sample and heated in a sealed, strong-walled tube with nitric acid. Upon cooling, the sulfur or halogen was leached out and determined gravimetrically. In 1883, Kjeldahl published his method for determining the nitrogen content of proteins (1). The Soxhlet method for extracting fat from biological material was also developed during this time and became very popular because it made possible extracting multiple samples with minimal operator attention (3). Today’s analytical methods rely on instruments that have below part-per-billion detection limits and are capable of simul-
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A N A LY T I C A L C H E M I S T R Y / J A N U A R Y 1 , 2 0 0 1
Analytical chemists first began using MEC in 1975 to digest biological samples (4). A domestic microwave oven was used to rapidly heat a mixture of sample and digestion acids to its atmospheric boiling point in an Erlenmeyer flask. This new microwave process allowed sample digestions, which used to take several hours using a hot plate, to be completed in
