Analytical instrumentation helps environmental scientists measure chemical compounds in a wide variety of sample matrices such as wastewater, drinking water, air and soils. Heavy metals, hydrocarbons, polynuclear aromatic hydrocarbons (PAHs), chlorinated compounds, dioxins, herbicides and pesticides are some of the main chemical compounds that scientists look for through environmental analysis.
The diversity of these compounds dictates that environmental chemists use a broad arsenal of instrumentation. The flame ionization detector (FID) and the electron capture detector (ECD) are just two examples of the many types of instrumentation used for specific environmental applications. However, one combination of two analytic techniques - the gas chromatograph/mass spectrometer (GC/MS) - stands out as having had a significant impact during the early days of the U.S. Environmental Protection Agency (EPA).
EPA's top pick
During the 1970s, many chemists relied upon the GC with a conventional detector such as FID or ECD to identify and measure organic compounds. Gas chromatography is a method of separating gases, liquids or dissolved substances, by adsorbing the gases, vapors or substances of a mixture onto a column and then driving each off, one by one. This permits the separation of the mixture into individual compounds. A detector is usually employed in conjunction with the GC to identify and quantify the separated components. Gas chromatography is advantageous as a means of analyzing minute quantities of compounds in complex chemical mixtures
By the end of the 1970s, the Clean Water Act's stricter water analysis requirements mandated the development of analytical instrumentation with greater accuracy than the GC. After examining several analytical techniques, EPA selected the computerized GC/MS. The combination of MS and GC techniques for analyzing water was recognized by EPA in 1978 for its superior level of accuracy. A mass spectrometer is an instrument that analyzes samples by sorting molecular or atomic ions according to their masses and electrical charges and can be used to detect very low concentrations of a chemical. The resulting pattern is compared to those in a library in order to determine the identity of the compound. The GC/MS's end result is a more sophisticated tool for detecting contaminants.
According to Bill Budde of EPA's Office of Research and Development, in Cincinnati, Ohio, "One of the reasons EPA went into MS was because there were a lot of mistakes with GC." The first EPA GC/MS methods for wastewater analysis were developed in 1979 and finalized in 1984.
The GC/MS also had the advantage of being one of the first types of instrumentation to become computerized. "Mass spectrometers benefited from computer technology more than a lot of instruments because of the volume of data that they generate," Budde said. Computerized instrumentation allowed users to perform quantitative analysis and database searches.
Another factor that contributed to the GC/MS's success was that it was less expensive to use than the GC on a cost-per-analysis basis. According to Budde, "As the number of target compounds increases, the cost of the GC/MS approach remains relatively constant, while the costs of methods using conventional detectors increase significantly."
Innovators of instrumentation
The inception of the MS that became so popular in the last decades of the 20th century can be traced to research performed in the early 1900s by English physicist Joseph John (J.J). Thomson, who also won the 1906 Nobel Prize in physics for unrelated scientific work. Called by many the father of mass spectrometry, he concentrated on "positive rays" constructed from ions in a discharge tube that passed through a perforated cathode. He developed the first instrumentation that employed the technique of mass spectroscopy. Through the use of this instrument, he discovered in 1913 that neon consists of a mixture of two different isotopes (masses 20 and 22) rather than only a single isotope.
According to Dr. Robert W. Kiser of Kansas State University, Thomson's observation of the existence of stable isotopes is perhaps the greatest achievement of mass spectroscopy. Since Thomson's seminal work, scientists have used mass spectroscopy to find and study many other isotopes. The consequence of Thomson's discovery was the realization that the chemical properties of an element are determined by the atomic number rather than by the atomic weight of the element.
Thomson was assisted in his research by Francis W. Aston who later perfected the mass spectrograph, and received a Nobel Prize in physics for his work. Aston observed that masses of all isotopes are not simple multiples of a fundamental unit. Isotopes therefore do not have integral masses.
In his article Introduction to Mass Spectrometry and its Applications, Kiser points out that beginning around 1915 mass spectrometry developed along two main lines: one concerned with precise determination of masses, and the other concerned with measuring the relative abundance of ionic species. The mass spectrograph that Aston used in many of his studies of stable isotopes was readily adapted to measurements of isotopic mass to a precision of 0.1 percent, but it was not suited to accurate determinations of the relative abundance of these isotopes because it used photographic recording.
In 1918, A.J. Dempster announced that he had constructed an electronic bombardment ion source mass spectrometer of simpler design than Aston's mass spectrograph. Dempster's mass spectrometer could be used for precise mass measurements, but it surpassed Aston's instrument in measuring the relative abundance of the ionic species and was suitable for studying electron impact processes in gas.
Many other scientists have contributed to the refinement of the MS throughout this century. A more complete listing of these pioneers of MS development can be found at the Scripps Research Institute's Web site at http://masspec.scripps.edu/hist.html.
The evolving laboratory
The GC/MS is just one example of the many breakthroughs that have taken place in the field of environmental analysis. Today's broad array of high-speed analytical instruments has its origins in the hard work of generations of scientists throughout this century.
Kirk-Othmer's Encyclopedia of Chemical Technology
(John Wiley & Sons Inc.)
Dictionary of Scientific Biography (Charles Scribner's Sons)
Chemical Heritage Foundation
American Chemical Society
Today's Chemist at Work magazine
EPA's History Office
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This article originally appeared in the December, 1999 issue of Environmental Protection magazine, Vol. 10, Number 12, pp. 14-16.
This article originally appeared in the 12/01/1999 issue of Environmental Protection.