Gas Chromatograph-Mass Spectrometer
█ LAURIE DUNCAN
The gas chromatograph-mass spectrometer (GC/MS) is an instrument used to analyze the molecular and ionic composition of chemical compounds. GC/MS technology combines two widely used laboratory techniques: gas chromatography (GC), which separates and identifies compounds in complex mixtures, and mass spectrometry (MS), which determines the molecular weight and ionic components of individual compounds. The combination of these two powerful tools into a single instrument—the chemical separates produced by the gas chromatograph become the input for the mass spectrometer—allows for quick, precise analyses of solid, liquid and gaseous chemical compounds.
Scientists from a wide range of fields currently use GC/MS to identify and analyze inorganic, organic, and bio-organic chemicals. Academic researchers have long used either gas chromatography or mass spectrometry to assess experimental outcomes, analyze biochemical reactions, and age-date geological samples; many of these theoretical and experimental scientists have adopted the newer, more precise, and faster GC/MS technology to replace the two separate instruments. Industrial applications of GC/MS include pharmaceutical drug discovery and testing, process monitoring in the petroleum, chemical, and pharmaceutical industries, and identification of unknown chemicals in applied forensic, military, and environmental sciences.
Gas chromatography is a technique for separating closely-related compounds (solutes) from a liquid or gaseous mixture. (Solids must be vaporized or liquefied before analysis.) GC is most commonly used to separate and detect volatile and semi-volatile organic compounds (VOCs and SVOCs) with molecular weights less than 500 atomic mass units (amu). Although chemists have probably used rudimentary chromatography to separate mixtures since the Middle Ages, the modern chromatograph was not developed until 1941 when British biochemists Archer Martin and Richard Synge invented a chromatographic method that allowed for precise partitioning and detection. Martin and Synge were awarded the 1952 Nobel Prize in chemistry for their efforts.
The GC component of a GC/MS system includes a carrier gas supply, a sample introduction inlet, a capillary column coated with a stationary liquid or solid, and an outlet to the detection system, in this case a mass spectrometer. To begin analysis, a GC/MS technician vaporizes the sample, or analyte, and introduces it into the chromatograph by syringe injection through a rubber septum. A flow of inert carrier gas like helium, argon, or nitrogen moves the analyte into the separation column. Partitioning occurs as the gaseous components of the original analyte assume different velocities when confronted with the column's liquid or solid coating. Partitioning behavior is temperature-dependent, and precise temperature control is an important part of the GC process. A filter removes the separated compounds from the carrier gas at the end of the column before they are fed into the mass spectrometer for individual analysis.
Mass spectrometry is a method of determining the molecular weights of a chemical compound's component ions. (Ions are electrically charged atoms or groups of atoms, and sub-particles of molecules.) The MS instrument, known as the "smallest scale in the world", provides a graph, or mass spectrum, with peaks that indicate the relative amount of each type of ion within a compound. Today's MS systems are based on Sir J. J. Thomson's research at the Cavendish Laboratory at the University of Cambridge. Thomson discovered the electron in 1897, and went on to observe that the parabolic paths of ions traveling through electrical and magnetic fields vary according to the ions' mass-to-charge (m/z) ratios. His experimental instruments were the first mass spectrometers, and he was awarded the 1906 Nobel Prize in physics for his discoveries.
MS instrumentation has become increasingly accurate and complex since Thomson's time, but the principles of the technique and its basic components have remained the same. The MS component of a GC/MS system includes a sample inlet into a vacuum-sealed chamber that houses an ionization source, a mass analyzer, and an ion detector. In a GC/MS system the input sample is always a chemically homogenous gas produced by the GC component that can be introduced directly to the ionizer. Once ionized, the partitioned compound moves into the mass analyzer where the ions travel through an electrical or magnetic field that sorts them according to their m/z ratios. The detector measures the beam of now-separated ions arriving at the end of the analyzer, and converts changes in its intensity to produce the mass spectrum. A sample's mass spectrum is then displayed, catalogued, and compared to a library of known mass spectra by a computer data
system. For many applications, environmental monitoring or drug testing at sporting events, for examples, an unknown sample can be identified using a fairly short list of possible spectra. Other applications, like theoretical chemistry, organic chemistry, or planetary exploration may require an enormous library of possible molecules for identification, and may even produce previously unknown molecules.
Improvements in the individual GC and MS components, electronic automation, and computer data analysis and storage have led to machines that can analyze ever more complex, fragile, and tiny chemical components; GC/MS can now be used to quickly analyze proteins, DNA, and even viruses, and has become a common technique in molecular biology and medical science. GC/MS instruments are also becoming smaller and less expensive, and field laboratory and even portable, suitcase-sized systems that can be used to analyze forensic samples, environmental contaminants, and unknown agents of chemical and biological warfare on site now exist. Remotely operated GC/MS systems are planned components of future space exploration expeditions that hope to characterize the chemical makeup of extra-terrestrial environments, and to search for organic material elsewhere in our solar system.
█ FURTHER READING:
Massachusetts Institute of Technology. "Present Life: Spectroscopic Analysis Gas Chromatography/Mass Spectrometry (GC/MS)." Mars Mission 2004, student final presentation. December 10, 2000. < http://web.mit.edu/12.000/www/finalpresentation/experiments/index.html >(January 5, 2003).
Scripps Center for Mass Spectrometry (BC-007), 10550 North Torrey Pines Rd., La Jolla, CA 92037. (858) 784–9596. Gary Suizdak, director. < http://masspec.scripps.edu/information/intro/index.html .> (January 5,2003).
Signature Science, LLC8329 North Mopac Blvd Austin, TX 78759. (512) 533–2022. Cassandra Hutson, staff chemist. < http://www.signaturescience.com .> (January 8, 2003).
United States Environmental Protection Agency. "Technology: Gas Chromatography." January 2001. < http://fate.clu-in.org/gc.asp?techtypeid=44 > (January 9, 2003).