Gas Chromatograph-mass Spectrometer

In a forensic examination, some sample material can be evaluated at the scene of the accident of crime. Other material, however, needs to be collected and taken to a dedicated laboratory for more sophisticated analyses using a variety of analytical instruments. In 1976, scientists William Keith Hadley and J. A. Zoro, in the United Kingdom, first suggested the use of gas chromatography/mass spectroscopy for forensic purposes and the instrument is now used for a variety of forensic purposes.

The GC/MS instrument helps separate and determine the individual elements and molecules in a sample. The GC/MS provides forensic investigators the ability to identify individual substances that may be found within a very small test sample. Forensic applications of GC/MS include identification and detection of explosives; investigations of arson, fire, and blasts or explosions; environmental analysis; and drug detection. In recent years, GC/MS has begun to be used in airport security areas as a means of detecting dangerous substances in luggage, on animals who are traveling, or on human beings. Because of its great sensitivity, gas chromatography/mass spectroscopy can be utilized to identify trace elements in either minute amounts of substances or in substances that were believed to be contaminated or to be degraded beyond usability.

The GC/MS is comprised of two parts: the gas chromatograph and the mass spectrometer. The gas chromatograph functions by separating the molecules within the sample compound into their most elemental particles, allowing some types of molecules to pass into the mass spectrometer more rapidly than others. When the molecules move into the mass spectrometer, they are broken down into ionized fragments, and then each molecule is specifically identified based on mass and ionic charge.

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.) Gas chromatography 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 spectroscopy 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, which has been called 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.

Mass spectroscopy 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.

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 on site now exist.

One of the reasons that the GC/MS test has a great deal of value in the world of forensic substance identification has to do with its specificity: the GC/MS can positively identify the presence of a suspected poison or other substance. The portability of GC/MS test equipment allows it to be taken directly from a crime scene to the area where a suspect is detained to immediately perform tests on a suspect's tissue, serology (blood), clothing, etc.