Gas chromatography-mass spectrometry
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Gas chromatography-mass spectrometry (GC-MS) is a method that combines the features of gas-liquid chromatography and mass spectrometry to identify different substances within a test sample. Applications of GC-MS include drug detection, fire investigation, environmental analysis, and explosives investigation. GC-MS can also be used in airport security to detect substances in luggage or on human beings. Additionally, it can identify trace elements in materials that were previously thought to have disintegrated beyond identification.
The GC-MS has been widely heralded as a "gold standard" for forensic substance identification because it is used to perform a specific test. A specific test positively identifies the actual presence of a particular substance in a given sample. A non-specific test, however, merely indicates that a substance falls into a category of substances. Although a non-specific test could statistically suggest the identity of the substance, this could lead to false positive identification.
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[edit] History
The use of a mass spectrometer as the detector in gas chromatography was developed during the 1960s. These sensitive devices were bulky, fragile, and originally limited to laboratory settings. The development of affordable and miniaturized computers has helped in the simplification of the use of this instrument, as well as allowed great improvements in the amount of time it takes to analyse a sample. In 1996 the top-of-the-line high-speed GC-MS units completed analysis of fire accelerants in less than 90 seconds, whereas first-generation GC-MS would have required at least 16 minutes. This has led to their widespread adoption in a number of fields.
[edit] Instrumentation
The GC-MS is composed of two major building blocks: the gas chromatograph and the mass spectrometer. The gas chromatograph uses the difference in the chemical properties between different molecules in a mixture to separate the molecules. The molecules take different amounts of time (called the retention time) to come out of the gas chromatograph, and this allows the mass spectrometer downstream to evaluate the molecules separately in order to identify them. The mass spectrometer does this by breaking each molecule into ionized fragments and detecting these fragments using their mass to charge ratio. Each molecule has a specific fragment spectrum which allows for its detection.
These two components, used together, allow a much finer degree of substance identification than either unit used separately. It is possible to make an accurate identification of a particular molecule by gas chromatography or mass spectrometry alone. The mass spectrometry process normally requires a very pure sample while gas chromatography can be confused by different molecular types that both happen to take about the same amount of time to travel through the unit (i.e. have the same retention time). Sometimes two different molecules can also have a similar pattern of ionized fragments in a mass spectrometer (mass spectrum). Combining the two processes makes it extremely unlikely that two different molecules will behave in the same way in both a gas chromatograph and a mass spectrometer. So when an identifying mass spectrum appears at a characteristic retention time in a GC-MS analysis, it is usually taken as proof of the presence of that particular molecule in the sample.
[edit] Analysis
The primary goal of chemical analysis is to identify a substance. This is done by comparing the relative concentrations among the atomic masses in the generated spectrum. Two kinds of analysis are possible, comparative and original. Comparative analysis essentially compares the given spectrum to a spectrum library to see if its characteristics are present for some sample in the library. This is best performed by a computer because there are a myriad of visual distortions that can take place due to variations in scale. Computers can also simultaneously correlate more data (such as the retention times identified by GC), to more accurately relate certain data.
Another analysis measures the peaks in relation to one another, with the tallest peak receiving 100% of the value, and the others receiving proportionate values, with all values above 3% being accounted for. The parent peak normally indicates the total mass of the unknown compound. This value can then be used to fit to a chemical formula containing the various elements assumed to be present in the compound. The isotope pattern in the spectrum, which is unique for elements having many isotopes, can also be used to identify the various elements present. Once a chemical formula has been matched to the spectrum, the molecular structure and bonding can be identified, and needs to be consistent with a substance with the characteristics recorded by GC/MS. The fitting is normally done automatically by programmes which come with the machine, given a list of the elements which could be present in the sample.
A “full spectrum” analysis considers all the “peaks” within a spectrum. However, selective ion monitoring (SIM) which looks only at a few characteristic peaks associated with a candidate substance, can also be done. This is done on the assumption that at a given retention time, a set of ions is characteristic of a certain compound. This is a fast and efficient analysis, especially if you have some prior information about a sample or are looking for a specific compound. When the amount of information collected about the ions in a given gas chromatographic peak is reduced, the sensitivity of the analysis goes up. So, SIM analysis allows a smaller quantity of a compound to be detected and measured, but the degree of certainty about the identity of that compound is reduced.
[edit] Applications
[edit] Environmental Monitoring and Cleanup
GC-MS is becoming the tool of choice for tracking organic pollutants in the environment. The cost of GC-MS equipment has fallen significantly, and the reliability has increased at the same time, which has contributed to its increased adoption in environmental studies as cost is always a major consideration in this kind of work. There are some compounds for which GC-MS is not sufficiently sensitive, including certain pesticides and herbicides, but for most organic analysis of environmental samples, including many major classes of pesticides, it is very sensitive and effective.
[edit] Criminal Forensics
GC-MS can analyze the particles from a human body in order to help link a criminal to a crime. The analysis of fire debris using GC-MS is well established, and there is even an established American Society for Testing Materials (ASTM) standard for fire debris analysis.
[edit] Law Enforcement
GC-MS is increasingly used for detection of illegal narcotic, and may eventually supplant drug-sniffing dogs.
[edit] Security
A post-September 11 development, explosives-detection systems have become a part of all US airports. These systems run on a host of technologies, many of them based on GC-MS. There are only three manufacturers certified by the FAA to provide these systems (citation needed), one of which is Thermo Detection (formerly Thermedics), which produces the EGIS, a GC-MS-based line of explosives detectors. The other two manufacturers are Barringer Technologies, now owned by Smith's Detection Systems and Ion Track Instruments, part of General Electric Infrastructure Security Systems.
[edit] Astrochemistry
Several GC-MS have left earth. Two were brought to mars by the Viking program.[1] Venera 11 and 12 and Pioneer Venus analysed the atmosphere of venus with GC-MS.[2] The Huygens probe of the Cassini-Huygens mission landed one GC-MS on Saturn's largest moon, Titan.[3] The material in the comet 67P/Churyumov-Gerasimenko will be analysed by the Rosetta mission with a chiral GC-MS in 2014. [4]
[edit] Medicine
In combination with isotopic labelling of metabolic compounds, the GC-MS is used for determining metabolic activity. Most applications are based on the use of C13 as the labelling and the measurement of C13/C12 ratios with an isotope ratio mass spectrometer (IRMS); an MS with a detector designed to measure a few select ions and return values as ratios.
[edit] See also
[edit] External links
[edit] References
- Eiceman, G.A. (2000). Gas Chromatography. In R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry: Applications, Theory, and Instrumentation, pp. 10627. Chichester: Wiley. ISBN 0-471-97670-9
- Giannelli, Paul C. and Imwinkelried, Edward J. (1999). Drug Identification: Gas Chromatography. In Scientific Evidence 2, pp. 362. Charlottesville: Lexis Law Publishing. ISBN 0-327-04985-5.
- ^ The Development of the Viking GCMS
- ^ V. A. Krasnopolsky, V. A. Parshev (1981). "Chemical composition of the atmosphere of Venus". Nature 292: 610 - 613. DOI:10.1038/292610a0.
- ^ H. B. Niemann, S. K. Atreya, S. J. Bauer, G. R. Carignan, J. E. Demick, R. L. Frost, D. Gautier, J. A. Haberman, D. N. Harpold, D. M. Hunten, G. Israel, J. I. Lunine, W. T. Kasprzak, T. C. Owen, M. Paulkovich, F. Raulin, E. Raaen, S. H. Way (2005). "The abundances of constituents of Titan’s atmosphere from the GCMS instrument on the Huygens probe". Nature 438: 77-9-784. DOI:10.1038/nature04122.
- ^ Goesmann F, Rosenbauer H, Roll R, Bohnhardt H (2005). "COSAC onboard Rosetta: A bioastronomy experiment for the short-period comet 67P/Churyumov-Gerasimenko". Astrobiology 5 (5): 622-631. DOI:10.1089/ast.2005.5.622.
