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 be 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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History
The use of a mass spectometer as the detector of gas chromatography was developed during the 1960's. Affordable and miniaturized computers were being developed at the same time, and rapidly were applied to GC-MS to process the information produced. Originally, these sensitve devices were bulky, fragile, and limited to laboratory settings. New innovations in GC-MS technology are bringing the technology into the field.
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.
Instrumentation
The GC-MS is composed of two major building blocks: the gas chromatograph and the mass spectrometer. The gas chromatograph uses chemical differences to separate the molecules within the compound sample by controlling the time required for different kinds of molecules to arrive at the mass spectrometer. The mass spectrometer then breaks each molecule into ionized fragments, and identifies each molecule from the charge and mass of the ionized fragments.
These two components, used together, allow a much finer degree of substance identification then either unit used separately. It is possible to make an accurate identification of a particular molecule by gas chromatography alone. The same is true for mass spectrometry, although this identification requires a very pure sample compared to gas chromatographic identification. 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 (retention time). Sometimes two different molecules can have a similar pattern of ionized fragments in a mass spectrometer (mass spectrum). But it is 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 a particular molecule in the sample.
Analysis
The primary goal of analysis is to identify a substance. This is done by interpreting the generated spectrum by comparing the relative concentrations among the atomic masses. Two kinds of analysis are possible: comparative and original. Comparative analysis essentially compares the spectrum associated with various compounds from a spectrum library to see if its characteristics are present within the sample. This is best performed by computer because there are a myriad of visual distortions that can take place due to variations in scale. Also, because the conditions used to generate the library will probably not be the same visually, human analysis is error prone. Computers can 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, the tallest receiving 100% of the value, and the others receiving proportionate values—all values above 3% must be accounted for. The parent peak indicates the total mass of the unknown compound and reflects the highest mass on the spectrum. Based on this value, the analyst must attempt to fit all the total masses of the other particles into that value. Beyond that, a molecular structure and bonding must be defined, consistent with a substance with the characteristics recorded by GC/MS. This method too is best performed by computer because of possibility for human error and because the computer can exhaust all mathematical possibilities in making the analytical determinations. A computer can effectively and rather instantly do both processes with the lowest rate of error and the maximum confirmation.
A “full spectrum” analysis considers all the “peaks” within a spectrum. However, another method is selective ion monitoring (SIM), which looks only at a few characteristic peaks associated with a candidate substance. A lab does this on the assumption that at a given retention time, a set of ions is characteristic of a certain subject. It is fast and efficient which makes the analysis less expensive for the lab, but this is a scientific hypothesis in itself. It should not be surprising that 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.
Applications
Environmental Monitoring and Cleanup
GC-MS is becoming the tool of choice for tracking organic pollutants in the environment. Cost is always a major consideration in environmental work, and other less-expensive methods of organic analysis were once commonly used. But the cost of GC-MS equipment has fallen significantly, and reliability has increased at the same time. There are some compounds for which GC-MS is not sufficiently sensitive, such as pesticides and herbicides. But most organic analysis of environmental samples is now performed by GC-MS.
Criminal Foresics
GC-MS can analyze the particles from a human body in order to link a criminal to a crime. The analysis of fire debris using GC-MS is “well established” and there has even been established American Society for Testing Materials (ASTM) standard for fire debris analysis.
Law Enforcement
GC-MS is increasingly used for detection of illegal narcotics, and may eventually supplant drug-sniffing dogs.
Security
A post-September 11th development, explosives-detection systems will soon be part of all US airports. The post-9/11 Transportation Security Agency is installing hardware to screen of 100% of passenger baggage at all of the US’s 429 airports by December 31, 2002. This will involve the deployment of 4,700-6,000 trace detection systems nation-wide, and will require $427 million in additional spending. While full-blown explosive detection systems cost roughly $1,000,000 each, explosives trace detection systems cost roughly $50,000. These bargain-priced 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, one of which is Thermo Detection (formerly Thermedics), which produces the EGIS, a GC-MS-based line of explosives detectors.
See also
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 0327049855.