Gas chromatography–mass spectrometry
Gas chromatography–mass spectrometry (GC–MS) is an analytical method that combines the features of gas-chromatography and mass spectrometry to identify different substances within a test sample.[1] Applications of GC–MS include drug detection, fire investigation, environmental analysis, explosives investigation, food and flavor analysis, and identification of unknown samples, including that of material samples obtained from planet Mars during probe missions as early as the 1970s. 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. Like liquid chromatography–mass spectrometry, it allows analysis and detection even of tiny amounts of a substance.[2]
GC–MS has been regarded as a "gold standard" for forensic substance identification because it is used to perform a 100% specific test, which positively identifies the presence of a particular substance. A nonspecific test merely indicates that any of several in a category of substances is present. Although a nonspecific test could statistically suggest the identity of the substance, this could lead to false positive identification. However, the high temperatures (300°C) used in the GC–MS injection port (and oven) can result in thermal degradation of injected molecules,[3] thus resulting in the measurement of degradation products instead of the actual molecule(s) of interest.
History[edit]
The first on-line coupling of gas chromatography to a mass spectrometer was reported in the late 1950s.[4][5] An interest in coupling the methods had been suggested as early as December 1954.[6] 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 analyze a sample. In 1964, Electronic Associates, Inc. (EAI), a leading U.S. supplier of analog computers, began development of a computer controlled quadrupole mass spectrometer under the direction of Robert E. Finnigan.[7] By 1966 Finnigan and collaborator Mike Uthe's EAI division had sold over 500 quadrupole residual gas-analyzer instruments.[7] In 1967, Finnigan left EAI to form the Finnigan Instrument Corporation along with Roger Sant, T. Z. Chou, Michael Story, Lloyd Friedman, and William Fies.[8] In early 1968, they delivered the first prototype quadrupole GC/MS instruments to Stanford and Purdue University.[7] When Finnigan Instrument Corporation was acquired by Thermo Instrument Systems (later Thermo Fisher Scientific) in 1990, it was considered "the world's leading manufacturer of mass spectrometers".[9]
Applications[edit]
Environmental monitoring and cleanup[edit]
GC–MS is becoming the tool of choice for tracking organic pollutants in the environment. The cost of GC–MS equipment has decreased significantly, and the reliability has increased at the same time, which has contributed to its increased adoption in environmental studies.
Criminal forensics[edit]
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 and Materials (ASTM) standard for fire debris analysis. GCMS/MS is especially useful here as samples often contain very complex matrices and results, used in court, need to be highly accurate.
Law enforcement[edit]
GC–MS is increasingly used for detection of illegal narcotics, and may eventually supplant drug-sniffing dogs.[1] A simple and selective GC–MS method for detecting marijuana usage was recently developed by the Robert Koch Institute in Germany. It involves identifying an acid metabolite of tetrahydrocannabinol (THC), the active ingredient in marijuana, in urine samples by employing derivatization in the sample preparation.[22] GC–MS is also commonly used in forensic toxicology to find drugs and/or poisons in biological specimens of suspects, victims, or the deceased. In drug screening, GC–MS methods frequently utilize liquid-liquid extraction as a part of sample preparation, in which target compounds are extracted from blood plasma.[23]
Sports anti-doping analysis[edit]
GC–MS is the main tool used in sports anti-doping laboratories to test athletes' urine samples for prohibited performance-enhancing drugs, for example anabolic steroids.[24]
Security[edit]
A post–September 11 development, explosive 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, 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.
Chemical warfare agent detection[edit]
As part of the post-September 11 drive towards increased capability in homeland security and public health preparedness, traditional GC–MS units with transmission quadrupole mass spectrometers, as well as those with cylindrical ion trap (CIT-MS) and toroidal ion trap (T-ITMS) mass spectrometers have been modified for field portability and near real-time detection of chemical warfare agents (CWA) such as sarin, soman, and VX.[25] These complex and large GC–MS systems have been modified and configured with resistively heated low thermal mass (LTM) gas chromatographs that reduce analysis time to less than ten percent of the time required in traditional laboratory systems.[26] Additionally, the systems are smaller, and more mobile, including units that are mounted in mobile analytical laboratories (MAL), such as those used by the United States Marine Corps Chemical and Biological Incident Response Force MAL and other similar laboratories, and systems that are hand-carried by two-person teams or individuals, much ado to the smaller mass detectors.[27] Depending on the system, the analytes can be introduced via liquid injection, desorbed from sorbent tubes through a thermal desorption process, or with solid-phase micro extraction (SPME).
Chemical engineering[edit]
GC–MS is used for the analysis of unknown organic compound mixtures. One critical use of this technology is the use of GC–MS to determine the composition of bio-oils processed from raw biomass.[28] GC–MS is also utilized in the identification of continuous phase component in a smart material, magnetorheological (MR) fluid.[29]
Food, beverage and perfume analysis[edit]
Foods and beverages contain numerous aromatic compounds, some naturally present in the raw materials and some forming during processing. GC–MS is extensively used for the analysis of these compounds which include esters, fatty acids, alcohols, aldehydes, terpenes etc. It is also used to detect and measure contaminants from spoilage or adulteration which may be harmful and which is often controlled by governmental agencies, for example pesticides.
Astrochemistry[edit]
Several GC–MS systems have left earth. Two were brought to Mars by the Viking program.[30] Venera 11 and 12 and Pioneer Venus analysed the atmosphere of Venus with GC–MS.[31] The Huygens probe of the Cassini–Huygens mission landed one GC–MS on Saturn's largest moon, Titan.[32] The MSL Curiosity rover's Sample analysis at Mars (SAM) instrument contains both a gas chromatograph and quadrupole mass spectrometer that can be used in tandem as a GC–MS.[33] The material in the comet 67P/Churyumov–Gerasimenko was analysed by the Rosetta mission with a chiral GC–MS in 2014.[34]
Medicine[edit]
Dozens of congenital metabolic diseases also known as inborn errors of metabolism (IEM) are now detectable by newborn screening tests, especially the testing using gas chromatography–mass spectrometry. GC–MS can determine compounds in urine even in minor concentration. These compounds are normally not present but appear in individuals suffering with metabolic disorders. This is increasingly becoming a common way to diagnose IEM for earlier diagnosis and institution of treatment eventually leading to a better outcome. It is now possible to test a newborn for over 100 genetic metabolic disorders by a urine test at birth based on GC–MS.
In combination with isotopic labeling of metabolic compounds, the GC–MS is used for determining metabolic activity. Most applications are based on the use of 13C as the labeling and the measurement of 13C-12C 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.