Gas Chromatography Mass Spectrometry

GC-MS is still the mainstay confirmation technique in drugs of abuse testing, although in recent years there has been a greater use of LC and tandem MS. The successful interface of a GC to the mass spectrometer offers the advantage of combining chromatographic separation and spectral analysis for identification of analytes. For confirmation of drugs of abuse, electron ionization is the most common form of ionization. The molecules in the ion source, which is under vacuum, are exposed to a beam of electrons, resulting in the loss of an electron from a molecule to form a positively charged molecular ion (M+). The molecular ion is usually fragmented due to the high energy of the electrons (70 eV). The fragmentation pattern (the mass spectrum) is characteristic of a molecule and can be used for identification of the compound. This can be accomplished by comparing the unknown spectrum to a library of mass spectra of known reference standards. A search algorithm helps to identify the unknown and provides an indication of quality of match of the mass spectra ("full spectral matches" or "full scan mode"). As the typical confirmation procedures in drugs of abuse testing are for a limited number of drugs or metabolites, the structural characteristics of which are well known, identification can be based on retention time data combined with the monitoring of several selected characteristic ions (usually 3) in place of full spectral matching. This mode of identification, called selected ion monitoring (SIM), is based on comparing the ratios of these ions ("ion ratios") with those obtained from a known reference standard. Because only a few mass fragments, not the whole spectrum of fragments, are analyzed, the data acquisition time for each of the selected ions is longer, allowing collection of more data points and improving signal-to-noise ratio. Therefore, SIM mode analysis is a more sensitive technique than full scan mode. For quantitative analysis in the SIM mode, the internal standard method is most often used. Because a mass spectrometer is capable of separating ions by mass, the best internal standards are the stable-isotope analogs of the analyte of interest. If the isotope is deuterium, then each molecule or fragment containing a deuterium atom instead of hydrogen will have one additional mass unit which can easily be discerned by the mass spectrometer.

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