NMSU: GC/MS

New Mexico State University

College of Arts and Sciences

Department of Chemistry and Biochemistry

Introduction to Gas Chromatography Mass Spectrometry

5873 + autosampler The capabilities of integrated gas chromatography mass spectrometry are almost unique in meeting the requirements for analytical methods which are exquisitely sensitive but also specific and reliable in providing information about the biological effects of specific compounds as a function of their concentrations. The instrument at the left is used for instructional and research and is located in room 190A.

Gas chromatography is generally recognized as one of the most powerful separative methods available, and is widely employed in biomedical and environmental research. Although several selective detectors have been described, the identity of the separated components which elute must generally be inferred from their retention times relative to known standards. Mass spectrometry is a complementary technique in that it can provide a great deal of information about the identity of a compound provided that a pure sample is available, and may be sufficient to identify it rigorously. The combined system (GC/MS) can utilize both instruments to maximum advantage and, under favorable conditions can identify every major component of a complex mixture.

The GC portion of this system provides high resolution separation of volatile organic solutes in a mixture in the gas phase. As each solute exits the GC column, it is diverted into a mass spectrometer which is capable of both monitoring the amount of and identifying the chemical nature of the solute. In this way, both quantitative and qualitative information about the mixture can be obtained.

The MS portion of the system takes each gaseous solute exiting the GC and ionizes it in an electron
beam. The ions formed by a specific solute will depend on the nature of the bonds in the molecule, and both ionized molecules and ion fragments of the molecule are possible. The ions are then directed down a separator which isolates and counts the ions according to mass. The sequence and relative intensity of the mass peaks give information about the chemical identity of the solute. The absolute intensity of the peaks provides information about the amount of substance present.

Background for GC/MS

Historical Information on Gas Chromatography/Mass Spectrometry

The use of a mass spectrometer as the detector in gas chromatography was developed during the 1950s by Roland Gohlke and Fred McLafferty. 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 analyze a sample. In 1996 the top-of-the-line high-speed GC-MS units completed analysis in 90 seconds, whereas first-generation GC-MS would have required at 15 minutes. This has led to their widespread adoption in a number of fields. Several reference are good good general sources and visual descriptions.

The Gas Chromatography/Mass Spectrometry (GC/MS) instrument separates chemical mixtures by GC and identifies the components at a molecular level by MS. It is one of the most accurate tools for analyzing environmental samples. The GC works on the principle that a mixture will separate into individual substances when heated. The heated gases are carried through a column with an inert gas (such as helium). As the separated substances emerge from the column opening, they flow into the MS. Mass spectrometry identifies compounds by the mass of the analyte molecule. A “library” of known mass spectra, covering several thousand compounds, is stored on a computer. Mass spectrometry is considered the only definitive analytical detector.

GC/MS can handle mixtures. The GC effluent is directed into the mass spectrometer, where a spectrum of each component is obtained as it elutes from the column. Just as an FID gives a response based on the number of ions formed as sample elutes from the column, so does GC/MS, in the form of a total ion chromatogram (TIC). The difference is that each point in the chromatogram is actually a mass spectrum. Thus a mass spectrum can be retrieved for any peak in the chromatogram. Conversely, a selected ion chromatogram can be retrieved for any mass. For example, aromatic compounds tend to give tropylium ion ([C₇H₇]⁺) at mass 91. A selected ion chromatogram of mass 91 will show the chromatographic peaks attributable to aromatic compounds. EI is usually used for ionization, but CI and FI can also be used. In EI or CI, high-resolution measurements are possible. The components of a GC/MS instrument are shown below

1. Sample Introduction through the gas chromatograph.

2. The mass spectrograph components are kept under a vacuum, the ion source, mass analyzer, and detector information processed by a computer as

3. Date Output.

GC/MS Block Diagram

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