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The coupling of gas chromatography (GC) and mass spectrometry (MS) has resulted in the extremely powerful technique of GG/MS, which is used extensively in research laboratories, particularly in synthetic organic chemistry and the development of pharmaceuticals. It is also used for the identification and quantification of trace amounts of environmental contaminants. A GC/MS method is used for the urinalysis screening carried out on Olympic and other athletes to check for drug use.
For many years GC/MS techniques were too expensive and complicated to be used by undergraduates, except perhaps for individual research projects. However, with the development of reasonable-cost, computer-controlled GC/MS systems it has become feasible to introduce GC/MS methodology into the undergraduate curriculum. A number of articles published in this Journal describe GC/MS experiments designed for use in undergraduate teaching laboratories (1-14). These include experiments in which GC/MS is used to identify individual substances, mixtures, or reaction products (1-9), or to follow the kinetics of certain reactions (10-12). One experiment allows the identification of amino acids in a small peptide (13) and another describes the extraction and identification of plasticizers from a number of different polymers (14).
This article describes a laboratory experiment that was developed for advanced undergraduate students. In it the students are provided with a mixture of unknown brominated and chlorinated compounds. They separate the compounds and obtain mass spectra of the individual components in the mixture using a GC/MS method. Armed with the information that each of the unknowns contains at least one and possibly as many as four chlorine or bromine atoms, the students interpret the mass spectral data to obtain the identity of the unknowns. The interpretation of the mass spectra (identification of molecular ions and major fragment ions) is simplified by the presence of the varying numbers of chlorine and bromine atoms, which produce patterns of peaks in the mass spectra. The relative intensities of the peaks within the patterns are characteristic of the number of chlorine or bromine atoms.
Experimental Section
Chemicals
The unknown compounds used in the experiment are listed in the table. Tetrachloroethylene was obtained from NMR Specialties.^sup 1^ The solvent was GC Resolv grade hexane and was obtained from Fisher.^sup 2^ The rest of the chemicals were obtained from Aldrich.^sup 3^
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