GCMShandout_000 - Gas Chromatography-Mass Spectrometry Gas

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Gas Chromatography-Mass Spectrometry
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Gas Chromatographic-Mass Spectrometry Is Olive Oil the healthiest oil? Introduction Gas chromatographic-mass spectrometry (GC-MS) is a very powerful and ubiquitous analytical technique. It is often the analytical method of choice in toxicology, forensics, food science, and environmental research. In essence, this hybrid instrument replaces the traditional thermal-conductivity detector (TCD) or flame-ionization chromatographic detector (FID) with a very sensitive and information-rich mass spectrometer (MS). Not only can a GC-MS separate the volatile components of complex mixtures, but it can also record a mass spectrum of each component. This hybrid instrument provides two separate dimensions of information about the components in the sample, GC retention times and electron ionization (EI) mass spectra. GC retention time is related to specific chemical properties of the molecules in question (e.g. volatility, polarity, presence of specific functional groups) while molecular weight (derived from the mass spectrum) is indicative of atomic composition. Chromatographic techniques separate mixtures of species based on their interactions with a stationary phase and a mobile phase. In the A315 gas chromatography experiment, volatile and semi-volatile species are vaporized into a flow of helium (mobile phase) and blown through a fused silica open tubular capillary (stationary phase). The capillary is usually derivatized to control its polarity (ranging from hydrophobic dimethylsiloxane to hydrophilic polyethylene glycol or any of a wide array of specialty coatings). The gas phase molecules interact with the surface and exist in dynamic equilibrium between being adsorbed onto the inner wall of the column and dissolved in the carrier gas. Gas chromatographs most often use temperature to control this equilibrium (higher temperatures shift the equilibrium more towards the mobile phase while lower temperatures shift it to the stationary phase). This is analogous to changing the polarity of the solvent mixture in thin-layer chromatography (TLC). Various components of mixtures will have different equilibria with the stationary phase and will consequently move through the column at different rates. The temperature can be left constant for maximum resolution or varied to minimize analysis time. A detector at the end of the column records the amount of time required for a given compound to elute off of the column (cf. RF values in TLC). With the proper conditions and column chemistries, very similar compounds can be separated by GC (e.g. isomers of dichlorobenzene, cis- and trans- fatty acids, racemic mixtures, etc.). The GC-MS places an electron ionization quadrupole mass spectrometer at the end of the column. The mass spectrum provides two valuable pieces of information; the retention time and the EI mass spectrum. Molecules eluting from the column are directed into the source of the
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This note was uploaded on 07/01/2011 for the course A 315 taught by Professor Arnold during the Fall '09 term at Indiana State University .

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GCMShandout_000 - Gas Chromatography-Mass Spectrometry Gas

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