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GAS CHROMATOGRAPHY | Mass Spectrometry
106 GAS CHROMATOGRAPHY / Mass Spectrometry
Mass Spectrometry D J Harvey, University of Oxford, Oxford, UK & 2005, Elsevier Ltd. All Rights Reserved.
Introduction The coupling of gas chromatography (GC) with mass spectrometry (MS), techniques that both employ the sample in the gas phase, was first achieved in 1957. Since then, GC–MS has developed into one of the most sensitive and selective analytical methods for the separation, identification, and quantification of components of complex organic mixtures. It gives a two-dimensional identification consisting of both a GC retention time and a mass spectrum for each component of the mixture. This dual approach is particularly useful for isomer differentiation where good GC separation frequently compensates for difficulties encountered as a result of indeterminate mass spectra. Conversely, the mass spectrum allows structural features to be assigned to the compound producing the GC peak and often provides unambiguous structural assignment. Although the method is limited to the analysis of those compounds that can be made volatile, without thermal decomposition, many compounds that initially fail this requirement can be successfully handled after chemical derivatization. The early success in the development of this socalled hyphenated technique owes much to the general compatibility between the two methods in terms of sample size and volatility. The only major difference is that the exit from the gas chromatographic column is at low (10 6 Torr), rather than at atmospheric pressure. Nevertheless, modern systems provide excellent GC and MS performance without significant compromise to the behavior of either technique.
Instrumentation The instrumentation consists essentially of three components: the gas chromatograph, the mass spectrometer, and a data system (Figure 1). The latter component can be used both for data acquisition and processing, and for instrument control. In systems where packed GC columns are used, an interface is included between the column and the spectrometer to reduce the carrier gas flow from B30 ml min 1 to B1.5–2 ml min 1 so that it can be handled by the mass spectrometer’s vacuum system. However,
modern systems usually employ capillary columns made up of polyimide-coated fused silica that are interfaced directly with the mass spectrometer. In addition to the above equipment, GC–MS instruments may also be provided with additional features such as an autosampler, an infrared spectrometer, or a flame-ionization detector. The Gas Chromatograph
Almost any commercial gas chromatograph can be interfaced to a mass spectrometer and, although its operation is similar to that when used in the standalone mode, there are certain features that are specific to successful GC–MS operation. Column dimensions and carrier gas flows for packed columns tend to be similar to those normally used for stand-alone GC. However, with capillary columns that are interfaced without a separator, the carrier gas flow needs to be fairly low and, consequently, narrow-bore columns with internal diameters of 0.35 mm or less are generally used. Typical columns are 5–30 m long with internal diameters of 0.25 or 0.32 mm. Although columns with internal diameters of 0.53 mm can be used, these usually require a splitter to reduce the flow into the mass spectrometer. Care must be taken with the metal-coated columns, used for high-temperature work, as the metal can short out the ion source voltage. This problem can be avoided by removing the metal coating from the terminal 2–3 cm of the column. Both gas/solid adsorption and gas/liquid partition chromatography can be used for GC–MS, but GC is by far the most common. Because, in GC, the stationary phase is a liquid, usually a polymer, its vapor pressure will cause a continual low flow, or bleed into the ion source of the mass spectrometer. This bleed, which usually consists of decomposed stationary phase, will produce a spectrum whose intensity increases with column temperature. Stationary phases should therefore be of the high-boiling, low-bleed type. Most currently used stationary phases for routine GC–MS are based on alkyl-polysiloxanes or alkylphenyl-polysiloxanes that are chemically bonded to the column wall to increase stability. Columns containing such phases can, in some cases, be used at temperatures of up to 4001C. One advantage, however, to the presence of bleed peaks in the spectrum is that they enable a continual check to be made on the mass spectrometer calibration. For the alkyl siloxanes, ion peaks are present, in decreasing relative abundance, at m/z 73, 207, 281, 355, 429,
Mass Spectrometry D J Harvey, University of Oxford, Oxford, UK & 2005, Elsevier Ltd. All Rights Reserved.
Introduction The coupling of gas chromatography (GC) with mass spectrometry (MS), techniques that both employ the sample in the gas phase, was first achieved in 1957. Since then, GC–MS has developed into one of the most sensitive and selective analytical methods for the separation, identification, and quantification of components of complex organic mixtures. It gives a two-dimensional identification consisting of both a GC retention time and a mass spectrum for each component of the mixture. This dual approach is particularly useful for isomer differentiation where good GC separation frequently compensates for difficulties encountered as a result of indeterminate mass spectra. Conversely, the mass spectrum allows structural features to be assigned to the compound producing the GC peak and often provides unambiguous structural assignment. Although the method is limited to the analysis of those compounds that can be made volatile, without thermal decomposition, many compounds that initially fail this requirement can be successfully handled after chemical derivatization. The early success in the development of this socalled hyphenated technique owes much to the general compatibility between the two methods in terms of sample size and volatility. The only major difference is that the exit from the gas chromatographic column is at low (10 6 Torr), rather than at atmospheric pressure. Nevertheless, modern systems provide excellent GC and MS performance without significant compromise to the behavior of either technique.
Instrumentation The instrumentation consists essentially of three components: the gas chromatograph, the mass spectrometer, and a data system (Figure 1). The latter component can be used both for data acquisition and processing, and for instrument control. In systems where packed GC columns are used, an interface is included between the column and the spectrometer to reduce the carrier gas flow from B30 ml min 1 to B1.5–2 ml min 1 so that it can be handled by the mass spectrometer’s vacuum system. However,
modern systems usually employ capillary columns made up of polyimide-coated fused silica that are interfaced directly with the mass spectrometer. In addition to the above equipment, GC–MS instruments may also be provided with additional features such as an autosampler, an infrared spectrometer, or a flame-ionization detector. The Gas Chromatograph
Almost any commercial gas chromatograph can be interfaced to a mass spectrometer and, although its operation is similar to that when used in the standalone mode, there are certain features that are specific to successful GC–MS operation. Column dimensions and carrier gas flows for packed columns tend to be similar to those normally used for stand-alone GC. However, with capillary columns that are interfaced without a separator, the carrier gas flow needs to be fairly low and, consequently, narrow-bore columns with internal diameters of 0.35 mm or less are generally used. Typical columns are 5–30 m long with internal diameters of 0.25 or 0.32 mm. Although columns with internal diameters of 0.53 mm can be used, these usually require a splitter to reduce the flow into the mass spectrometer. Care must be taken with the metal-coated columns, used for high-temperature work, as the metal can short out the ion source voltage. This problem can be avoided by removing the metal coating from the terminal 2–3 cm of the column. Both gas/solid adsorption and gas/liquid partition chromatography can be used for GC–MS, but GC is by far the most common. Because, in GC, the stationary phase is a liquid, usually a polymer, its vapor pressure will cause a continual low flow, or bleed into the ion source of the mass spectrometer. This bleed, which usually consists of decomposed stationary phase, will produce a spectrum whose intensity increases with column temperature. Stationary phases should therefore be of the high-boiling, low-bleed type. Most currently used stationary phases for routine GC–MS are based on alkyl-polysiloxanes or alkylphenyl-polysiloxanes that are chemically bonded to the column wall to increase stability. Columns containing such phases can, in some cases, be used at temperatures of up to 4001C. One advantage, however, to the presence of bleed peaks in the spectrum is that they enable a continual check to be made on the mass spectrometer calibration. For the alkyl siloxanes, ion peaks are present, in decreasing relative abundance, at m/z 73, 207, 281, 355, 429,