ABSTRACT
From earlier chapters about HPLC (Chapters 2, 3, and 4), you will be familiar with the general concept of chromatography and with the theory that underpins it. When a gas is used as the mobile phase, the process is described as gas chromatography (GC). The other (stationary) phase is usually a liquid, although this is not apparent as it is invariably very viscous and spread very thinly over the walls of a capillary tube. The word liquid is nowadays generally omitted from the earlier title of gas-liquid chromatography. Only a simple explanation of why GC works will be included here
6.1 Introduction ......................................................................................................................... 123 6.2 Why Gas Chromatography Works ......................................................................................124 6.3 Factors That Affect the Chromatography ........................................................................... 125 6.4 Choices in GC ..................................................................................................................... 125
6.4.1 Stationary Phase....................................................................................................... 125 6.4.2 Mobile Phase ............................................................................................................ 127 6.4.3 Column Length ........................................................................................................ 128 6.4.4 Column Diameter ..................................................................................................... 128 6.4.5 Film Thickness ........................................................................................................ 128 6.4.6 Flow Rate ................................................................................................................. 128 6.4.7 Temperature ............................................................................................................. 129 6.4.8 Some Rules of Thumb ............................................................................................. 129
6.5 GC Hardware ...................................................................................................................... 130 6.5.1 Pneumatics ............................................................................................................... 131 6.5.2 Sample Introduction ................................................................................................. 131
6.5.2.1 Split Injection ............................................................................................. 131 6.5.2.2 Splitless Injection ....................................................................................... 132 6.5.2.3 Cool/Direct On-Column Injection ............................................................. 133 6.5.2.4 The Programmed Temperature Vaporisation Injector (PTV) .................... 133
6.5.3 Detectors .................................................................................................................. 134 6.5.3.1 Flame Ionisation Detector (FID) ................................................................ 134 6.5.3.2 Nitrogen Phosphorus Detector (NPD) ........................................................ 135 6.5.3.3 Electron Capture Detector .......................................................................... 136 6.5.3.4 Mass Selective Detector ............................................................................. 137
6.6 GC-MS Inlets ...................................................................................................................... 138 6.7 Derivatisation for GC .......................................................................................................... 138 6.8 A GC Strategy for Bioanalysis ............................................................................................ 139 Bibliography ................................................................................................................................... 140 Further Reading ............................................................................................................................. 140
because there are many textbooks devoted to the subject, most of which carry a detailed theory of the chromatographic process as it applies in GC. The book Analytical Gas Chromatography (Jennings 1987) is particularly recommended. Capillary GC will be described here. Packed columns are now largely defunct because of the considerable improvement in efciency that the open-tube systems provide. Although capillary GC continues to be used as a term of reference, purists argue that the newer wide-bore tubes can no longer be described as capillaries. Open-tubular (OT) GC is the more correct term and will be used here.
