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The terminology that will be used throughout this chapter will refer to the subject as wall-coated open tubular (WCOT) columns, expressing the fact that the openness of these columns is their most important characteristic.1 Further, the term WCOT is now used almost universally to specify columns in which the liquid phase is spread directly on the inner wall of a glass capillary tube without the addition of particles that might be considered solid support. Columns in which the coating is deposited on a surface that has been considerably extended by macro-particle deposits during the drawing process are known as porous layer open tubular (PLOT) columns. The term support-coated open tubular (SCOT) columns refers to columns in which the wall has been coated with a mixture of finely divided microparticles, usually silicagel and alumina and a known amount of a liquid phase.2 Comprehensive reviews have been published.3,4

The retention behavior of a compound contains the qualitative information available from gas chromatography. This has long been recognized and stimulated many efforts to establish useful concepts for developing systematic data broadly applicable to analytical problems. The concept of specific retention volume was suggested to make retention data essentially independent of the variable chromatographic parameters.1 Its calculation of the volume of carrier gas reduced to 0 °C required to elute a compound takes into account the column temperature, carrier gas flow rate, column pressure drop, and the amount of liquid phase present. Although it is theoretically correct, specific retention volume is not used in practice because it is awkward to calculate, does not allow for retention changes with temperature, and calls for values that may change. The use of relative retention data is at present the most accepted method. In the determination of relative retention, the retention of both a reference compound and the sample components are determined under identical chromatographic conditions. Since it is almost impossible to establish only one compound to be a reference, a more universal method is needed for wide usage. It is known that under isothermal conditions a plot of the logarithm of the adjusted retention time t′ R versus molecular weight of a homologous series is linear.