Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column  As the pressure falls along the column length, the velocity changes and, as the solute diffusivity depends on the pressure, the diffusivity of the solute will also change. The multi-path term, which contains no velocity or gas pressure dependent parameters, will be unaffected and the expression that describes it the same. The other terms in the  HETP equation, however, all contain parameters that are affected by gas

The Compressibility of the Mobile Phase By measuring the retention volume of a solute, the distribution coefficient can be obtained. The distribution coefficient, determined over a range of temperatures, is often used to determine the thermodynamic properties of the system; this will be discussed in The Thermodynamics of Chromatography . Thermodynamic studies are used largely as a diagnostic tool to investigate the nature of the distribution. Thus, an accurate measurement of (V'r) can be

functions contained in it, are well substantiated by experiment. The Knox equation is obtained from an empirical fit to experimental data and the individual functions of other pertinent variables contained in the equation are not all substantiated by experiment. The Golay equation accurately described dispersion in capillary or open tubular columns but in GC the compressibility of the mobile phase must also be taken into account (Golay in his original derivation did not accommodate gas compressibility). It would appear from the data available at this time, that the Van Deemter equation (for packed columns) or the Golay equation (for capillary or open tubular columns) would be the most appropriate to use in column design and in the interpretation of column properties

Peak Dispersion in a Chromatographic Column The first comprehensive approach to dispersion in chromatographic columns was taken by Van Deemter (8) who developed the dispersion equation for a packed GC column. Van Deemter's development did not take into account the compressibility of the mobile phase which was dealt with later by Katz, Ogan and Scott (9). A simple form of this theory will be given that does not accommodate the compressibility of the mobile phase but a more detailed and comprehensive treatment is given in Dispersion in Chromatography Columns and Extra Column Dispersion. Van Deemter et al. assumed that there were four band spreading processes responsible for peak dispersion, namely, multi-path dispersion, longitudinal

;  or              Thus,                                                   (27) Thus, the complete HETP equation for a packed GC column, that takes into account the compressibility of the carrier gas, will be          (28

It is seen that using the average velocity data, the extracted value for the multi-path term is negative, which is physically impossible (for a capillary column should be zero or very close to zero). In contrast, the values obtained from data involving the exit velocity give small positive, but realistic values for the multi-path term. In all aspects of column evaluation and column design in GC, the compressibility of the mobile phase must be taken into account or serious errors will be incurred.                     Figure 10. De-Convolution of the HETP Curve Obtained Using the Average Mobile Phase Velocity