Dispersion in Chromatography Columns - Summary > Page 97


In summary, it can be said that there are a number of dispersion processes that take place in a column and for packed columns the dispersion can be best described by the Giddings equation. This equation gives the total dispersion as function of the mobile phase velocity and other pertinent physical properties of the column and distribution system. However, for columns operated in the vicinity of the optimum velocity (where the best performance is to be realized) the van Deemter equation is simpler and give equally accurate and precise calculated data. In GC columns, the compressibility of the mobile phase must be taken into account and the exit mobile phase velocity (not the mean velocity) employed in the dispersion function. In addition, the diffusivity of the solute must be taken at atmospheric pressure. Only the Van Deemter equation, the Giddings equation and the Knox equation fit experimental (H) versus (u) data accurately and only the Van Deemter equation and the Giddings equation correctly account for other physical properties of the chromatographic system. The Van Deemter equation appears to be a special case of the Giddings equation, which simplifies to the Van Deemter equation when the mobile phase velocity is close to, or around, the optimum mobile phase velocity. The form of the Van Deemter equation and, in particular, the individual 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.