Dispersion in Chromatography Columns - Effect of Mobile Phase Compressibility On the HETP Equation for a Packed GC Column > Page 42
Equation (30) gives the variance per unit length of a GC column in terms of the outlet pressure (atmospheric); the outlet velocity; and physical and physicochemical properties of the column, packing, and phases and is independent of the inlet pressure. However, equation (28) is the recommended form for HETP measurements as the inlet pressure of a column is usually known, (and consequently (g), the inlet/outlet pressure ratio is also known) and the equation is less complex and easier to use. The important aspect of this development is that the resistance to mass transfer in the stationary phase is seen to be a function of the inlet-outlet pressure ratio (g).
Regrettably, the average velocity is the variable that is almost universally used in constructing HETP curves in both GC and LC. This is largely because it is simple to calculate from the ratio of the column length to the dead time. Unfortunately, in GC, the use of the average velocity provides very erroneous data and, for accurate column evaluation and column design, the exit velocity must be employed together in conjunction with the inlet-outlet pressure ratio. An example of the errors that can occur from the use of the average velocity, as opposed to the exit velocity, is shown in figure 9 from data obtained for a capillary column.