Dispersion in Chromatography Columns - Experimental Validation of the Van Deemter Equation > Page 78
When a satisfactory fit of the experimental data to a particular equation, is obtained the constants, (A), (B), (C) etc. must then be replaced by the explicit functions derived from the respective theory and which incorporate the respective physical properties of solute, solvent and stationary phase. Those physical properties of solute, solvent and stationary phase must then be varied in a systematic manner to change the magnitude of the constants (A), (B),(C) etc. The changes predicted by the equation under examination must then be compared with those obtained experimentally. The equation that satisfies both requirements can then be considered the true equation that describes band dispersion in a packed column.
The identification of the pertinent HETP equation must, therefore, be arrived at from the results of a sequential series of experiments. Firstly, all the equations must be fitted to a series of (H) and (u) data sets and those equations that give positive and real values for the constants of the equations identified. The explicit form of those equations that satisfy the preliminary data, must then be tested against a series of data sets that have been obtained from different chromatographic systems. Such systems might involve columns packed with different size particles or employ mobile phases or solutes having different but known physical properties.
Katz, et al. (24) measured the efficiency of two different solutes (benzyl acetate and hexamethylbenzene) on a silica column 25 cm long and 9 mm I.D. packed with Partisil 10 (actual mean particle diameter 8.5m) employing six different solvent mixtures. Measurements were made in triplicate and further replicate measurements made if the mean of the three differed by more than 3% from the extreme. They employed a specially designed chromatographic system with low extra column dead volume to ensure that the contribution of extra column dispersion to the values of (H) was less than 2%. The properties of the system are shown in table 1.