EXTRA-COLUMN EFFECTS IN HIGH-PERFORMANCE LIQUID-CHROMATOGRAPHY .1. THEORETICAL-STUDY OF THE INJECTION PROBLEM

The effect of the injection process on the column efficiency depends on the injection time, the injection volume and the technique of sampling (syringe on-line or stop-flow injection through a septum-type injector port or valve injection in either the normal or the split mode). The injection process can be described by a contribution to the plate height that is proportional to the square of the length of the solute zone at the column inlet (Δx) and iversely proportional to the square of the column length and to a constant describing the concentration profile in the solute zone. A simple model based on the combined effect of solute displacement due to the injection flow and to the mobile phase flow makes possible the calculation of Δ. The effects of injection time and volume and volume are complex and depend on the sampling system and on geometrical parameters such as the column internal cross-sectional area, the sampling canal intenal and external cross-sectional areas and two co- efficients related to the flow pattern at the column inlet. In some conditions, the model proposed predicts, in agreement with experimental results, a non-linear dependence of the plate height on the square of the injection volume. Finally, the influence of particle size on the extra-column effect due to injection depends cosely on the experimental conditions: the results are different when the column is operated at maximal efficiency and when we want to achieve a given number of theoretical plates in a given time. In some instances the injection effect is independent on the particle size and in others it is inversely proportional to dp4. © 1979.