Effects of modifiers in packed and open-tubular supercritical fluid chromatography

The applicability of packed and open columns for supercritical fluid chromatography using pure carbon dioxide for the elution of a number of selected test components was investigated. It is shown that the number of solutes that can be eluted as symmetrical peaks is much larger in open-tubular capillary supercritical fluid chromatography. For strong hydrogen donors and acceptors, however, poor peak shapes were observed on both types of columns. In both cases, modifiers are needed. Modifiers may effect retention by increasing the mobile phase polarity and density, and by deactivating active sites. The relative influence of these processes was studied in packed and open columns. The effects of various modifiers in packed and open columns were compared under identical operating conditions. The intluence of temperature on retention and peak shape was studied for pure carbon dioxide and carbon dioxide-modifier mixtures. The influence of modifiers on retention was found to be larger at lower operating temperatures. For components with high affinities for silanol groups the influence of polar modifiers on the capacity factor and the peak shape was found to be of comparable magnitude in packed and open columns. Columns packed with alkyl-modified poly(styrene4ivinylbenzene) and with porous carbon coated with poly(ethylene glycol) were used to study the influence of modifiers without interference from silanol groups. Polar and non-polar modifiers were used to study the relative influence of the polarity and density effects of the modifiers. In supercritical fluid chromatography (SFC) both packed and open-tubular columns are routinely used. Packed columns generally offer an increased speed of analysis in comparison with open-tubular columns. However, when high plate numbers are necessary, open columns are preferable [1,2]. Plate numbers in excess of approximately 20 000 are not readily attainable in packed-column SFC, because the maximum length of the column is limited by the maximum allowable pressure drop. With open-tubular columns much higher plate numbers can be obtained. Here the pressure drop is lower owing to the higher permeability of the column. 0021-9673/91/$03.50