The gas—liquid chromatographic stationary phase properties of liquid organic salts: Anomalous selectivity variation when employing the Rohrschneider/McReynolds system

Abstract

The liquid organic salts studied here have wide stable liquid temperature ranges and act as efficient, highly selective gas—liquid chromatographic stationary phases. The effect of carbon number on the gas liquid chromatographic stationary phase properties of this series of tetra- n-butylammonium n-alkylsulfonate salts was evaluated by the well-known Rohrschneider/McReynolds system. Several problems arose when attempting to characterize these polar liquid organic salts employing this system. The specific retention volumes for the polar selectivity probes were generally not affected by an increase in the carbon number for the series studied here. However, the specific retention volumes for the n-alkane retention index markers increased dramatically as the anion carbon number was increased. The overall effect was a net decrease in the calculated McReynolds constants with increasing carbon number, although the true selectivity of the different stationary phases remained constant. Additionally, the specific retention volumes of the basic test probe, pyridine, showed large erratic variations and, in some cases, was not recovered from the columns. The results suggest the possibility of on-column chemical reactions occurring with some of these salts, and an alternative test probe, 2,6-dimethylpyridine (lutidine) is proposed to eliminate this problem. As McReynolds constants are presently the most commonly used parameters for predicting retention and gas chromatographic stationary phase selection, it is important that workers are aware of the inherent limitations of this scheme.