Elution of Cationic Species with/without Ion Pair Reagents from Polar Stationary Phases via SFC

Abstract

Traditionally, the application of supercritical fluid chromatography has been limited to compounds of relatively low polarity. Thus, we report here that one secondary amine and two quaternary amine salts were successfully eluted from a Deltabond Cyano-bonded silica column with the addition of sodium alkylsulfonate to the methanol-modified, carbon dioxide-based mobile phase. A possible ion-pairing interaction between the positively charged analytes and the anionic part of the sulfonate additive has been proposed. In another set of experiments, the three amine salts readily eluted from both Ethyl-pyridine-bonded silica and Amino-bonded silica phases without the need of additive although the peak shapes were less than desirable. The addition of sulfonate salt to the mobile phase again sharpened the peaks. In the presence of a mixture of methanol and CO2, we suggest that these stationary phases are positively charged. We describe here also for the first time the employment of a strong silica-based anion exchange (SAX) column for supercritical fluid separation of cationic species. Two elution mechanisms were proposed for the three amine salts depending upon whether the ionic additive was incorporated into the mobile phase. Separation as the ion-pair was proposed with an ionic additive in the mobile phase; whereas separation of the intact amine salt from the positively charged basic stationary phases was suggested to be operational without the ionic additive.