Selectivity and applications of supercritical fluid chromatography
thesisposted on 27.07.2017, 15:22 by M. Marsin Sanagi
Supercritical fluid chromatography (SFC) has been undergoing tremendous developments in recent years as an attractive technique that is complementary to both gas and liquid chromatography. The main aim of this study has been to examine selectivity in SFC by using retention indices as the means of recording retention and to explore the potential applications of this novel technique. A simple supercritical fluid chromatograph was constructed based on a Pye-Unicam gas chromatograph and a HPLC pump. SFC separations were carried out using HPIC packed columns with pure or modified carbon dioxide as the mobile phase and using ultraviolet and flame ionization detection. The effects of different parameters on the retention of compounds. of different functional groups have been studied. At a given temperature, the capacity factors of the compounds generally decreased with increases in pressure (density). Little change in selectivity was observed between compounds in a homologous series but selectivity differences were observed between compounds of different functionalities and anomalous behaviour was seen for acetophenone on a cyano-silica column. Temperature can also be used to selectively modify the retention of the analytes. The use of methanol or acetonitrile as the organic modifier generally improved the peak shapes and reduced the retentions, and this was particularly marked for polar compounds. Similar trends in retentions were observed for PS-DVB, ODS-, and cyano-silica columns although the compounds were generally more retained on the PS-DVB column, suggesting that the selectivity was controlled mainly by the properties and compositions of the mobile phase. Using different sets of homologues, alkylarylketones, n-alkanes, and alkylbenzenes, the possibility of the application of retention indices in SFC was investigated. The use of retention indices have been shown to have advantages over capacity factors, but unlike HPLC, retention indices in SFC are much more susceptible to selectivity changes caused by variations in the operating parameters. Studies were carried out to demonstrate the viability of SFC for the separation of several drug groups, barbiturates and benzodiazepines. With no modifier in the mobile phase, these compounds were highly retained on the columns, particularly on ODS- and cyano-silica. Successful separations of the drug compounds were achieved with methanol or acetonitrile as the modifier in the mobile phase. The application of the SFC system was extended to the separations of several terpenes and the leaf essential oil of Cymbopogon martinii (Roxb.) Wats on a PS-DVB column. The results were compared with those obtained using GC and GC-MS methods and it was evident that the GC methods gave better resolutions and sensitivity and would still be the method of choice.