Comparison of supercritical fluid chromatographic and high-performance liquid chromatographic separations of p-tert-butylcalix[n]arenes

Supercritical fluid chromatography (SFC) using packed analytical columns is compared with high-performance liquid chromatography (HPLC) for the separation of the readily available calixarenes, p-tert-butylcalix[4]arene, p-tert-butylcalix[6]arene, and p-tert-butylcalix[8]arene. With methanol as a modifier in supercritical carbon dioxide, p-tert-butylcalix[8]arene fails to elute from any of the four different columns used (ODS, RP-18, Cyano, and Diol), although the smaller calixarenes elute; p-tert-butylcalix[4]arene always precedes p-tert-butylcalix[6]arene. By using chloroform modifier under gradient conditions, all three can be resolved, and a retention time as short as 1.4 min is obtained for p-tert-butylcalix[8]arene on an RP-18 column. The total run time including re-equilibration is slightly over 3 min. In contrast, the shortest retention time obtained for p-tert-butylcalix[8]arene by HPLC (150-mm ODS column, 3-μm particle size, isocratic conditions, acetonitrile-ethyl acetate mobile phase [80:20]) is approximately 5 min. Peaks are both narrower and more symmetrical under SFC rather than HPLC conditions.