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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.

Comparison of Supercritical Fluid Chromatographic and High-Performance Liquid Chromatographic Separations of p-tert -Butylcalix[ n ]arenes