Anal. Chem. 1987, 59, 1039-1043
occur is approximately compensated by an increase in silanophilic interactions as more bare silica becomes available. At about 18 h the k’appears to decrease more abruptly in coincidence with a rapid increase in column back pressure and pressure fluctuation with each pump cycle. This is a considerably shorter time than with the hydrocarbon analogue column. This difference in stability likely results from one of two factors. First, the HFD column has a lower carbon load (8.6%) than the Clo column (10%). Thus on average there is more bare silica open to attack by base. Second, the HFD ligand is far more rigid than the hydrocarbon ligand and it is very probable that it assumes a fully extended conformation. In contrast, the hydrocarbon may be collapsed, thereby forming a protective blanket over the surface. CONCLUSIONS It is apparent that fluorinated bonded phases are much less retentive than the analogous hydrocarbon materials. In view of the great differences in chemical and physical properties of hydrocarbons and highly fluorinated materials (17), it is remarkable that the columns, particularly the Clo and HFD columns, are so similar and have very analogous chromatographic selectivities and that, as shown in other work, the solute parameters of most significance are their size and hydrogen bond basicity (18). We had anticipated that differences in dispersive interactions would be more significant, or at least more readily observable. The fluorinated phase retentivity increases in the sequence PFP < HFIPP
