Investigation of the retention mechanism for brush type bonded phases in gas and liquid chromatography

The mechanism of retention for chemically bonded stationary phases of the SiOR type, where -R is an organic molecule with a polar functional group, has been investigated. Sample size studies in conjunction with thermodynamic parameters have been utilized to determine the correct mechanism. Comparisons between conventionally coated columns and bonded phase columns have been made for the gas chromatographic study also. The effect of the support surface has been studied for both gas and liquid chromatography. In the absence of surface -OH groups for the gas chromatographic case, retention is controlled by adsorption on the brush as shown by comparison of regular support to silanized support. If surface hydroxyls are present, retention of certain solutes is controlled by interaction with these groups. Sample size studies and thermodynamic parameters show that retention of other solutes is controlled by brush adsorption even in the presence of surface hydroxyls. The results on the bonded oxypropionitrile (OPN) are especially significant in illustrating this point. Acetone exhibits a sample size curve indicative of brush adsorption even in the presence of these surface hydroxyl groups, acetone shows almost identical retention volume values. A plot of |H| versus |AS| illustrates the dominance of one mechanism on the bonded OPN. It is also possible that in some cases a combination of both surface and brush adsorption occur. Sample size studies were the only means of studying the liquid chromatographic case. Comparison of regular support material to silanized support indicates the importance of surface hydroxyls for polar solutes. The adjusted retention volume (Vr) for chloroform on bonded Carbowax 400 possibly indicates brush interaction, as the value for Vr is twice that obtained for the regular support. Alcohol solutes have high Vr values on the Carbowax column but the contribution of the brush cannot be separated from that of the support. Comparison of bonded OFN and silanized OPN shows the importance of the surface hydroxyls. Solutes such as the alcohols could only be eluted from the bonded OFN with polar carrier solvent, but on the silanized OPN they could be eluted with heptane. The retention behavior of acetone showed an eight-fold decrease on the silanized OPN, also indicating the importance of the surface hydroxyl groups.