Analysis of higher polyamide-6 oligomers on a silica-based reversed-phase column with a gradient of formic acid as compared with hexafluoroisopropanol

The analysis of polyamide-6 oligomers and polymer is usually performed with expensive fluorinated alcohols like 2,2,2-trifluoroethanol (TFE) or 1,1,1, 3,3,3-hexafluoroisopropanol (HFIP). Formic acid is well known as a mobile p hase additive to adjust pH in reversed-phase high-performance liquid chroma tography. However, formic acid is seldom used as a modifier to perform grad ient elution chromatography on octadecyl-modified silica-based columns. Her e we demonstrate the determination of cyclic and linear polyamide-6 oligome rs using formic acid as a modifier on an octadecyl-modified silica-based co lumn. This column was shown to be stable for more than 5000 column volumes, even when a mobile phase of 65-95% formic acid in water at a flow of 1 ml/ min is applied. With formic acid under the conditions used (65-95% formic a cid in water) the oligomers are retained on the column, while the polymer d oes not precipitate. In comparison, during adsorption and separation with a HFIP gradient, precipitation of the polymer occurs. The implications of th e different separation mechanisms, i.e., adsorption vs. precipitation chrom atography are discussed. Loadability is shown to be much better with the fo rmic acid system. However, with formic acid as a modifier UV detection belo w 250 nm is not feasible. The less sensitive evaporative light scattering d etector is used to detect the polyamide oligomers in the formic acid phase. In addition it is shown that capillary zone electrophoresis (CZE) with UV- absorbance detection using HFIP is an attractive combination as HFIP is UV- transparent and CZE allows low modifier consumption. (C) 2000 Elsevier Scie nce B.V. All rights reserved.