Journal of Chromatography A, 871 (2000) 259–268 www.elsevier.com / locate / chroma Analysis of higher polyamide-6 oligomers on a silica-based reversed-phase column with a gradient of formic acid as compared with hexafluoroisopropanol a, a b b * Y. Mengerink , Sj. van der Wal , H.A. Claessens , C.A. Cramers a DSM Research, P .O. Box 18, 6160 MD Geleen, The Netherlands b Laboratory of Instrumental Analysis, Eindhoven University of Technology, P .O. Box 513, 5600 MB Eindhoven, The Netherlands Abstract 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 phase additive to adjust pH in reversed-phase high-performance liquid chromatography. However, formic acid is seldom used as a modifier to perform gradient elution chromatography on octadecyl-modified silica-based columns. Here we demonstrate the determination of cyclic and linear polyamide-6 oligomers using formic acid as a modifier on an octadecyl-modified silica-based column. 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 acid in water) the oligomers are retained on the column, while the polymer does not precipitate. In comparison, during adsorption and separation with a HFIP gradient, precipitation of the polymer occurs. The implications of the different separation mechanisms, i.e., adsorption vs. precipitation chromatography are discussed. Loadability is shown to be much better with the formic acid system. However, with formic acid as a modifier UV detection below 250 nm is not feasible. The less sensitive evaporative light scattering detector 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.  2000 Elsevier Science B.V. All rights reserved. Keywords: Mobile phase composition; Polyamides; Nylon; Formic acid; Hexafluoroisopropanol 1. Introduction not soluble in common chromatographic solvents, the use of exotic mobile phase modifiers is necessary Polyamide-6, also known as nylon-6, is a poly- to determine the oligomers and polymer [2]. m- condensate of caprolactam. Although low-molecular- Cresol [3], benzylalcohol [4–6], m-cresol–chloro- mass cyclic structures exist, the majority of the benzene [7], hexamethylphosphorotriamide [8] and condensation reaction product is linear (Fig. 1) [1]. methyl chloride–dichloroacetic acid [9] have been As the higher oligomers and the polymer itself are usedtodeterminethemolecularmassdistributionofpol- yamides. 1,1,1,3,3,3-Hexafluoroisopropanol (HFIP), introduced by Drott, simplified the determination of *Corresponding author. Tel.: 131-46-4761-632; fax: 131-46- the molecular mass distribution of the polyamide, as 4761-127. E-mail address: ynze.mengerink@dsm-group.com (Y. Mengerink) the analysis could be performed at room temperature 0021-9673 / 00 / $ – see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S0021-9673(99)01072-9