Analysis of linear and cyclic oligomers in polyamide-6 without sample preparation by liquid chromatography using the sandwich injection method I. Injection procedure and column stability

We report a method for reliable routine polymer sample introduction with mi nimal bias, a separation method of the first six linear and cyclic oligomer s by liquid chromatography, quantification using group equivalents and long term method performance. Injecting a polymer sample in a mobile phase cont aining an aqueous non-solvent often results in blocked systems as the polym er precipitates in the connecting capillaries. In this first part we focus on a new injection technique, in which the dissolved polyamide is placed be tween two zones of formic acid, preventing the polymer to precipitate befor e it reaches the column. Development of this sandwich injection method make s direct injection of the polymer into an aqueous acetonitrile gradient fea sible. The oligomeric polyamide recovery of this technique, extraction, dis solution/precipitation and direct injection on a hexafluoro-isopropanol (HF IP) gradient were compared. With the sandwich injection method the polymer remains on the column, slowly changing the stationary phase. The influence of this on resolution and retention was studied. Column stability allows si xty injections before cleaning or replacing the column is necessary. (C) 20 00 Elsevier Science B.V. All rights reserved.