Evaluation of different injection techniques in the gas chromatographic determination of thermolabile trace impurities in a drug substance

Abstract

A gas chromatographic method for the determination of an epoxide in a drug substance was validated. Hot splitless injection of the sample extract was identified as a critical step with unsatisfactory repeatability. Therefore, the gas chromatographic procedure was re-evaluated using an instrument equipped with an on-column inlet and a split-splitless inlet with a programmable column head pressure. Further, the re-evaluated method also included the determination of the corresponding chlorohydrin that might occur as a reaction product of the epoxide. Three different sets of parameters were chosen: splitless injection using the same parameters as were used in the validation experiments, splitless injection with a high initial column head pressure and cool on-column injection. Generally, the precision of repeated injections of standard solutions and of the analysis of spiked sample extracts was improved using the newer instrument. On-column injection gave the best results in terms of both precision and absolute peak areas. The limits of detection and quantification for the overall method were 0.09 and 0.29 μg/g, respectively, for the epoxide and 0.09 and 0.31 μg/g, respectively, for the chlorohydrin. On hot splitless injection the chlorohydrin formed the epoxide, and also losses of the epoxide were observed. Owing to a shorter residence time in the hot injector block, a high initial column head pressure could successfully reduce the degradation of the analytes. However, the precision was not improved because of occasional leakage of the septum immediately after injection, which resulted in uncontrolled losses and discrimination.