High-performance liquid chromatography procedure for the determination of purity of di-N-n-propylamine

Abstract Di-N- n -propylamine (DNP) is used in the final step of the synthesis of ropinirole (SK&F 101468-A). Trace impurities in DNP give rise to significant impurities in the raw drug substances. A high-performance liquid chromatographic (HPLC) method for the trace analysis of the impurities in DNP was developed and validated. The trace enrichment method uses a simple derivatisation reaction with oxindole analogous to the final step of the ropinirole synthesis. DNP samples of varying purity were reacted with oxindole and were used in the preparation of ropinirole. The products were then assayed by the HPLC method. A specific impurity in ropinirole was shown to correlate well with the product of the reaction with oxindole. From the correlation data a calibration curve was constructed which enabled a specification for DNP to be determined based on the reaction with oxindole. Thus, the HPLC method could be used to assess the acceptability of DNP for use in the ropinirole synthesis.