3. Results and discussion
3.1. HPLC method development and optimization
3.1.1. Optimization of chromatographic conditions
An HPLC method for the analysis of process-related impuritiesin VTXT using an Acquity UPLC BEH C18 column at 60◦C has beenreported [7]. The reported method [7] cannot be applied directly forseparating the impurities described in Section 2.1 because only twoout seven impurities were analyzed. Considering the conditions inlab, a relatively universal HPLC method was developed to deter-mine all the seven impurities. Besides, a low column temperaturewas selected to avoid potential long-term risks in the chromato-graphic efficiency. Because VTXT contains a basic group with apKaof approximately 8.85, trifluoroacetic acid, which would fullyprotonate the compound and increase the retention of high polarsubstances, was selected as the mobile phase additive. The mostwidely used HPLC column was tentatively selected to separate andquantify the process-related and degradation impurities in VTXT.A solution spiked with process-related impurities was injectedto the HPLC system to investigate the HPLC conditions in eachtrial. ACN and 0.05% trifluoroacetic acid in different gradient elu-tion modes were initially chosen to optimize the HPLC conditions.The results showed that the resolution (R) of VTXT and Imp-A waslow (R = 1.51).To further optimize the performance, ACN and trifluoroaceticacid mixtures, with the acid concentration adjusted over the range0.03–0.07%, were used as the mobile phase in the gradient mode.In this trial, all process-related impurities can be observed withadequate separation. The acid concentration of the mobile phasehas a significant effect on the capacity and tailing factors of all thecompounds investigated, and the optimum symmetry for all theanalytes was achieved with an acid concentration of 0.05%. Opti-mum separation was observed with a Shimadzu Inertsil ODS-3C18column (250 mm × 4.6 mm, 5 m) (Tokyo, Japan), with the gradientelution mode as described in Section 2.2.