Ultra-high performance supercritical fluid chromatography in impurity control: Searching for generic screening approach.

In this study, 63 compounds measured as 10 individual mixtures containing in each case an active pharmaceutical ingredient and its relevant impurities, and additional set of 7 basic beta blockers were analyzed using ultra-high performance supercritical fluid chromatography with UV and mass spectrometry detection. The separations were accomplished using 8 different stationary phases (diol, diethylamine, 2-picolylamine, 1-aminoanthracene, BEH 2-ethylpyridine, BEH, CSH pentafluorophenyl, and HSS C18 SB), 6 modifiers (methanol, ethanol, isopropanol, methanol/acetonitrile, methanol/ethanol, ethanol/acetonitrile) and 5 additives in methanol (0.1% formic acid, 10 mmol/L ammonium formate, 10 mmol/L ammonium acetate, 0.4% ammonium hydroxide, and 2% water). The resulting chromatograms were evaluated using 6 selected parameters: number of eluted peaks, number of separated peaks, resolution between active pharmaceutical ingredient and following impurity, peak symmetry, peak width at 50% of the peak height, and selectivity. Volatile additives such as ammonium formate, ammonium acetate, and especially ammonium hydroxide provided overall generic approach with very good chromatographic performance. Combined modifier methanol/acetonitrile (1:1) was important for separations of some critical pairs in case of neutral compounds. Diol stationary phase proved its suitability for wide range of applications when the separations of all mixtures were satisfactory on this column, except for vardenafil and beta blockers mixture. HSS C18 SB stationary phase offered unique complementary selectivity especially for analysis of structurally close compounds and/or isomers.
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