Enantio- and chemo-selective HPLC separations by chiral–achiral tandem-columns approach: the combination of CHIROBIOTIC TAG™ and SCX columns for the analysis of propionyl carnitine and related impurities

We describe a new tandem-columns chiral–achiral HPLC arrangement by using a chiral column (CHIROBIOTIC TAG™) connected in series with an achiral column (Spherisorb S5 SCX), based on a strong cationic exchange mechanism; this approach is very useful for the analysis of chiral molecules, containing cationic groups in their structures. We used this special combination to develop an easy and convenient procedure for the enantio- and chemo-selective dosage of propionyl l-carnitine (1) and relative impurities (2–6), which allowed for the simultaneous separation and quantitation within 30 min. Under the best chromatographic conditions (acetonitrile–10 mM sodium dihydrogen phosphate 65:35, v/v (pHa 6.80) as the mobile phase and UV detection at 205 nm], all the individual peaks were well separated. The applicability of the method, fully validated, was demonstrated by the analysis of a pharmaceutical batch of propionyl l-carnitine, where we found the following contents: 98.5% for 1 (drug substance); 0.15% for 3; 0.1% for 5 and 0.2% for 6. The enantiomeric excess (e.e.%) measured for the drug substance was 98.9%. Finally, a single mixed-bed column, packed with a binary mixture of the chiral and achiral phases, in a 1:1 ratio, gave similar chromatographic results as the tandem-columns approach, and thus, offered an easy alternative solution to the separation of the considered mixture.