Corrected retention indices in HPLC: their use for the identification of acidic and neutral drugs.

Thirty-seven selected acidic and neutral drugs were analyzed by HPLC with two-step gradient elution and nine different ODS silica columns. The retention index values (RI) for drugs were calculated against a series of alkyl-arylketones and corrected according to the procedure described earlier. The correction procedure decreased the intercolumn variability of the RI values, and the differences between the RI values obtained for the reference column and other columns substantially declined after correction. The mean standard deviation of uncorrected RI values, calculated for all drugs analyzed on 9 columns, amounted to 25 RI units (range 8 to 40). After correction, the mean standard deviation was 15 RI units (range 3 to 26). The method proved to be suitable for the comparison of results obtained with commercially different ODS silica columns.