Impurities at a level of 0.01% in foscarnet sodium determined by capillary zone electrophoresis with indirect UV detection and sample self-stacking

Low detection limits were obtained by the use of sample self-stacking in combination with careful selection of pH, co-ion and control of the baseline noise. The major component in the sample, foscarnet sodium, was used as a stacker. By choosing sulphanilic acid as co-ion, good peak shapes of the impurities were obtained in combination with low detection limits due to the strong absorption properties of this ion. To obtain a good peak shape for the slowest migrating impurity, a second stacker was added, partly to the sample and partly as a second injection. Rinsing the bare silica with Triton X-100 had a beneficial effect on the baseline noise and helped to produce a constant migration time. By means of this procedure a detection limit of 0.003% (w/w) was obtained for the impurities. The precision at a level of about 0.03% (w/w) in foscarnet was 3-4% (RSD). Linearity for the impurities was shown in the tested range 0.0085-0.085% (w/w). (C) 1999 Elsevier Science B.V. All rights reserved.