Capillary isotachophoretic determination of flufenamic, mefenamic, niflumic and tolfenamic acid in pharmaceuticals

Anionic capillary isotachophoresis (ITP) with conductimetric detection has been used for determining selected non-steroid anti-inflammatory and analge sic drugs of the phenamate group. namely tolfenamic (I), flufenamic (II), m efenamic (III) and niflumic (IV) acid. Initially the pK(a) values (proton l ost) of I-IV were determined as 5.11, 4.91, 5.39 and 4.31, respectively, by the UV spectrophotometry in aqueous 50% (w/w) methanol. The optimised ITP electrolyte system consisted of 10 mM HCl + 20 mM imidazole (pH 7.1) as the leading electrolyte and 10 mM 5,5'-diethylbalbituric acid (pH 7.5) as the terminating electrolyte. The driving and detection currents were 100 mu A ( for 450 s) and 30 mu A, respectively (a single analysis took about 20 min). Under such conditions the effective mobilities of I-IV varied between 23.6 and 24.6 m(2) V-1 s(-1) (evaluated with orotic acid as the mobility standa rd). The calibration graphs relating the ITP zone length to the concentrati on of the analytes were rectilinear (r = 0.9987-0.9999) in the range 10-100 mg l(-1) of the drug standard. The R.S.D.s were 0.96-1.55% (n = 6) when de termining 50 mg l(-1) of the analytes in pure test solutions. The method ha s been applied to the assay of the phenamates in six commercial mass-produc ed pharmaceutical preparations (Mobilisin gel and ointment, Lysalgo capsule s, Nifluril cream, Niflugel gel, and Clotam capsules). According to the val idation procedure based on the standard addition technique the recoveries w ere 98.4-104.3% of the drug and the R.S.D, values were 1.25-3.32% (n = 6). (C) 2000 Elsevier Science B.V. All rights reserved.