Chemiluminescence determination of antibacterial drug trimethoprim by automated sequential injection technique with permanganate and hexametaphosphate as reagents

Antibacterial drug trimethoprim [2,4-diamino-5-(3,4,5-trimethoxybenzyl)-pyrimidine] (I) was determined in pharmaceutical formulations by using a lab-made PC-controlled SIA analyser linked to conventional HPLC fluorimetric detector equipped with a chemiluminescence module. The chemical principle is the oxidation of I by KMnO4 in acid medium; the reaction is accompanied by the emission of chemiluminescence, which is enhanced in the presence of hexametaphosphate (HMP). The optimum sequence and the flow parameters and concentrations and volumes of reagents aspirated optimised by a computer-aided simplex method were, 100 μl of 5 mM HMP, 40 μl of a test solution of I, 2 μl of 0.5 M H2SO4 and 20 μl of 1 mM KMnO4; the luminescing zone was pushed into the detector at a flow rate of 49 μl s−1. The calibration graph relating the intensity of luminescence to concentration of I was parabolic (r=0.9994) in the range 0.5–100 μg ml−1 of I with rectilinear part (r=0.9999) in the range 20–100 μg ml−1 of I; the limit of detection was 0.1 μg ml−1 of I. The method was used for the assay of Triprim® tablets (with nominal content 100 or 200 mg of I) for the active substance as well as for content uniformity tests; the R.S.D. values did not exceed 1% (n=5). The SIA results did not show statistical difference from those obtained by pharmacopoeial acidimetric titration in non-aqueous medium; the excipients such as microcrystalline cellulose, maze starch, povidone, talc, magnesium stearate and gelatin did not interfere.