Photochemical reactivity of sulfamethoxazole and other sulfa compounds with photodiode array detection

Abstract

The photochemical activities of six sulfa compounds [sulfacetamide (CET), sulfadiazine (DIA), sulfaguanidine (GUA), sulfamerazine (MER), sulfamethoxazole (SMX), and sulfamethizole (MET)] under different experimental conditions such as photolysis time, solvent and buffer pH are investigated by photodiode array (PDA) spectrophotometry. With no photolysis, the sulfa drugs CET and DIA show no absorbance at 332 nm and the other compounds only modest absorbance. Upon photolysis for 4 min, absorbance enhancements at 332 nm of three to four times for GUA and MET and 12–15 times for SMX and MER are observed. For CET and DIA after photolysis, the (absorbance) l/mg is now approximately 0.01–0.02 units. Although two pH optima of approximately 3–4 and 7 are noted, the optimum solvent for photolysis is ethanol without pH adjustment. For flow injection (FI) with on-line photolysis and PDA detection, a mobile phase of 100% ethanol with a step flow rate from 0.1 to 1 ml/min is used providing a 4-min reaction time. The FI detection limit for SMX with photolysis at 330 nm is 1 mg/l. The relative standard deviation data ( n=4) of seven individual points in a calibration curve from 5 to 150 mg/l are 0–4%. The recovery of SMX from pharmaceutical tablets is 99.7% indicating no interference from trimethoprim which is not photochemically active.