Ion chromatography of azide in pharmaceutical protein samples with high chloride concentration using suppressed conductivity detection

Methods based on reversed-phase liquid chromatography with UV detection of 4-nitrobenzoyl- or 3,5-dinitrobenzoyl azide derivatives lack in accuracy and stability of derivatives to be applied for azide determination in pharmaceutical protein samples with high sodium chloride concentrations. This paper describes a sensitive and selective ion chromatographic method, with simple sample preparation and suppressed conductivity detection, developed for trace determination of azide in protein samples containing sodium chloride in concentrations as high as 11.6 g L−1. Anion exchange stationary phase with quaternary alkyl amine functional groups and gradient elution with sodium hydroxide enabled good resolution of anions with similar retention times: azide, bromide and nitrate, as well as chloride whose retention time was shorter than azide's. Anions with high affinity to stationary phase (phosphate and citrate) were also eluted within acceptable analysis time of 32 min. The stability of sample solutions and the method selectivity, accuracy, precision and sensitivity satisfied the validation criteria of international organizations competent for pharmaceutical industry. The detection and quantitation limit ranges of sodium azide in protein samples were 0.007–0.02 mg L−1 and 0.02–0.06 mg L−1, respectively. Both limits increased with the concentration of sodium chloride.