Vapour phase Fourier transform infrared spectrometric determination of l-cysteine and l-cystine

A novel and selective procedure for the determination of l-cysteine and l-cystine based on vapour-generation Fourier transform infrared spectrometry is described. Potassium iodate solution was injected into a glass vessel containing l-cysteine and/or l-cystine. The evolved CO was swept by a stream of nitrogen to an infrared gas cell. The vapour phase FTIR spectra were continuously recorded, as a function of time, between 2240 and 2000 cm−1, which includes the CO absorption band. The maximum absorbance at 2170 cm−1 was selected as a measurement criterion. The calibration curve was linear over the range 6–300 mg L−1. The method provided a limit of detection of 2 mg L−1 of l-cysteine, a throughput of 12 samples h−1 and an R.S.D. of 1.76% for five independent analyses of a 75 mg L−1 l-cysteine solution. For the measurement of l-cysteine and l-cystine separately, after measuring total concentration of l-cysteine and l-cystine, l-cysteine was masked with p-benzoquinone at a pH of 3 and individual l-cystine was determined. The amount of l-cysteine was obtained by difference. The method was applied to the determination of l-cysteine and l-cystine in pharmaceutical and urine samples. Results obtained for real samples compared well with those obtained by a reference spectrometric method.