Indirect determination of cysteine in pharmaceutical formulations by high-resolution continuum source graphite furnace molecular absorption spectrometry

An indirect method for the determination of cysteine in pharmaceutical formulations involving the monitoring of CS (carbon monosulfide) molecular absorption bands using high-resolution continuum source graphite furnace molecular absorption spectrometry (HR-CS GF MAS) is reported. The best conditions for the determination were based on the sum of the absorbance signals of the molecular transition lines for CS at 258.034 and 258.056 nm with a pyrolysis temperature of 1000 °C and a volatilization temperature of 2400 °C using Pd/Mg as a chemical modifier. The linear range for cysteine was 1.4 to 100 mg kg−1 with a limit of detection and limit of quantification of 0.4 and 1.3 mg kg−1, respectively. The precision, evaluated as relative standard deviation (RSD), was 3.7% (n = 10) for a sample containing 25.0 mg kg−1 of cysteine. The accuracy of sulfur determination was evaluated using a standard reference material (NIST-SRM-1573a tomato leaves) and concordant values were observed within the 95% confidence interval. The method was successfully applied to the analysis of pharmaceutical formulation samples containing cysteine, lysine, and alanine. The concentrations of cysteine found in the samples were between 9.8 and 20.1 mg kg−1.