Voltammetric determination of volatile free sulfide and alkylthiols as contaminants in parenteral amino acid solutions

A method for the simultaneous voltammetric determination of free sulfide and volatile alkythiols (methanethiol/ethanethiol) existing as contaminant in parenteral nutrition (PN) solutions was developed. The volatile sulfides (dihydrogensulfide and alkylthiols) formed in the formulations were distilled over 45 min at 47°C in a purpose-made Conway cell. The analytes were quantified by differential pulse cathodic stripping voltametry (DPCSV) at the hanging mercury-drop electrode and determined, simultaneously as dihydrogensulfide and alkylthiols using a 60 s preconcentration time at −300 mV (versus Ag/AgCl, Cl⁻ 3.0 mol l⁻¹). The voltammetric signals were directly linear proportional to the sulfides concentrations in the PN solutions in the range 3–20 μmol l⁻¹ and detection limits of about 2 μmol l⁻¹ were calculated. Recoveries of sulfides from PN solutions in the range 90–110% were calculated using the method and nine commercial PN formulations were analyzed. Methanethiol and ethanethiol were quantified from their composite signals, and the mutual influence of the analytes on the DPCSV measurements is discussed.