Hydrochloric and nitric acids, potassium nitrate, and sodium hydroxide were purchased from POCh (Poland). Silver nitrate was obtained from Honeywell (USA). Hydroxylamine hydrochloride, L-Cysteine (Cys-cabn), and L-Cysteine-13 C3, N (Cys-CABN) were delivered by Sigma-Aldrich (USA). Nitrogen gas (99.9999%) was purchased from Air Products (Poland). All reagents were at least of the analytical grade and were used as received without any further purification.