Determination of sulfur-containing amino acids by capillary electrophoresis dynamic reaction cell inductively coupled plasma mass spectrometry

Capillary electrophoresis dynamic reaction cell™ inductively coupled plasma mass spectrometry (CE-DRC-ICP-MS) for the determination of sulfur-containing amino acids is described. The sulfur-containing amino acids studied include L-cysteine, L-cystine, DL-homocystine and L-methionine. The species studied were well separated using a 70 cm length×75 (mu)m i.d. fused silica capillary while the applied voltage was set at +22 kV and a 10 mmol l^-1 disodium tetraborate buffer (pH 9.8) containing 0.1 mmol l^-1 EDTA and 0.5 mmol l^-1 Triton X-100 was used as the electrophoretic buffer. The sulfur-selective electropherogram was determined at m/z 48 as 32S16O+ by using its reaction with O2 in the reaction cell. The method avoided the effect of polyatomic isobaric interferences at m/z 32 caused by 16O16O+ and 14N18O+ on 32S+ by detecting 32S+ as the oxide ion 32S16O+ at m/z 48, which is less interfered. The detection limit of various species studied was in the range of 0.047–0.058 (mu)g S ml^-1, which corresponded to the absolute detection limit of 1.3–1.6 pg S based on the injection volume of 27 nl. We determined the concentrations of selected sulfur-containing amino acids in urine and nutritive complement samples. The recovery was in the range of 92–128% for various species.