Quantitative analysis of proteins via sulfur determination by HPLC coupled to isotope dilution ICPMS with a hexapole collision cell

Quantitative analysis of proteins is an essential part and also constitutes a major challenge in modern proteomics. Quantification of proteins by inductively coupled plasma mass spectrometry (ICPMS) offers an alternative method for quantitative proteomics. In this study, we developed a method of absolute quantification of proteins via sulfur by size exclusion chromatography (SEC) coupled to ICPMS with a collision cell (ICP-CC-MS) and postcolumn isotope dilution. Bovine serum albumin (BSA), superoxide dismutase (SOD), and metallothionein-II (MT-II) served as model proteins. Enriched ³⁴S, ⁶⁵Cu, and ⁶⁷Zn isotopic solutions were continuously mixed with the eluate from the SEC. Oxygen was added as a reactive gas into the collision cell where sulfur reacts with oxygen to form sulfur-oxygen ion, the ratio of ³²S¹⁶O⁺/³⁴S ¹⁶O⁺ thus representing ³²S⁺/ ³⁴S⁺. The absolute quantity of proteins could be calculated by the isotopic dilution equation and the content of sulfur in the proteins. The detection limits for BSA, SOD, and MT-II are 8, 31, and 15 pmol, respectively. The relative standard deviations for the proteins are less than 3%. The ratios of S/Cu and S/Zn in the proteins were also determined. The quantitative method was validated by comparing with gravimetric results.