Tools that provide absolute quantification of biomolecules, particularly of proteins and their post-translational modifications, without needing suitable specific standards, are urgently demanded nowadays. To this end, we have significantly improved the recently introduced strategy based on CH4 addition to the plasma for absolute quantification of biomolecules using HPLC-ICP-MS. Addition of CO2 has been optimized and finally selected as a safer, more efficient quantitative strategy that is able to provide constant (<6% error) signal response factor for the six elements assayed (S, P, As, Se, Br, I) under compromised conditions. In the particular case of absolute protein quantification, accuracy and precision attainable for S-based absolute determination of intact proteins using internal and external S-generic standards were compared. Potential for real sample analysis was demonstrated by the high-sensitivity analysis of toxins present in snake venoms. Finally, multielemental speciation capabilities of the approach have been also demonstrated through P and S simultaneous analysis in phosphoproteomics. Simultaneous accurate determination of both absolute protein amount and corresponding phosphorylation degree for intact β-casein, and even impurity traces of κ and α-s1 isoforms present, has been successfully achieved using a simple mixture of inorganic P and S standards. The lowest detection limits (<1 fmol protein) ever published for S- and P-based intact protein quantification with ICP-MS are reported.