Simultaneous determination of Fe/S and Mn/S ratios on transient signals was performed by size exclusion chromatography, hyphenated to inductively coupled plasma mass spectrometry with dynamic reaction cell technology (SEC-ICP-DRCMS), in order to characterize metalloprotein samples by their metal/sulfur ratio. Oxygen was used as the cell gas. The method eliminates the effect of polyatomic isobaric interferences at m/z = 32 by detecting sulfur as the product oxide ion (SO)-S-32-O-16 which is less interfered. Using the same reaction gas conditions, Fe and Mn were measured at m/z 54, 56 and 55, respectively. SEC-ICP-DRCMS measurements (injection volume 20 muL, SEC flow 300 muL min(-1)) resulted in excellent limits of detection (LOD). 4.3, 0.4, 2 and 0.6 ng g(-1) were assessed for (SO)-S-32-O-16, Mn-55, Fe-54 and Fe-56, respectively. Reference measurements were carried out by size exclusion chromatography inductively coupled plasma sector field mass spectrometry (SEC-ICP-SFMS), setting the mass resolution at 4500. LODs of 14, 0.5 and 0.4 ng g(-1) were obtained for S-32, Mn-55 and Fe-56, respectively. The metal/sulfur ratios of 5 commercially available metalloproteins were determined (myoglobin, haemoglobin, cytochrom c, arginase and Mn superoxide dismutase from E coli). Two proteins were characterized after in-house heterologous expression in a host organism (Mn superoxide dismutase from Anabaena PCC 7120, catalase-peroxidase from Synechocystis PCC 6803). Different calibrants (i.e., Fe3+, Mn2+, SO42-, methionine, myoglobin) for the assessment of inter-elemental ratios have been employed. It was found that calibration using metalloprotein myoglobin is preferable to inorganic standards in terms of uncertainty of measurement. However, all metal/sulfur ratios of the investigated proteins obtained by the different methods agreed within their total combined uncertainty.