Accurate quantitative measurement of selenoproteins GPx, SelP, and SeAlb in blood serum was achieved by using affinity HPLC‐ICP/MS with Post Column Isotope Dilution (PCID). D₂ was used as a collision gas to avoid BrH⁺ interference that arises from the combination of matrix Br and the H₂ collision gas normally used in octopole reaction cell (ORC) ICP/MS. The advantages of using D₂ were to be free from isobaric interference, and the use of the ratio of ⁷⁸Se/⁸⁰Se which has the higher abundances for higher precision in ID techniques. Ammonium formate, rather than ammonium acetate or ammonium nitrate, was selected as an eluent for affinity chromatography to accomplish an efficient separation of the three selenoproteins and to provide the least baseline change during the gradient elution. The baseline change in PCID during the peak elution was compensated by active background subtraction. The accuracy was verified against the reference blood serum BCR 639 (133 ± 12 ng/g) to give 130.1 ± 6.4 ng/g (97.7% accuracy) with 4% RSD. The distributions of three proteins were 41%, 48%, and 11% for GPx, SelP, and SeAlb, respectively. The abstract should briefly state the problem or purpose of the research, indicate the methodology used, summarize the principal findings and major conclusions.