Here, we present a quantitative method for H₂ detection by gas chromatography–selected ion monitoring–mass spectrometry (GC-SIM-MS) using a single quadrupole instrument. Additionally, the developed method was applied to the detection of H₂ in human blood by GC-SIM-MS analysis using the existing ²²Ne in air as an internal standard (IS). H₂ was analyzed by GC-SIM-MS using a single quadrupole instrument with double TC-Molsieve 5A capillary columns for the separation of permanent gases. The detections of H₂ (analyte) and ²²Ne (IS) were performed at m/z 2 and 22, respectively, by GC-SIM-MS. The analyte and IS were separated using He as the carrier gas. The ratio of the peak area of H₂ to ²²Ne was employed to obtain a calibration curve for H₂ determination in the gas phase. The proposed GC-SIM-MS method exhibited high sensitivity in terms of the limits of detection (LOD) (1.7 ppm) and quantitation (LOQ) (5.8 ppm) for H₂ analysis. The developed quantitative assay of H₂ in the headspace of blood samples achieved high repeatability with a relative standard deviation (RSD) of 1.4 – 4.7%. We successfully detected and quantified H₂ in the headspaces of vacuum blood-collection tubes containing whole blood from 11 deceased individuals with several causes of death by employing the developed GC-SIM-MS method. The quantitative value of H₂ ranged from 5 to 905 ppm. The proposed GC-SIM-MS method was applicable to the quantitative assay of H₂ in biological samples without tedious pretreatment requirements.