Helium is considered as an inert gas that has no flammability. It has been reported that deaths due to suffocation by excess helium inhalation are becoming an increasing serious social issue [
1‐
7]. However, the lack of an appropriate analytical assay for the helium in the biological samples results in the difficulty for clarifying the unequivocal cause of death in the helium-related cases.
Gas chromatography–thermal conductivity detection (GC–TCD) has been commonly used as a convenient method for helium assays so far. Some studies have reported the detection of helium accumulated in the organs (lung, stomach, brain, liver, and trachea) and blood by GC–TCD [
7‐
11]. However, the GC–TCD method suffers from poor sensitivity and selectivity for helium detection [
12]. Although headspace analysis of the helium in the blood during autopsy may solve the problem of insufficient detection [
8,
9], it still suffers from poor reproducibility for the helium assay due to the unstable volatility of the helium found in the blood matrix. Norimine et al. [
10] improved the poor GC–TCD detection of helium by increasing the helium volatility in the headspace using a reduced-pressured vial. To date, gas chromatography–mass spectrometry (GC–MS) has been extensively used for the selective detection of helium in biological samples including the lungs, stomach, trachea, and blood. Auwaerter et al. [
13] and Musshoff et al. [
14] qualitatively assayed helium using nitrogen and hydrogen as carrier gases, respectively, on a nonpolar capillary column by the selected-ion monitoring (SIM) GC–MS technique. Alternatively, Malbranque et al. [
15] proposed a quantitative GC–SIM-MS assay of the helium in the organs with the aid of an external standard (nitrous oxide; N
2O). Although the use of N
2O as a standard in GC–SIM-MS analysis might be useful for quantitative helium assay, it required a mixing of the headspace gas with N
2O in another vial owing to its high solubility in biological samples, causing difficulty in direct helium assay in the target headspace samples.
Because of the aforementioned disadvantages of the reported GC–SIM-MS methods for complex biological samples, in this study, we tried to develop an internal standard (IS)-aided GC–SIM-MS method for quantitative helium assays. Blood was targeted for the present assay, since the blood of the deceased is the first priority matrix for judging the asphyxiation by helium inhalation. The application of the neon-21 naturally existing in air (0.049 ppm [
16,
17]) as the IS, but not as the external standard, to the present GC–SIM-MS assay is described in this article.