Comparison of Helium-Alternative Carrier Gases for Gas Chromatography/ Mass Spectrometry of Standard Test Methods for Indoor Air Quality Guidelines in Japan

Helium is the most frequently used carrier gas for GC/MS, which is the official standardized test method in Japan to assess chemical substances in indoor air. However, recent global challenges in the supply chain for helium have led to a need to validate GC/MS using alternative carrier gases. In this study, we examined the applicability of hydrogen and nitrogen as helium-alternative carrier gases in the standardized GC/MS analytical test method for volatile organic compounds (VOC) and phthalate esters in indoor air. Comparison of the signal-to-noise ratios of standard solutions showed that detection sensitivities of hydrogen and nitrogen analysis were enough for the standard test method, although these gases, especially nitrogen, were less sensitive than helium. Measurements using these alternative carrier gases showed good linearity and could quantify around 1/100 th of Japanese guideline values for indoor air concentrations. Therefore, hydrogen and nitrogen gases can be applied to the standard GC/MS analysis test method for VOC and phthalate esters in indoor air as alternative carrier gases to helium.

The targeted chemicals were toluene, xylene, styrene, ethylbenzene, 1,4-dichlorobenzene, tetradecane as VOC and di-n -butyl phthalate (DnBP), and di-2-ethylhexyl phthalate (DEHP) for SVOC ( Fig. 1 ). These target and qualifier ions are shown in Table 1 . These chemicals were measured using Selected Ion Monitoring (SIM) modes and were quantified based on internal standard methods using toluene-d 8 for VOC and DnBP-d 4 and DEHP-d 4 for phthalate esters. The concentration range of the calibration curve was 0.5-100 ng. m -, p -Xylenes, parts of the three isomers of xylene, were quantifi ed as overlapping peaks. The chromatograms were processed with PRISM 9 (GraphPad Software, CA, USA). The limits of detection (LOD) and those of quantifi cation (LOQ) were calculated as 3-fold, 10-fold the standard deviation of fi ve analyses of the lowest concentration samples (0.1 ng), respectively, and were further divided by the expected sampling volume according to the standard test method specifi ed by the CIAP, MHLW. 10) The details of the measurement conditions are shown in Table 2 .

RESULTS AND DISCUSSION
The SIM chromatograms of the standard solutions for the targeted chemicals showed that the peaks had good separation for qualitative and quantitative analysis for all tested carrier gases ( Fig. 2 ). The signal-to-noise (S/N) ratios of the measurements with 100 ng of the targeted chemicals are shown in Table 3 . The S/N ratio was the highest for helium, followed by hydrogen, which was 55%-93% lower than that of helium, and nitrogen, which was 93%-98% lower than that of helium. The reason for the extremely low sensitivity of nitrogen is suspected to be the lower vacuum pressure.
The coefficients of determination (R 2 ) of the calibration curves are shown in Table 4 . Most of the chemicals showed good linearities, even the least sensitive nitrogen carrier gas with an R 2 of 0.9917-0.9998 in the concentration range of 0.1-100 μg/mL for VOC and 0.1-5 μg/mL for phthalate esters. However, as the R 2 of tetradecane was below 0.99, the concentration range was changed to 0.1-20 μg/mL to give an R 2 of 0.9948.
A comparison of the LOD, LOQ, and guideline values for indoor air concentrations is shown in Table 5 . The LOQ of all chemicals was less than 1/100 th of guideline values, except for xylene with helium carrier gas and for toluene, xylene, and tetradecane with nitrogen. However, the LOQ of xylene with helium was 2.40 μg/m 3 , and that of toluene, xylene, and tetradecane with nitrogen were 3.40 μg/m 3 , 7.20 μg/m 3 , and 5.90 μg/m 3 , respectively, revealing that these LOQs were close to 1/100 th of the guideline values. Therefore, measurements using any of these carrier gases can quantify around 1/100 th of the guideline values for the indoor air concentrations of all tested chemicals.
Herein, we found that the analysis results using hydrogen as a carrier gas were comparable to those of helium, suggesting that hydrogen is a promising alternative to helium in the standard GC/MS test method for chemicals in indoor air. Meanwhile, nitrogen was clearly less sensitive than helium, and it might be diffi cult to conduct non-targeted analyses at the current level of an analytical equipment. However, target analysis using SIM measurements showed the good linearity. Moreover, around 1/100 th of guideline values for indoor air concentrations were quantifiable. Therefore, nitrogen can be also applied to the standardized GC/MS test method for chemicals in indoor air, which is a targeted analysis.
In conclusion, in this study, we examined the applicability of hydrogen and nitrogen as helium-alternative carrier gases for the standard GC/MS test for indoor air quality. Comparison of the S/N ratios of the standard solutions showed that detection sensitivities of hydrogen and nitrogen analysis were enough for standard test method, although these gases, especially nitrogen, were less sensitive than helium. Measurements using all of these carrier gases showed good linearity, and around 1/100 th of guideline values could be quantified in indoor air concentrations. Therefore, hydrogen and nitrogen gases can be applied to the GC/MS analysis of standard test methods for indoor air quality guidelines in Japan as heliumalternative carrier gases.