Hydrogen isotope analysis of benzene and toluene emitted from vehicles

Abstract

The isotopic analysis of atmospheric volatile organic compounds (VOCs), and in particular their hydrogen isotope ratio (δ2H), has the potential to be an effective tool for clearly identifying sources of VOCs. However, to date there have been very few such analyzes. Here, we have analyzed the δ2H values of VOCs using thermal desorption and chromatography, thermal conversion, and isotope ratio mass spectrometry (TD-GC/TC/IRMS). After determining the analytical conditions needed for high precision and accurate analysis, we adopted minimum peak area thresholds of 10 Vs for the low concentration samples and 15 Vs for other samples. We also confirmed that breakthrough during adsorption of samples would have only minimal effect. We found that the collected samples could be stored for at least 7 days. Precisions of 1.1‰–5.3‰ (n = 7) were obtained for 28 standard compounds in a standard gas containing 58 VOCs (C6–C11). Next, we collected the exhaust gas produced in cold mode and hot mode from five vehicles, and measured the δ2H values. For benzene, we found that the δ2H value for the hot mode vehicle emissions was 19.3–104.7‰ lighter than that for the cold mode, while the δ2H value of the vaporized gasoline was 0.7–25.2‰ close to that in the cold mode. It should, therefore, be possible to distinguish cold mode vehicle emissions from those of the hot mode by analyzing the hydrogen isotope ratio. For benzene, particularly, the difference in δ2H values between 2 modes is important since emitted in large quantity from vehicles generally. Additionally, we measured VOCs in vaporized gasoline and roadside air, and compared the results with those for vehicle emissions. The roadside samples were characterized mainly by the hot mode. It has been shown that the hot mode has a significant impact on roadside VOCs, if no isotopic fractionation in the atmosphere is assumed. The results suggest that our approach could improve our understanding of the origin and fate of atmospheric VOCs, by allowing measurement of the δ2H values of further target compounds and sources.