A technique for atmospheric measurements of stable carbon isotope ratios of isoprene, methacrolein, and methyl vinyl ketone
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A technique was developed to measure stable carbon isotope ratios (13C/12C) of light volatile organic compounds (VOCs) such as isoprene, methacrolein (MACR) and methyl vinyl ketone (MVK) using gas chromatography combustion isotope ratio mass spectrometry (GCC-IRMS). An automated sampling and cryofocussing system allowed for the extraction of VOCs from air samples of up to 140 L of air collected over 3 h, and the subsequent 13C/12C analysis of the VOCs by GCC-IRMS. Chromatography using selective transfer between two columns was used to improve the separation for selected compounds, increasing peak resolution and attaining less noisy baselines. Still, some target compounds could not be completely separated from co-eluting peaks. To reduce the bias of isotope ratio determinations, which can result from incomplete peak resolution, a peak-fitting procedure has been developed. In cases of overlapping peaks or substantial baseline drift, this peak fitting allows more accurate determination of isotope ratios than conventional integration schemes. Comparisons between off-line IRMS measurements and a peak-evaluation procedure using a prepared VOC gas-phase standard show that isotope ratios derived from large (>1 ng of carbon per peak) and well-resolved peaks have a reproducibility of ±0.3‰. With smaller masses in the range of 0.1–1 ng of carbon, reproducibility decreased to ±(0.5–0.8)‰. For a 140 L sample of air, such small masses of carbon correspond to mixing ratios in the low pptV range. The developed measurement technique was applied to a small set of ambient air samples taken during hot, sunny periods from late May to early August, 2005, at Forschungszentrum Jülich, Germany, a semi-rural area. The range of δ13C values determined for isoprene, benzene, and toluene are consistent with those reported in the literature. GCC-IRMS results of δ13C for ambient samples of isoprene, MACR, and MVK, measured at mixing ratios of 15–280 pptV, are presented and discussed.