δ¹³C stable isotope analysis of atmospheric oxygenated volatile organic compounds by gas chromatography-isotope ratio mass spectrometry

We present a new method for analyzing the δ¹³C isotopic composition of several oxygenated volatile organic compounds (OVOCs) from direct sources and ambient atmospheric samples. Guided by the requirements for analysis of trace components in air, a gas chromatograph isotope ratio mass spectrometer (GC-IRMS) system was developed with the goal of increasing sensitivity, reducing dead-volume and peak band broadening, optimizing combustion and water removal, and decreasing the split ratio to the isotope ratio mass spectrometer (IRMS). The technique relies on a two-stage preconcentration system, a low-volume capillary reactor and water trap, and a balanced reference gas delivery system. The instrument's measurement precision is 0.6 to 2.9% (1s), and results indicate that negligible sample fractionation occurs during gas sampling. Measured δ¹³C values have a minor dependence on sample size; linearity for acetone was 0.06% ng C^-1 and was best over 1-10 ng C. Sensitivity is ∼10 times greater than similar instrumentation designs, incorporates the use of a diluted working reference gas (0.1% CO₂), and requires collection of >0.7 ng C to produce accurate and precise results. With this detection limit, a 1.0 L sample of ambient air provides sufficient carbon for isotopic analysis. Emissions from vegetation and vehicle exhaust are compared and show clear differences in isotopic signatures. Ambient samples collected in metropolitan Miami and the Everglades National Park can be differentiated and reflect multiple sources and sinks affecting a single sampling location. Vehicle exhaust emissions of ethanol, and those collected in metropolitan Miami, have anomalously enriched δ¹³C values ranging from -5.0 to -17.2%; we attribute this result to ethanol's origin from corn and use as an additive in automotive fuels.