A thermal desorption system for measuring δ13C ratios on organic aerosol

We developed a system that links a thermal desorption oven for aerosol filter samples to an isotope ratio mass spectrometer (IRMS) via a combustion interface. Organic compounds are desorbed from the filter sample in He at 7 temperature steps between 100 and 400 °C and subsequently oxidized to CO2, which is analyzed for δ13C. We tested the system for isotopic fractionation, reproducibility and linearity using organic test compounds and aerosol filter samples. An oxalic acid standard with known δ13C value shows no isotopic fractionation when it is desorbed at temperatures well above its melting point. When organic acids are heated at a temperature close to their melting point only a fraction of the compound (10–80%) evaporates which shows depleted δ13C values at this temperature step. At the next higher temperature step, when the rest of the compound is desorbed, δ13C values are enriched. This effect is stronger for compounds with a lower molecular weight than for compounds with a higher molecular weight. In ambient samples this should only have a moderate effect on the overall δ13C value, since hundreds of different compounds are desorbed at each temperature step. Choosing different values for temperature steps does not strongly change the δ13C values for ambient aerosol filter samples. For typical ambient samples the reproducibility lies below ±0.3‰ for oven temperatures below 200 °C and below ±0.5‰ for oven temperatures above 200 °C. Tests with oxalic acid and ambient filter samples show that the IRMS is linear for peak areas in the operational range of 1–20 V s.