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A novel method has been developed for the compound-specific carbon isotope analysis of atmospheric formal-dehyde using gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS). The method allows the determination of the [[sigma].sup.13]C value for atmospheric formaldehyde at nanogram levels with higher precision and lower detection limit. In the present work, atmospheric formaldehyde was collected using NaHS[O.sub.3]-coated Sep-Pak silica gel cartridges, washed out by water, then derivatized by cysteamine of known [[sigma].sup.13]C value, and the [[sigma].sup.13]C value of its derivative (thiazolidine) determined by GC/C/IRMS. Finally, the [[sigma].sup.13]C value of atmospheric formaldehyde could be calculated by a simple mass balance equation between formaldehyde, cysteamine, and thiazolidine. Using three formaldehydes with different [[sigma].sup.13]C values, calibration experiments were carried out over large ranges of formaldehyde concentrations. The carbon isotope analysis method achieved excellent reproducibility and high accuracy. There was no carbon isotopic fractionation throughout the derivatization processes. The differences in the carbon isotopic compositions of thiazolidine between the measured and predicted values were always