Analysis of the stable carbon isotope composition of formic and acetic acids

Formic and acetic acids are ubiquitous in the environment and in many biological processes. Analysis of the stable carbon isotope composition (δ13C) of formic and acetic acids is important to understanding their biogeochemical cycles. However, it has been faced with poor accuracy and high detection limits due to their low carbon number, high hydrophilicity, and semi-volatility. Here we developed an analytical technique by needle trap and gas chromatography–isotope ratio mass spectrometry (GC–IRMS). The organic acids in aqueous solution were extracted using a NeedlEx needle through purge-and-trap and were analyzed by GC–IRMS for δ13C. The procedures incur no isotope fractionation. Defined as the point at which the mean δ13C is statistically the same as the given value and the analytical error starts rising, the method’s detection limits are 200 and 100 mg/L for formic and acetic acids, respectively, with an uncertainty of approximately 0.5‰ in direct extraction and analysis. They were lowered to 1 mg/L with precision of 0.9‰ after samples were subjected to preconcentration. The method was successfully applied to natural samples as diverse as precipitation, vinegars, ant plasma, and vehicle exhaust, which vary considerably in concentration and matrix of the organic acids. It is applicable to the organic acids in not only aqueous solution but also gaseous phase.