Carbon-isotopic analysis of dissolved acetate

Abstract

Heating of dried, acetate-containing solids together with oxalic acid dihydrate conveniently releases acetic acid for purification by gas chromatography. For determination of the carbon-isotopic composition of total acetate, the acetate-containing zone of the chromatographic effluent can be routed directly to a combustion furnace coupled to a vacuum system allowing recovery, purification, and packaging of CO2 for mass-spectrometric analysis. For analysis of methyl carbon, acetic acid can be cryogenically trapped from the chromatographic effluent, then transferred to a tube containing excess NaOH. The tube is evacuated, sealed, and heated to 500 degrees C to produce methane by pyrolysis of sodium acetate. Subsequent combustion of the methane allows determination of the 13C content at the methyl position in the parent acetate. With typical blanks, the standard deviation of single analyses is less than 0.4% for acetate samples larger than 5 micromoles. A full treatment of uncertainties is outlined.