Method development for origin assignment of volatile organic chemicals (VOCs) using C-14 liquid scintillation spectrometry

A procedure for discrimination between fossil versus non-fossil components in volatile organic compounds, VOCs, was developed. The protocol measures the amount of the radioactive isotope of Carbon, C-14, in the sample via low level liquid scintillation counting. The fractional amount of fossil versus non-fossil components can be uniquely discriminated from each other based on the C-14 abundance in the samples. As a baseline study, ten representative volatile organic compounds were studied to determine whether VOCs encountered in the environment have natural or synthetic origins, or both. Each of the samples were mixed with Ultima Gold scintillation cocktail in a low potassium glass scintillation vial, and analyzed in a low-level Wallac 1414 scintillation counter. Prior to the conduct of a program of analysis, the optimum counting protocol was established to achieve the highest figure of merit (FOM) or signal to noise ration. The FOM is determined from the efficiency and background, which depends on the counting window as set by the lower and upper level discriminators. The optimum ratio of sample to scintillant was 5:15, in terms of stability, ie., homogeneity of the solution was maintained throughout the counting. This ratio was used for the routine analysis of samples. Counting efficiencies were determined using external C-14 standard in Ultima Gold, which was counted for 2 hours in a predetermined optimum window. Samples were counted for 2 hours in a chamber temperature maintained at 14 degreecentigrade. Samples with sufficient 'modern' or 'contemporary' C-14 levels (higher than 12 disintegrations per minute per gram Carbon, dpm/C) are classified as biogenic and include: acetonitrile, acetone, and methylene chloride; those which turn out to be fossil-derived (14C-free), include: benzyl alcohol, methanol, ethylene glycol, petroleum benzene, toluene, cyclohexane, chloroform and methyl isobutylketone. (author)