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Source identification of trichloroacetic acid with preparative capillary gas chromatography and accelerator mass spectrometry
(2006)
- A method for radiocarbon source identification of environmental chemicals present in soil at ppb levels using trichloroacetic acid (TCA) as an exemplary compound was developed in the present thesis. A powerful approach to differentiate between biogenic and anthropogenic sources of environmental chemicals is to determine the radiocarbon content: an organic chemical derived from the biological carbon pool, and thus resulting from assimilation of atmospheric carbon dioxide, contains the radioactive carbon isotope 14C at the typical atmospheric equilibrium abundance of 14C/12C = 1.2 x 10-12, while a chemical derived from fossil carbon over 200 million years old is free of 14C (half life 5730 years). The only method being sensitive enough for direct quantitative determination of a few thousand atoms of radiocarbon in a minute amount of a pure environmental trace chemical is accelerator mass spectrometry (AMS). In the present thesis an AMS equipped with a titanium-target gas source was used. Therefore, 1 microgram carbon in the form of carbon dioxide was sufficient for the measurement. To determine TCA in soil the total amount of soil sample needed depends on its concentration in this compartment. For sampling, sites of the Bavarian spruce site observational network were selected where TCA concentrations were relatively high. Two sampling sites were compared and the one with the higher TCA soil burden was chosen. Soil samples from Oberthulba were collected, and TCA was isolated in sufficient quantity in pure form, by a combination of differential liquid extraction steps, followed by derivatization, and preparative capillary-gas chromatography (PC-GC). In environmental samples the actual analyte constitutes usually only a minute fraction, mostly less than 0.1 % of the primary extract. Taking this into consideration, sequential (two-dimensional) PC-GC-separation on two different columns was necessary emphasizing high chromatographic capacity and high stationary phase selectivity in the first step and high separation efficiency in the second one. This was achieved by using two columns of appropriate dimensions. In the first dimension a wide-bore high-capacity thick film capillary coated with a non-polar stationary phase was used. For further removal of by-products the fraction containing the analyte was cryogenically collected, and the final purity refinement was effected on a short thin-film, high-efficiency capillary coated with a medium-polar stationary phase. From a sequence of runs, the required amount of trichloroacetic acid methyl ester (TCA-OMe) was collected with a cryogenic fraction collector, combusted to carbon dioxide, water and chlorine were separated, and carbon dioxide was fed into the gas ionization source of the AMS-instrument. Great care was taken to avoid blank contamination during sample preparation steps for AMS because at such low levels significant complicating factors are the ubiquitous chemical trace pollutants in laboratory air and the presence of atmospheric carbon dioxide containing natural occurring 14C. The radiocarbon determination of a real sample of TCA collected in the mineral horizon of forest soil in Northern Bavaria yielded a value of 2.2 percent modern carbon (PMC) for TCA-OMe corresponding to 3.3 PMC for TCA indicating the anthropogenic origin of TCA. If TCA were of biogenic origin, a value of 66.7 PMC would be expected as one additional carbon from a petrochemical source is introduced during derivatization to the methyl ester. The result proved that the TCA found in the soil at the particular sampling location was an atmospheric breakdown product of tetrachloroethylene and not a product of any purported soil microbial process. This result is important for risk assessment procedures performed for TCA and tetrachloroethylene under OECD and EU chemical programs. To control emissions of environmental chemicals and to reduce their influence on the various environmental compartments it must be known if they originate from anthropogenic sources.
