The sources of chloroacetates in the environment are only partly known. Photodegradation of volatile chlorinated hydrocarbons have been suggested as anthropogenic sources, but in the last few years natural production has also been proposed. An experimental approach to disentangle anthropogenic and biogenic contributions is based on 14C/12C measurements via AMS, since fossil materials contain no 14C whereas biogenic materials contain the equilibrium concentration of 14C typical for modern carbon. The main problem with this method is the very low concentration of haloacetates in environmental samples, e.g. trichloroacetic acid (TCA) has a concentration of only few µg/kg in soil. Thus a large amount of sample material has to be collected to extract some microgram carbon for AMS measurements. With this small amount of carbon the problem of possible contamination with modern or old carbon during each step of sample preparation is very important. The aim of this project is the development of a method for radiocarbon measurements of organic environmental trace chemicals in the microgram range and to apply it to TCA occurring in soil. The soil samples were collected from a spruce site in Bavaria where relatively high TCA concentrations have been observed in needles in previous studies. Chemical treatment of soil samples and extraction of TCA are performed in Bayreuth. The separation of the chloroacetic acids is performed by 2D preparative capillary gas chromatography using an apolar column as first dimension and a medium polar column as second dimension [1]. The conversion to CO2 was done by vacuum oxidation at 900 C with CuO. The production of negative carbon ions was performed with a hybrid sputter ion source [2]. With the tiny amount of carbon of 1.5 µg a 12C current of 4µA has been maintained for 8 minutes at the Faraday cup close to the detector. The 14C content of 2.2 ± 0.2 pMC indicates that the TCA extracted from forest soil is of anthropogenic origin. For routine measurements a direct coupling of GC with the AMS ion source is necessary, first tests with the EA were successful and showed promising results [2].
[1] K. Jakubovska, Dr. thesis, University of Bayreuth, 2005. [2] T. Uhl, W. Kretschmer, W. Luppold, A. Scharf, Radiocarbon Vol. 46, Nr 1(2004) 65.
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