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The theoretical basis for use of CSIA in degradation monitoring of contaminants is the establishment of representative isotope fractionation factors, which describes the shifts in isotope ratios during reactions or other processes affecting the target compound. Experiments have been performed to constrain this isotope fractionation for the carbon, hydrogen, chlorine and nitrogen (when applicable) constituents of several common contaminants: e.g. dichloro methane (DCM - Heraty et al., 1999), TCE and PCE (Numata et al., 2002), MTBE (Hunkeler et al., 2001a), benzene (Hunkeler et al., 2001b; Mancini et al., 2003), and nitroaromatic compounds such as TNT (Hartenbach et al., 2006). Such experiments are specific and the obtained data is in a strict sense only valid for the studied system. Recently, the CSIA research community has begun to adopt a concept that allows for approximate general isotope fractionation factors to be calculated from case-specific data. isoSoil will develop this further by means of the theoretical framework and software, and also perform experiments to fill knowledge gaps by constraining selected isotope fractionation factors. Efforts will also be made to develop models to accurately predict isotope fractionation factors in cases where no experimental data exists. This expanded assortment of available isotope fractionation factors and their calculated derivatives will apply to a wider range of situations and types of contaminated sites, thus improving the usefulness of CSIA. Furthermore, the complex calculations will be automated to allow end-users outside the CSIA research community to perform site evaluations with CSIA data.

CSIA of carbon and hydrogen - “the established isotopes”
The analytical technique of CSIA is well established for carbon, which can be analyzed at nanogram levels by on-line Gas Chromatography Combustion Isotope Ratio Mass Spectrometry (GC-C-IRMS) (Merritt et al., 1994; Ricci et al.,1994). This type of analysis is commercially available (e.g. from EIL:www.uweilab.ca and Hydroisotop Gmbh: www.hydroisotop.de). Similarly, compound-specific on-line analysis of hydrogen isotopes is now on the rise due to serial production of equipment for this purpose, and is also available as a commercial service.

CSIA of nitrogen, halogens and radiocarbon - “the emerging isotopes”
Nitrogen isotope analysis is now also being developed as an on-line technique (Berg et al., 2007). CSIA of Cl isotopes have been performed with off-line prep. methods followed by IRMS (e.g., Holt et al., 1997). Another isoSoil partner developed an alternative off-line method that instead used thermal ionization mass spectrometry (TIMS) and thereby lowered the detection limit by about a factor 50 to around 1 ug Cl (Holmstrand et al. 2004). Recently, Shouakar-Stash et al. (2006) reported on an automated online method to analyze TCE and PCE for chlorine isotopes, indicating the imminent boom of this type of analysis at remediation sites. In addition, the use of ICP-MS for halogen-isotope measurements has been demonstrated (van Acker et al., 2006; Sylva et al., 2007) and is being adopted and developed by Stockholm university (SU). It is anticipated that ICP-MS will improve the cost-efficiency of halogenisotope analysis, and make it more accessible since ICP-MS instruments are getting more commonplace at analytical service laboratories.

Compound-specific radiocarbon analysis (CSRA) has been successfully employed both to differentiate between biomass and fossil-fuel combustion sources of PAHs in sediments and air (Mandalakis et al., 2004, 2005) and to differentiate between natural and industrial sources of methoxylated polybrominated diphenyl ethers (PBDEs; Teuten et al., 2005). CSIA of Cl isotopes was used to elucidate the source of high PCDD levels in ball clay (Holmstrand et al., 2006). Again, multi-dimensional CSIA has rarely been applied for source apportionment on contaminated sites, and never for more than two combined isotope systems (e.g. Jendrzejewski et al., 2007). isoSoil will explore this un-tapped potential, since additional dimensions add further source-resolving power.
CSRA, primary tool for source apportionment, is based on labor-intensive preparative capillary gas chromatography (pcGC; Eglinton et al., 1996; Mandalakis and Gustafsson, 2003) followed by microscale accelerator mass spectrometry (AMS). The full CSRA sequence has recently been evaluated and optimized to avoid trace C contamination and method-induced isotope fractionation (Zencak et al., 2007).

The research has received funding from the European Community's Seventh.
Framework Programme FP7/(2009-2012) under grant agreement no 212781.