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A Working Group on Quality Assurance/Quality Control of analyses in laboratories active in the chemical analysis of atmospheric deposition and soil water has been created within the framework of the Integrated Co-operative Programme on Assessment and Monitoring of Air Pollution Effects on Forests (UN-ECE/ICP Forests) and the EU/Forest Focus Programme (Regulation 2152/2003). This paper is a follow up to an earlier paper dealing with the validation of chemical analyses, in which validation techniques (ion balance, comparison between measured and calculated conductivity, Na/Cl ratio and relationship between different forms of N) were tested on a set of real analysis data obtained from different laboratories. This paper focuses on the validation of chemical analysis of samples containing high dissolved organic carbon (DOC) concentrations (> 5 mg C L-1), where the ion balance criterion fails because of the presence of weak organic acids. About 6000 chemical analyses of bulk open field, throughfall and stemflow samples, which contained complete sets of all ion concentrations, conductivity and DOC, produced in 8 different laboratories, were used to calculate empirical relationships between DOC and the difference between the sum of cations and the sum of anions, with the aim to evaluate a formal charge per mg of organic C. Regression coefficients were obtained for data from each laboratory, as well as for all the data combined. The coefficients were further tested using an independent set of data from each country. The differences between the individual laboratory and the overall regression coefficients are discussed. The results are also considered in the light of formal charge values for DOC/TOC obtained in studies on freshwater. The formal DOC charge proved to be useful for estimating the contribution of organic acids in the ion balance test, thus considerably improving the applicability of the ion balance as a validation criterion for samples with high DOC values.
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