The merits of the high-temperature combustion method for determining the amount of natural organic carbon in surface freshwater samples

The merits of the method for the quantitative analysis of carbon of natural organic matter (NOM) in aquatic freshwater samples, using the high-temperature catalytic combustion technique, have to our knowledge not previously been documented. Although a large intercalibration study concerning marine NOM have documented and improved the analytical merits of this technique in marine chemistry, these results cannot directly be adapted to freshwater analysis. This article presents the findings from an intercalibration on the determination of carbon in freshwater NOM, between 25 laboratories participated in using the high-temperature catalytic combustion technique. The laboratories analysed the amount of total- and dissolved organic carbon (i.e. TOC and DOC, respectively) on a set of 10 samples. The samples consisted of fresh surface water, reverse osmosis (RO) isolates of dissolved NOM from natural freshwater locations, as well as synthetic standards. General merits (i.e. accuracy, precision etc.) of the analytical method are presented and problems with detection limits, high blank values, contamination from filters, experience in the use of RO-isolates and analysis of the more refractory (i.e. not readily oxidizable) NOM are discussed. The focus of the study is on the analytical merits achieved on the natural samples compared to the commonly used and possibly readily oxidizable standard material (i.e. potassium hydrogen phthalate). It is demonstrated that the method's merits generated using the readily oxidizable potassium hydrogen phthalate are too optimistic when applied to the analysis of natural aquatic samples.