Understanding what we are measuring: standards and quantification of natural organic matter

Some of the problems involved in quantifying operationally-defined parameters in natural waters are generally overlooked. In particular, the implications of the fact that standards have an effect on the results obtained that differs from usual analytical determinations are often ignored. On the basis of a revision of published data and personal experience, the case of natural organic matter types is discussed here for four different categories: carbohydrates, thiols, TEP (transparent exopolymers), and humic substances in freshwaters. In all cases, the results obtained are noticeably dependent on the standard, though the type of dependency varies. Key aspects are discussed in detail and advice is given about the procedure to follow when reporting data. Conclusions are applicable to other aquatic systems and natural organic matter fractions.