Dissolved Oxygen Concentration Interlaboratory Comparison: What Can We Learn?

Dissolved oxygen concentration is a key parameter for characterizing natural and wastewaters and for assessing the global state of the environment in general. The decrease of dissolved oxygen levels in the world's oceans, which is becoming increasingly obvious, is expected to have an impact on the whole ecosystem of the Earth, including the carbon cycle, the climate, etc. Dissolved oxygen measurements by sensors are often deemed easy measurements by routine laboratories. In reality, the physical and chemical processes underlying the measurements are complex and these measurements are not at all as robust as often considered. Recently an international in situ interlaboratory comparison measurement of dissolved oxygen concentration took place at the University of Tartu. The results revealed that the routine laboratories as a rule still do not fully master the art of dissolved oxygen concentration measurement: Out of altogether 63 measurement results obtained by the participants 52% were unacceptable according to the En numbers. Based on the analysis of the results of the intercomparison a set of tools and recommendations are given to the participants of how to improve the quality of their results.