Water – an important parameter for the preparation and proper use of certified reference materials

Property values of powdered certified reference materials (CRMs) are very often related to dry mass. The dry mass is indirectly determined by measuring the moisture content of the sample. The most commonly used methods are the drying oven method and the Karl Fischer titration (KFT). It is well known that these two methods may give different values for the moisture/water content. In this study thermogravimetry was used to simulate the drying oven method and to investigate the reasons of the differences between both moisture/water content determination methods. A mass spectrometer coupled to a thermobalance (TGMS) added further useful information regarding the release of volatile substances and the decomposition of the material during the drying process.The relative humidity of the laboratory may influence the water content of powdered CRMs because of their more or less pronounced tendency to take up water (hygroscopicity). This fact can lead to biased property values. The water adsorption kinetics and the water uptake capacity vary, depending on the constitution of the sample. This was studied analysing several powdered food CRMs and individual constituents thereof. The speed of water uptake and the water uptake capacity were described using an exponential function.The findings have consequences both for the producer and for the user of certified reference materials.