Ohaus MB45 MOISTURE ANALYZER Manual - Manual (Page 49)

MB45 47
8.2 Sampling and sample preparation (cont)
Use of quartz sand
To ensure an optimum drying process, samples should always have as large an area as possible.
Results of substances which form crusts (e.g. glucose syrup) or pasty substances (e.g. butter) can be
considerably improved by mixing with quartz sand. Sample pans with a large volume and relatively high
walls are needed for this.
Pasty, fat-containing and melting substances
For pasty, fat containing and melting substances, use of a glass fiber filter is advantageous to increase
the surface area of the sample. The glass fiber filter is tared together with the sample pan. The liquid
contained in the substance is uniformly and extensively distributed in the interstices between the fibers
throughout the available area. The same also applies to melting fats and fat containing samples. This
increase in the surface area results in faster and complete vaporization of the moisture. Pre-drying of the
glass fiber filter and storage in a desiccator is necessary only for highly precise measurement results.
Liquid substances
Liquid substances (e.g. dispersions) often tend to form drops on the sample pan owing to the surface
tension of the liquid. This prevents a rapid drying process. The use of a commercial glass fiber filter
shortens the drying time by a factor of 2 to 3. The glass fiber filter distributes the liquid sample over a
wide area as a result of its absorbent action. Pre-drying of the glass fiber filter and storage in a
desiccator is necessary only for highly precise measurement results.
Skin-forming and temperature sensitive substances
The use of a glass fiber filter can be useful for temperature-sensitive and skin forming substances. In this
case, the sample to be dried is covered by the filter and thus receives a “new surface”. This shields the
surface of the sample against direct IR radiation. Gentler heating of the samples is based on convection
rather than on IR radiation. Experience with this type of preparation has been good; particularly for
products containing sugar. Further, the shielding of the sample against direct IR radiation by covering the
test substance can make a considerable contribution to improving the reproducibility with temperature
sensitive samples.
Sugar-containing substances
Samples containing a large amount of sugar tend to caramelize on the surface. In such cases, ensure a
thin layer is applied. Also select a moderate temperature.
Sample application on the drying pan
To obtain reproducible results, it is essential to ensure uniform distribution of the sample on the pan. An
uneven distribution can result in homogeneous heat distribution in the sample. As a result, the sample
could be incompletely dried in the center owing to excessive piling. Thick layers have an adverse effect
on the escape of moisture. The resulting lengthening of the measurement time promotes decomposition
at the surface of the sample by prolonged heat action.
Film formation on the sample can prevent complete escape of the moisture. With such samples, ensure
application of a thin and uniform layer thickness.