16
Application
The viscosity of the sample plays an important role in determining how the sample
is spread onto the pad for analysis. If a sample is thin, it can be pipetted onto the
pad and can be tested at high power levels. A thin sample spread over a large area
permits moisture to evaporate rapidly with little residual heat buildup. Glass fiber
pads may be stacked for increased absorbency.
High viscosity samples (samples that do not flow well) require a different
technique. If the sample can be easily spread across the glass fiber pad, no dilution
is required. A thick sample will tend to puddle and form a crust, sometimes trap-
ping volatiles within the bubble. Overheating or burning can also occur on thick
samples. If repeatable results cannot be obtained by spreading the sample, a
dilution may be necessary. The sample must be soluble in the diluting agent, and
the diluting agent must be a microwave absorber.
To generate the appropriate amount of heat and achieve optimum moisture results,
different types of samples must be spread to different thicknesses over different
areas of the glass fiber sample pad. As a general guideline, the sample will
generate heat in direct proportion to the thickness of the sample layer on the pad.
A sample spread too thickly can cause sample burning; a sample spread too thinly
can cause insufficient drying. The prepared sample should be applied to a glass
fiber sample pad. Once the sample is properly applied to the sample pad, it should
be covered with a second glass fiber pad to ensure heat retention and to eliminate
splattering.
Liquid samples should be pipetted onto the pad. Solids samples should be spread
onto a pad placed on the edge of a clean, flat surface, such as a countertop. The
spatula should be held level with the pad when spreading the sample to ensure a
smooth, uniform application of sample to the pad surface.
If the sample is an organic solvent-based liquid, special handling is required.
Equipment and method modification may be required. CEM recommends using
the SMART System
5
Exhaust Kit when analyzing solvent based samples.
Non-polar organic solvents do not absorb microwave energy; therefore, samples
containing this type of solvent will not dry unless they are mixed with a diluent
which absorbs microwave energy and has a higher boiling point than the non-polar
solvent.
If the sample is a polar organic solvent-based liquid, samples will absorb
microwave energy and dry as readily as a water-based liquid. If the organic solvent
is an alcohol, a ketone, or an ester, samples should be tested with water-based
techniques.