Considerations For Wet Samples; Choice And Preparation Of The Dispersant - Malvern Mastersizer Series Getting Started

C H A P T E R 9
measurement. Obviously care should be taken with delicate materials where
drying in the oven may damage the sample.
A fresh sample that has not had time to absorb moisture from the atmosphere is
always preferable and will usually give better results.
Some samples can only be measured in a dry state as they react with all wet
dispersants, for example they may dissolve or the particles may swell when in
contact with a liquid.

Considerations for wet samples

The above are considerations to make when analysing a dry sample. Even more
care should be taken when analysing a sample in a wet medium as there are more
choices to be made.

Choice and preparation of the dispersant

The first choice to consider when measuring a wet sample is the choice of
suspension medium (dispersant). When analysing a sample for the first time, it is
always best to check the dispersion before hand. Add the selected dispersant (for
an initial measurement it is usual to use water) to a little of the sample in a beaker
and visually note the result. The sample may dissolve, this can usually be seen
visually, or if unsure, analyse the sample and observe the obscuration figure. If the
obscuration figure is seen to decrease then the sample is probably dissolving.
The dispersant may itself contain impurities or particles that could be significant.
You are recommended to filter your dispersant before use either with an in-line
pipe filter or, for small quantities, a syringe based disposable type. Filtration to 1
micron is generally adequate with 0.22 microns being commonly available and an
ideal size.
If your dispersant is stored under pressure, or low temperature then you may also
have to consider degassing before use. The pressure release, or temperature rise
will reduce the solubility of gasses resulting in the formation of bubbles in the
pipes and tanks etc. Bubbles are a problem as they are measured with the sample
and are counted as particles - biasing the results. This is particularly a problem
with some mains water supplies. The simple answer is to store sufficient
dispersant at room temperature and pressure for several hours before use to allow
the de-gassing to occur. It should also be noted that the use of cold dispersant in a
warmer environment can also give rise to condensation on the outside surfaces of
the cell windows. For systems plumbed into the mains supplies a small header
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