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Sample preparation guidance
Considerations for wet samples
This section covers sample preparation considerations for wet samples. When ana-
lysing a sample in a wet medium there are even more choices to be made than for
dry samples.
Choice and preparation of the dispersant
The majority of samples will allow water to be used as the dispersant. If the parti-
cles are soluble in water or react chemically an alternative must be used. For exam-
ple, flour added to water will float on the surface but it disperses well when added
to propan-2-ol (IPA).
The first choice to consider when measuring a wet sample is the choice of suspen-
sion medium (dispersant). When analysing a sample for the first time, it is always
best to check the dispersion beforehand. Add the selected dispersant (for an initial
measurement it is usual to use water) to a little of the sample in a beaker and visu-
ally note the result. The sample may dissolve; this can usually be seen visually. If
unsure, analyse the sample and observe the obscuration figure. If the obscuration
figure decreases the sample is probably dissolving.
The dispersant may itself contain impurities or particles that could be significant.
We recommend filtering the 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 the dispersant is stored under pressure or at low temperature it may be necessary
to consider degassing before use. A pressure release or a temperature rise will
reduce the solubility of gasses, resulting in the formation of bubbles in pipes and
cells, 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 degassing to
occur. Also see the section on bubbles later in this chapter.
Note 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 tank may overcome this problem. Filter this
water prior to use. Another solution is to warm the dispersant (for water typically to
60-80 °C) and then allow it to cool before use.
Mastersizer 3000
Warning!
Do not warm a dispersant to allow re-gassing if the dispersant is volatile.
Never allow dispersants to reach their boiling points.
Chapter 6
Page 6-5
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