Considerations For Dry Samples - Malvern Mastersizer Series Getting Started

C H A P T E R 9

Considerations for dry samples

P a g e 9 . 2
G e t t i n g S t a r t e d
As the bottle vibrates in transit Á, large particles part to allow fine particles to fall
through  and then close together over the fine particles - being lifted in the
process.
In most samples there are some large particles and some small, but the majority
fall in-between these two. If a sample is taken from the top of the container, then
only the large particles will be measured. If this is then compared to a
measurement with the sample taken from the centre of the container then the
results will be different.
If the sample is stored in a container then mix the sample thoroughly. Do not
shake the container as this often increases the separation of the particles. Instead,
hold the container in both hands and gently roll the container, continually
changing its orientation.
If the distribution of particles within a sample is particularly broad, then
representative sampling can be difficult. If you are experiencing problems then the
use of a "spinning riffler" may be beneficial. A spinning riffler uses the same
principle that causes the sample to separate when it is kept in a container. The
riffler comprises of a vibrating hopper which vibrates the sample down a shute.
The act of vibrating the sample causes the larger particles to separate out and
travel down the shute first. At the end of the shute is a collection of rotating plates
that collect the sample evenly. When all of the sample has passed down the shute
then each collecting plate will contain a representative sample.
Liquid samples can also separate out if stored in containers, with larger particles
sinking to the bottom. Again the sample should be thoroughly mixed if you are to
get a representative sample. Sample splitters/rifflers are also available for liquid
samples.
The first step when analysing a sample for the first time is to decide whether to
analyse the sample in a wet or dry state. This is usually determined by the nature
of the end use of the sample. If the product is to be used and stored in a dry form,
for example sand, then perhaps a dry analysis is preferred. Another consideration
is whether the material in its dry state is free flowing. Good pouring
characteristics indicate a non-cohesive powder which will usually disperse well in
a dry powder feeder without any difficulties, whereas a highly cohesive material
tends to stick and clump together giving biased readings.
The sample clumping together can often be overcome by drying the sample in an
oven or by placing in a desiccator until dry and then continuing with the
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