Sample Preparation; Cleanliness Of The Optical System - Malvern Mastersizer Series Getting Started

C H A P T E R 4
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P a g e 4 . 2
G e t t i n g S t a r t e d

Sample preparation

Firstly the most important thing to consider is the preparation of your sample
before it is measured. A representative sample must be taken. Dry powders, for
example, tend to separate out if stored for some time or vibrated. The larger
particles tend to rise to the top and the smaller particles collect at the bottom of
the container. If you were to take the sample from the top of the container it will
not contain the smaller particles, giving you a biased measurement. The sample
should be correctly mixed before a measurement is taken.
Wet samples have also to be correctly dispersed in a liquid dispersant. Using the
wrong dispersant can cause the sample to stick together in lumps, float on the
surface or even dissolve. The sample and dispersant should be checked to see if
they are suitable before a measurement is made. There are many ways to prepare
your sample to ensure a perfect measurement.
Details on sample preparation are given in chapter 9.
N o t e
. It has been found that over half the problems encountered in measuring
the sample have been caused by bad sample preparation. It should be
obvious that if you make a bad measurement that no amount of
analysing will give you a good result.

Cleanliness of the optical system

Laser scattering is a high resolution optical method in which the lenses and cell
windows are an integral part of the measurement zone. Dust or smears on the
lenses will scatter light that will be measured with the sample scattering. In
general the process of measuring both a background and a sample and then
subtracting the two corrects for such contributions. However this correction is
first order only and excessive dirt on the optics degrades the instruments
sensitivity.
You are recommended to ensure at all times that your lenses and cell windows are
clean. Use the procedures described in chapter 11 for advice on cleaning the
system.
If you are in doubt about the optical cleanliness then you can use the live display
of scattered light to view the background measurement. By viewing this screen
you will be able to judge if the optics are clean or not. This procedure is described
in chapter 11.
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