Chapter 4 Making measurements - a tutorial
Page 4-2 MAN 0384
4
General measurement advice
Before making a measurement, note the important general advice given here.
Sample preparation
Good sample preparation is critical. 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 the sample is taken from the top of the container it will not
contain the smaller particles, giving a biased measurement. The sample should be
correctly mixed before a measurement is taken.
Wet samples also have to be correctly dispersed in a liquid dispersant. Using the
wrong dispersant can cause the sample to stick together in clumps, float on the sur-
face, or even dissolve. Check the sample and dispersant to see if they are suitable
before a measurement is made. There are many ways to prepare the sample to
ensure a perfect measurement.
Details on sample preparation are given in Chapter 8.
Note
Over half of the problems encountered in measuring a sample are caused
by bad sample preparation. If the user prepares badly for the measurement,
no amount of subsequent analysis will give a good result.
Adding the sample
The software reports exactly what level of signal the sample generates and whether
this is ideal, too low, too high, etc. The technique has a wide range of concentra-
tions that are ideal so concentrations do not have to be precise.
The sample concentration is controlled by monitoring the obscuration of the laser
beam caused by the sample. The obscuration is simply the fraction of light “lost”
from the main beam when the sample is introduced. For example, an obscuration
of 30% means that 30% of the incident laser beam (recorded during the Measure
Background step) has been lost through scattering or absorption.
The range of concentrations over which the instrument can be used can be
expressed in obscuration terms as the following table shows.
i
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