TESTING REPRODUCIBILITY
There are currently two internationally accepted standards written on the use of
laser diffraction: ISO 13320 (ref. 9) and USP<429> (ref. 10). Both standards state
that samples should be measured at least three times and reproducibility must meet
specified guidelines. Note that this means three independent measurements (i.e.
prepare the sample, measure the sample, empty the instrument, and repeat). The
coefficient of variation (COV, or (std dev/mean)*100) for the measurement set must
be less than 3% at the D50 and less than 5% at the D10 and D90 to pass the ISO
13320 requirements. These guidelines change to less than 10% at the D50 and less
than 15% at the D10 and D90 when following the USP<429> requirements. Finally,
the guidelines all double when the D50 of the material is less than 10µm.
While following the ISO or USP guidelines to test reproducibility is suggested, it is
typically part of an internal specification or procedure. The specifications shown to
potential customers typically don’t include the reproducibility values.
figure 15
|
REPORTING PSD PERCENTAGE
SMALLER THAN THE GIVEN SIZE
In this example, percentage of
the PSD is reported at 100nm.
figure 14
|
MEASUREMENT ERROR
Error appears exaggerated on
the Y axis because of the
narrowness of the PSD
12
undersize error of +/-20%
size error
of +/-5%
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
20
40
60
80
100
SIZE IN
µm
%
UNDER
120
140
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