Chapter 9 Advanced features
Page 9-4 MAN 0384
Obscuration filtering
Obscuration filtering rejects data that does not fall within the set limits. To enable
it click on the Only include measurements…. check box and enter a Time-out
period. Measurement will stop if the required number of snaps has not been made
in this time-out period. The software records the number of snaps measured and
this figure can be used in reports to indicate the completeness of a measurement.
For example, a report can be configured to say “1750 of 2000 snaps measured”.
This facility is particularly useful where the sample concentration is liable to some
degree of fluctuation, as with some dry powder measurements. For dry powder
measurements practical limits would be between 0.5% and 6%.
In case a totally inappropriate feed rate is set, so that the obscuration seldom (if
ever) falls within the desired limits, a time-out is provided to allow the user to exit
the measurement. This should not be set so long as to give the measurement the
appearance of having “hung up”, but sufficiently long to allow a fair chance of sam-
ple of the required concentration passing through the measurement cell.
Clicking on the Raise alarm if time out period expires check box will advise
how many good snaps were collected in the event of a time-out, and offers the
option of using them as the basis for the measurement
Autodilution
Autodilution allows the sample concentration to be automatically adjusted by add-
ing more dispersant. This feature is not available in manual measurements. Good
Laboratory Practice dictates that autodilution is an inferior procedure to the addi-
tion of a correctly predetermined sample quantity.
If operational requirements make the use of autodilution necessary, run a test to
ensure that no modification of the particle size distribution is caused by the proce-
dure.
A typical test is to perform a manual measurement of the sample at a higher
obscuration than required, say 20%, and then run the sample at the same starting
obscuration using autodilution with the target obscuration threshold set to a lower
value than the ideal, say 5%.
If the two sets of results show a high level of comparability, autodilution is an
acceptable procedure. Users who are required to conform to Good Laboratory
Practice should keep the results of this test on file.
As a general rule, emulsions and fine suspensions (0.1 to 10 microns) will be suita-
ble candidates for autodilution. Extreme caution must be exercised with materials
having particle size distributions broader than this as fines could be lost and results
biased.
MAN0384-1.0 Mastersizer 2000.book Page 4 Tuesday, March 20, 2007 8:56 AM
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