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GEA Niro Method No. A 8 d |
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Revised: September 2005 |
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1.
Definition
A suspension of powder in
isopropanol is measured with a low angle laser beam, and the particle size
distribution is calculated.
2.
Scope
This is a fast method for
measuring particle size distribution of powders.
3.
Principle
The method can be used on all
powders containing less than 10% fat.
4.
Apparatus
1. Malvern Instrument,
Mastersizer Basic, equipped with software version B.0 or similar
equipment.
2. Malvern QS Small Volume
Sample Dispersion Unit.
3. Malvern in/out measuring
cell, beam length 2.0 mm.
4. Dispenser 0-50 ml with
container.
5. Filling
knife.
6. Waste
container.
5.
Reagents
1. Isopropanol, IPA (technical
quality).
6.
Procedure
1. Look at the particle size in
a microscope and choose a lens capable of measuring the largest particles, see
Remarks 7.1.
2. Prepare the instrument for
measuring in wet mode using IPA as the liquid, as described in the user
manual.
The stirrer regulator should be set at 2000 rpm on the Malvern
unit.
3. Measure the background for
IPA.
4. Quickly add a sufficient
amount of milk powder and measure as soon as the powder is dispersed and not
later than 20 seconds after addition of the powder. For detailed instructions
about measuring, see the Malvern user manual.
5. Rinse twice with
IPA.
All
measurements are made in duplicate
7.
Result
The following calculations are
done automatically:
1. The volume median diameter
D(v,0.5) is the diameter where 50% of the distribution is above and 50% is
below.
2. Two determinations of mean
particle size should not differ by more than 5% relative. The shape of the
curves in the two determinations should be the same.
3. D(v,0.9), 90% of the volume
distribution is below this value.
4. D(v,0.1), 10% of the volume
distribution is below this value.
5. The span is the width of the
distribution based on the 10%, 50% and 90% quantile.
8.
Reproducibility
N/A
9.
Remarks
1. The lens should be chosen
according to the actual particle sizes. All lenses are capable of measuring a
specific area of microns; therefore, ensure that the lens measures 100% of the
largest particle. The software estimates the particles below the lens capacity,
but cannot account for larger particles. The most precise results are obtained
when the lens cuts off just above the largest particles because the estimate of
the particles below is more precise.
2. The obscuration is the
amount of sample added to the system or more correctly the light intensity
absorbed by the sample. The optimal obscuration is 0.1-0.3. The results are
unreliable outside this interval.
3. The powder particles should
be insoluble in the dispersing agent (IPA).
4. Fat filled products,
containing more than 10% fat, and milk powder containing sugar cannot be
measured by this method.
5. D(4.3) is the equivalent
volume mean diameter or the De Broncker mean diameter.
6. D(3.2) is the equivalent
surface area mean diameter or the Sauter mean diameter.
7. Non-agglomerated milk
powders will give the same mean diameter measured in wet or dry mode; whereas
agglomerated powders will give considerably finer mean particle sizes when
measured in dry mode due to wear of the particles.
8. It is possible to use
ultrasound in-line. The use of ultrasound affects the particles. Agglomerates
and primary particles are separated and it is possible to measure the primary
particle size.
10.
Literature
·
GEA Niro Research Laboratory
·
The Malvern Mastersizer Basic
user manual.
QS Small Volume Sample Dispersion Unit user
manual.