Evaluation of Fine Particles in Distributions and
the Relationship to Microscopic Evidence.
(Understanding the impact of microscopic observations
compared to mass/volume distribution relationships)
Philip E. Plantz
Application Note
SL-AN-09 Rev C
Provided By:
Microtrac, Inc.
Particle Size Measuring Instrumentation
Introduction
Microscopy is a technique that has been used for many years to determine the chemical composition, shape,
morphology and size of particles. Many types of microscope exist including light microscopy, electron
microscopy (useful in determining elemental composition) and atomic force microscopy. Their application
encompasses many materials including examination of tissue, bacteria, fungi, minerals, liposomes, pigments,
blood cells, viruses, and nanoparticles. Of special concern to this discussion is its use in the determination of
particle size distributions especially within the context of the presence of distribution tails or fines when
compared to laser light scattering data. This paper describes an experiment in which Microtrac data and
competitive light scattering data were evaluated and compared to optical microscopy and the issues that
arise.
It is important to note that volume distributions explain more relatively about weight or mass of product.
One definition of size that it is the amount of space taken up by an object and can be expressed in terms of
volume. Thus volume may be considered a more important indicator of size than the number of particles
present. The volume is important in particle size since product amount is generally determined by weight and
not by how many particles are present. It would be sorely difficult to specify an amount needed in a
formulation by counting particles when it is much more easily accomplished by weighing. An analogy to this
is in chemistry where the use of moles substitutes for the laborious need to count molecules related to
Avogadro’s number (6.02E 23 molecules/mole). Using moles assists calculations in stoichiometry and is
directly related to mass (weight) of a compound. This is akin to using weight rather than count when product
quantities are determined. Modern light scattering instruments afford a direct relationship to weight through
the volume distribution.
Experimental Approach
Customer asked to have a particle size measurement performed using a Microtrac S3500 low angle light
scattering instrument (range 0.020 to 2800 microns). Data showed that particles were present up to
approximately 150 microns (µ). The other instrument showed particles to the largest size as Microtrac. The
interesting point of the comparative data was that the amount of fines present did not agree. The Microtrac
S3500 reported 0.01% smaller than 0.7 microns while the other instrument showed 2.5%. To resolve this
issue, optical microscopy was employed using an Olympus BH microscope at 400X magnification.
To provide a rational comparison to the light scattering (diffraction) data where the fines can be emphasized
for proper comparison, calculations were performed to draw a picture that is representative of what should be
expected to be observed by light microscopy. This rationality includes a first step to unbias the light
scattering data of the two instruments. This was accomplished by assuming no particles were present above
20 microns. This assumption allowed for a nearly identical starting point for developing a drawing of a
population distribution based upon the volume distribution of each instrument’s data. The 20-micron point
was chosen since the volume amount at 20 and 10 microns were nearly identical. The resulting volume
distributions were then renormalized to equal 100%. The results of this calculation are shown in the right
column of data in the figures. The sizes corresponding the volume amounts were determined by interpolation
of the actual distributions to the sizes shown. This allowed a direct comparison of sizes and volume amounts.
Further Calculations
Number percent (%) was determined by assuming that the distribution is comprised of spherical particles
having a volume of d3/6. The number % is backed out using the following formula for each of the sizes
shown:
N% = [Volume / (d3/6) (Total volume)] X 100%
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SL-AN-09 Revision C
A calculated number of particles corresponding to the sizes was determined assuming that one 20-micron
particle was present that represented 40 volume %. This provided a starting point or basis for calculating the
relative number of particles that should be present for each size assuming one 20-micron particle. The
“number” is shown in the table accompanying the figures.
The number or count of the particles was used to develop a drawing of what should be expected when
examining the sample by microscopy. In addition, the drawings were adjusted in size to represent a 400 X
magnification as was used for actual microscopy. This provides good perspective when comparing drawings
and actual photomicrographs.
Microscopy Procedure
An Olympus HB optical microscope was used at a total 400X having an eyepiece containing a reticule.
Sample was prepared as for introduction to the Microtrac S3500 particle size analyzer. For microscopy, a
cover slip was used having dimensions that allow for exact focusing. Fourteen samples were obtained and
examined to obviate representative sampling problems providing at least 300 particles.
Drawings
For diagrammatic purposes, MS PowerPoint was used to draw a circle to represent a spherical particle for a
single 20-micron particle. The 20-micron particle was used as the starting or basis as shown in the calculated
particle size distributions. The drawing of the 20-micron particle was reduced proportionally to obtain
drawings of the sizes shown in the tabular calculated distribution for number of particles. Each new particle
drawn was then multiplied to provide the number of particles related to each size listed.
Results
The object of the experiment was to evaluate data obtained from two instruments that reported different
amounts of particles smaller than 0.7 microns. Calculations were performed on volume distributions of each
set of data so that a proposed drawing could be developed showing how the particulate mass would appear
during actual microscopic examination. A starting point for the calculations was selected to be 20-microns in
order to provide a means of direct comparison. Since the distributions from the two instruments contained
nearly identical volume amounts in the 20 and 10-micron channel sizes and since the desire is to include as
much of the original data as possible, 20-microns was selected.
It is clear from a comparison of the two drawings that emulate the distributions from the two instruments,
that the Microtrac data are substantiated by microscopy. The photograph shows arrows pointing to particles
that are 20 and 2 microns as reference points. The photo has little to no particles below 2 microns suggesting
that particles at or below 0.7 microns are not present in the sample. The photo is representative of many that
were taken and is offered as a demonstration of experimental evidence.
According to competitive data and the ensuing drawing, there should be an overwhelming number of
particles smaller than 2 microns. The large number of small particles less than 1-micron should have formed
a “cloud” of particles in the photomicrograph obscuring the larger particles. In all microscopic examinations
of a variety of representative samplings, this did not occur.
Of special note is comparison of the drawing for the Microtrac S3500 to the photomicrograph image. When
particles larger than 20 microns are eliminated, the 20 and 10-micron particles become a highly abundant
species in terms of volume as reported by both light diffraction instruments. That fines smaller than 1 micron
occur in the distribution in the distribution is but one issue but one must also consider the amount. The
volume amount below 1-micron is approximately 6.43% for the other instrument while it is 0.41% for the
S3500 (a factor 10 less powder weight reported).
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Such small sizes relate to respirable particles, bag house design, powder specifications, relationship to
powder performance and the potential expensive engineering and time allocated to address the issues. The
extent of the cost of these issues (that may not exist) may hamper opportunities for business growth, R&D
and efficient process design. Use of the Microtrac S3500 obviates theses concerns by providing a more
realistic view of the presence of fines and distribution tails.
Figure 1. Competitor Analysis Converted to Microscopy
Basis for comparison is the number % of large particles corresponding to the
volume %.The number percent was normalized to one
20 micron particle to get the “count”. Then we will compare the
Magnification X 400 –(Relative)
20 micron
particle
2.5% < 0.7uM (volume %)
Size
0.5
0.6
0.75
1u
2u
3u
4u
5u
6u
10u
20 u
Calculated
Number
665
948
417
773
120
35
2
2
4
7
1
Calculated
Number %
22.0
32.0
14.0
26.0
4.0
1.2
0.08
0.05
0.14
0.24
0.03
Measured
Volume %
0.43
1.0
1.0
4.0
5.0
5.0
0.8
1.0
4.6
37.0
39.0
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SL-AN-09 Revision C
Figure 2. Microtrac S3500 Analysis Converted to Microscopy
Using the same 20-micron basis, the total number of particles is much less
and the number of small particles is far less.
Magnification X 400 –(Relative)
20 micron
particle
0.01% < 0.7 uM, (volume %)
Size
0.5
0.6
0.75
1u
2u
3u
4u
5u
6u
10u
20 u
Calculated
Number
0.0
0.0
55
79
80
50
20
7
3
7
1
Calculated
Number %
0.0
0.0
18.2
26.1
26.5
16.5
6.6
2.3
1.0
2.3
0.3
Measured
Volume %
0.0
0.0
0.01
0.4
3.6
7.4
7.4
4.9
0.9
35.1
40.1
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SL-AN-09 Revision C
Figure 3. Microscopy – Bright Field Photomicrograph
Magnification X 400 (Relative)
20 uM
10 uM
2 uM
Microtrac Data Represent Fines in Similar Amount to the
2 uM
20 uM
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SL-AN-09 Revision C