POWER July 2017 - 37

ENVIRONMENTAL ISSUES
3. Visible like stars in the sky. The
same particles shown in Figure 1 are shown
here, but using backscattered electron imaging
mode. Courtesy: MVA Scientific Consultants
Table
1. Details tell the story. This table shows a sample image analysis output for the
first 20 particles examined in one example. Courtesy: MVA Scientific Consultants
Particle # Area (µm2)
1
2
3
4
5
6
7
8
9
4. Computer analyzed. The same particles
shown in Figure 3 are shown here after
recognition by the automated image analysis
system algorithm. Courtesy: MVA Scientific
Consultants
10
11
12
13
14
15
16
17
18
19
20
best. In those cases, a more heavily loaded
filter is preferred.
In such cases, a portion of the collected
particles can be removed and redispersed
onto a polycarbonate or similar filter. Care
must be taken in this step because some
components of the sample particulate may
be soluble in liquids used for redispersing
the sample. For example, samples with high
sulfate or other salt concentrations cannot be
dispersed in water because those salts will
dissolve and then will not be included in the
particle size measurements.
Scanning Electron Microscopy
Analysis
Figure 1 shows a polycarbonate filter that
has been used in a Method 5 train to collect
particulate material from a power plant stack.
The pores in the filter are dark and the filter
surface itself is smooth and featureless. Particles
on the filter are easily recognized by
eye and can also be recognized by image processing
algorithms.
Figure 2 shows stack particulate collected
on a quartz glass filter. While the particles
are still easily recognized by eye, the complicated
structure of the filter itself will confuse
image processing programs, and this particulate
must be pointed out to the computer alJuly
2017 | POWER
Perimeter
(µm)
0.3
0.6
2.7
0.5
0.3
0.6
0.5
0.3
1.0
1.1
0.6
0.4
0.4
3.7
1.1
1.2
0.6
0.8
0.5
0.4
1.7
2.6
8.2
2.2
1.9
2.6
2.4
1.7
3.5
3.7
2.4
1.8
2.1
8.8
4.1
4.0
4.3
3.1
2.4
2.7
Minimum
diameter
(µm)
0.4
0.4
0.3
0.5
0.3
0.4
0.1
0.3
0.6
0.5
0.6
0.2
0.4
1.7
0.7
0.9
0.6
0.4
0.6
0.3
Maximum
diameter
(µm)
0.8
1.1
2.5
1.0
0.9
0.8
1.2
0.5
1.3
1.3
1.0
0.8
0.8
3.1
1.8
1.5
1.4
1.2
1.0
0.8
Mean
diameter
(µm)
0.6
0.7
1.5
0.7
0.6
0.7
0.7
0.4
1.0
1.0
0.8
0.6
0.6
2.4
1.3
1.1
1.1
0.8
0.8
0.6
Aspect ratio
1.0
1.0
2.4
1.0
1.7
1.0
1.1
1.0
1.0
1.0
1.0
1.0
1.0
1.8
1.2
1.0
2.6
1.0
1.0
1.7
Table 2. Getting the full picture. The particle size distribution data from one sample
is shown here in terms of number and mass percentages. This particular sample had 19,102
particles. Courtesy: MVA Scientific Consultants
Diameter range (µm)
0.5 - ≤1.0
>1.0 - ≤2.5
>2.5 - ≤5.0
>5.0 - ≤7.5
>7.5 - ≤10.0
>10.0
gorithm by a human analyst or removed to a
more suitable substrate for image processingbased
analysis.
Samples that have been collected on or
redispersed onto polycarbonate or similar filters
can be analyzed by automated SEM, allowing
for the unattended analysis of several
tens of thousands of individual particles. In
MVA Scientific Consultants' laboratory, the
analysis of 20,000 particles is standard. More
particles can be analyzed, if better statistics
are required, especially for samples with a
very low fraction of particles greater that 10
µm.
For automated particle sizing, the image
must contain sufficient contrast between the
background and the particles so that an image
analysis algorithm is capable of differwww.powermag.com
Number
(%)
55.4
34.4
7.1
2.0
0.6
0.5
Mass (%)
1.5
7.3
16.0
21.0
16.3
37.9
entiating between them. In image analysis,
a " particle " is defined as a set of contiguous
pixels all of which are brighter (or more
rarely, darker) than the threshold brightness
used to define the surrounding " background "
pixels.
Polycarbonate filters are ideal for this purpose.
In Figure 1, the particles are easily recognizable
as bright features; however, there
is not sufficient contrast between the particles
and the background for automated image
analysis algorithms to accurately distinguish
the particles from the filter.
A variety of techniques are used to increase
the contrast. Instrument parameters
may be varied and/or different imaging
modes of the microscope may be employed.
The same field of view, but in backscattered
37
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POWER July 2017

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