X VS. Y AXIS
Other published specifications are based on the percent below a given particle
size such as: 50% below 20µm and 90% below 100µm.
This type of specification
is based on points along the y axis (which reports frequency percent) as opposed
to the x axis (which reports diameter) as in the previous examples. Although
this approach has been used in many specifications, it is important to realize the
difference between using the x (size) and y (percent) axes. All measurements
include an error which should always be considered when setting a specification.
For the example shown in Figure 14, the D50 is 100µm with an error of +/- 5%
on the x (size) axis. This error includes all sources such
as sampling and sample
preparation. The same error becomes +/- 20% when translated to the y (percent)
axis. Stating an error of +/- 5% is more attractive than +/- 20%, even when
expressing the same actual error range. The degree to which the y axis error is
exaggerated vs. the x axis depends upon the steepness of the distribution curve.
There are applications where the percent below a given particle size is an important
result. Recently there has been interest in the presence of “nanoparticles” (at least
one dimension smaller than 100nm) in products such as cosmetics. The
software
which calculates the PSD should be capable of easily reporting the percent under
any chosen size—in this case the percent below 100nm (Figure 15). In the LA-960
software this is displayed as “Diameter on Cumulative %”. In the example below the
value for percent less than 100nm is reported as 9.155%.
Several points are worth mentioning in regards to setting a specification on the
percent below 100nm as in this example specifically and
for sub-micron materials
generally. The particle size distribution is dependent upon many factors including
the sample preparation method. The laser diffraction technique works best within
a certain particulate concentration range. This sometimes requires that samples
undergo dilution. In some cases this dilution may change
the state of the particles
and affect the apparent size distribution. Additionally, ultrasonic energy can be
applied to improve the dispersion of agglomerates which can significantly change
the result.
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