Setting particle size specifications

The creation of a meaningful and product-appropriate particle size
specification requires knowledge of its effect on product performance in
addition to an understanding of how results should be interpreted for
a given technique. Today's blog provides guidelines for setting particle size
specifications on particulate materials—primarily when using the laser diffraction
technique, but also with information about dynamic light scattering (DLS), acoustic
spectroscopy, and image analysis.

DISTRIBUTION BASIS
Different particle sizing techniques report primary results based on number,
volume, weight, surface area, or intensity. As a general rule specifications should
be based in the format of the primary result for a given technique. Laser diffraction
generates results based on volume distributions and any specification should be
volume based. Likewise, an intensity basis should be used for DLS specifications,
volume for acoustic spectroscopy, and number for image analysis. Conversion to
another basis such as number—although possible in the software—is inadvisable
because significant error is introduced. The exception to this guideline is converting a number based result from a technique such as image analysis into a volume basis. The error involved is generally very low in this scenario.

DISTRIBUTION POINTS
While it is tempting to use a single number to represent a particle size distribution
(PSD), and thus the product specification, this is typically not a good idea. In
nearly every case, a single data point cannot adequately describe a distribution of
data points. This can easily lead to misunderstandings and provides no information
about the width of the distribution. Less experienced users may believe that the
“average particle size” can adequately describe a size distribution, but this implies
expecting a response based on a calculated average (or mean). If forced to use a
single calculated number to represent the mid-point of a particle size distribution,
then the common practice is to report the median and not the mean. The median
is the most stable calculation generated by laser diffraction and should be the
value used for a single point specification in most cases. Rather than use a single point in the distribution as a specification, it is suggested to include other size parameters in order to describe the width of the distribution. The span is a common calculation to quantify distribution width: (D90 – D10)/D50. However, it is rare to see span as part of a particle size specification. The more common practice is to include two points which describe the coarsest and finest parts of the distribution. These are typically the D90 and D10. Using the same convention as the D50, the D90 describes the diameter where ninety percent of the distribution has a smaller particle size and ten percent has a larger particle size. The D10 diameter has ten percent smaller and ninety percent larger. A three point specification featuring the D10, D50, and D90 will be considered complete and appropriate for most particulate materials. How these points are expressed may vary. Some specifications use a format
where the D10, D50, and D90 must not be more than (NMT) a stated size.
Example:
D10 NMT 20μm
D50 NMT 80μm
D90 NMT 200μm
Although only one size is stated for each point there is an implied range of
acceptable sizes (i.e. the D50 passes if between 20 and 80μm). Alternatively, a range of values can be explicitly stated.
Example:
D10 10 – 20μm
D50 70 – 80μm
D90 180 – 200μm
This approach better defines the acceptable size distribution, but may be
perceived as overly complicated for many materials. It may also be tempting to include a requirement that 100% of the distribution is smaller than a given size. This implies calculating the D100 which is not recommended. The D100 result (and to a lesser degree the D0) is the least robust calculation from any experiment. Any slight disturbance during the measurement such as an air bubble or thermal fluctuation can significantly influence the D100 value. Additionally, the statistics involved with calculating this value (and other “extreme” values such as the D99, D1, etc.) aren’t as robust because there may not be very many of the “largest” and “smallest” particles. Given the possible broad spread of D100 results, it is not recommended for use in creating specifications involving a statement that 100% of the particles are below a stated size.