prepare the sample, measure the sample, empty the instrument, and repeat). Note that this means three independent measurements (i.e. Both standards state that samples should be measured at least three times and reproducibility must meet specified guidelines. There are currently two internationally accepted standards written on the use of laser diffraction: ISO 13320 (ref. Additionally, ultrasonic energy can be applied to improve the dispersion of agglomerates which can significantly change the result. In some cases this dilution may change the state of the particles and affect the apparent size distribution. This sometimes requires that samples undergo dilution. The laser diffraction technique works best within a certain particulate concentration range. The particle size distribution is dependent upon many factors including the sample preparation method. 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. In the example below the value for percent less than 100nm is reported as 9.155%. In the LA-960 software this is displayed as “Diameter on Cumulative %”. 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 2). Recently there has been interest in the presence of “nanoparticles” (at least one dimension smaller than 100nm) in products such as cosmetics. There are applications where the percent below a given particle size is an important result. the x axis depends upon the steepness of the distribution curve.įigure 2: In this example, percentage of the PSD is reported at 100 nm. The degree to which the y axis error is exaggerated vs. Stating an error of +/- 5% is more attractive than +/- 20%, even when expressing the same actual error range. The same error becomes +/- 20% when translated to the y (percent) axis. This error includes all sources such as sampling and sample preparation. All measurements include an error which should always be considered when setting a specification.įor the example shown in Figure 1, the D50 is 100μm with an error of +/- 5% on the x (size) axis. 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. 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. Other published specifications are based on the percent below a given particle size such as: 50% below 20μm and 90% below 100μm. Some specifications use a format where the D10, D50, and D90 must not be more than (NMT) a stated size.įigure 1: Error appears exaggerated on the Y axis because of the narrowness of the PSD. A three point specification featuring the D10, D50, and D90 will be considered complete and appropriate for most particulate materials. The D10 diameter has ten percent smaller and ninety percent larger. 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 more common practice is to include two points which describe the coarsest and finest parts of the distribution. However, it is rare to see span as part of a particle size specification. The span is a common calculation to quantify distribution width: (D90 – D10) / D50. 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 median is the most stable calculation generated by laser diffraction and should be the value used for a single point specification in most cases. 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. 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). This can easily lead to misunderstandings and provides no information about the width of the distribution. In nearly every case, a single data point cannot adequately describe a distribution of data 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.
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