Why are my laser diffraction results not repeatable or reproducible?

It is perhaps human nature to “blame“ the instrument when our results are not repeatable or reproducible. Interestingly enough it is rarely the instrument that is to blame!

Let’s start with the basics, that is, what is repeatability and reproducibility in terms of results?

Repeatability is measuring the same sample multiple times under the same measurement conditions. We simply repeat the measurement on the same sample in the same instrument. In the case of a laser diffraction measurement that is repeating the measurement on the same sample allready placed in the instrument.

Reproducibility is measuring a new sample using the same measurement conditions and then comparing the results between these individual samples. Of course there is instrument to instrument reproducibility, operator reproducibility etc.. But let’s keep it simple. 

Repeatability

In terms of laser diffraction particle sizing repeatability will depend on the nature of your sample. How wide is the size distribution ? If it is less than 1 decade (e.g. 10 -100 um) you would expect better repeatability than say for a sample spanning 3 decades (1 – 1000 um)!  

Is the sample stable/ inert or soft and fragile? A simple glass powder will be very stable under a wide range of measurement conditions while an agglomerated organic powder will change in size depending on its measurement conditions. We say a sample is robust when the size stays the same under a range of different measurement conditions.

If the instrument is in good working order (see the How do I know my instrument is working correctly? for further details) and an appropriate method has been developed for the sample then you would expect your results to be repeatable.

What is good repeatability?

ISO 13320:2009 states the following

 • ±3% for x10 (and all other values of cumulative undersize distribution between the 10th and 30th percentiles)

• ±2.5% for x50 (and all other values of cumulative undersize distribution between the 30th and 70th percentiles)

• ±4% for x90 (and all other values of cumulative undersize distribution between the 70th and 90th percentiles).

Reproducibility

It is clear that we cannot evaluate reproducibility unless we have good repeatability allready for our sample. Assuming the particle size result is allready repeatable then reproducibility is “simply “ due to sampling. For samples with an x90 greater than 100um sampling is the biggest contributing factor to variance in the results. The best technique to sample from your bulk sample will largely depend on the sample itself.

If it is dry, tumbling the sample container 20 times before taking a representative sample is best practise. Making the sub sample into a paste on a watch-glass is also recommended as it prevents segregation of fines in your sample.

Sampling from an slurry can be challenge and will depend on the solid content and density of the sample. Experimentation with various techniques might be needed to generate the best protocol.

The good news is that the Malvern Mastersizer 3000 software assists users with data quality and repeatability via its Expert report under the Data Quality tab that will test highlighted results for various quality issues as well as repeatability.

Contact us if you need specific assistance with repeatability or the reproducibility of your results!