Wet dispersion is the most widely used technique to separate sample particles ready for laser diffraction and while it is not overly complicated, it does require some thought.

The unsettling facts…(ha ha)

Unlike colloids, suspensions will eventually settle. If you have ever had to paint a wall you will no doubt have experienced the need to stir a tin of newly opened paint before using it to ensure it can be applied smoothly – for me it is precisely at this point that my initial enthusiasm wears off! Likewise, by the time we have reached adulthood, we are likely to automatically shake a bottle of liquid medicine, orange juice ‘with  suspended solids or salad dressing.

Luckily, when remixing paint, salad dressing or medicine we never have to worry about breaking the suspended particles. We can shake as hard as we like because when suspended in a liquid even fine, fragile particles are less likely to break. In some cases, we can be particularly assured of this fact because manufacturers carry out multiple quality tests measuring particle size by laser diffraction. The shear in a sample dispersion unit is similar to that imparted by shaking – and we have the ability to regulate this energy input in a defined manner and so can be certain that our shaking will not affect the performance, taste or efficacy of that final product. After all, wet dispersion for laser diffraction analysis makes particular sense in the case of a product specifically designed to behave as a suspension! However, the basic principles of suspending particles in a liquid to disperse them as evenly as possible, while protecting them from unwanted damage, can also be used for a great many other materials.

Wet method development

To achieve control and ensure results are both robust and reproducible, method development for wet measurement requires the systematic consideration a number of factors:

o        Dispersant selection

o        Measurement settings

o        Dispersion energy

o        Representative sampling

Fortunately, there’s a lot of basic information available to help guide best practice towards valid results. So, rather than try to cover such a detailed subject in a short blog I shall use the space to link you to key documents of use so this might act as a research platform.

Laser diffraction sample dispersion guidance and standards

USP 429 [General Chapter <429>, “Light Diffraction Measurement Of Particle Size”, United States Pharmacopeia, Pharmacopoeial Forum (2005), 31, pp1235-1241] states that laser diffraction involves the measurement of “a representative sample, dispersed at an adequate concentration in a suitable liquid or gas”. The dispersion process, whether wet or dry, must ensure that these goals are met.

Extensive guidance for sample preparation is offered in the form of international standards. For example:

A great place to start…

As well as the pharmacopoeial guidance and international standards, my colleague Anne Virden has produced two very good summary documents on this subject, which together might act as a very good starting point for more detailed research:

1.       An application note; “Wet method development for laser diffraction”

2.       An application note; “Dry Method Development”

3.       An article; “Method development for laser-diffraction particle-size analysis” published in Pharmaceutical Technology in October 2010