Coincident Events

I’ve mentioned it before, but I think it’s well worth repeating, that Coulter Counter users can open up a mine of useful background information and application advice if they go to the Beckman Coulter website, www.beckmancoulter.com. Once you’re on the site, go to the relevant product section and then in the right-hand column click on ‘Literature’. In my latest search I came across some notes on ‘coincident events’ in relation to use of the Coulter Principle.

A coincident event, when it comes to particle characterisation using a Coulter Counter, is what happens when two or more particles pass through the sensing zone at once and are counted as just one.

To reduce the chances of this happening, it’s important to have the suspension’s concentration low enough to keep the particles separate. In practice, though, it may still occur even when the sample is very dilute. Whether it does will depend on factors including the sensing zone volume, the distribution of the particles – i.e. how homogeneously they are suspended – and to some extent how quickly the electronics respond.

Another name for this effect is coincident particle passage, and it can be subdivided into primary coincidence and secondary coincidence.

In primary coincidence, the instrument counts two particles as one – and so the recorded results reflect a lower particle concentration than is in fact present. As long as its occurrence is within the 10% coincidence limit – this being the situation where one in every ten particles is not counted – it is quite easy to apply a mathematical correction. The correction, which the instrument’s software can make automatically, depends on knowing how many particles are counted in the measured volume, and what the diameter of the aperture is, and using empirical data relating to the sensing zone’s effective volume.

In secondary coincidence, two particles which are too small to be detected individually are seen by the instrument as one larger particle. It is not easy to correct for this. To minimise it, you can continue diluting the sample until no significant difference in the distribution of particle size is apparent. You can also keep the concentration below a coincidence limit of between 5 and 10%.

To find out more, do please have a look at the document on the website.

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