Coulter Counter Used for Particle Counting and Sizing in Drinking Water

Drinking water

Here is a good example of how a Coulter Counter can be used. In this case, it’s to find out about the particulate content of water – which is essential information for those involved in water treatment and management.

Even if the water is very clean, in terms of particles, Beckman Coulter’s Multisizer 3 Coulter Counter will analyse that content – quickly, easily, accurately and reliably. Ultimately, it is important that the analyst should be confident that the water sent for drinking is actually free of solid particles and does not simply contain too few particles for the testing method to detect.

A recommended method for this analysis involves use of a 50 μm aperture tube. An aperture’s linear dynamic range is between 2 and 60 per cent of its diameter. With a 50 μm aperture, a tube will be able to analyse particle size distribution and concentration from 1 to 30 μm. Should a different range be needed, an alternative size of aperture could be substituted.

Calibration of the Multisizer 3 Coulter Counter is straightforward, following the instructions in its operator’s manual. In the procedure described here, to determine particle concentration and size distribution, results are recorded in terms of number per ml. The control mode is set at ‘Volumetric’ and the run volume is set at 500 μl.

The procedure involves use of Beckman Coulter’s Electrolyte Preparation Isoton, which provides a balanced electrolytic solution. Its salt concentration (sodium and potassium, together with a phosphate buffer system) is about 1 per cent in total. Given that the particulate content of the water will be very low, it is best to avoid diluting the sample too much. The following steps can be used to prepare a sample with 1 per cent salt concentration:

a)    Prepare a custom-made saline solution with 20 per cent salt content. Filter this using a 0.2 μm filter.

b)    Mix 19 ml of water and 1 ml of the custom saline solution in an Accuvette.

The following details should be entered in the sample information dialog of the Coulter Counter’s software: sample volume 1 ml, electrolyte volume 500 μl, sample volume 19 ml.

To run a blank, 19 ml of 0.2 μm-filtered DI water and 1 ml of the custom saline solution are first mixed in an Accuvette. This preparation is run using the same settings as for the sample, and then this run is set as the background in the Coulter Counter’s software.

To run the actual analysis, a 50 μm aperture tube is used with an Accuvette containing the prepared sample. This tube is flushed before each analysis, while both aperture and electrode are also rinsed each time before moving to the next sample run.

The Coulter Counter expresses its results graphically, measured in particles per ml, showing size distribution and statistics for each sample of water. As well as the total particle concentration, it is possible – using the software’s Interpolation Points feature – to assess the concentration of particles above a pre-set size.

When it comes to sizing and counting particles to a high level of resolution, there is really no technology to compete with that of the Coulter Counters. This makes these instruments invaluable for monitoring drinking water processes and assessing filtration efficiencies.