Particle Counting in Liquid using a Coulter Counter

Particle Counting in Liquid using a Coulter Counter

The Multisizer 4™ (Beckman Coulter) provides highly resolved counting and size analysis of particles or cells suspended in liquid. The technique (Coulter Counter) is unaffected by particle color, particle shape, composition, or refractive index, and can count and sizing particles at very low concentration levels (single particle count). It is also many times faster than manual analysis using a microscope and have an excellent resolution (400 size channels).

In addition to measuring particle distribution (number and size) in a stable and non-changing suspension it is also possible to measure the change of the distribution over time thanks to the fast measurement time. For instance, the solvent can be modified during the measurement. Even polar electrolyte solvents can be used to study water soluble suspensions.

A Coulter Counter automatically counts individual particles and determine particle size with the technique of resistive pulse sensing. Basically, a Coulter Counter consists of two chambers containing weak electrolyte solutions. Between the chambers there is a microchannel with an orifice of a defined diameter. Two electrodes are immersed in the electrolytes, one in each chamber with the orifice in between them. When a voltage is applied over the electrodes, there will be a current in the circuit and the electrolyte solution will be a resistance to the current. A suspension of particles or cells is drawn through the system at a steady rate with an isolated piston. 

Particle Counting in Liquid

Figure 1. Number of particles vs particle size bins in sample containing calibration particles. Error bars indicate variability between samples.

When the particles are drawn from one of the chambers to the other through the microchannel, there is a displacement of electrolytes enough to cause a small momentarily increase in impedance. The increase in impedance of the solution results in an electrical pulse with an amplitude proportional to the volume of the particle. The particle size measured is thus the volume equivalent particle size, i.e., the diameter of a spherical particle with the same volume as the actual particle measured. The analysis also yields size distributions in number and specific surface area for the sample.

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