What Are Particle Counters?
A particle counter requires each particle to be counted directly as opposed to other methods of estimating numbers, volumes and sizes. There is an overlap with other methods of analysis though as many particle counters use optical techniques involving direct imaging, light scattering or light obscuration. Variations of these approaches can be used for counting the actual number of particles in a liquid, or in an aerosol, or simply in a ‘dry’ powder.
If a high-energy light source, such as a laser or halogen light, is shone through a chamber, which only allows single particles to pass through it, then the particles can be measure by a variety of techniques. Direct imaging produces an image, via a high-resolution camera, that is then analysed automatically by a computer programme. Obscuration techniques use the light scattering properties of the particle, as measured by a photo detector, to tabulate the particles as they pass.
Other approaches to particle counting include Condensation Particle Counting, which detects particles in an aerosol by first increasing them in size to produce droplets in a supersaturated gas. Tunable Resistive Pulse Sensing (TRPS) technology uses a small nanopore, through which individual particles (colloids/biomolecular analytes) are propelled. This is picked up as a small change in the ionic current flow, which is then correlated with particle numbers.
The importance of particle counting impinges on a number of areas. For liquids it’s important in the medical world, both for injection of products and in the manufacturing of device such as artificial hips. A similar need for quantifiably pure water exists in the microelectronics industry. Air particle counters are, of course, vital in clean room technology. Again semiconductor manufacturing and the pharmaceutical industry relies on this technology during many critical processes.
For counting biological particles such as bacteria, cells or viruses, which are suspended in electrolytes, a Coulter counter can be used. This relies on detecting electrical resistance changes caused by particles passing between two different chambers and is thus related to the volume of the particle.