While liquids that are tested in pharmaceutical manufacturing are expected to be free from contamination, particles can enter as a result of handling and managing the water supply. Therefore, it is vital to measure particle contamination in these processes.
Particle counters are primarily used to test finished products, but a second and equally-important application is monitoring quality of water for injection.
Currently, two methods accomplish this: light obscuration and light scattering.
This method directs a laser through a narrow capillary tube with a flowing stream of liquid. Particles that pass through the beam will block light, casting shadows across the photo-detector. A calibrated particle counter determines the sizes of the particles based on how much light is blocked.
This method is best used to measure relatively large particles that are not quite visible to the naked eye. These fall into a range of 1.5 microns to over 150 microns.
This method also directs a laser through a narrow capillary tube with a flowing stream of liquid. However, rather than cast shadows, passing particles scatter the laser beam via refraction, reflection, and diffraction. Mirrors collect the scattered light and focus it onto a photodetector to be analyzed.
Again, the amount of disruption to the beam is equivalent to the size of the particle. In this case, the more light that is scattered, the bigger the particle. Like with obscuration, known particle size standards are used to calibrate particle counters.
Light scattering is appropriate for volumetric instruments down to 0.2 µm. It’s effective up to 20 µm.
Pharmaceutical products are comprised mostly of water, especially products that are injected. Controlling the quality of this water is extremely important. For a full description of the technology and applications of liquid particle counting, see this article in Controlled Environments.
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