ISO 21501 — A Standard Methodology To Optical Particle Counter Calibration And What It Means To Cleanroom Owners

By Tony Harrison and Bob Latimer, Hach Ultra Analytics

ISO 21501 is a new family of standards describing the instruments and calibration requirements for determining particle size distribution using light interaction methods. It represents the culmination of work by instrumentation manufacturers and industry users and comes at a critical time for the life science industry with the increasing trend for real-time air particle monitoring in cleanrooms using light scattering air particle counters.

In comparison to liquid particle counters, the calibration of air particle counters presents greater challenges due to the need to generate air samples containing sub-microscopic particles of homogenous size and distribution. Although the technology of air particle counting is well understood, the ability to calibrate any two air particle counters so that they produce the same results when sampling the same air sample has proven to be challenging, bringing into question the accuracy of these instruments. ISO 21501 now delivers a calibration method that can significantly improve the repeatability and reproducibility of air particle counters.

Optical instrumentation has been used to determine particle contamination in air and liquids in the life science industry for many years. In addition, the correlation between airborne particles and final product quality has long been recognized in the semiconductor, flat panel display and hard disk storage manufacturing industries, where improvement of air quality (reduction of particulate contamination) has led to increases in final product yield.

ISO 21501 also applies to liquid particle counters used for determination of particulate contamination in infusions and injections. Until recently, the calibration requirements [known as "IST" methods] for liquid particle counters used to test infusions and injections were described in detail in the United States Pharmacopoeia (USP) chapter <788>. However, in the interest of international harmonization of the pharmacopoeias, the details of these IST calibration methods have been removed in order to simplify the text of USP <788>. ISO 21501 now offers an alternative to these IST tests and establishes calibration methods to ensure accurate and repeatable performance of liquid particle counters.