An alternative to the HSE/NPL Mark II Phase Contrast Test Slide for airborne asbestos fiber analysis laboratories

More than 55 years ago, an analytical method employing phase contrast optical microscopy was developed for the quantitative estimation of airborne asbestos fiber concentration, and it was found that the detection limit of these microscopes was dependent upon microscope design, microscope set-up, and observer performance. In the early 1980s, the HSE/NPL Mark II Phase Contrast Test Slide was developed which facilitated standardizing detection limits of microscopes and observers in laboratories around the world. This paper describes the development and testing of an alternative test slide, known as the Pickford Phase Contrast Test Slide, which employs state-of-the-art nano-fabrication technology. Each Pickford Test Slide is certified by the Environmental Analysis Laboratory of the Southern Cross University stating that it is equivalent in performance to that of the HSE/NPL Mark II test slide, which aligns with the United Kingdom Health and Safety Executive HSG248 Asbestos: The Analysts' Guide (2021) requirement. Users report that the Pickford Test Slide is much easier to use than the HSE/NPL Slide. Since the early 1980s, the certification of phase contrast microscope test slides has been based on subjective visibility testing, which is less than ideal because of variations between certifiers, reference standards, and microscopes. Hence, a unique objective visibility test was developed with the aim of replacing subjective testing, and also of conducting routine testing of the test slide phase objects following nano-fabrication. Routine testing has been useful because each Pickford Test Slide is tested and documented so that intricate nano-fabrication chip processes can be controlled over years of production. However, thousands of comparisons of both forms of testing have shown that it is very unlikely that objective visibility testing will ever replace subjective testing because valid and relevant objective testing depends upon standardizing several dozen microscope camera set-up parameters that vary from one camera to another. Further, because the ultimate use of a test slide has always been based on human visibility, validation may not be achievable.