Phase Contrast Microscopy (PCM) Analytical Service
Phase Contrast Microscopy (PCM) was one of the first methods used in the analysis of fibers in air samples. While it does not identify fibers as asbestos, it is the industry standard for measuring the number of fibers that workers are exposed to in the air.
This is the standard method used to sample and count fibres in air. Air sampling involves drawing air at a known flow rate through a filter for a measured time, so that airborne particles are collected. The filter is then mounted on a glass slide and rendered transparent for microscopy examination. A known fraction of the filtered deposit is examined using phase contrast microscopy (PCM) of at least 500x magnification, to count all fibres seen (particles >5 µm long, <3 µm wide and a length to width (aspect ratio) of >3:1) in a known number of microscope graticule areas. The fibre concentration (in terms of fibres per millilitre of air (f/ml)) is determined by dividing the total number of fibres collected on the exposed area of the filter by the volume of air sampled.
Limitations with the PCM method which may affect the result and decrease the precision:
- Some types and batches of membrane filters can give high background counts on blank filters.
- If too many particles are sampled on the filter it will prevent or bias the fibre count.
- There is significant moisture in the air being sampled.
- Fibre counting is subject to human error and fatigue.
- The visibility of fibres depends on a number of variables.
- The performance of sampling and analytical equipment.
- Fibre counting has poor statistical precision especially for low numbers of fibres.
- The PCM does not count all the fibres sampled; it cannot readily detect fibres <0.2 µm in width.
- In mining and quarrying, the relatively small (>3:1) aspect ratio used to define a fibre will allow many elongated mineral fragments to be counted.