This method does not allow the fibres to be distinguished according to their chemical composition and is therefore not fibre type specific. The opaque measuring filters are made optically transparent using acetone vapour and triacetin. The evaluation is carried out in the light microscope under phase contrast conditions, i.e., very thin fibres below 0,3 µm cannot be seen.
Since the use of asbestos has been banned, this method is used almost exclusively to determine the concentration of artificial mineral fibres. Artificial mineral fibres (AMF) are inorganic, synthetic fibres produced from mineral raw materials such as glass, natural stone or slag. They include continuous fibres, mineral wool, ceramic fibres and special fibres. In contrast to natural mineral fibres, AMFs are amorphous silicates. The chemical composition of AMFs varies widely, with the SiO2- Al2O3 system being the most significant. AMFs are a large group of different classes of fibres with different physical, chemical and biological properties. It should be noted that in many workplaces the fibre dust in the air can originate from different types of AMFs, and often mixing with other, different fibre dusts such as gypsum fibres occurs.
Depending on the task, sampling can be person-related, the sample holder is placed in the breathing zone of the worker or stationary. For this purpose, a measuring device is placed in the area to be assessed. The sampling time must be chosen in such a way that the measurement results are representative for the assessment of the situation according to the measurement task.
A cellulose nitrate filter with a pore size of 0,8 µm with a grid is used for sampling. There are two suitable systems for sampling according to DGUV Information 213-531:
- Monitor (25 mm Ø)
With this system, a volume flow of 1 l/min must be set on the GSA sampling device (SG5100ex or SG5200).
- PGP cassette (37 mm Ø)
With this system, a volume flow of 2 l/min must be set on the GSA sampling device (SG5100ex or SG5200).
The sampled filters are prepared on a glass slide in the laboratory and analysed in a phase-contrast light microscope.
The detection limit of the measurements performed depends directly on the sampling time. For a duration of two hours, it is 0.032 fibres/m³.
(Source: DGUV Information 213-531)