Principles of the membrane filter method for the determination of airborne asbestos fibre concentrations by light microscopy

• Pages: 62-63

B. Principles of the membrane filter method for the determination

of airborne asbestos fibre concentrations by light microscopy

1. A sample is collected by drawing a measured quantity of air through a membrane

filter by means of a battery-powered sampling pump. The filter is later transformed from an

opaque membrane into a transparent optically homogeneous specimen. The fibres are then

sized and counted using a phase contrast microscope. The result is expressed as fibres per

millilitre of air, calculated from the number of fibres on the filter and the measured volume of

air sampled.

2. Personal samples are taken within the worker's breathing zone, i.e. within. a

hemisphere of 300 mm (12 in) radius extending in front of the face and measured from a line

bisecting the ears.

3. An open-faced filter holder fitted with a cylindrical cowl is fastened to the jacket

lapel of the worker with the cowl pointing downwards.

4. Static samples are taken at fixed locations.

5. Membrane filters (mixed esters of cellulose or cellulose nitrate) of 1.2 µm pore size

with printed grids and a diameter of 25 mm (1 in) should be used.

6. A portable battery-operated pump – carried on the worker's belt or in a pocket for

personal sampling – must be used. The flow rate should be adjusted to 1 l/min ± 5 per cent, i.e.

approximately 4 cm/s face velocity. The difference of the flow rates before and after sampling

should not be greater than 10 per cent.

7. The minimum filter loading should exceed 50 fibres/mm² (i.e. approximately 40

fibres/100 Walton-Beckett graticule areas).

8. The maximum filter loading should normally not exceed 5 fibres/graticule area

(average value for all counted fields), but should never exceed 10 fibres/graticule area.

9. Sampling time should be measured accurately.

10. It is preferable to mount the total filter on a microscope slide using the acetone-

glycerol triacetate (acetone-triacetin) method.

11. A binocular microscope in phase-contrast should be used for counting and should be

assessed by means of detection limit test slides. As reference method, the United Kingdom

Health and Safety Executive Test Slide1 should be used and Block 5 should be achieved.

12. The microscope should have the following features:

(a) Koehler illumination;

(b) Abbé or achromatic phase-contrast condenser incorporated into a substage unit;

(c) built-in mechanical specimen stage fitted with slide clamps and x-y displacement;

(d) rotating nose-piece fitted with 10 × and 40 × parafocal phase-contrast achromatic

objectives. The 40 × objective must have a numerical aperture (NA) of 0.65, achromatic.

The phase ring absorption should be in the range 65-85 per cent. Either positive or

negative phase-contrast is suitable;

1 Health and Safety Executive (United Kingdom) and National Physical Laboratory (United Kingdom):

HSE/NPL test slide (mark II) for the determination of detection limit when using phase-contrast microscopy.

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