6.8 - SOUND MEASUREMENT METHODS
-- Introduction
A variety of sampling and analytical techniques have been used to quantify fibrous dust in occupational atmospheres and in bulk materials. These include optical and electron microscopy, X-ray diffraction, infrared spectroscopy, differential thermal analysis and light scattering diffraction patterns resulting when light is passed through a fibrous particle dust cloud aligned by passing through a high-intensity electric field (see Table 1).
The quantification of fibrous dusts in occupational and environmental air samples is difficult for several reasons:
€ The mass of fibrous dusts present in air is relatively low even though the fibre number concentrations may be high.
€ Many instrumental analytical techniques cannot differentiate the fibrous-shape of particles from their non-fibrous mineralogic polymorphs. (e.g. The chemistry of a chrysotile fibre can be the same as that of a non-asbestiform serpentine flake particle).
€ The identification of some fibrous dust types requires expensive instrumentation and analytical methodology such as electron microscopy, electron diffraction and microchemical analyses.
The routine technique, applied for the measurement of concentrations of respirable fibres in air, is the membrane filter method using phase contrast optical microscopy. A widely used reference membrane filter method is the one based on the Asbestos International Association/RTM 1 (1982), which was adopted by the International Labour Office (ILO, 1984). For most inorganic materials with refractive indices greater than 1.51, the following methods can be used:
€ Reference method for the determination of airborne asbestos fibre concentrations at workplaces by light microscopy (Membrane filter method). RTM 1, AIA (1982).
€ Reference methods for measuring airborne man-made mineral fibre. WHO/EURO (1985).
€ Determination of the number concentration of airborne inorganic fibres by phase contrast optical microscopy. Membrane filter method. Third revised version DIP 8672. ISO (1986).
€ NIOSH Method 7400. Revision #3 (1989).
-- Samples collection and record
Prior to and after any sampling day, the sampling pump calibration (see Appendix-I) shall be checked.
To meet the objective of the occupational exposure measurements, the appropriate sampling strategy will be of prime importance.
Different sampling schemes are available:
€ Long term:
- Full-shift consecutive samples
- Partial-shift consecutive samples
€ Short term:
- Random samples
- Systematic samples
The filter size, flowrate and sampling time are the three parameters which should be used to have acceptable fibre loadings on filters. Taking into account filter loading considerations, sampling duration time for each sample will be determined.
With the sampling details, all data necessary for the determination of the fibre concentration must be recorded. Appendix-II gives an example of a dust sampling record.
-- Preparation and analysis
We must first classify the fibres according to their refractive index and then select the right mounting technique to ensure a proper analysis.
Refractive Index > 1.51
i.e. with PCM: Fibres darker than background
Clearing with Acetone
Mounting with triacetine
Refractive Index > 1.51
i.e. with PCM: Fibres become darker with Becke Line, moves away when lowering the stage
Plasma Ashing 7 minutes, 8 ML./MIN. of 02 at 2 Watts
Mounting with distilled water
Note: Analysis of mounted filters according to the counting criteria of the reference membrane filter method. Appendix-III gives an example of a fibrous dust counting record.
Sampling can be easily learned and accomplished but the counting of fibres using the membrane filter method can only be performed by well trained counters with a rigorous quality control program.
-- Quality assurance of fibre counts
The quality control program should contain at least the following elements:
€ Laboratory blanks (4% of the filters).
€ Field blanks (at least 2 or 10%).
€ Reference samples (one every counting day).
€ Blind filters (10% of the samples).
€ Check samples (e.g. specific field training slides, sample of known concentration from experienced counters).
€ Inter-laboratory exchanges (NIOSH, AFRICA, AIA, ...).
All laboratories engaged in asbestos counting should participate in an asbestos proficiency testing program to ensure good reproducibility.
€ NIOSH - Proficiency Analytical Testing (PAT).
For information on the PAT Program, contact:
NIOSH
Proficiency Analytical Testing (PAT)
Program R-8
4676 Columbia Parkway
Cincinnati (OH) 45226
Tel.: (513) 684-4357
€ IOM - Asbestos Fibre Regular Informal Counting Arrangement (A.F.R.I.C.A.).
For information on the A.F.R.I.C.A. exchange, contact:
Mr. P.W. Brown
Institute of Occupational Medicine
8 Roxburgh Place
Edinburgh
EH8 9SU
U.K.
€ AIA - Asbestos Counting Trial - DAP.
For information, contact:
Mr. Alain Baujon
Laboratoire d'Hygiene et de Contrôle
des Fibres Minérales (L.H.C.F.)
10, rue de la Pépinière
75008 Paris
FRANCE
€ QAMA - Interlaboratory Exchange Scheme.
For information, contact:
Mr. Jacques LeBel
Quebec Asbestos Mining Association
4125 Garlock St.
Sherbrooke (Quebec)
Canada J1L 1W9
€ Any other recognized interlaboratory exchanges.
Gravimetric methods
The workplace, gravimetric measurement methods of total or respirable airborne dust are often used to supplement or to replace the fibre count membrane filter method. Gravimetric results can be found by:
€ Weighing the dust collected on the filter
€ Quantifying the filter using infra-red spectroscopy:
- KBr after low-temperature ashing of the filter
or
- Total internal reflection phenomenon
€ Direct-reading instruments:
- Light scattering, e.g.: Miniram PDM-3 and T.M. Digital µP.
- Beta-radiation, e.g.: RDM-101, 201 or 301; APM and RAM.
- Piezobalance, e.g.: T.S.I.
particles are smaller than 1.0 µm.
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