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| Module 1: Occupational Hygiene - Section 3: Evaluation of Airborne Contaminants |
| OH3.4: Analysis of Samples |
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| Module 1: Occupational Hygiene - Section 3: Evaluation of Airborne Contaminants |
| OH3.4: Analysis of Samples |
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Sampling and analysis of airborne contaminants are comnnplementary activities. Analytical sensitivity governs the minimum volume of air to be sampled and choice of medium for capturing air contaminant is partly governed by its suitability for subsequent analysis. The National Institute for Occupational Safety and Health (NIOSH), provides an on-line manual for the analysis of various substances.
Analysis entails determining the amount of contaminant that is collected or trapped on a sampling medium.
Analysis of gases and vapours:
The two most common analytical methods for gases and vapours are gas chromatography and colorimetry.
Gas chromatography is the most common analytical technique used in occupational hygiene. It is ideal for analysis of volatile organic compounds. Chromatography is based on separation of molecules as they move at different rates up a column. GC is used to separate and quantify known compounds with distinctive patters of migration in the column.A gas chromatograph consists of an injection port, a heater, a column, a detector and a printer or recorder.
The variation of colour of a liquid forms the basis of colorimetric analysis. The colour may be due to formation of a coloured compound when a reagent is added, or it may be inherent in the sample collected.
Analysis of particulates:
There are various techniques that are used to analyse particulates deposited on a filter. The method used depends on the type of contaminant that is sampled and the method of analysis desired.
Gravimetric analysis involves weighing of the filter prior to and immediately after sampling in the same temperature and humidity controlled room. The difference between the two weights is the amount of dust deposited on the filter. A chemical balance that can measure to at least 0.1 mg is used to accurately weigh the filter.
A filter that is not easily affected by moisture or that can withstand handling is ideal for gravimetric analysis.
Optical microscopy can be used to analyse particle size distribution, for example for asbestos fibres. The sample is collected on a membrane filter using an open face cassette. A portion of the filter is cut, mounted on a microscope slide, cleared, and particles (fibres) are counted and sized using a calibrated graticule. Phase contrast illumination is used for asbestos to improve visibility of fibres.
The amount of contaminant trapped or dissolved and the sampling duration are used to calculate the concentration of airborne chemical contaminants in the workplace atmosphere. The formula used is:
The units of concentration are expressed as milligrams of contaminant per cubic metre of air (mg/m3) for dusts, fumes or other particles in air; parts per million of contaminated air (ppm) for gases and vapours; or the number of fibres per cubic centimeter of air (f/cc) for asbestos fibres.
