Introduction to Air Sampling

 

The basis for air sampling is to sample a volume of air through a suitable sampling medium, usually paper or solid filter media for particulates and fumes and through sorbent materials for gases and vapours. Another is via impingers and bubblers or a grab sample taken into a gas sampling bag.
The volume of air is measured and calculated as a concentration expressed as mg/m³ or parts per million (ppm).

Sampling for particulates, dusts and fumes

A complete range of sampling media is available. These filters are used in conjunction with sampling heads or as integral sampling cassettes.

Always refer to the relevant sampling guidance and methodologies (eg. MDHS, OSHA or NIOSH) to ensure selection of the correct sampling media for the analyte to be sampled.

Sampling Train

A typical sampling train includes a sampling head (or cassette) complete with filter inserted, connected via a length of tubing to the air sampling pump. Each sampling head requires a
specific flow rate and this is set via a calibrator prior to and after sampling. These may be via a simple rotameter or digital flowmeter:

Example of an inhalable head with rotameter

 

Example of a respirable head with digital flow meter

 

The sampling head should then be mounted on the individual to be sampled, within 30 cms of the breathing zone.

Sampling Heads and Cassettes

A variety of sampling heads and cassettes are available for asbestos, total inhalable, respirable and other dust fractions. Please refer to the relevant section of the catalogue for further details.

Sampling Filter Media

PTFE Filter Membrane

• Used for post sampling chemical analysis
• Strong, chemically resistant membranes for air monitoring and sampling in aggressive environments

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PVC Membrane Filter

• For sampling airborne metals, silica, and dust
• Post sampling chemical analysis
• Assures gravimetric stability. Low moisture pick-up and low tare weight
• Low ash. Provides interference-free silica determinations

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Glass Microfibre Filter (GFA)

• Fastest flow rate and high load capacity
• Retention of particles to sub-micron size
• Temperature range up to 500°C
• Most commonly used media for gravimetric dust samples
• Inert fibre composition
• Free of binders or additives

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Quartz filter

Suitable for a broad range of air monitoring applications

• Heat treated for reduction of trace organics and superior chemical purity
• High temperature use for analysis of acidic gases and stack sampling aerosols
• High flow rate and filtration efficiency

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MCE filter

Developed from a mixture of cellulose acetate and cellulose nitrate, MCE membrane filters are one of the most widely used filters in both air monitoring and analytical applications. Not subject to moisture issues and an economical membrane for air monitoring applications

• Suitable for air monitoring applications
• Dissolves completely using standard digestion procedures

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Polycarbonate Membrane Filter

• Smooth glass-like surface with cylindrical pores for maximum particulate capture
• Precise pore sizes and pore distribution for absolute filtration and separation
• Very low extractables
• Biologically inert
• Excellent chemical resistance and thermal stability
• Superior strength

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Silver Membrane Filters

• 99.97% pure silver
• High temperature resistance
• High chemical resistance
• Economical – can be cleaned & reused
• Smooth surface for particle capture and easy observation

 

Material	Main Properties	Air Sampling Applications
MCE	Readily soluble for atomic absorption analysis Readily rendered transparent for transmitted light microscopy Dissolve and clear easily	Metal dust analysis Asbestos & man-made fibers Air Quality
PVC	Low moisture pick up. Non-oxidizing surface Silica-free Low ash membrane Low tare weight for gravimetric analysis	Gravimetric analysis Hexavalent chromium Silica
PTFE	Hydrophobic Inert to solvents, acids & bases Aggressive environments	Alkaline dusts PAH's Pesticides Isocyanates Ambient Air quality
Poly-carbonate	Hydrophobic Microscopically smooth surface Precise pore sizes Inert Strong	Scanning electron microscopy Asbestos fibers
Silver	Wide solvent compatibility Higher temperature tolerance Autoclavable Uniform porosity and thickness	Bromine Asbestos by TEM Silica by x-ray diffraction
Glass Fibre	Up to C500 degree range Inert composition High particulate retention	General gravimetric Isocyanates Ethylene glycol Air Quality Fractions
Quartz	Heat treated Low level metals content High temperature 300°C Autoclavable High filtration efficiency	General air sampling High temperature applications Stack sampling Diesel particulates Acidic gases

Sampling for solvents and vapours

Solvents, gases and vapours are sampled actively using air sampling pumps and sorbent tubes, into bubblers, gas bags, direct reading tubes or passively using diffusive sampling badges.

Sampling Train

Typical sampling train for gas and vapour adsorbent tube sampling requires a tube with ends broken off inserted into a tube holder and connected to a pump. The pump may be a specific low flow pump or a medium flow pump (Tuff or Apex) with a Constant Pressure Controller (CPC) and low flow adaptor assembly connected. (as shown below). Flow rate (typically 10-120ml/min) needs to be set prior to and post sampling.

 

Solid Sorbent Sampling Tubes

Sorbent materials are used which adsorb the vapour or gas onto their surface during sampling and this is then desorbed from the material (via other chemicals or thermally) and subjected to further chemical analysis typically via a gas chromatograph.

Organic vapors and gases can be collected using several different sampling media including charcoal sampling tubes in conjunction with Casella CEL low-flow sampling pumps. For each analyte, a suitable reference work or methodology is consulted. You can refer to Casella Air Sampling Solutions (www.airsamplingsolutions.com) or the relevant MDHS, NIOSH or other in territory sampling methodologies for specific details on required sampling media, sampling rates, and sampling times for specific chemicals.

Flow rates vary but generally have to be slow enough for the vapours to be absorbed and are in the range 20–100ml/min. There are generally two sizes of tubes, small and large. For most routine sampling the small size will suffice but in suspected high concentration areas the larger size may be more appropriate to avoid saturation of the tubes. Always refer to the sampling methodologies for the media and the required sample volume and flow rate.

Typical sorbent sampling tube.

Various adsorbent materials are used including Charcoal, Chromopak, Tenax and others. Silica gel is also used as the adsorbent material for polar hydrocarbons, low molecular weight mercaptans, methanol, amines and inorganic acids. Some specially reagent treated tubes are also used eg 2,4-dinitrophenylhydrazine for aldehyde sampling, sulphuric acid treated for ammonia.

Most tubes have several layers of material, where the smaller layer is the back up layer and should be closest to the sampling pump inlet (If printed arrows are present, point arrow towards pump).
Both ends of the tube are broken off, the sample taken with the tube in an approximately vertical position on the body and the tube sealed with the cap ends and sealing tape and sent off for analysis.

The two layers will be analysed separately, and if the back up layer is more that 10% of the main layer the tube is deemed as saturated, and the sample will be discarded.

Charcoal tubes are the most common type of tube for organic vapour sampling, but are not always suitable.

Bubblers and Impingers

These are used for adsorption of particles acids, alkalis, vapours and gases via a liquid sorbent. The detection is by dissolution or by chemical reaction.
Impingers have a tapered outlet which allows particulate matter to exit at high velocity, hit the flask bottom and be captured into the solution, whilst bubblers are open or fritted to increase surface area so the gases can be absorbed into solution, these are generally the preferred solution for gas and vapour sampling.

Impingers and bubblers can be positioned in an area, held or worn in a holster.

Typical sampling train for bubblers:

Diffusive (Passive) Sampling

Diffusive or passive samplers are also used for background sampling. Badges are opened for sampling, worn on the person, sealed and sent off for analysis where the substances are desorbed via another solvent and subjected to gas chromatography.

Gas Sampling Bags

These are constructed from a range of materials. The bags are used with a sampling pump and can be used for grab sampling or for longer term sampling at lower flow rates. A range of capacities is available.

Direct Reading Colour Detection Tubes

These are glass vials, filled with chemical reagents that react to a specific chemical or family of chemicals. A known volume sample is drawn through the tube via a hand syringe for a set number of pulls (indicated on the tube).

If the targeted chemical is present, the reagent in the tube changes colour, with the length of the colour read against a concentration scale on the tube. Select the appropriate tube for the required hazard.