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Sampling for airborne contaminants
Whether you are already using sorbents in some format, or whether this application is a new concept to you, it is interesting to see how the simple sorbent tube started out – the work and research involved to create a device many of us now take for granted.
Today all over the world, both in workplace situations and environmentally, solid sorbent material is used to capture and hold gas and vapour samples. With the analysis of the exposed sorbent matter, levels of contaminants are ascertained and the potential to cause harm to human health is assessed. Sorbent technology arose to fulfil a purpose – to enable the quantifying of gaseous airborne hazards. With We take a close look at some of the key moments in history behind sorbents and sorbent air sampling devices a monitoring program, steps can be taken to stay below the level deemed hazardous to health.
Increasing interest in worker health and safety into the twentieth century led to guidelines and legislation. Countries throughout the world have followed a similar path, and have significant dates, when key safety legislation was implemented. Some are not yet at that point. For Britain, the Factory Act of 1901 sought to protect workers in industry, many exposed to harsh and dirty conditions, and possibly facing high exposures to harmful chemicals. More recently C.O.S.H.H., the Control of Substances Hazardous to Health, emerged and in 1988 it caused companies to think even more carefully about their responsibilities to the worker. Complying with C.O.S.H.H. regulations meant assessing risk to workers and this would generally mean finding a way of monitoring exposures.
In the U.S.A. from the 1930s the Journal of Occupational Hygiene was already discussing and reporting on worker health issues including exposure monitoring. In the 1970s the Occupational Safety and Health Act created OSHA out of the Public Health Service to conduct research and recommend safety and health standards. It would have the authority to issue regulations and standards with the force and effect of law.
Clearly there was a growing need for accurate monitoring techniques to show compliance.
What are the key properties of sorbents?
- They trap and retain the contaminant
- Allow for desorption of the contaminant
- Have sufficient capacity to retain the contaminant
- Do not cause a chemical change in the contaminant
The sorbent sampling tube is a simple concept manufactured in glass or metal, filled with a solid sorbent appropriate to the hazard being collected. Sorbents are processed to clean them, and are produced or ground to meet a specific granular size range (mesh size) ready for packing. Sorbents are measured by weight, packed into the tube and held in place with sterile glass wool, mesh or springs. Finally the tube is sealed or capped for storage to avoid contamination.
For correct use, tubes are opened to at least 2mm, or half of their diameter, and should always be positioned vertically during use, whether for personal or environmental sampling.
A sorbent tube patent?
A US Patent was never received on sorbent tubes. The legal authorities, at the US Department of Health, Education and Welfare, thought that sorbent tubes were a ‘novel’ concept but would never be used to a large degree.
History has proven them wrong!
For the past 35 years sorbent tubes have been at the forefront of air sampling for both health and environmental applications. The variety and choice is still increasing for new applications, and at SKC alone we estimate well in excess of 10 million tubes have passed through production over the years.
The origins of charcoal in air sampling
- March 1935 Journal of Industrial Hygiene reports on method for the collection of Benzene vapours from the air
- April 1936 Journal of Industrial Hygiene and Toxicology describes activated charcoal method
- 1964 ACGIH meeting discusses progress with charcoal
In 1935, the above-mentioned journal reported on Benzene collection methods. Activated charcoal and silica gel were found to retain the vapours very effectively, but ‘releasing’ the collected vapours was difficult.
Benzene was frozen out from the air onto glass beads and then released by raising the temperature.
However by 1936, the Journal of Industrial Hygiene and Toxicology described tubes, filled with previously dried charcoal which were analysed by pre and post weighing. By 1964 the American Conference of Governmental Industrial Hygienists (A.C.G.I.H.) described how to desorb and analyse individual contaminants by gas chromatography once collected onto activated charcoal. This is the basis for most current sorbent sampling, although the range of methods is now diverse, and appropriate to the contaminant. Gas Chromatography, (GC) especially using a Flame Ionisation Detector (GC FID) Mass Spectrometry, (GC MS) and High Pressure Liquid Chromatography (HPLC) still feature in the majority of analyses carried out on sorbent sampling tubes and badges.
April 17th 1972 – First draft method using charcoal tubes issued by N.I.O.S.H.
This method from the National Institute of Occupational Health & Safety (N.I.O.S.H.) set out parameters for the measurement of benzene, and included a range of measurement, limit of detection and a percentage precision.
Meeting commercial needs
- N.I.O.S.H. approaches Lloyd Guild (founder of SKC) on an application using tubes
- 1973 Scientific Kit Corporation manufactures a commercial tube for N.I.O.S.H.
N.I.O.S.H. believed that a tube incorporating 2 sections of sorbent was required to guarantee that no sample was lost during the sampling period. Researchers at N.I.O.S.H. had developed a charcoal tube with 2 beds that allowed for chemical desorption. In talks with the Scientific Kit Corporation, N.I.O.S.H. specified this design and samples for the first commercial tube were submitted by SKC. Having received these samples, N.I.O.S.H. placed an order for 6000 tubes.
Developments in sorbent devices
- Speciality sorbents
- Multiple stage tubes
- Sorbent Filter combinations
- Thermal Desorption tubes
To collect a wider range of contaminants, research led to exciting developments in sorbent technologies. This brings us to the wide and commercially available range on the market today.
Beaded and micro porous
- Specially washed sorbents
- Chemically coated sorbents
Combination tubes have extended tube use:
- Multiple stages in a tube allow for screening or for complex mixtures
- Combining sorbents with a prefilter allow trapping of aerosol and gaseous phases of a contaminant
Maintaining quality in sampling products has meant regulation of sorbent and tube production, with manufacturers needing to comply with various standards. Examples of these are:
BS ISO 16000-3 Formaldehyde using active sampling BS ISO 16200-1 Workplace Air Quality – sampling and analysis of volatile organic compounds, the pumped method (section 4.3 Sorbents)
Thermal desorption tubes use heat and a flow of inert gas to desorb the sample from the sorbent. This allows lower levels of contaminant (such a ppb – parts per billion) to be quantified which can be useful in environmental or indoor air quality applications.
To be suitable for thermal desorption, sorbents must meet exacting specifications that include low contaminant background, high thermal stability, and sufficient absorptive strength to retain contaminants of interest, but release them quickly when heat is applied.
Pumps as your sorbent tube partner
Tubes require a known flow rate of air to travel through the sorbent. This function, drawing air through the sorbent, is fulfilled by a personal air sampling pump, and is set by calibrating the flow.
For workplace personal exposure monitoring, the pump should be small and light enough to be worn. In the U.K., the quality and performance of sampling equipment is regulated, and as an example sampling pumps should conform to the standards laid down in the British Standard BS EN 1232.
Flow rates are determined by the sampling method, as in the original N.I.O.S.H. guidelines for benzene, and care should be taken not to exceed the flow rate guidelines for tube use.
Today a variety of methods exist in the U.K. from the Health and Safety Executive – there are over 100 in the M.D.H.S. (Methods for the Determination of Hazardous Substances) range, many specifying tubes for gas sampling.
Laboratories regularly carry out quality assurance, by spiking sample media with known concentrations and desorbing these. Prespiked tubes are a quick and convenient way to quality control laboratory analysis.
Advantages of passive samples
Minimal training and instruction required
Perform large studies at lower cost
Deploy samplers quickly with virtually no set-up
While sampling tubes continue to be the main use of sorbents in air sampling, passive badges are gaining in popularity.
Following extensive testing and validation many passive badges, filled with sorbent, can now be used for sampling, without the need or an air sample pump. Badges work via diffusion onto a bed of sorbent with a large surface area. They are then analysed in the same way, as the sorbent in a tube would be if used for the same contaminant.
The collection of a wide variety of organic vapours onto sorbent badges has been validated, with both O.S.H.A. and N.I.O.S.H. bringing out methodology. In the U.K. there is acceptance of badge use and they can be found documented in the Health and Safety Executive method M.D.H.S. 88 and parameters specified for the requirements and test of a diffusive sampler in British Standard BS EN 838.
Further information on diffusive sampling is available in both the British and International Standards, including:
BS EN 13528-3 Guide to selection and use
BS EN ISO 16107 Diffusive Sampling of Volatile Organic Compounds
ISO 16000-4 Determination of Formaldehyde
ISO 16200-2
Workplace Air Quality
Sorbent use is still the most common method of air sampling today.
Years of extensive research and validation of sampling methods mean that sorbents can be relied on. This research into both active (with a pump) and passive sampling continues to increase the efficiency, accuracy, and range of contaminants that we can sample onto sorbents.
The European norm EN1076 lays out exacting specifications for the production of tubes, and their use in sampling to better protect the consumer.
Extensive research has produced the variety and quality of sorbent products we have today. Sorbents in a variety of devices continue to be specified in M.D.H.S. methods from the Health & Safety Executive, U.K, N.I.O.S.H. and O.S.H.A. methods in the U.S.A. and many other countries worldwide.
Now, through these methods and with British and International Standards laying out guidelines, the consumer should expect validated, consistent and effective products.
Sorbents may have a long history but they clearly have a long future ahead fulfilling the needs of occupational hygiene and environmental professionals.
Published: 10th Jun 2005 in AWE International
