Air Sampling Techniques - A Guide to the Different Types

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Image Credit: nEwyyy/Shutterstock.com

Monitoring air quality is necessary for health and safety as well as ensuring a pleasant workplace environment. For example, in breweries or dry food storage areas, where fermentation produces carbon dioxide, making sure that the gas levels do not exceed safe levels is critical for the workers’ health.1 ­Even in office buildings, where chemical processing does not pose a risk, monitoring the chemical composition of the air can help prevent ‘sick building syndrome’ and keep employees working comfortably.2

Air sampling is a method of monitoring air composition as a function of time, and there are a variety of techniques that can be used to achieve this. Each approach has its advantages and disadvantages and the best approach comes down to a combination of available resources, the necessary application and the potential health risks posed by the workplace. For example, for workplaces that are at risk of high levels of asphyxiant or toxic gases, constant, online monitoring of air composition may be a legal health and safety requirement.3

Grab Sampling

Grab sampling involves taking a sample from the local atmosphere at a specific time, then taken away for analysis. This typically involves taking the sample to a different location, as the necessary equipment for compositional analysis may not be on site.

While it produces relatively small datasets, as the number of samples will be the number of grabs taken during the day, it offers convenience, as well as other advantages in the form of offline analysis.

For air analysis, the most widely used techniques for the analysis of grab samples include gas chromatography or hyphenated versions of the technique that include additional gas chromatography or mass spectrometers for more accurate identification of compounds.4 These analyzers excel for complex mixtures and provide very high-quality information.

Passive or Active Sampling

A significant downside to grab sampling is that it is very labor-intensive and to accurately characterize a site, large numbers of samples may be necessary. An alternative to this is to place a device in situ that can continually collect samples that can then be analyzed later. This approach is known as continual monitoring.

Gas monitoring devices for this type of sampling are one of two classifications: active or passive. Passive sampling technology is a catch-all term for devices that monitor gas concentrations by just allowing air to pass over the sensor rather than it being pumped. The random movement of the gas molecules means there is a certain probability that they will collide with the sorbent where they can then be detected.5

The difference in active sampling is that, rather than relying on gas diffusion into the device, the gas is pumped into the sorbent medium. The added bulk of the pumping equipment means that active sampling devices tend to be larger and more complex. However, the measured gas concentrations are not as sensitive to environmental factors such as changes in wind speed or humidity.

There is still on one key disadvantage in using sorbent tubes to collect air samples though: the tubes must first be removed before they can be analyzed. The ideal solution would be to have an on-site sampler with an attached detector. This could be connected to a data stream for fully online, automated monitoring.

Edinburgh Sensors

The need for continual data logging, constant monitoring of air quality in potentially hazardous areas is the reason Edinburgh Sensors offers nondispersive infrared-based detectors (NDIR) for gas monitoring products.6

NDIR technologies allow for continual online gas analysis all within one discrete device. Edinburgh Sensors offer several ‘boxed’ units such as the Guardian NG7 and Boxed Gascard8 that require only a connection to a power supply and the reference gas for immediate use.

The Guardian NG series comes with a dedicated on-device display which can output live readings, plot some historical data, and has a menu interface for quick and simple settings changes. As it can be used as a stand-alone device, the Guardian NG has a built-in alarm system so that a built-in alarm can be sounded if gas concentrations go beyond a critical point.

Edinburgh Sensors NDIR devices are suitable for the detection of a wide range of gases, including carbon dioxide, carbon monoxide, nitrous oxide, and various refrigerants. The most versatile of these is the Guardian NG and GasCard9, which are both capable of detecting the largest variety of gases as well as offering excellent sensitivity, accuracy and rapid response times.

Guardian NG

Guardian NG

For air sampling in challenging environmental conditions, each of Edinburgh Sensor’s devices provides a pressure and humidity compensated readout over the humidity range of 0 – 95 %. The Guardian NG boasts a ± 2 % accuracy over its full detection range, which is 0 – 100 % in the case of commonly monitored gases such as methane and carbon dioxide. Some air sampling applications may find it particularly advantageous to implement the gas monitoring on unmanned aerial vehicles or drones, rather than wall-mounted, static ‘boxed’ devices. The Gascard NG is a highly flexible, lightweight sensor that draws sufficiently low power that it can be fixed to such devices.

In the spirit of true continual air sampling, all of Edinburgh Sensors gas monitors can be interfaced with external networked data logging.

References and Further Reading

  1. Hatice Pekmez. (2017). Cereal Storage Techniques: A Review. Journal of Agricultural Science and Technology B, 6(2), 1–6. https://doi.org/10.17265/2161-6264/2016.02.001
  2. WHO on Sick Building Syndrome (2019) https://www.wondermakers.com/Portals/0/docs/Sick%20building%20syndrome%20by%20WHO.pdf
  3. HSE on Carbon Dioxide, (2019) http://www.hse.gov.uk/carboncapture/carbondioxide.htm
  4. Lerner, B. M., Gilman, J. B., Aikin, K. C., Atlas, E. L., Goldan, P. D., Graus, M., … De Gouw, J. A. (2017). An improved, automated whole air sampler and gas chromatography mass spectrometry analysis system for volatile organic compounds in the atmosphere. Atmospheric Measurement Techniques, 10(1), 291–313. https://doi.org/10.5194/amt-10-291-2017
  5. Zabiegała, B., Kot-Wasik, A., Urbanowicz, M., & Namieśnik, J. (2010). Passive sampling as a tool for obtaining reliable analytical information in environmental quality monitoring. Analytical and Bioanalytical Chemistry, 396(1), 273–296. https://doi.org/10.1007/s00216-009-3244-4
  6. Edinburgh Sensors (2019) https://edinburghsensors.com/about/our-history/
  7. Guardian NG (2019) https://edinburghsensors.com/products/gas-monitors/guardian-ng/
  8. Boxed GasCard (2019) https://edinburghsensors.com/products/oem/boxed-gascard/
  9. Gascard NG, (2019), https://edinburghsensors.com/products/oem/gascard-ng/

This information has been sourced, reviewed and adapted from materials provided by Edinburgh Sensors.

For more information on this source, please visit Edinburgh Sensors.

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