An introduction to the different types of air sampling techniques

Monitoring air quality is both an essential part of health and safety and for ensuring a pleasant workplace environment. For example, in breweries or dry food storage areas, where carbon dioxide is produced as part of the fermentation process, monitoring gas levels is key for protecting workers’ health. ­Even in office buildings, where chemical processing may not be a risk, monitoring the chemical composition of the air can help avoid ‘sick building syndrome’ and maximise employee comfort.

Air sampling is a way to monitor air composition as a function of time but there are is range of different approaches that can be used to perform air sampling. Each approach has advantages and disadvantages and the best approach is dependent on a combination of available resources, the necessary application and the potential health risks posed by the workplace. For example, for workplaces were high levels of asphyxiant or toxic gases are a risk, constant, online monitoring of air composition may be a legal health and safety requirement.

Grab Sampling

Grab sampling is where a sample of air is taken at a specific time and analysed. Often, the sample is removed from the environment and taken to a different location for testing, if it is not feasible to do the types of composition analysis required in situ.

While grab sampling is convenient and produces relatively small datasets, as the number of samples will just be the number of grabs taken during the day, the ability to perform offline analysis has some advantages.

For air analysis, techniques that are often used 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. Such analysers provide very high-quality information and excel for complex mixtures.

Passive or Active Sampling

Grab sampling though is very labour-intensive and large numbers of samples may be necessary to accurately characterize a site. One alternative to this is to use continual monitoring where a device is placed in situ and can continually collect samples that can then be analysed later.

Gas monitoring devices for this type of sampling can be classified as either active or passive. Passive sampling technology is any device that monitors gas concentrations by simply allowing the air to pass over it rather than being pumped. The motion of the gas molecules means they will collide with a certain probability with a sorbent where they can then be detected.

In active sampling, rather than relying on gas diffusion into the device, the gas is pumped into the sorbent medium. Active sampling devices tend to be more bulky and complex due to the need for pumping equipment as well as the detector, however, the measured gas concentrations are less sensitive to environmental influences such as changes in wind speed or humidity.

Using sorbent tubes to collect air samples though does have one key disadvantage, the tubes still need to be removed for later analysis. Ideally, an on-site sampler would also contain a detector which could be connected to a data stream for fully online, automated monitoring.

Edinburgh Sensors

The need for continual data logging 24/7 monitoring of air quality in potentially hazardous areas is why, for air sampling, Edinburgh Sensors offers nondispersive infrared-based detectors (NDIR) for the gas monitoring products.

NDIR technologies allow for continual online gas analysis all within one small device. Edinburgh Sensors offer several ‘boxed’ units, such as the Guardian NG and Boxed Gascard, that simply need a connection to a power supply and reference gas and can immediately be used. The Guardian NG series comes with its own on-device display which can show current readings, plot some historical data and has a menu interface for easy change of settings. As it can be used as a stand-alone device, the Guardian NG has a built-in alarm system so that if any gas concentrations exceed a certain range, a built-in alarm can be sounded.

Edinburgh Sensors has NDIR devices suitable for detecting a wide range of gases, including carbon dioxide, carbon monoxide, nitrous oxide, and various refrigerants. The Guardian NG and GasCard are the most versatile of the gas monitors offered, capable of detecting the largest variety of gases and both offer excellent sensitivity, accuracy and rapid response times.

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