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- My emissions of VOC vary considerably over time. How can I take relevant air samples in this case?
My emissions of VOC vary considerably over time. How can I take relevant air samples in this case?
We can achieve this using time-weighted sampling.
What is time-weighted sampling?
Time-weighted sampling is sampling performed over a longer period of time. The length of this period depends on the purpose of the measurement. If you have a process that takes several hours, for example, where several steps are performed that may release different VOC each time,
you can sample over the entire period of the process to identify the total emissions from the process. Or you can in fact pick a specific phase to highlight it. Another type of measurement is exposure measurements in which VOC exposure is measured over 8 hours.
What techniques are possible?
There are several ways to take time-weighted samples. When filling sampling bags, the suction speed can be set to collect a specific volume over a specific time. Canisters can be equipped with a flow regulator. In this way, as with the sampling bag, the suction rate is determined so that a specific volume of air is collected over a specific time.
A third technique is using what is called passive sampling. In this process, an adsorbent is exposed for a certain period of time in the environment to be sampled, trapping the VOC.
So which technique should you choose?
The purpose of the measurement will initially determine which technique we will propose. Adsorption cartridges are quite compact, and can be used both on-site and in-person. Canisters and sampling bags are not suitable for personal monitoring.
The possibilities of taking time-weighted samples via sampling bags are rather limited due to the nature of the technique. This is a viable options over shorter periods, such as less than 1 hour.
The advantage of canisters is that they sample the entire air where adsorption cartridges depend on the affinity of the VOC and are affected by temperature.
Advantages and drawbacks
Besides the practical choice of technique, there is also a difference in processing of the analyses. When passive sampling is carried out via adsorption cartridges, a complex calculation follows to determine the concentration of VOC in the air. In this case, a number of parameters are component-specific and not always available.
The uptake rate in particular, which is the speed of uptake of a component on the adsorbent, is highly decisive and also depends on the duration of sampling and on the concentration present.
Sampling with canisters and flow regulators requires some preparation.
Comparable results
Whichever technique you use, the results should be more or less comparable. We have therefore tested this extensively and provide our findings here. The total concentration in the graph is quite different.
This is due to the presence of components not captured by the adsorption cartridge especially the VVOC. If VVOC is not included in the calculations, the difference between the two techniques already becomes much smaller. If uptake rates were available for each component, the results would match even better.
Conclusion
If conditions allow, time-weighted sampling with canisters and flow regulators is the most straightforward and most complete method. When canisters are not suitable due to their dimensions, such as for personal monitoring, adsorption cartridges are an excellent alternative, provided due consideration is given to the complexity of determining concentrations.
The ultimate purpose of the measurements will also factor into the choice of the right technique. We will be happy to put our knowledge at your disposal to jointly draw up the most suitable measurement plan using time-weighted Sampling.
How can we help?
Do you have questions about VOC analysis? We can be reached via the channels below or leave a message on our contact page.