In most gas detection applications, you are dealing with either the ambient open air, source effluent of a very high volume, or a confined entry location which still has no practical limit of the air sample available.
In certain cases, however, an application may arise in which the available sampling volume is limited. Most of these limited sample situations take place in research environments, whereby some process (often biological) is generating a gas of interest—and this process is occurring in a chamber of limited volume.
If you happen to be using a gas detection instrument set up for diffusion sampling (rather than using a sample-draw pump), your first inclination might be to simply arrange it so that your sampling “head” can be placed inside the chamber. Perhaps this would be possible, and the chamber apparatus would allow such a configuration. However, this may not be allowable in that the sampling head could contaminate the process.
If you are using a sample draw method, you could arrange a flow pattern such that the exhaust from your instrument is re-directed back into the chamber. Thus, there is no apparent net sample loss from the chamber. In such a setup, you need to make sure that there are no leaks in any of the interconnect tubing or fittings between the chamber and your instrument—and within your instrument. Here again, though, having the sample pass through your instrument could also introduce some contamination.
Moreover, if the gas concentration in the chamber is changing with time, then re-introduction of an older sample from your instrument could be spoiling the data.
Likewise, if you were to place a diffusion sampling head in a flow-through arrangement, to be recycled back into the system, you would still have the potential problems of both contaminating the sample, as well as re-introducing older sample back into the chamber.
Gas chromatography aficionados would tout their pet technology as requiring only a very small amount of sample (typically microliters). This tiny amount could be removed periodically from the chamber with little ill effect, and readings taken for the target analyte. This approach seems to have few drawbacks, beyond the high price of a laboratory-grade GC.
Certainly, the difficulty of these applications increases as the available volume decreases. It is most important to determine at least these parameters:
1. The volume available.
2. How sample is to be accessed from the chamber, and any special contamination concerns.
3. In this specific case, could the sample be recycled without damaging the data? Or, are relatively small errors acceptable? Perhaps you merely want to establish a trend, so absolute quantification is less of a concern.
Once again, it is important to pin down all the application details. And, we can help you do that! Contact one of our experienced engineers for help on your limited sample project.