TO-15 Canister Relative Humidity: Part I (Introduction)

From time- to-time I receive inquiries on how to humidify a TO-15 canister blank and/or standard. See… Method TO-15 clearly states the use of humidified zero air several times throughout the document. For example: “10.7.3 Procedure. Fill a cleaned and evacuated canister with humidified zero air (RH >20 percent, at 25 °C). Pressurize the contents to 2 atm.” However, it never indicates why humidity matters; how one should go about humidifying their air; and how do you calculate the theoretical relative humidity (RH). I say theoretical, because my hypothesis is that some of the water introduced into a canister may be lost to the canister valve and/or wall; thereby resulting in less water vapor and a lower than theoretical RH. Stay tuned for future blogs that address this… In the meantime, I want to cover the following in this blog (as it pertains to canister sampling/analysis): 

1. What is relative humidity?
2. Why do we care about relative humidity?
3. How do we calculate the theoretical relative humidity?

# 1: Relative humidity is defined as the ratio of the partial pressure of water vapor to the saturated vapor pressure of water at a set temperature. In laymen’s terms: relative humidity is the ratio of how much water vapor is in the air, relative to the maximum amount of water vapor the air can hold. We all know that humid days feel “wet” and that is because the air has been saturated with a relatively higher amount of water vapor.

#2: When talking about whole air sampling with canisters, relative humidity is of concern for the following reasons:

1. If the relative humidity in the sample stream is high, coupled with warm air temperatures, the water vapor can condense out on the relatively cooler canister valve/wall. This can be problematic, because the condensed water vapor may now act as a sink for polar compounds and their recovery drops off.
2. The other potential problem with high relative humidity is on the analytical side. Sample preconcentrators have two primary tasks: 1) Concentrate/focus the sample analytes out of the air stream such that there is a sufficient amount of mass to detect and 2) remove the now concentrated water vapor from the sample stream so as to not saturate your mass spectrometer. Obviously, high RH and large concentration volumes may bump up against this obstacle.
3. On the flip side, if the relative humidity in the sample stream is low enough, some compounds may not be recovered as efficiently from SUMMA canisters (see below)

Notice that SUMMA canister recoveries drop off below ~10% RH (see graph above), but remain~100% in a SilcoCan regardless of RH (see graph below). Well… the reason for this is that SUMMA canisters do not have as inert of interior surface as a SilcoCan. So a little bit of water vapor actually helps in this case by “priming” the interior surface of the SUMMA canister. And by that I mean the water covers up active sites; the same active sites that have already been capped in a SilcoCan by the chemical vapor deposition (CVD) of silica. In any event, the practical implications of reduced VOC recoveries at low RH are very limited unless of course you do a lot of sampling in the Gobi desert.

#3 Sorry… but it looks like you will have to wait for the next blog in this series.