Cryogenic Cooling for Air Analysis – Interferences from N2, CO2, and O2

Spring is already here and summer is just around the corner and soon we’ll be trying to stay cool.  For those of us doing ambient air analysis though, keeping cool can sometimes mean cryogenic cooling. While our VMS columns can easily separate the TO-15 compounds with no need for cryo cooling (see https://blog.restek.com/air-columns-part-iii-my-favorite-column-phase-for-epa-method-to-15/) there are some compounds that need a little chill to get separated, not from other target analytes, but from air itself.

 

What do I mean by that? First, let’s consider . Ethylene oxide (EtO) is a commonly used chemical in manufacturing as well as a sterilant for medical devices, and has both acute and chronic health effects in humans (https://www.epa.gov/sites/production/files/2016-09/documents/ethylene-oxide.pdf). The problem with early eluters (like EtO) is when we preconcentrate our air sample, we concentrate everything. By everything we mean N2, O2, argon, and CO2; which are present in air at ~78, 21, 1, and 0.04%, respectively. We also preconcentrate the water vapor in the air sample as well, which is outside the scope of the current blog. The good news is that despite the high concentrations of N2 and O2, these two do not preconcentrate very well; however, CO2 does. To put things in perspective CO2 is present at 0.04%, which is 400 ppm. This is a relatively large concentration when we are trying to look at VOCs down at the mid-part-per-trillion levels (e.g., 50 pptv).

 

Ultimately, this means that when an air sample is analyzed you can see the large peak at the beginning from the CO2 and residual nitrogen from the sample that is adsorbed on the preconcentrator trap. While this could be removed with a sufficient pre-desorb purge with helium carrier gas, this also risks removing very volatile analytes.  A high split ratio can also help decrease the size of the air peak, but that’s counter to any attempts to analyze down to pptv levels, as many labs wish to do for ethylene oxide.

Fig 1 – Extracted Ion Chromatogram (EIC) for m/z 29 and 44 in air sample indicating the need to chromatographically resolve early eluting compounds from the air.

Now if we look at the EtO mass spectrum, we see that two of the most abundant ions are 44 and 29. Ion 44 is present from CO2, while 29 comes from the nitrogen isotope (15N). Our dilemma becomes quite apparent in figure 2.

Fig. 2 – Ethylene oxide (EtO) mass spectrum which shares ions with carbon dioxide (CO2) with a m/z 44 and Nitrogen (N2) with a m/z 29.

 

To have good signal using m/z 29 and 44 for ethylene oxide, cryo cooling is required to separate these ions from the large CO2 and nitrogen peaks. In the figure below you can see that even with cryogenic cooling m/z 44 has an elevated baseline after 8 minutes, indicating trace amounts of air in the carrier gas.

Fig. 3 – Ethylene oxide (EtO) at 0.5ppb. Black trace=m/z 29, blue trace =m/z 44.

 

Hydrogen sulfide is another compound with similar issues. Its mass spectrum is composed mainly of ions 32, 33, and 34, with interferences from O2 ( isotopes 17O and 18O). Again, good signal requires the hydrogen sulfide to be well separated from the oxygen peaks.

Fig. 4 – Hydrogen sulfide mass spectrum where separation is required from the oxygen, found in the initial air peak.

Fig. 5 – Extracted ion chromatogram (EIC) of oxygen with m/z 32 (top) and m/z 33 and 34 (bottom)

Fig 6 – Hydrogen sulfide (H2S) at 50ppb. Black trace = m/z 34, blue trace = m/z 33

 

Many labs are trying to move away from cryogenic cooling due to increased cost and safety issues. Other methods can be used to separate these compounds from the air peak, although they also have their downsides. The air peak should be mostly unretained on the column, so increasing retention for target compounds using longer columns or thicker phases could work. However, this does increase your analysis time, especially if your samples have  heavier compounds such as naphthalene. Plot columns could also be used, but require the use of particle traps for MS use and are not suitable for most TO-15 compounds, limiting the number of GC/MS methods on a single instruemnt.  Also, many plot columns have issues with common air compounds such as CO2 and water, which cause deactivation and changes in retenetion (https://blog.restek.com/what-you-should-not-inject-on-alumina-al2o3-columns/).

So remember, as the air starts to warm up outside sometimes it pays off to keep it cool in the lab.

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