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Can I use a GC/MS to analyze permanent (fixed) gases?

1 Mar 2012

This is a common question we get asked in tech service, and although this is a better question for the instrument manufacturer, I will try and provide our blog readers some general information. First, let me define my definition of permanent (fixed) gases; I consider hydrogen, helium, oxygen, nitrogen, argon, carbon monoxide and carbon dioxide the common permanent gases.

So, can these be analyzed using GCMS? Well, in theory, I am guessing that if you can adequately tune your mass spec to accurately detect these very low mass/charges (m/z’s), then I guess it may be possible, but this is not something I was ever successful at doing; probably because many interfering compounds produce these low m/z’s.

Then the question arises; which column should one use? Well, even mass spec analysts would prefer to obtain separation among compounds. The only columns capable of providing some separation (without cryo cooling) are molecular sieve columns (zeolite or carbon; just remember that carbon dioxide and moisture will get stuck in the pores of many zeolite molecular sieves like the 5A and 13X). Due to most mass spec carrier gas flow rate limitations, packed columns usually cannot be used, so the analyst is usually limited to PLOT columns. Because these (PLOT) columns contain particles, one needs to be careful not to get these particles into their mass spec, especially turbo pumps. Use of a particle trap will catch most of these particles, but I’m sure some will break through the trap and end up inside the mass spec. In addition, the connector used to connect the particle trap to the end of the PLOT column can become another source of leaks.

So, as you can see, the mass spec may not be the best detector for this analysis. What other detectors are usually used? The most common (that I am aware of) are TCD’s (Thermal Conductivity Detectors), µTCD’s, DID’s (Discharge Ionization Detectors) and HID’s (Helium Ionization Detectors). Any of these detectors are considered universal, but keep in mind that they won’t detect the compound that is also the same as the carrier gas (e.g. nitrogen if nitrogen is the carrier gas), and that most DID’s and HID’s (the ones I am familiar with) are not an appropriate detector for detecting helium because they require it (helium) as the carrier gas (for the detector to operate properly).

If anyone uses a GC/MS routinely to analyze permanent gases, drop me a note; I would like to know how you are accomplishing this challenging analysis. Thanks.