We often get asked in tech service for a column recommendation for a specific analysis, but before we provide an answer, we ask what GC detector is going to be used. This is important to know because certain dimension columns work better with certain detectors. I tried to elaborate on this topic in one of my previous posts.
How to choose the correct GC column – Part 3
One of the more common questions is which column should be chosen for analyzing fixed (permanent) gases using a GC/MS. As I had mentioned in another post, I never had much luck trying to do this analysis using a mass spec, and therefore recommend using another universal detector (like a TCD, HID, or DID) along with possible column choices (depending upon the specific analysis).
What are some of the other analyses where a (GC) mass spec may not be the best detector to use? My list would look something like this (and by no means is it all-inclusive).
- Light hydrocarbons including methane, ethane, propane and butane (and related compounds/isomers) – Best with FID
- Formaldehyde (and water vapor) – Best with TCD, HID & DID
- Ammonia – Best with TCD, HID, DID & IRD
- Hydrogen Sulfide (and other light sulfur gases) – Best with SCD & FPD
- Methanol and/or Acetaldehyde (and other light alcohols/solvents) – Best with FID
- Ethylene (and Propylene) Glycol – Best with FID
- High boiling point compounds (approx published boiling points greater than 450C) – Best with FID
- TCD = Thermal Conductivity Detector
- HID = Helium Ionization Detector
- DID = Discharge Ionization Detector
- IRD = Infrared Detector
- SCD = Sulfur Chemiluminescence Detector
- FPD = Flame Photometric Detector
- FID = Flame Ionization Detector
So now you may be asking why I would recommend another detector (like a TCD, FID, or a sulfur-specific detector) for the compounds listed above. Well, there are a couple of different reasons.
Let’s start with #7, high boiling point compounds. If possible, my first detector choice would be a FID. It’s too easy for compounds with boiling points over 450C to condense out in the mass spec transfer line (if they even make it out of the split/splitless injection port). You may be thinking “I’ll just max out my transfer line temperature”, but fair warning, exposing fused-silica tubing to temperatures greater than approximately 300C around-the-clock may cause the polyimide coating on the fused-silica tubing to flake off over time, leaving the column brittle. In addition, even though we also manufacture MXT columns which can withstand these high temperatures without becoming brittle, we normally do not recommend them for mass specs because of the concern over possible arcing issues inside the source.
Now let’s look at the other compounds in my list above. The reason I don’t recommend using mass spec for #1 though #6 is because of the spectra these compounds generate. Each will produce primary m/z’s less than 45, which as any mass spec analyst knows, is far from ideal; there are just too many possible interferences when scanning this low (very high baseline and loss of sensitivity are commonly reported). Before deciding if a mass spec can/should be used, I always search the NIST Chemistry WebBook for spectra, and if all the ions in the compound of interest are less than m/z 45, I look at alternative detectors. Below are a few spectrum examples from the NIST Website which illustrate this point.
In summary, before you decide to use a GC/MS for your next project, ask yourself if the mass spec is really your best detector choice. Thanks for reading.
