Chromatography Forum

Got a project for detecting ethylene gas released by plants. I've reviewed a few papers and only found detection of ethylene by GC-FID or ETD detectors, not by GC-MS.

GC/GC-MS is new to me. Just wondering, is it possible to detect ethylene with GC-MS? Previously when I used LC-ESI-MS, since ESI is a 'soft' ionization, some molecules cannot be ionized so there is no mass spectrum for them. With GC-MS's EI, I generally think most molecules can be ionized. Correct me if I was wrong. So why no published work? Higher cost of MS detection compared to FID? Safety concerns as ethylene is flammable? Your comments would be great appreciated.

Molecular weight for Ethylene is 28, which would yield ions of 28 with m+1 of 29 and if you lose 1 hydrogen in ionization you get 27, cleave the double bond and you will see 14.

NItrogen(N2) mass is 28 (N) mass is 14.

You can never get a GC/MS totally leak free or your gasses totally pure without nitrogen impurities so you will always have a background for m/z 14 and 28 which would interfere with detection of Ethylene. FID would be much better for analyzing Ethylene as it would not have these interferences. Also some of the better columns for gas analysis are packed columns and those have too high flow rates for carrier gas to be used with the GC/MS without using a jet separator.

Molecular weight for Ethylene is 28, which would yield ions of 28 with m+1 of 29 and if you lose 1 hydrogen in ionization you get 27, cleave the double bond and you will see 14.

NItrogen(N2) mass is 28 (N) mass is 14.

You can never get a GC/MS totally leak free or your gasses totally pure without nitrogen impurities so you will always have a background for m/z 14 and 28 which would interfere with detection of Ethylene. FID would be much better for analyzing Ethylene as it would not have these interferences. Also some of the better columns for gas analysis are packed columns and those have too high flow rates for carrier gas to be used with the GC/MS without using a jet separator.

Thank you for your information. I was thinking that the low molecular weight of ethylene was a big issue, but never though of other aspects.

When talking about "jet separator", are you referring to the GC-MS interface which is used to interface the packed columns, rather that the direct coupling of the capillary column? Currently I only has a HP-5MS column. If I just lower the flow rate of the carrier gas (He), will it be a problem?

Molecular weight for Ethylene is 28, which would yield ions of 28 with m+1 of 29 and if you lose 1 hydrogen in ionization you get 27, cleave the double bond and you will see 14.

NItrogen(N2) mass is 28 (N) mass is 14.

You can never get a GC/MS totally leak free or your gasses totally pure without nitrogen impurities so you will always have a background for m/z 14 and 28 which would interfere with detection of Ethylene. FID would be much better for analyzing Ethylene as it would not have these interferences. Also some of the better columns for gas analysis are packed columns and those have too high flow rates for carrier gas to be used with the GC/MS without using a jet separator.

Thank you for your information. I was thinking that the low molecular weight of ethylene was a big issue, but never though of other aspects.

When talking about "jet separator", are you referring to the GC-MS interface which is used to interface the packed columns, rather that the direct coupling of the capillary column? Currently I only has a HP-5MS column. If I just lower the flow rate of the carrier gas (He), will it be a problem?

Yes, the jet seperator was the interface used with packed columns when the flow rates are 5ml/minute and above.

You really can't get good results from a mass spec if you have more than 2ml/minute carrier flow into the mass spec, and you get much better results when it is below 1ml/min. If you are using an HP5 column it should be 0.25mm ID or smaller to achieve the correct flow rate.

As others have already said, analyzing ethene using EI on a low res mass spec is not going to work. You could do it with a high res instrument (Ethene and N2 do not have the same exact mass) but that's not likely to be a possibility.

Can you use CI? CI will give you MH+ at 29. Methane will not work as the CI gas due to a huge C2H5+ peak from the CI gas itself. Maybe use ammonia as the CI gas ?? (I don't personally have any experience with that)

On balance, FID is likely the easiest option!

The system background is relatively constant. As long as you are using a PLOT column or something similar you can separate the ethene peak from nitrogen in the injection. This will give you a quite nice peak of ethene. The sensitivity is not that dissimilar from FID. You won't get much improvement over FID, but you will be in the same ballpark.

Using GC-MS for this does seem to be a bit like driving a nail with a sledgehammer, but with the appropriate column it will work.

There must be some in tube headspace derivatization you could do to increase the molecular weight to something that gives a better signal maybe bromination like with acrylamide. or dimethyldisulfide adducts as with unsaturated fats.

http://lipidlibrary.aocs.org/ms/ms04/file.pdf

Thanks everyone for your suggestion and comments! really appreciate.

We ended up with a GC-FID... as it is easier and lots of well established protocols to follow.