Chromatography Forum

I am quantifying CH4 emission from salt marsh incubations and have observed unexpected, gigantic peaks elluting at the same time as my CH4 standards. In these ecosystems, methylated sulfur compounds are produced in large quantities. I am using a Shimadzu GC14-A with FID.

Is it possible that a methylated sulfur compound (e.g. dimethyl sulfide) could be contributing to the large peaks?

Would a FID detect any sulfur compounds in addition to hydrocarbons?

Question 1

YES. COS, Methyl Mercaptan, DMS, H2S.

Question 2

Yes, they most certainly detect quite well.

Would the detection of methylated sulfur compounds along with any methane be due to the breakdown of the sulfur compounds upon injection?

Or, are they able to ellute just as quickly?

Question 1

Possibly, but less than likely. Sulfur gases usually combine to form larger molecules. A large molecule could be decomposing into volatile components in the injector.

Question 2

This depends upon the column ID, GC phase, and film thickness, unless you are using packed columns where it depends upon headpressure (carrier flow rate), GC phase, the phase loading, and the type of GC support used.

Thanks so much for your comments.

Are there many gaseous molecules that would breakdown upon injection into a GC and show up as an artifact peak?... (or uncommon).

My injector temperature is set at 120. My column is a Poropak 80/100 N, 6' 1/8'' ss, stock 2716PC. I'm running N2 carrier at 58sccm, purified compressed air through moisture, particulate and hydrocarbon traps at 498 sccm, and H2 at 50sccm. The column is kept at 50 degrees, and my FID is 200.

I am injecting environmental samples from Exetainers, and getting some peaks (only from certain geographic areas) containing over 1100ppm CH4 (supposedly)!

Now your analysis is explained (except for sampling technique and injection volume) .

At 50°C you should have no peaks that should elute around methane with the following exceptions:

Oxygen
Nitrogen
Carbon monoxide
Pressure upset from injection introduction

Since you are using an FID, I suspect you need to run blanks. If you see no peaks (and I suspect that will be the case) then I believe you are seeing Carbon Monoxide.

I hope this helps. Replace the Porapak Q with a molesieve 5A or 13X column properly activated and determine if the broad peak you are seeing is CO. Co will elute after O2, N2, and methane. CO2 will elute on the 13X but will not elute from a 5A mole sieve column.

It may be a decomposition product from something in your sample that is thermally sensitive that is producing the CO ( perhaps a peroxide? ).

Good luck and best wishes,

Rodney George
Senior Research and Development Scientist
Gas Separations Research
Supelco
595 North Harrison Road
Bellefonte, PA 16823

814-359-5737 voice
814-359-5459 fax
rgeorge@sial.com