Detection of Methane and Co2 by GC-MS

[song jack]

Hi Guys,

As you know, GC-FID/TCD has a good sensitivity for methane and Co2 detection. But I am wondering if it is possible to measure them by GC/MS as their molecular weight is quite small. I can only access GC/MS at my lab center.

Thanks in advance

Song

[haiedc]

Theoretically, you can do it, at least with 5973/5 because that instrument can get down to 10 amu. But you may have some difficulties: noise at low mass and when you analyze gas, you need large column and that's not so good for MSD.

[Don_Hilton]

The important question is methane and CO2 at what concentration range and in what.

If you are using GC/MS, I would expect the supply of columns includes the usual selection of capillary columns. And these are not great for separating the components of air at temperatures close to room temperature.

But, when I've checked linear velocity by injecting air, there is always a bit of separation on a DB-5 column. I've not done this for several years, so I don't remember which component seemed to be slighly retained. (it was not a resolved peak, just an ion trace or two delayed slightly from the others)

If 1) you can run subambient and 2) the concentrations of methane and CO2 are high enough and 3) the matrix will allow for injection at low column temperatures - you have a chance at doing this.

Two cautions: 1) there is always air background in a mass spectrometer - thus CO2 Expect CO2 to be displaced by components coming through the column, resulting in an increase in signal at the leading and back ends of large peaks for other compounds. Be sure you peak for injected CO2 is away from these effects. 2) Running subambient adds cost to the analysis.

[Ron]

The matrix can have interferences , for example N2 and CO are both mass 28, and as Don has pointed out the separation on a standard capillary column may not be too good unless you go subambient. Some mass spectrometers can handle the flow from a 0.32 mm id column, and this would allow the use of a PLOT column IF a particle trap is used. This will give a better gas separtion.

I usually advise against the use of a mass spectrometer for the analysis of light gases, it can be done, but it is a lot of work, and is generally much easier to use a GC with a TCD or a discharge ionization detector.

Is there a possibility of adding a detector and second injection port to the GCMS you have access to? That might be the best solution to your problem

[song jack]

Thanks Guys. Methane concentration is pretty high. I have a the CarbonPLot column which is supposed to not emit any particles at all. Can I run at isothermal like 35 instead of subambient ? I am not sure why GC/MS need to run at subambient?

[haiedc]

Thanks Guys. Methane concentration is pretty high. I have a the CarbonPLot column which is supposed to not emit any particles at all. Can I run at isothermal like 35 instead of subambient ? I am not sure why GC/MS need to run at subambient?

You can run at 35 deg with that column. Only concern is the 0.32mm id column may not suit you MSD or you have to choose quite a long column (50-60m).