Missing Methane

[elleraim]

Hello!
I am a novice graduate student hoping to find some help. My goal is to use a Varian Saturn 2100T GC/MS to find methane in our gas standards; a mix of air, methane and CO2, used as a set up for future methane analysis. So far, no peaks of methane have shown up but O2 and CO2 create peaks. The column is capillary and it was recommended I use a PLOT (25m) column. Do I need to purchase a new column? Or can the GC/MS detect the methane as is? Any advice would be great. Thanks.

[AICMM]

elleraim,

The forum has lots of suggestions about not using MS for gas analysis. You should take a peek at these to get a handle on some of the issues. Very hard to do well for lots of reasons. (My standard disclaimer applies here but the advice still holds.) Is there any way you can possibly do this by GC? Do you have an available FID electrometer for your GC?

Best regards,

AICMM

[elleraim]

Unfortunately the only machine available for use is the GC/MS. I have noticed that a GC-FID is the most common, but I was hoping to work with what we have. Thanks so much for the advice. I will comb the topics more.

[npaz]

Make sure you have the right column. Perhaps a nonpolar RTX-5 from Restek. Second with what you are describing I would recommend SPME ( solid phase microextraction ). Set the fibers in your sample with a tight cap with septa tops. They are fairly easy to use. When you run this with a GC/MS do not use a solvent delay set it to 0 minutes since your gases will elute well below a minute.

[Bigbear]

Make sure you're scanning "low" enough. In my world ( Agilent) the default low mass is 50. You may want to set uo a sim method or scan a naarrow range.

[chromatographer1]

You can check with Supelco about SPME for methane, but if I remember correctly, it is not feasible.

best wishes,

Rod

[nvalentine]

Check with the column manufacturers, but from what I recall, they do not recommend using PLOT columns on a mass spec.

A normal phase column (i.e. DB-5, RTX-5) will not separate that light of gases. You definitely need the phase from a PLOT or some type of molecular sieve to do it.

An FID or TCD would be ideal.

[Bigbear]

Oh contrare, we use a 30M X 0.32 plotQ column for a CDC CN method using a 5973.

[AICMM]

elleraim,

If you are stuck with MS, my best suggestion is a really thick film DB-1/SPB-1/RTX-1 type column. I would also check to see if your GC has the option of cryo (CO2 or LN2) or look at the possibility of throwing a piece of dry ice in there and see if it hits a low temp set point.

Bigbear, what is CDC CN?

Best regards,

AICMM

[nvalentine]

Oh contrare, we use a 30M X 0.32 plotQ column for a CDC CN method using a 5973.

Yes, I had a conversation previously with Restek and they said you should be safe with a diameter less than 0.53 on a PLOT for mass spec. Safe, but I got the feeling not "recommended".

[Bigbear]

CDC Center For Disease Control CN cyanide

[AICMM]

So there's two issues with PLOT's and MS. The first is that they sometimes spit off column. Much better than the bad old days but still happens once in a while. This lands somewhere and that somewhere is your source. May or may not be an issue. The second issue is that a 0.32 optimal flow is somewhere between about 2 and 5 mL/min but most MS's only like about 1 mL/min. If your MS can handle more (the newer MS's with turbo's often can), then a 0.32 might work okay, if only 1 mL, then the plot is not operating at optimum and the curve is very steep on that side. But what can you do, right?

Bigbear, regarding CN, you mean HCN I assume?

Best regards,

AICMM

[nvalentine]

http://blog.restek.com/?p=4520

Good timing.