Chromatography Forum

Hi everybody,
I am new to the forum and a newbie in GC so please be patient. I am currently using a Shimadzu GC-2014 equipped with a CTR-1 concentric packed column, and so far I've managed to separate a mixture of gases (He carrier) and to calibrate the system for CO2 via headspace GC. All good.

As I might be interested to measure the methanol content of my samples (gases in water), I tried to add a small amount of it, and I managed to readily detect it with the FID (the relative chromatogram is not a beauty, still working on it, but it's there). No signal on the TCD, even after switching off the FID (I was thinking that the two detectors are in series and FID might prevent TCD detection). I know CTR-1 is not made for methanol but as I managed to get methanol out I would think TCD would pick it up.

So where is my reasoning wrong? As methanol is coming out of the column, why TCD doesn't pick it up? Is it just a matter of sensitivity?
I tried increasing the current of the bridge up to the max allowed value for the oven T (I tried several ramps, isotherms, etc) but no methanol signal.

As a newbie I feel like I am missing an important piece of information, I read a few manuals and did my homework but I need some directions here.

Thanks for reading this, any hint is greatly appreciated.

It is probably just the sensitivity. TCD is not a fantastically sensitive detector. Certainly not even close to a flame.

Probably sensitivity.

Try running a sample with a much higher concentration of methanol to see if you can get a peak.

Thanks, I am trying in a bit, I'll let you know.

Sensitivity of TCD not that great and I suspect you are losing 1/2 of the methanol to the sieve on the CTR anyway. So, if you injecting, say, 1 cc then half of that is going onto the porous polymer and the other half going onto the sieve not to be seen again, at least not anytime soon.

Best regards,

AICMM

So it appears you were right as it was a detectability limit. FID is definitely the way to go so far, calibration works nicely so we are happy with that.

I am using an autosampler with the vial oven temperature set to 80C, so well above methanol bp. By decreasing this temperature I was expecting to minimize the large interfering water peak, but with no luck. I checked methanol partition coefficient vs water partial pressure and I still think it should work. Where's my reasoning wrong?

You may have best results with a PLOT column instead of a packed.
About the water peak, are you talking about TCD or FID? Because water should not apear at the FID.