Chromatography Forum

Hello all,

I have a tank sample that I am being told SHOULD be mostly methanol with trace amounts of chlorobenzene, however when I run the neat sample on the GC as per the method, i see the a big MeOH peak and a big CB peak and the report says 44% MeOH, 46% CB. HOWEVER this method is not designed to run methanol only pure CB, and it is not calibrated in any way or via response factors. Its solely off area count.

So could there be a reason why methanol will skew low and CB high with an FID? I am going to spike the sample with various amounts of methanol shortly and see if the numbers match up.

Thanks,

Hello all,

I have a tank sample that I am being told SHOULD be mostly methanol with trace amounts of chlorobenzene, however when I run the neat sample on the GC as per the method, i see the a big MeOH peak and a big CB peak and the report says 44% MeOH, 46% CB. HOWEVER this method is not designed to run methanol only pure CB, and it is not calibrated in any way or via response factors. Its solely off area count.

So could there be a reason why methanol will skew low and CB high with an FID? I am going to spike the sample with various amounts of methanol shortly and see if the numbers match up.

Thanks,

You are going to spike your sample with solvent. IMHO it will lead you nowhere.
Calibration is neccessary.

Unless you are injecting very small quantities with a very high split your peaks are probably way above the detector linear range, so their areas are not proportional to quantity. If the benzene is heavily chlorinated it will have a low response anyway.

Peter

Yes thats what I was afraid of, its mono chlorobenzene. Split ratio 10/1, 0.2 uL injection vol. The peaks look pretty sharp.

Indeed when I check raw material purity I inject 0.2 ul with a 500:1 split and still get huge peaks of the main component. 10:1 is probably detector saturation and column overload.

AZBio,

While an FID is a good carbon counter, it has a reduced RRF for both methanol and for aromatics, especially benzene. In addition, it has a reduced RRF for halogens, getting worse with increasing number of halogens. So, your guess is as good as mine as to the response factors for these components. Calibration is essential here.

A 200:1 split would not be at all unusual. If you set up a 5890 with two pressure's feeding the flow controller you can affect a crude gas saver option. Let me know if you want more details.

Best regards,

AICMM

This will be on a 7890b which is already setup with a gas saver method, plus this method won't be used very frequently.

Tomorrow I will up the split ratio and try a calibration. Thanks all.

Yeah, make up series of knowns using low levels of chlorobenzene made up in clean methanol, get close, use external standard quantification.

Calibration at 200:1 split showed good linearity, results were more in line with what was expected. Thanks everyone.