Chromatography Forum

Sir,

I am using Varian 3800 for last 4years to analyse natural gas samples. But 2months back we were missing IC5 peak in TCD. Now we are loosing IC4 peak in TCD. But when we run Calibration standard, IC4 and IC5 peaks are visible. Is this means the Natural Gas sample composition has changed? Or Oxidization happened to TCD? We use FID for IC5 and higher components.

Regards,
_keshu

Assuming your valve timing does not change from sample to the calibration sample, if the peak is present in your std then it should also be in your sample, IF IT IS PRESENT.

Perhaps, it is winter where you are, and the heavier components are dropping out of the the pipeline?

If your peaks have changed in their retention times, perhaps you should check for leaks in your system, especially if you are using EPC, or make adjustments in your valve activation settings.

Thankyou very much for the reply. I am using same method for nalysing both Calibration standard and pipeline samples. So change of valve timing can be eliminated. The problem seems to be leaks or cold sample. It was observed that when we heat the sample the result is better, C4 peaks did show up. But still it is under observation.

You should have heated lines if you are using a std in a cylinder that is outside of a controlled environment.

Are you using bagged samples of gas? Or are you on a pipeline?

High pressure will 'dew out' heavier components of a gas mixture.

Sounds like you have a cold spot transferring your NG sample to your analyzer.

Would like to help more but you have not furnished enough information for me to make any other comments.

Good luck.

Rod

Hello Rod,

We use 1000CC sample bombs to carry the pipeline sample to Lab for analysis. The line pressure is about 1500Psi and sample is collected up to line pressure. The sample bombs are evacuated before taking samples. After bringing the sample to lab, we keep it in Oven and heat it up to 140DegF and injected at 20psi. A pressure regulator is used for reducing the pressure.
The cal standard is at room temperature (75DegF) and is at 80Psig (cylinder pressure).

I hope these deatils will help you in troubleshooting the problem.

Thanks and Regards,
_keshu

You may have the proper probe in the pipeline to remove a characteristic sample of the pipeline contents and you may not.

A heated probe is often used to volatilize components quantitatively into a bomb at reduced pressure, say 100 psi.

If you are sampling at 1500 psi and then heating the sample in an oven I believe you may not be volatilizing the heavier components completely.

If they remain liquid on the surface of the bomb you will never see them on your GC detector.

I suggest you reduce the pressure used to fill the bomb when sampling the gas.

Did you ever wonder why the calibration cylinder is only at 80 psi and not 1500 psi?

Good luck

Rod

Hi Rod,

Thanks a lot for the advice.

Now a days we are heating the sample bomb in the oven for 5-6Hrs and analysis result looks good. So for time being it seems the problem is solved. Anyways, I will contact you if I still face problem.

Once again millions of thanks for the guidance and help.
Regards,
_keshu

Hi Keshu74
at the pressure you sample the C4 and C5 will remain in liquid phase at all the tempratures you use. reduce the pressure at the sample point and heat the sample during sampling.
Good luck

Hi Keshu74

Mr. arie2044 is quite correct. You should be aware if you are not, that if you do not have the proper probe on your pipeline you will not be sampling its contents in an accurate manner.

And if that is your goal, and I think it is, you should inquire with the powers that be about making sure your analysis is correct. That would include taking a representative sample from the pipeline, and making certain you are getting the same representative sample into your GC.

best wishes,

Rod