Chromatography Forum

I am trying to analyse the gaseous products of a water gas shift experiment. I have a variety of compounds including H2, N2, O2, CO, CO2, CH4. I am currently using an Agilent GC-MS setup with TCD as the front detector. The column is a Restek Shincarbon column and the carrier gas for the TCD is ultra high purity argon. My problem is that I cannot get an accurate measurement of CO since the CO peak overlaps with the N2 peak. Any suggestions?
Thanks

Are you sure that it is the CO peak that is eluting close to N2. My experience with this column is that it would not separate O2 and N2 when they are present in in the percent levels.

Gasman

Are you sure that it is the CO peak that is eluting close to N2. My experience with this column is that it would not separate O2 and N2 when they are present in in the percent levels.

Gasman

Yes I am pretty sure that it is the CO peak. I also have a simulated syngas mixture containing only N2, CH4, CO, CO2 and H2 which I used to calibrate the GC for these compounds. The peaks for N2 and CO for this mixture also overlap partially. I can see the two peaks but since they overlap, i cannot get accurate area measurement. Would setting a temperature ramp improve/worsen the situation?

I would drop the temperature from whatever you have it at. What are the conditions of your analysis. I have seen separation of N2 and CO on Shincarbon, but at ppm levels. My guess is that if you have a high concentration of N2, it will tail and your CO will come out on the tail.

Gasman

I would drop the temperature from whatever you have it at. What are the conditions of your analysis. I have seen separation of N2 and CO on Shincarbon, but at ppm levels. My guess is that if you have a high concentration of N2, it will tail and your CO will come out on the tail.

Gasman

That is exactly what's happening. Since I am using air as the source of oxygen, the nitrogen content in the gas stream is about 60%. The CO peak which sits on the tail of the nitrogen is roughly 1% but the actual value could be anything since there is no way of extrapolating the actual peak. Currently I am running the GC at an over temperature of 40C. I also tried a temperature ramp of about 15K/min but it didn't make much difference...

How big is your CO2 peak with Ar carrier?

You have the wrong column for the sample you have. Do you want to buy another?

best wishes,

Rod

How big is your CO2 peak with Ar carrier?

You have the wrong column for the sample you have. Do you want to buy another?

best wishes,

Rod

24% CO2 corresponds to 1.7e9 peak area. w.r.t. changing the column, I am open to suggestions.

Chromatographer1 is correct that this is not the best column. I would do this analysis with two columns and two valves. You need a gas sampling valve and a column isolation valve. You would inject into something like a Porapak Q column which would allow all of your peaks except the CO2 to pass through quickly into a Mole Sieve 5A column. This would be isolated to let the CO2 elute from the porapak column, and then you would switch the mol sieve column back into the flow stream for the gases trapped on that column.

Believe me, anybody doing your type of analysis has been praying for years for a single column that could separate H2,O2,N2, CH4, CO and CO2 at high concentrations. Shincarbon may have been touted as being able to do this, but only at very low concentrations. It may do the separartion at sub ambient temperatures, but I have never tried this.

Gasman

I assume you are using a 2m Imm ID Shincarbon column.

You could add another column ahead of this column and get complete separation of the analytes.

Restek does not sell this column as far as I know, and I don't know if my former employer will make it or sell it. They did not wish to sell it when I worked for them.

There is another column which should work but there are many suppliers of this packing and not all of them have material that will work.

The only certain method is the one using multidimensional chromatography using two valves. Even then conditioning of the columns outside of the GC oven would be necessary when needed or damage to your valves would result.

I know you are seeking a simple single column analysis but there is not any unless your sample does not contain water vapor. In this case with a program temperature for your oven and 20 to 30 minutes per run it is possible.

Good luck, I recommend you get the two valve system.

Rod

Restek chromatogram uses a 5 microliter injection using a HID, not a TCD. If you bought a detector like AIC sells you may be able to do the analysis using your present column.

I am sure Matthew would discuss this with you.

http://www.detectorsbyaic.com

Rod

I'm analysing the same syngas with an Agilent MC-GC. I have two columns, Molsieve 5A and Porapak Q. I have no problem in detecting CO but H2. I suppose it is due to the use of Helium as carrier gas with a TCD detector. If you're using Argon, maybe you can change the column and get some achivements. I'm thinking in using Argon to detect every compound

The main factor here is cost. Cost of helium. Cost of argon. Cost of nitrogen. Cost of a second TCD or pHID. Secondary factor is the quality of results.

Do the cost evaluation in your location and project the future costs.

The only downside is argon is less sensitive than helium or nitrogen for your gases.

Detectors cost money. Better detectors give better results. Better results cost money.

Your result requirements and your cost limitations will decide the best solution for you.

Do what works best FOR YOU.

Rod

So, lets see if I can follow directions and post a chromatogram.

So, that would be a "no..." One more try.

Hah, I can handle a computer!