Chromatography Forum

Hi,

I am new to gas chromatography and therefore have a question that I have been struggling with for some time. My research group has an old Shimadzu GC-8AIT. We have a moseive 5A and a chromosorb 102 installed. We are doing gasification studies and want to detect the concentrations of O2, H2, N2, CO, CH4 and CO2. Currently, I run a sample through the molsieve to quantify all but CO2. Then I run another sample through the chromosorb to quantify the CO2.

My problem is understanding how to interpret and relate the data from the two chromatograms. For instance, I am currently getting 1% CO2 when separating the sample using the chromosorb 102. How is that 1% CO2 included in the results of the chromatogram from the molsieve run?

Thanks

The results cannot be compared.

Your error is not uncommon. You are assuming that 1% by area in the peak report actually indicates a content of 1 mole percent.

It does not.

To determine the actual amount of gas one has to use an external standard of a prepared and known content to compare any peak of a gas in a sample.

For example, look at the peaks in the following chromatogram. While the nitrogen peak is the primary peak, the hydrogen, oxygen, methane, and CO peaks are actually each 4 mole % of the sample. Notice how the peak area are quite different for each gas. Likewise, if you try to take the area percent of any peak from the chromatogram report you will find none of them will measure 4%.



Thought experiment: Imagine that your gas sample contains 10% helium and you are using helium as a carrier gas. How can you measure and determine that all your peaks are 10% less than they should be?

best wishes,

Rod

Rod,

Let's say that I use a calibration gas with the following composition: 1% H2, 1% O2, 1% CO, 1% CO2, 1% CH4, balance N2.

Since H2, O2, CO and CH4 elute at together through the chromosorb 102 column, I would create a calibration graph using the actual concentration of CO2 (1%) and the integrated area of the chromatogram to find the calibration function.

However, how can I account for the CO2 that will not elute when I inject the mixture into the molsieve 5A? Do I recalculate the actual species concentrations by subtracting the CO2 mole fraction, so my new concentrations are: 1.01% H2, 1.01% O2, 1.01% CO, 1.01% CH4 and 95.96% N2 ?

Thank you for your help.

You run the calibration gas through the mole sieve column.

You note the area for each gas when it is 1 mole percent.

You run your test sample and compare each peak against its match in the calibration mix by dividing each peak area by the area that a 1% composition will produce, and multiply that ratio times 1% to give you the mole % of each component.

I hope I have been clear.

best wishes,

Rod