Chromatography Forum

Hi... I use a 7890 A GC System from Agilent Technologies. Thus far I have been using helium as carrier gas coupled with TCD Detector for my gas analysis. I do want to analyze for hydrogen but any time I switch to argon gas at the same conditions as helium, the chromatogram turns out very bad. The peaks were more jagged rather than the normal gaussian peak expected. What could be the problem?

My GC conditions are described below:

Carrier: Helium or Argon

Column: HP PLOT Q column

Inlet:Temperature;55 C

Oven: 50 C (10 mins) at 10 C/min to 120 (5 mins)

Detector: 180 C (TCD).

Thank you.

Are you changing the makeup and reference gases on the tcd to Argon as well?

Hi,
There are some great articles stickied at the top of the forum that address factors (such as carrier gas) which affect chromatography. Argon is expected to produce poorer chromatography compared to helium. Certainly you should try to optimize your method for argon, if you must use argon (rather than use the same conditions as helium).

In particular, read part 2 of the articles on "Impact of GC Parameters..." at the top of the page.

Aidan

Argon is expected to produce poorer chromatography compared to helium

Hello Makafui14,

Like the change from any carrier gas to another, the change from helium to argon can produce poorer chromatography. However, nothing in GC theory suggests that argon is "expected" to produce poorer chromatography compared to helium if each gas is used at its own optimal flow rate, or if the relative departure from the optimal flow rate is equal for both gases. So, there is a reason not to give-up right away.

The optimal flow rate of argon is about 3.85 time lower than that of helium. This implies that, in order to preserve the resolutions of all peak pairs when switching from helium to argon, the flow rate of argon should be 3.85 times lower than the flow rate of helium. As a result, the analysis time with argon would be longer than that with helium. How much longer depends on the ratio of the hold-up times with the two gases. That ratio depends not only on the ratio of the flow rates, but also on the difference in viscosities of helium and argon.

In order preserve the resolutions of all peak pairs when switching from one gas to another in temperature-programmed analysis, it is also necessary to re-scale the temperature program in proportion with the ratio of the hold-up times so that the longer is the hold-up time, the proportionally longer is the temperature program.

Summary:

To preserve the resolutions of all peak pairs when switching from helium as a carrier gas to argon in the same column, do the following:
1. Before switching, measure the initial flow rate and the initial hold-up time in the analysis with helium (initial means at initial temperature, 50 C in your case)
2. Choose the initial flow rate of argon 3.85 time lower than the initial flow rate of helium
3. Measure the initial hold-up time in the analysis with argon
4. Calculate the ratio (G) of the measured hold-up times with argon and with helium
5. Rescale the temperature program with argon in proportion with G.
Example. Suppose that G=3 (the hold-up time with argon is 3 times longer than with helium). In this case, instead of the temperature program with helium (50 C (10 mins) at 10 C/min to 120 (5 mins)), use this one: 50 C (30 mins) at 3.33 C/min to 120 (15 mins).

To re-scale the temperature programs, one can also use several GC Method Translation software packages available free of charge from Restek (http://www.restek.com/ezgc-mtfc), Agilent (http://www.chem.agilent.com/en-US/Techn ... ation.aspx), and from other sources.

One more thing:

Strictly speaking, the temperature program re-scaling described here is expected to preserve the resolutions of all peak pairs only if both temperature-programmed analyses, with helium and with argon, run in a constant pressure mode. In the constant flow mode, the resolutions of some peak pairs might get better or worse. Usually, the changes in the resolution are very minor. If not then please post your results and we can go from there.

Good luck

Hi Imb,
I basically agree with your method of optimizing for argon, and have no issues with many other bits of your post. However, I disagree with your assertion that the resolution will be preserved. The main point being, the longer your analytes sit in the column, the more loss of resolution due to diffusion. Therefore 'faster' gases (Hydrogen and Helium) are -- yes -- expected to produce better chromatography.

Regards,
Aidan

the longer your analytes sit in the column, the more loss of resolution due to diffusion. Therefore 'faster' gases (Hydrogen and Helium) are -- yes -- expected to produce better chromatography.

Hi aidnai,

Unless you insist (please let me know), I would not speak about dispersion of analytes in the column. However, I would agree that the longer the analytes stay in the column the wider are the peaks in the chromatogram. But this is not the only consequence of the longer analyte stay in the column. The longer they stay the further apart in time from each other they elute. After all, this is why an optimized analysis with argon is longer than an optimized analysis with hydrogen.

In summary, the longer the analytes stay in the column the wider are the peaks and proportionally larger is the time distance between the peaks. The net effect on the resolutions is a wash or, (rephrasing your words) all gases are expected to produce chromatography with nearly identical resolution of all peak pairs. It has been verified many (hundreds?, thousands?) times. Try it. It works!