Carrier gas changeover from helium to hydrogen

[Mike @ AgCan]

Although this is normally a simple procedure for newer GC's, we have a Carle 211 that has been less than co-operative in this change. We would have laid this beast to rest but it has been such a great workhorse for a very simple analysis. As helium supply is becoming limited and prices are going up here in Canada, we considered the use of a hydrogen generator for both the FID and carrier supply.

The GC is used isothermally on a Alumina packed column (6' x 1/8") for analysis of ehtylene. Following the nearly ancient manual we have set the carrier flow to ~25 ml/min, FID air to ~300 ml/min and the former port to the FID hydrogen is now the diluent gas, nitrogen. Flame lights and sensitivity has increased but so has the detector's variability.

Has anyone used hydrogen as the carrier on a Carle 211 before?

Does anyone have recommendations on the flow scheme we have used?

Is the dulient gas necessary or is there another option?

We have already tried removing the diluent gas and plumbing partially dividing the 25 mL/min hydrogen flow to column and FID. This resulted in lower sensitivity.

[chromatographer1]

This is speculation. Do you really think your detector has variability?

Could it be that the impurities are being absorbed on the alumina due to changes in the water content of the carrier.

Perhaps the problem is running a generated hydrogen carrier gas on a alumina column? I refer to the possibility of trace amounts of water getting into the carrier from the generator.

Do you have a hydrocarbon and water trap on your hydrogen carrier supply?

But in any case, since you are doing such a simple analysis why don't you use nitrogen carrier? Can you get clean dry nitrogen in Canada inexpensively? (I am in the US, so I really don't know your situation.)

Just a suggestion. best wishes,

Rod

[Mike @ AgCan]

Good point, we are not certain that there isn't adsorption on the Alumina. We'll test the theory and see if there is an effect.

As for changing to nitrogen as a carrier, the hope was to use the hydrogen generator as dual duty for this GC and a 2nd GC, both for carrier and FID. We had planned to move away from bottled gas. Perhaps we shall need to consider a small nitrogen gen to supply this make-up flow. Need to consider the ROI.

Thanks for your comments.

[Peter Apps]

Hello Mike

The nitrogen make-up gas has two functions; to sweep the space below the flame tip, and to increase the sensitivity. The effect on sensitivity is small, and you have enough flow out of your column to sweep the space under the flame tip, so you should be able to operate without make-up.

When you say that sensitivity has increased, what are you comparing ?, I presume hydrogen vs helium as carrier gas. If you are measuring peak areas I would be surprised if there was a change with carrier gas, if you are measuring heights, and hydrogen gives you narrower peaks, they will also be taller. Being narrower and taller, they might not be as robust to changes in injection band width.

What was your hydrogen flow to the flame before you changed the carrier gas ?

25 ml/min of hydrogen is on the low side for an FID. can you increase the hydrogen carrier rate without compromising the chromatography ?

Peter

[chromatographer1]

Mike,

If you use nitrogen carrier there shouldn't be any need for makeup gas. The nitrogen from the carrier already is acting as a make up as far as the sensitivity of the FID is concerned. Just check to determine that there is a good gas sweep of your flame tip just using the air and hydrogen fuel flow. I don't know the design of the FID on the Carle to tell you one way or the other. Good luck,

best wishes,

Rod

[Peter Apps]

Something else just occurred to me - you have an unsaturated hydrocarbon in a hydrogen atmosphere in a hot injector and column - could you be getting hydrogenation ?? Do you see any other peaks ?, or distortion of peak shape ?

Peter

[chromatographer1]

Peter has a point.

The hydrogenation with alumina and hydrogen carrier and the reduction of sites on the alumina are certainly a possibility.

That was another concern but water and the possibility of it deactivating the alumina inconsistently was my primary concern.

This analysis is done at a rather low temperature and I don't think it would be an issue but nevertheless Peter's concern has merit.

A long porous sil B column (Ohio Valley is the supplier) might be a good substitute for the alumina, especially if a 1% or 2% coating of TCEP is added. Ethane elutes before ethylene and acetylene elutes much later. Propane will be very close to ethylene but propylene will be fine.

best wishes,

Rod