Helium or nitrogen as make-up gas for capillary GC-FID?

[Tony]

Hello,

We are in the process of installing a fast capillary GC-FID system for fatty acid methyl ester analysis using hydrogen as the carrier gas. With the narrow column diameter (0.1 mm) and low flow rates, a make-up gas is required at the FID for optimum sensitivity. We already have air and helium (in addition to hydrogen as carrier) plumbed into the site, and could use either of these as make-up gas.
After consulting several GC setup manuals and guides, it seems that nitrogen or helium are recommended as make-up gases, with nitrogen being preferred. Is the preference for nitrogen solely down to cost, or is FID sensitivity increased with nitrogen compared to helium? If it's purely a cost issue, it is cheaper for us to run 30-40 mL/min helium from our existing supply rather than install a new gas delivery system for nitrogen. However, if sensitivity is actually better with nitrogen, then we will install and use this. Any comment would be appreciated.

thanks
Tony

[Consumer Products Guy]

We use helium as make up gas mostly, because it's convenient to plumb in on most systems (as you have stated). Nitrogen as make up gas is less expensive, provided you have the set up, and can deliver better sensitivity (but you don't need that in your application).

[Tony]

Any idea about what "better sensitivity" means with nitrogen? Are we talking <2, >2, or >>2 X better than helium?

[chromatographer1]

The amount of improvement depends upon your hardware and the hydrogen air ratio. I remember that changing this ratio made a difference and that nitrogen gave better sensitivity but exact numbers elude me. It may have been 50% better response.

[DR]

If you're just using GC to check ratios of different carbon chain lengths (how much palmitate is in this stearate?), He should work well enough, if you're checking for these things where they shouldn't be (trace analysis), I'd suggest using N.

[Tony]

Yes, OK, but why is N2 better than He? What's the thory/mechanism here? Are there any published comparison(s) on these as alternate make-up gases? Up to now on our older GC we have been using H2 or He as carrier at fairly high flow rates (2 mL/min) with no make-up gas and have been getting satisfactory FID sensitivity. Would I be right in assuming that going to lower flow rates and adding He as make up gas would give us the same or slightly higher sensitivity as we are now achieving (ignoring the additional obvious sensitvity increases one would get from narrower peaks in fast GC)? If this is the case, then we wouldn't need the (possibly) even higher sensitivity from using N2.

[chromatographer1]

Listen to DR.

He is correct, Tony.

Want a theory? Here is one I have read:

The ionization of the flame is more effective with nitrogen, Helium is essentially an insulator.

Good enough?

Have a Happy New Year !

[JI2002]

There is an article about FID in LC.GC north america (Dec 2005). It didn't mention the effect of the type of makeup gas on the sensitivity of FID. But it did mention that hydrogen, air and makeup gas flow rates have big effect on the sensitivity of the detector. My guess is that setting the flow rates correctly is more important than what type of makeup gas you use.

The mechnism of how FID works is very complicated. I'm not sure if it's fully understood by now.

Just another note: to make the setup simpler, I think you can use hydrogen as the makeup gas so that you don't need to plumb helium to GC. Just to remember to reduce the detector hydrogen flow rate so that the total flow rate of hydrogen (including H2 carrier gas flow, H2 makeup flow and H2 detector flow) is the H2 flow rate recommended by the manufacture.

[chromatographer1]

Good points, JI2002.

I might suggest that while simpler to use H2 as a makeup, it might not be a good idea for the POSSIBLE impact it may have depending upon the FID design:

The flame shape might be distorted and the sensitivity might be reduced.


But one never knows until it is tried. That is what research is all about.

I would not spend the money Tony, until you know you benefit from the additional plumbing. Try it with a cylinder supply for the makeup first, and then proceed as you determine to be best.

Good luck.

Rod

[carrolr]

Nitrogen gives higher sensitivity. Older manuals had comments that nitrogen gave a sensitivity as much as 4 times helium as carrier / makeup.

The nitrogen flame runs hotter due to higher heat conductivity. Assume the higher temperature gives more efficient compound breakdown / ion generation. Shifts to leaner (air rich) flames generally gives higher sensitivity for simple hydrocarbons.

The differential in cost between helium and nitrogen is narrowing as the purification / handling cost are outweighing material costs.

[2152812]

I am new to GC in general and FID detection in particular. Can you see any peak when injecting DI water and using FID, assuming no carryover and contaminant present in the system?

Thanks,
2152812

[chromatographer1]

Yes you most certainly do see a response especially if the temperatures and column used produce a sharp water peak through the column, such as when injecting water onto a porous polymer packed column at oven temperatures above 180°C.

The response is small and generally has a tail.

best wishes,

Rod

[2152812]

Based on FID principle I would guess the peak is negative? I do, however, see positive peak all the time and like you said, the peak intensity is a lot lower (than say when MeOH is injected).
Any idea Chromatographer1? Do you do consulting work too? I would need to help with deciding which is best GC technology to bring in my company!
Thanks,
2152812

[chromatographer1]

The peak should be positive

[Peter Apps]

Make-up gas in an FID has two functions; first to purge the plumbing between the end of the column and the flame tip so that there is no dead volume in that area. This was mainly an issue in the era when packed column instruments were being converted to glass capillaries, and the columns could not be inserted up into the detector. All (?) modern GCs have the column going up inside the flame tip so this flushing function is largely obsolete, Beware though that on Agilents the flame tip and the column have to have compatible diameters so that the column can get up inside the tip without blocking the hydrogen flow.

Since this is purely a flushing action, any gas can be used (the hydrogen flow actually does the job if the column is inside the flame tip).

The second function of FID makeup is to enhance sensitivity. Nitrogen works better than helium for reasons to do with flame temperature and ionisation, as well as being very much cheaper. If you do not have nitrogen on tap, remember that air is 80% nitrogen and does this job extremely well. Its only drawback (a really minor one) is that the air-hydroen mixture inside the flame tip is flammable, which nitrogen - hydrogen is not. Since the flame tip is so narrow the flame cannot burn back through it, but to be safe make sure that you always shut off the hydrogen before you shut off the make up gas (which you do in any case, right ?).

I would definitely use air as make up rather than going to the expense of installing nitrogen.

Peter