Hi

This might seem to elementary a question for most of you out there.

Im trying to analyse mixtures of N2-O2, and N2-CO2.

Im using a Pressure-Lok syringe to take samples and then inject them into the GC.

Im just wondering what would be the right method to calibrate.

This question arises because, Im unable to measure accurately the quantity of the sample I inject .

Can a relative calibration work. ie. Say I takeair , and inject that and then use

( Area of O2/Areaof N2) * X=(21/79)
and backout X?

CAn any one suggest if this method would be accurate, or is there a better way to do it.

And Im using a molsieve 5A to analyse the N2 O2 samples, and am running it at ambient conditions. The peaks seem to overlap, Is that Ok, or should I do something about it

Im a begineer and hence , my qyestion might be too naive.

Thanks in advance.

Arvind

Calibrating an O2 and N2 measurement using room is reasonable, assuming your unknown samples have O2 and N2 in the 0-100% range each. The relative amounts of O2 and N2 in air are fairly consistant. However, you need to know the volume you are injecting, or at least keep the volume constant.

If you are trying to measure the volume injected by using the ratios of the O2 and N2 peak sizes, I don't think you will be very happy. O2 and N2 have different responses on a TCD, and so the ratio of O2/N2 peak areas for any given volume of air will not be 21/79.

Try a valve. Valco sells very nice vapor injection valves that are easily mounted on virtually any GC. This way, the injected volume is constant. Alternately, rig some kind of "stop" on the plunger of your gastight syringe, so that when you pull back the plunger, you always stop at the same point. For example, if you are using a 0.25 mL syringe, always pull the plunger all the way back. The volume injected will be fairly constant. Calibrate the O2 and N2 peaks against air, and drive on.

Don't forget that you are not accounting for water (your measurements are on a "dry basis", and don't forget that your O2 measurement includes about 0.932% Argon which Mole Sieve 5A will not separate from O2 unless you have about 30 ft of 1/8" column or run at subambient temperatures. Your Mole Sieve column adsorbs CO2 so strongly that you will never see it. If you must measure CO2, try about 2m x 1/8" OD Hayesep Q - 80/100 at room temperature. There is about 330 ppmv CO2 in room air, but this is much more variable than O2 or N2 in air, for obvious reasons. Try shooting your exhaled breath. This will have about 1-5% CO2, more or less.

Your Mole Sieve column is rapidly deactivated by water and CO2. It must be conditioned at about 275-300 C, with a flow of carrier, for at least 8 hours and preferably overnight. Assuming you are injecting 0.1-0.5 mL of sample, and your column is 1 to 2m x 1/8" OD tubing, it should separate O2/N2 nicely at room temperature when properly conditioned. Larger samples or shorter columns will be a problem.

Bill Jenko
Sr. Chemist
Siemens Applied Automation, Inc.