Chromatography Forum

I've found that using acetone near our GC with an FID causes the baseline to shift and produces many sharp, spiked peaks.

Is that normal? It strikes me as odd, but perhaps I'm missing something.

Mike,

I assume that as you work for a gas supply company, you are using tank air and not a compressor which is close to the GC. A compressor close to the acetone source could be sucking it in and then supplying it to the GC.

I would say that this problem is 'normal', but it could also indicate that there is a problem woth the FID. If you think about it, if you can smell the acetone, then it is in the air around the GC, and therefore it will diffuse down into the FID and it will be burnt. You could also have a sealing problem with the upper part of the FID, and it could be possible that the acetone is being drawn into the FID through a badly sealing 'O'ring.

I have seen similar problems with chlorinated solvents close to an ECD, the stuff just diffuses up the vent line when customers do not connect the vent of the ECD to a proper ECD vent line

Gasman.

I would say that this problem is 'normal', but it could also indicate that there is a problem woth the FID. If you think about it, if you can smell the acetone, then it is in the air around the GC, and therefore it will diffuse down into the FID and it will be burnt. You could also have a sealing problem with the upper part of the FID, and it could be possible that the acetone is being drawn into the FID through a badly sealing 'O'ring.

I have seen similar problems with chlorinated solvents close to an ECD, the stuff just diffuses up the vent line when customers do not connect the vent of the ECD to a proper ECD vent line

It's just that the FID has ca. 450 cc/min of gas flowing through it, between the fuel, air and make-up. I know that you can get diffusion against gas flow, but the effect seemed rather dramatic.

I'm probably over-reacting.

But, then again, over-reaction can be a good thing in a QA chemist.

Mike,

I think that you need to look at the molecular velocity. Digging out my old physical chemistry books I calculated that acetone at 25'C has a molecular velocity of 3.5 x 10^4 cm/sec. OK, this is only over a very short distance as there are collisions, but at least it gives you the idea that this can happen against the flow in the FID.

Gasman

The problem may be that your FID nozzle is acting like a venturi combined with extensive thermal convention of the combustion products, rather than diffusion.

Consider the nozzle cross-section, the flows, and and the flow paths of the gases in the detector jet/collector region and inside your detector shroud - if present.

I know the some detector/collector combinations inadvertently allows gases inside the shroud to be circulated around, because I've seen light silica deposits on the outer collector housing surfaces, and older collectors ( eg on Agilent 5792 ) used to generate water condensation spikes - before shrouds were installed to maintain a higher collector temperature.

It's very possible that some local ambient air is sucked downwards around the collector as the jet of hot combustion products and nitrogen hurtles out.

Bruce Hamilton

If somehow acetone gets into the detector, shouldn't that result in baseline increase? How come individual peaks show up? Do the peaks look like normal peaks? Does it happen for methylene chloride or hexane?
Do you have anything such as a fan or AC on near FID? Make a small tent and put it on top of FID and see if it makes any difference.