Chromatography Forum

My system uses two columns in series initially where one of the columns is isolated with a valve closure at 40C and then put back in series at 170C.

When the valve opens at 170C, the FID blows out evidently from a flow/pressure pulse of the gas trapped at 40C on the isolated column expanding at 170C. The FID seems robust, and clean, yet this flame out always happens.

What are effective ways to reduce flow disturbances on valve switching of columns? Would a few loops of deactivated column past the valve and before the FID help...i.e. a buffer volume? How about a spliced in short length of narrow bore tubing at the exit of the isolated column? Other ideas?

Thanks

Try to change hydrogen and air flows to get FID flame much less prone to carrier gas pulse. Just experiment systematically. Eventually decrease makeup flow if former measures are not sufficient.

The usual way to solve this is to add some restriction between the valve and the FID. What GC are you using and are you using packed or capillary columns?

Gasman

HP 6890 with 0.53mm Plot columns. I tried splicing in a 15 cm length of 0.01 mm bore tubing I had for HPLC as a restrictor, and while I didn't get flame out on the valve switching, I didn't get any peaks eluting either.

Apparently, the restriction is usually the methanizer (Ni catalyst tube) to covert CO & CO2 to methane for FID detection. I've tried packing the methanizer more tightly but can't get it to provide enough restriction. Would a short loop of 1/16" tubing provide a small buffer volume to attenuate the pressure pulse?

What's usually used as a restriction? I've tried a 0.25 mm zero dead volume union and that didn't work either.

You are going in the right direction with the 0.01 restrictor. The reason that you are not getting any peaks is that the presure drop across the restrictor is close to the pressure that you have as the inlet pressure for your column. Turn the flows off to the FID, and measure your column flow. Increase the pressure until you get your original flow that you had without the restrictor. You should then get your peaks, but the retention times will probably be a bit slower.

Gasman

Or a couple of feet of 0.25.

Best regards,

AICMM

Going from 40C to 170C the volume of gas in the column increases by about a half. With a couple of mls of gas in the column the flow pulse through the detector is only a ml or two. I would be surprised if this was enough to blow the flame out unless there is another problem. Some Agilent/HP GCs had flame tips if different diameter for different size columns - the larger bore is more resistant to the fame lifting off the flame tip when flows increase. Presumably you are not using make-up, if you arte then try reducing its flow. What is the total carrier flow going into the flame ?

Peter