Varian 3400 FID

[Gerela]

I have a surplus Varian 3400 GC that I'm trying to resurrect. The GC starts normally and the FID flame ignites but there is no solvent peak after injection.
The cathode voltage is -175 Volts. Is this low or normal?

There is only a tiny change in baseline post H2 ignition. I can get a small response by touching the anode with an unconnected multimeter probe so the detector amp seems to work.

[benhutcherson]

I will add a big asterisk and say that my familiarity is with later Varian GCs(3800 and 3900) and I've used neither of those with an FID(MS only) but basic troubleshooting still applies.

First of all, are you using a packed or capillary column?

With either column type, do the basics to make sure that sample is first getting onto the column, and that you're also getting flow through the column.

I usually check packed columns for flow by putting the hose from a bubble flow meter right on the end of the column. I check capillary columns by putting the end in a beaker or vial with methanol in it and seeing if I see bubbles from the end. If you're not getting flow out the column, try tightening the inlet end to see if you're leaking there. Also, check, double check, and if necessary reinstall the column in the inlet, making sure to position it correctly.

Failing that, you may need to trim the column. When in doubt, 6"-12" from the front of a capillary column is usually a good starting point. I have seen blockages at the detector end.

Beyond that, look at the column's attachment to the detector. Be sure it's inserted to the proper length. Also, be absolutely sure the jet isn't plugged-they can usually be cleaned with a fine wire(the ones that come with Hamilton syringes work great). Sonicating in methanol can work also.

If you are using a capillary column, remember your makeup gas. Usually if this is turned off, you'll just get a dramatically reduced response, but it's worth checking to make sure both it is set properly and actually flowing.

[Gerela]

Thanks Ben for your response. The FID anode and cathode were cleaned by sonication for 3 hours and the cathode bore was examined under a dissecting scope and looks open and clean. I am using a capillary column and did confirm that carrier is bubbling through the detector end. I changed the injector ferrules, septa and the glass liner was replaced with a new splitless liner. The column ferrules are also new. Makeup gas is on.

I'll recheck the flow and the column FID connection but I'm not sure how to test that the injector is working properly beyond that. I'll post again when I have new results.

[nipe]

Check real flows carrier(column+makeup)/H2/air. Must be about 1/1/10. And must be water spot on a mirror on the FID

[AICMM]

Gerela,

As noted by Nipe, check for water at the exhaust to confirm it is lit. Wonderful thing about an FID - in general if it is lit it is working. Likewise, you should not see much of an increase in signal when it is lit - wonderful thing about an FID, it generates almost no standing current.

If you confirm it is lit, the next thing I would do is take a little acetone (somesuch hydrocarbon) vapor and squirt it down the top of the detector. The signal should go crazy. If not, with a 3400 I would next be looking to see if the prongs on the bias and the collector are truly making contact. Then.... I would have to start looking at my old manual to give you further advice.

If it goes crazy, then I would pull the column off the inlet and put a little solvent physically on the column and re-connect it....

Be curious to see what you find out....

Best regards,

aicmm