TCD Issues

[kbrodw01]

I recently received a 5890 Series II GC as a donation from a nearby company. I needed the TCD it had to work, (it came to me in a very "manhandled" condition) I reinstalled the thermocouples and it heats up to temperature fine. The issue I'm having is that the baseline doesn't hover nearly as high as the other GC that I have with a TCD and any time I inject a sample (mostly air as a test) instead of a peak, I see a large elongated bump. I don't think the problem is with my column since this is a brand new duplicate of the column I have in the other working GC.

The column is a Micro-Packed Shin-T Carbon column. I'm pretty sure the filament in the TCD isn't burned out since I still hear the telltale clicking that means it's on, and I do see a baseline above zero (however slight). Any advice would be greatly appreciated since i have hit a wall as far as diagnosing this problem goes.

[Consumer Products Guy]

Install the known good column in the 5890 unit. And install the "new" column in the other GC. Report whether the issue stays with the GC or "travels" with the columns.

[chromatographer1]

The Restek column may be using wire bundles as termination of the micropacked column. These can become damaged easily and cause the problem you are seeing.

I agree with CPG that the column is the first place to check.

Rod

[kbrodw01]

I changed out the column today with the good one from the other instrument. I have the same problem as before, so I'm pretty sure it's not the column. Any ideas?

[chromatographer1]

perhaps the TCD is blocked with packing from columns and is not allowing the proper flow through the detector.

I have had to replace units like this before when a damaged column caused a loss of packing into the detector. You can try flushing the detector with methanol or with pressured gas, carrier for instance, in reverse direction to cure the problem.

Good luck.

Rod

[AICMM]

kbrodw1,

Time to start measuring flows. With the TCD off and the reference off you should have column + make-up if there is any. Then back on with the reference and the difference in flow is the reference flow.

Sounds to me like you may have way too much flow through the detector and this would cause a loss of sensitivity. You also don't mention what carrier gas you are using and this could cause your problem as well. Finally, you get a "bump" for the air peak but you don't say whether it is early or late which would be useful to know.

Best regards,

AICMM

[chromatographer1]

MM has good advice.

I assumed that flows were the first thing you checked and it was wrong for me to make that assumption.

Rod

[kbrodw01]

I have the flows set to about what they are on the other instrument: roughly 12 mL/min of carrier gas running through the column and approximately 24 mL/min of reference gas. The carrier I am using is Argon and the awkward bump occurs at the time air should elute out of the column. Like I said, this GC is basically going to be used for simultaneous analysis with instrument 1, so I am attempting to make them as identical as possible.

I'll try cleaning out the detector tomorrow when I get in. When i received the unit, the TCD was attached but out of the GC itself. I had to steal a thermocouple from another TCD on a different GC (it was beyond repair, but still had good parts) and connect it. It is possible that the TCD itself was clogged from the beginning.

[kbrodw01]

So, I blew out the TCD with compressed air and the problem I was having remained. Since then I have done a few things:

replaced the entire TCD cartridge and valve with another TCD on a scrap GC (this particular GC was never actually tested so I do not know if the TCD was ever in working order) I also double and triple checked my flows. I replaced the known working column into this GC and the problem remained.

I am still receiving a signal from the TCD on my ChemStation readout and it's not zero, so I don't think the TCD is burned out. I also checked the resistance across the leads that go into the TCD board while the detector was hot and found it to be 15.9 ohms (a little high, but still within workable range) I also replaced the TCD board with another working board from a different unit and still my problem remains.

At this point I really have no idea what else to do. my column is a micro-packed column with a flow of 12 mL/min and my reference gas flow is 33 mL/min, my carrier gas is Argon and I made sure the switch on the TCD board has been moved to Argon. Any Ideas?