FID baseline signal low

Question

I have an Agilent 7890A with a split/splitless injector and FID. The FID has been experiencing a falling baseline signal that triggers the instrument to relight the flame unless I lower the lit offset value. As it gets worse, I have to set the lit offset below 0.1 pA or even 0.0 to keep the problem from interrupting my chromatography. The documentation suggests that 1 or 2 pA is a good value for the lit offset. The first time this problem happened, I performed the FID maintenance using the rebuild kit (G1531-67000) and cleaned the jet, and the problem appeared fixed. FID gas flows were checked and verified. Then a month later the problem returned, and I tried to get by with a low FID lit offset value. A few days later the instrument complained about the igniter current out of spec and wouldn't even try to light anymore. I replaced the igniter and it ran great. I wrote the problem off to a faulty igniter. It's now another month later and the FID baseline is falling below 0.1 pA again.

I'm running H2 carrier through an OV-17 30m x 0.25mm x 0.25mm capillary column. Air and H2 to the detector are 300 and 30 ml/min, respectively, and the makeup gas is N2 at 25 ml/min. Operating FID temperature is 300C. My samples are dissolved in methanol or xylene and the analytes are chlorinated aromatics and related compounds. I'm starting to suspect dirty gases. I don't have gas purifiers in line, but I do use UHP gases. Before spending bucks on a gas purifier system, I'm curious what the community's thoughts are. Are these detector symptoms characteristic of dirty supply gases or high boiling chlorine containing analytes? Any other ideas? Thanks in advance.

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sruszkay · Answer

I don't believe your gas supply is the issue, as dirty gases would result in elevated FID baseline levels. A lit offset of 1-2 pA is very common. On many systems I have installed with high purity gases and traps, the baseline is typically between 2-5 pA. Occasionally I have seen baselines between 1.6-2.0 pA, in those cases the lit offset was adjusted to 1.5 pA. I suspect your issue is the electrometer on the FID. Is the spring on the electrometer bent or damaged?

joepo · Answer

Thanks for your response, sruszkay. Your point about dirty gases makes sense and dispelled my concern. If course, the day after I had this problem, it fixed itself and the baseline is in the expected range of 1-2 pA. I'm not sure how to check for damage on the electrometer spring. Last time I serviced the FID, I can't say that I noticed it bent or dirty, but I'll make a point to look next time I service the detector. Thinking along the lines of the electrometer spring, I think I might have had detector troubles on a day where the laboratory was particularly cold due to a need to leave a door open. Maybe/maybe not. I'll try to record environment temperature better in the future. But could varying lab temperatures have an effect? I would think that the detector would be fairly immune to environmental temperature swings since its heated to a controlled temperature, but maybe where the spring is secured isn't heated and can cause a bit of movement as the temperature varies? This theory seems unlikely as the baseline decay is characterized by a continuous decrease over the course of hours. My usual work mode is to put the GC in standby overnight: FID flame off, temperature to 150C, carrier gas at a low rate. The baseline always seems strong after starting up the GC for at least a few hours, but decays over the day when I've had this problem. But like I said, the baseline problem seems to have gone away since my original post. And wen I had this problem a long time ago, I had to overhaul the detector to fix it. Perhaps on disassembly and reassembly it affected the electrometer spring contact?

FID baseline signal low

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