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FID baseline signal low

Question asked by joepo on Nov 4, 2019
Latest reply on Dec 17, 2019 by james_jenkins

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.