Decreasing PID response

[kolter]

I'm running an OI 4450 PID/FID tandem detector. I have been having trouble specifically with the PID. I have been monitoring the response for my surrogate for the past few days. The response from the FID remains fairly constant around 1.4 million, while the PID response steadily decreases.

I installed a brand new OPI PID lamp yesterday. The first response was over 4.5 million. Under 23 hours later, the response is down to ~950,000. I swapped another power supply in to check if the primary was faulty. I'm running some tests on it now, but I don't believe the power supply is the issue.

Has anyone ever seen this issue before? Any suggestions on what I should check? It is clear from the stability of the FID responses that the column is probably not the root cause.

[chromatographer1]

The PID must have a clean lens for the response to be consistent. Contamination from carrier gas or from column, septum, or injection liner bleed can cause the optical path to become partially blocked.

Since you lost response quickly after a new lamp was installed, I suggest you look at these possibilities.

Rod

[kolter]

I made sure the entire PID housing and the new lamp were clean when I installed it. I have been noticing the drop in response over the past few months, however it has more recently become a bigger problem. That is why I purchased a new lamp as it seemed that was the most logical cure. I discovered my flow is a little bit high at 41 mL/min, but I wouldn't imagine that would cause the response to drop continuously. I'm led to believe that the response would bottom out and stabilize at some point even if the flow is a little high.

[chromatographer1]

You did understand me when I said: ?

" Contamination from carrier gas or from column, septum, or injection liner bleed can cause the optical path to become partially blocked. "

Carrier gas can be run as fast as needed if it is clean. If it is bringing in bleed then the faster it flows the quicker the lens is 'fogged up'.

It is the septum? the injection liner? the carrier gas itself? phase bleed from the column? Deposits on the pneumatics from sample injections?

Lots of possibilities exist. Sometimes a sample can contain or will form things that will slowly bleed from a column, and fog a lamp.

And if your FID has a higher than normal baseline, 15pA rather than 3pA, then that is a good clue, but sometimes bleed doesn't always give its presence away by a FID response, especially things like CS2 and silicones.

Good luck,

Rod

[AICMM]

kolter,

Your symptoms do describe a possible power supply failure. If I had a spare power supply, the power supply swap is what I would do next.

Best regards,

AICMM