Severe drop in sensitivity after replacing ECDs

[aliboren]

I am analyzing pesticides by GC-ECD, two columns and detectors (Agilent 6890N) and last spring one of our ECDs was leaking so we replaced both ECDs on the instrument.

Prior to the ECD replacement, the method worked very reliably. My 10 ppb calibrator for toxaphene gave a response of ~ 2000. Also, my endrin breakdown after inlet maintenance was always <5%.

Now, after installing the new ECDs, the response has dropped. The 10 ppb toxaphene calibrator gives a response of ~250, which is not much higher than my background. The change is the same on both columns and detectors. Also, now the lowest I can get my endrin breakdown is 12%.

I have been struggling to figure out what the issue is. I have gone over the detector installation instructions. I have tried replacing pretty much every part on the GC, redone all of the connections. I've cleaned the inlet weldment. It seems strange that brand new ECDs would really perform that poorly, and I don't see how changing the detector would have impacted breakdown. It seems like it has to be some stupid little thing that I'm overlooking.

Does anyone have a suggestion? I would really appreciate any advice!

[70 eV]

What are your signal 1/signal 2 responses? Do you know what they were on the old units? You should shoot for under 200, anything over 500 and I would look at preforming an ECD thermal cleaning.

I got a 6890 that another dept was going to chuck and they were using it as a drying oven with no makeup gas flow for 7 years. Detectors were saturated but after an overnight cleaning session they were back to humming around 140 Hz.

[Andy F]

I agree with 70ev.
We had a brand new GC-ECD (admittedly a different manufacturer) installed and the engineer had to perform a significant bakeout of the detector to get a reasonable baseline.

[aliboren]

My baseline is very nice right now. Before we replaced them, the old ones had a baseline of ~250 for the back detector and ~550 for the front detector (the front detector was leaking).

Now the two new detectors both have a baseline of 130-170.

[70 eV]

What do your signals look like if you set temps to the end of your GC run?

[aliboren]

By the end of the run, the background signal has gone up to ~250

[70 eV]

Those are healthy numbers, guessing you already went over all the new connections with a leak detector?

Is the chromatography the same peak shape wise?

[aliboren]

Yes, I have gone over all of the connections with a leak detector, and remade all of the connections at least once.

The pattern of the peaks is the same, just depressed.

[aliboren]

I did have one person tell me recently that it is normal for brand new ECDs to be less sensitive, and that they become more sensitive with age, although their linear range decreases. And that maybe what I'm seeing, reduced signal-wise, is normal.