Chromatography Forum

We're seeing this problem all of a sudden this morning with pulsed discharge helium ionization detector (PDHID or PDD). Strange baseline. We're checking the FID channel right now to see how it looks.

FID side looks OK. Power cycled the GC and the detector, but this didn't help.

larkl,

I don't have a PDD but since in these days I'm considering as possible replacement for an ECD I'm looking for more info about it.

I have a presentation from Vici and they write:
''Troubleshooting – negative peaks during sample run

This is usually associated with high standing current level and typically affects N2 and O2 peaks. It means that there is more N2 and O2 in the discharge and carrier gas streams than in the sample.
Check system plumbing for leaks''

Further in the presentation: ''a baseline with negative and positive spikes seems to be associated to a bad HV cable.''

Except for this problem, are you happy about your PDD? Which model you have?

Hope it helps.
bhuvfe

I had a month-long fighting with PDD just to make the baseline voltage work. This thing is highly sensitive to almost everything...
I am not sure what your background to start with, but after I brought the background level down to less than 3, everything works perfect...

So my suggestion is make sure your baseline looks good first, check all your flow rates and go from there...

It is very good to communicate with someone using PDD too. I am happy to share with you guys. Right now, I am using PDD-ECD mode...

Looks more like electrical problem to me rather than chromatographic. Changed anything recently ? Might be time to call an engineer in

larkl,

Not a PDD user since I make my own HID. However, it looks sort of like a power supply problem (possibly the HV cable that bhuvfe points out) to me.
This is based on its sharpness and on the fact that it is somewhat repetitive in your chromatogram.

It is not likely to be contaminated gas, in my opinion, since this usually shows up as either a very high baseline or as a discrete negative peak, for example at the retention time for oxygen or nitrogen or both.

Best regards,

AICMM

Also, it could be carrier gas pressure fluctuation. I've had similar chromatograms while someone was turning the helium on and off.

SOLVED

Replacing the ground pin fixed the problem. Note that the ground pin can be cleaned using 1000, 1500, 2000 grit sandpaper followed by a solvent rinse.

Thanks for letting us know.