Diminishing signal on NPD

Discussions about GC and other "gas phase" separation techniques.

5 posts Page 1 of 1
I'm new to NPD; for the most part, I get fantastic signal and everything works great. But then there's how sometimes a days' worth of work looks like this:


That's my solvent peak (I can't seem to find how to turn off the detector to save the bead), and each peak is a little lower than the one before it. Signal diminishes across the entire chromatogram, and my minor peaks are disappearing.

The detector itself is running at 3 volts, 330C, and 3.1 mL/minute hydrogen. Air is at 60, and for now I'm skipping the makeup gas since I don't need that much resolution. Samples are injected by an Agilent 6890 with an autosampler into an inlet at 250C.

I'm running hydrogen as carrier gas, from 90-330C with 0.60 minute and 3 minute holds respectively, using an Rtx-5 column, 30 meters, 1.1 mL/minute hydrogen which is about all my system can muster.

System was kept hot prior to use, i.e.: turned on and kept on for >24 hours.

Every now and again, the detector will just go flat, with virtually no response. At these times, I just turn everything off and let it think about what it's done. At start-up with >2 hours of warm up, it does OK again.

Any thoughts as to why the NPD response nosedives? When I first started the system up, I was getting 800-1000 mV response. Now I'm happy if it's at 250. Bead is new from Agilent with <100 hours on it.
Easy fix, as it turns out: replace the septum. D'OH!

I'm not used to running an autosampler, so the number of samples I pushed through in ~1 week were more like what I would normally do in 3-5 weeks. Not injecting manually led to neglecting the septum replacements I would normally do.
Nevermind; that's not it. It was just a spurious event. Back to low signal even after installing a new septum and ensuring the nut was tight enough.

Any ideas would be welcome.
Put the makeup gas back on. It doesn't affect resolution unless you have dreadful flow problems in the detector, but it does affect the complicated gas phase chemistry going on around the bead.

Peter Apps
NPD's slowly die. That is the nature of the beast. It is a reactive surface that depletes with time. As a result, you have to turn up current over time to obtain necessary sensitivity.

If you are running a 7890, I would think you can set up a timed event to turn the hydrogen or air on once past the solvent peak (a very good idea.)

Two suggestions to think about. 1) Don't use H2 carrier since this will react with the hot bead and generate water when not running. 2) Go contact Paul Patterson at DET and ask him for help. He makes NPD beads and is a wealth of knowledge about these puppy's.

Best regards,

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