Diminishing signal on NPD
Posted: Sun Jan 06, 2019 7:29 pm
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.
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.