Chromatography Forum

I'm refurbishing an old Thermo HPLC for a local university. I'm having problems with the UV6000LP's UV signal. The original UV lamp in the system had a poor signal in the 200-400nm range with a good signal above that. I believed this meant that the tungsten lamp was working great, and the UV lamp needed replaced.

So, I purchased a new UV lamp and installed it per instructions. When pumping HPLC grade methanol through the system at 1mL/min, looking at the system in intensity setting, from 2-177 diodes, There is a max signal at 20 diodes and then it drops off between 2-20 diodes. According to the manual, there should still be signal in this range. Overall, the signal is very weak compared to the tungsten signal when I look at the full detector range (200-800nm).

I adjusted the attenuation per manufacturing instructions and there was no change for my UV wavelength intensity as I tried to increase the signal. The intensity is about 10X too weak according to the ranges in the manual. At the higher wavelength that uses the tungsten lamp, I was able to get improvement in signal- low end of the passing range.

Calibration accuracy with the holmium oxide filter appears to work. Sometimes, the results are off by 1nm at most. And, when flip to the holmium oxide filter, the spectra looks correct.

Unfortunately, this system is not connected to the internet and I have no room on my cell phone to take a photo. I can try to take photos of spectrums on Monday if anyone wants to see something.

Does anyone have any ideas on where to look next? This is an old system that was sitting for about 7 years before I started helping out. My next plan is to blow out some dust and maybe take a peak at the mirrors and make sure they are all still attached. Thermo has said they will not work on the system due to its age; so, I would like to fix this myself.

Thanks,
Denise

It sounds like a faulty diode array.
Another possibility is dirty flow cell lenses , which passes visible light but cuts of uv light.

Thanks. I'll get cleaning.

What would cause the diode array to go bad? Just age? The attenuation wasn't fully open to saturate it.

Another possibility is a corroded mirror.

@DeniseHeinz

did you find out which diode array sensor is used in the device ?

We still have a couple of these detectors, we used to have more. I am quite sure that we have changed mirrors in all of them due to low intensity.

Not sure if it is corrosion, it seems to be the UV-light that kills them.

Another option is that you have an air bubble in the cell, but since you have flushed with MeOH I do not think this is the case. Just to be sure, put a precolumn or something after the cell (stay below the max pressure drop over the cell!!). That will make the bubble smaller and increase the intensity instantly.