Noise in PDA at 190 nm to 230 nm

[Angela]

hi,

We have a Shimadzu VP series HPLC. Recently, the PDA has been creating a lot of noise in the 190nm to 230 nm. The lamp is brand new.
 

Please find attached the output we generated running 60%ACN/water at 50uL/min.  We also looked at the noise in the flat parts of the chromatogram at different wavelengths and would be interested in your comments.  There is a column in place but it has been thoroughly washed after the last few days so don’t think it can be to blame.

 

Wavelength       noise

 

190nm              3mAU

200                   3.5

210                   3

220                   0.7

230                   0.1

240                   0.02

[/img]

[ConstantinS]

Mark "excellent"

[syx]

I have 'same' problem: wavy baseline at 200 - 220 nm. The mobile phase contains ACN - MeOH - water (bidestillated-demineralized water).
I do not know why ...

[ConstantinS]

It is due to air bubbles in your pump. Open its vavle and purge the eluent throgh the pump.

[syx]

Yeah... I hope the problem 'just' bubbles not in the electricity.

[Mark Tracy]

A "wavy" baseline with an identifiable period may be a pump problem. Is the period the same as your pump cycle? If you are using a high-pressure mixing type pump, the period will change with the %A/%B. Lots of things, including a bubble, dirty solvent, bad checkvalve, or contaminated pump head can do this.

A "noisy" baseline that is just random probably means that there is not enough light getting through. Is this a new problem, or has the detector always done this? It might be a dirty flow cell. Some micro-cells just don't pass that much light, and you can't fix that. Excess absorbance in the mobile phase can also do it, as can column bleed.

In a truly bizzare case, I once saw corroded gratings. The poor operator had two detectors with this same problem, and was really confused when a detector swap failed to fix the problem.