Does LU matter on FLD detector baseline monitoring?

I am very new to FLD detectors and have been seeing some issues between some of our newer units and older ones.  We are thinking the optical units may have some etching on the older ones but because of our company's structure we cannot service them ourselves.  What I am trying to understand is how the baseline monitoring works.  When we start our systems, our newer systems seem to monitor closer to zero.  However our older models (1100 series) do not behave the same way.  They tend to drift upwards of 5LU, some even to 10 or 20 LU.  Is this something that should be of concern? We have cleaned with IPA and still see the same issues.  When we do test runs, we notice better reproducibility of injections the longer the system equilibrates, however it seems that 4/5+ hours is sometimes necessary to achieve more consistent reproducibility.  Again, total newbie here, please feel free to provide with any advice, feedback or even resources that would be available for better understanding of how these detectors work and what signs to look out for when assessing proper performance.  Thanks!

  • With IPA you just clean the flow path. The optical path remains untouched.

    It often makes sense to check first what is working okay, before looking at an issue, so I'd do an Intensity Test first and possibly some checkout runs with the isocratic sample. Then I have an idea about the condition of the system.

    In your case, an engineer should look at the system, as drift and noise are less common on FLD. It's important to use the best possible quality of water and always flush the system thoroughly with fresh water and IPA, after using it. If you leave it sitting in water or buffer, you generate a nice growth of algae and those do fluorescent, so your background rises. Furthermore, the lamp house window, which is made of quartz, becomes foggy/milky over time, simply because of the strong UV light it sees constantly. The mirror can also degrade. Both parts need to be replaced by an engineer. Once the FLD is open, he can also look at the shape of the gratings, but there is stricly no cleaning in this area (also true for the mirror).

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