In an HPLC method of analysis, Copper (II) sulfate is used in the mobile phase which makes the mobile phase blue. However this affects adversely the PDA detector where its intensity is dramatically diminished causing problems in the detector and so it is not working anymore. however this problem is not encountered using the UV detector.
A PDA can in effect be used (and often because of the additional features present in a PDA or DAD, not found in single wavelength detectors, will work even better). Note: A "PDA" (or DAD; same thing) unit will usually cover a wide range of wavelengths in both the VIS and UV ranges. Please read up on the detector's operating principles to better understand their uses and limitations. Training in the basic concepts of light transmission and how to use the detector for the intended application(s) are critical to your success.
To maintain the maximum useful dynamic range of the UV/VIS detector we try to utilize a UV/VIS clear mobile phase solutions which will not absorb or obscure the region of analysis. The mobile phase may absorb light in some areas (esp the low UV region where just about everything absorbs) and less so in others. If you select a mobile phase which absorbs more light than can be measured in the range of the detector, than no useful data will be obtained.
You can use a mobile phase additive such as copper sulfate in low concentrations (and indeed we do for some specialized applications which require complexing agents), but in doing so you will decrease the available dynamic measuring range of the detector in the same region where the solution absorbs light.
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