How do I lower my signal in my DAD detector in my Agilent 1260 HPLC system?

Hello,

You will need to modify the sample concentration [less concentration] or make a smaller injection volume.  Alternatively, you can choose a wavelength where it absorbs less intensely and that will lower the signal as well.

I assume you are collecting the signal into a PC digitally and if true, lowering the attenuation will have no effect.  Attenuation is for analog signal manipulation.

In addition to what abasdeo said, what flow cell are you using?  Is it a PREP flow cell?  You may be able to use one that lets in less light.  What model DAD is this?

Here is the catalog  https://www.agilent.com/cs/library/catalogs/public/InfinityLab_5991-8031EN_Catalog_LR.pdf  starting on page 75