What type of analysis will you be doing? Building a standard curve off pure compounds would be preferred, but 1,3,5-trimethoxy benzene is a nice standard. Fairly unreactive, distinguishable elution time, and can be used in other analysis like 1H-NMR; the methoxy singlet is distinct from the aromatic and will integrate to 3:9 respectively.

The alcohol (more reactive) is also available and may be more water soluble. Phloroglucinol

For most applications I would not use a DAD detector for PAH analysis on HPLC, since a fluorescence detector gives much better sensitivity. Particularly good are those fluorescence detectors that allow several emission wavelengths to be measured in parallel. When we bought such a multiband detector several years ago, our analytical performance and robustness was significantly improved and the same happened to other research groups as well. You can read the details in some of our papers, for instance in:

https://www.researchgate.net/publication/51859155_Measuring_binding_and_speciation_of_hydrophobic_organic_chemicals_at_controlled_freely_dissolved_concentrations_and_without_phase_separation?ev=prf_pub

or

https://www.researchgate.net/publication/41507755_Controlling_and_maintaining_exposure_of_hydrophobic_organic_compounds_in_aquatic_toxicity_tests_by_passive_dosing?ev=prf_pub

Article Measuring Binding and Speciation of Hydrophobic Organic Chem...

Article Controlling and maintaining exposure of hydrophobic organic ...

My intention is to use fluorescence and PDA in parallel ... I'm waiting for the replacement of the fluorescent lamp ...

thanks for the replies and for the papers that suggested me ...

I read it now!