Inorganic salts aren't resolved, nor are simple acids (acetic acid, formic acid, for example). But many ionic compounds are resolved on reverse phase. Also small molecules such as methanol and ethanol can't be resolved. In general, having a few carbons that reduce the polarity of the molecule allows retention on reverse phase.

The compounds in the app note  link below are ionic, but resolve well on reverse phase:

http://www.teledyneisco.com/liquidChromatography/Chromatography%20Documents/Application%20Notes/RediSep%20Rf%20Gold%20C18Aq%20for%20Highly%20Aqueous%20Mobile%20Phases%20App%20Note.pdf#search=C18Aq

Any analyte that is not soluble in a 'reverse phase' solvents like Water, Methanol, Acetonitrile, IPA, THF....

Very difficult are hydrophilic compounds such as most amino acids or sugars. In these cases, a chemical derivatization might be a good approach.

Rather than derivatization I would try some other LC method 1st like HILIC, normal phase...  If the molecule has a tail 'grab it'.

I note some replies about solubility in polar solvents above- please consider "non-aqueous reverse phase"  as described here:

Snyder, L.R.; Kirkland, J.J.; Glajch, J.L.; Practical HPLC Method Development, 2nd Ed,

John Wiley & Sons, 1997, p.264

and demonstrated here:

http://www.teledyneisco.com/Documents/Liquid%20Chromatography/Posters/Facile%20Isolation%20of%20Carotenoid%20Antioxidants%20from%20Solanum%20Iycopersicum%20Poster.pdf#search=aqueous%20reverse

Thanks a lot for your help! And please excuse my late response!

I need the answer for my theoretical part of my Master thesis. It should give motivation to use polar modified RP chromatography or HILIC instead of conevntional RP.

So the analytes could be ionic/polar but shouldn't be too small to get interaction with the C18-Ligands and it should be something organic? Is this the right extract from your answers (beside solubility issues)?

Obviously, something like concrete cannot be analyzed by HPLC.  But HPLC is like the emf (electromotive frequency) and spans the range from gamma rays to microwaves.  Thus, reverse phase and C18 columns are a 'very' small spectrum of liquid chromatography.

But RP-Phase are the most used ones! Most labs would like to get all analytes in one run, sometimes with loosing quality of signal but saving time and money. But there are restrictions, some analytes are completely inaccessible for RP-Phases. Than HILIC or something other has to be used. These exceptions I search for. ;)