If you are looking for a specific metabolite, you can use the various papers that they have been published on HPLC (which that depend on the peak absorption range of the desired compound in different concentrations of the standard sample and using the calibration curve produced). However, if you are looking for a different range of various metabolites, it is necessary to prepare/purchase the standard of each samples, separately and it is prepare the calibration curve for each compound in different concentrations. Finally, it must be compared the unidentified peak made of output of (HPLC) and analyzed with standard of each of them, accurately. Then, it could be expected that the identity of the unknown/unidentified absorption peak is identified.
One identify phytochemicals from the HPLC retention time and peaks in absence of standards by matching the structure & Retention Time data available in library of computer.