HPLC is probably the most common method used for targeted analysis of plant materials, and for metabolic profiling of individual classes. A derivatisation step is not essential (unless needed for detection), since involatile and volatile substances may be measured equally well. Selection of compounds arises initially from the type of solvent used for extraction (as with all methods that use an extraction step), and then from the type of column & detector. For example HPLC/UV will only detect compounds with a suitable chromophore, a column selected for its ability to separate one class of compounds will not generally be useful for other types. HPLC profiling methods all rely to a great extent on comparisons with reference compounds. The full UV spectrum (measured for each peak when UV-diode array detectors are used) gives some useful information on the nature of compounds in complex profiles, but often indicates the class of the compound rather than its exact identity.