A sample which contains known quantities of the compounds of interest. Standards are used to help identify sample peaks by comparing the time in which they elute to the retention times obtained through the injection of the sample under the same conditions. For quantitation, external standards are compounds that are used to construct calibration curves of detector output (peak area or peak height) vs. concentration; the concentration of unknowns are determined by fitting the detector output to the calibration curve. Internal standards are compounds of known concentration with different retention times that are added to the sample and relative detector responses between the internal standard and the unknown are compared in order to quantitatively measure unknown compounds.