Figure 44 : The lower the concentration of a substance in a mixture is butMUST be quantitized, the more differing substances coexist in this low concentration range. No substance is absolutely clean. If we quantitize caffeine in coffee, we get the caffeine signal by capillary chromatography next to thousand other peaks of similar size - larger and smaller. But by PLC we may get a correct quantitative caffeine signal but cannot have any hope to see the other coexisting substances. The specific detectability does it often correctly in PLC but NOTE: some overlapping substances may change the signal strength. So correct results need quite some add-on, external and internal calibration. A well written book on PLC quantitation is missing. Have a look here.