AAS uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyze atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. This method could identify whether there is the interested atom in the sample or not by noticing the absorbance. The analyte concentration is determined from the amount of absorption. The concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration.