A calibration curve is a method used in analytical chemistry to determine the concentration of a substance in a sample by comparing the response of an analytical instrument to known concentrations of that substance. This process involves creating a graph that shows the relationship between the concentration of the substance and the instrument response. By measuring the response of the instrument to a series of standard solutions of known concentration, a calibration curve can be constructed to accurately quantify the concentration of the substance in an unknown sample. Calibration curves are commonly used in various analytical techniques such as chromatography, spectrophotometry, and mass spectrometry.