A calibration curve, also known as a standard curve, is a graph generated by experimental means that is used in analytical chemistry to determine the concentration of an unknown sample solution. It is a general method for determining the concentration of a substance in an unknown sample by comparing the unknown to a set of standard samples of known concentration. Calibration curves are often used in many fields, including analytical chemistry, biochemistry, and pharmaceutical chemistry. The curve is created from the instrumental response to a set of standard samples at a range of concentrations, and the data are then fit with a function to enable the prediction of unknown concentrations. Calibration curves are useful in any situation where a substance is unknown, and they can be used to understand the concentration of an environmental pollutant in a soil sample, determine the concentration of a neurotransmitter in a sample of brain fluid, vitamin in pharmaceutical samples, or caffeine in food. The most important part of making a calibration curve is to make accurate standard samples that are in a linear range.