The refractive index detector measures the refractive index of a substance. As all substances have a refractive index, the RID is a universal detector, which can detect any substance. It is especially useful for substances that do not absorb visible light as carbohydrates, lipids and polymers. As low as 10 ng of sucrose can be detected. As the solvent has a refractive index as well and the refractive index strongly depends on the temperature, the RID is sensitive to changes of the gradient and ambient temperature. This limits its applicability to isocratic conditions and requires a good equilibration to stable ambient conditions. The RID has no selectivity.