A material that is present or added to a sample, undergoing instrumental analysis, to serve as an intensity reference for to permit correction for inefficiencies. In order that the internal standard can be used for the correction of non-spectroscopic interferences, it should behave similar to the target analyte. Depending on the technique used (e.g. ICP-AES, ICP-MS) similarity can be expressed in terms of excitation energy, ionization energy or the sum of both.