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isotope dilution An analytical technique used to determine the concentration of an element in a sample by means of a mass spectrometer. The method is based on the determination of the isotopic composition of the element in a mixture. A known quantity of a compound containing an unknown quantity of a particular element is mixed with a spike (a known weight of a radioactive isotope of the element). The specific activity (disintegrations per second per kilogram) of the spike is known precisely, so the isotopic composition of the mixture can be used to calculate the amount of the element in the sample. A small amount of the mixture is isolated from the sample, weighed, and its specific activity measured. The concentration of the inactive element in the sample may be estimated by the dilution of the radiotracer. Isotope dilution analysis can be applied to all elements that have two or more naturally occurring isotopes (about 80% of all elements), provided that a spike enriched in one of the isotopes of that element is available. As a technique it has several advantages over other analytical methods. It is free of interference from other elements present and its accuracy is governed by the calibration of the spike solution.