Application of isotope dilution and double spiking in geochemistry

Isotope dilution is a highly accurate and precise technique for measuring element concentrations in a wide array of samples in the natural sciences. With isotope dilution, the element concentration is directly determined from the masses of the sample and an added element with non-natural isotope composition, the so-called spike, as well as the measured isotope ratios of the sample, spike, and sample-spike mixture. Isotope dilution is therefore a primary method of analysis, is directly traceable to SI units, and considered superior to methods that rely on calibration against external reference materials. Isotope dilution is, however, not applicable to some mono-isotopic elements. Another disadvantage is its destructive nature: It generally requires dissolution of the sample for homogenization with the spike, often limiting analysis of other elements in the dissolved sample aliquot. With the so-called ‘double-spike’ technique, isotope dilution can also be used to distinguish between the mass-fractionation that occurs before spike-sample equilibration from that which occurs afterwards. This allows the removal of isotope fractionation effects induced during analysis to reveal the stable isotope fractionation inherent in the sample.