Isotope double spikes, calibration with

Isotopic double spike. The most rigorous approach is to use an isotopic double spike , in which samples are doped with a known quantity of spike Mo which consists of two isotopes in a known ratio (Wetherill 1964 Siebert et al. 2001). These spike isotopes serve as an internal standard to monitor mass fractionation of the sample subsequent to spiking. The fundamental advantage over the element spike is that the spike isotopes follow exactly the same fractionation behavior as the isotopes of interest. A disadvantage is the need to carefully prepare and calibrate the double spike. 

Isotope amount ratio calibration with double spikes involves measuring the relative amounts of at least four isotopes of an element (there are over 30 elements fulfilling the condition of availability of >4 isotopes). For two of these isotopes, the relative isotopic abundance is significantly enhanced by the addition of an enriched isotopic spike to the sample. Such an approach circumvents the bias incurred... 

Practical disadvantages associated with use of the double-spike technique include the required availability of the high-purity enriched double spikes, the effort required to calibrate the isotopic composition of the spike, the need to avoid possible cross-contamination between the analysis of unspiked and spiked samples, the requirement for at least four interference-free isotopes of the analyte, and the need for two analyses of the sample - unspiked and spiked. 

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