Is a solution containing an isotopically enriched element with two isotopes with enhanced abundances in comparison to the natural abundance.
The term "double spike" was found in the following pages:
Chromium Isotope Ratio as a Tool to Track the Redox State of the Earth | EVISA's News
Experts in Speciation Analysis came together in Münster, September 4-7, 2007: Part 3 | EVISA's News
Double spiking species-specific isotope dilution result calculations simplified | EVISA's News