Measurement of the total U or Pu in a sample by introducing a known quantity of a tracer isotope normally absent or of low abundance in the sample (e.g. 233U for U, 242Pu or 244Pu for Pu), measuring the abundances of all isotopes present in the sample with a mass spectrometer, and then determining the unknown by multiplying the isotope ratios observed by the quantity of tracer. This technique is generally used for highly radioactive reprocessing dissolver solutions.