New application – Potential for use of double spike to correct for mass fractionation

The ability to accurately correct for instrumental mass fractionation is crucial in any TIMS analysis. Elements such as strontium and neodymium have naturally occurring stable isotopes which can be used to calculate a mass fractionation correction but uranium possesses no suitable isotope pairs. A normalising ratio can however be artificially produced by adding a double spike of known isotope ratio. For instance, if the 235U/238U  ratio is required, a 233U/236U spike of known ratio can be added to the sample during preparation and be used to calculate a correction that can be used for all other uranium isotopes.

In this application note we have used the SRM U500 in a series of tests to evaluate the mass fractionation behaviour of uranium and to test how well the double spike method works on a Phoenix TIMS instrument. The known 234U/238U ratio in SRM U500 (0.010425)  was used to correct the 235U/238U ratio with the result being within error of the certified value (0.99973) with a precision of 0.016% despite two orders of magnitude difference in the ratios.

This note highlights that accurate and very precise U isotope data can be obtained using a double spike methodology to correct for mass fractionation effects on the Phoenix TIMS system. Phoenix is the instrument of choice for many of the worlds leading high precision geochronology laboratories, laboratories that routinely use double U, and triple U-U-Pb spikes for fractionation correction during U-Pb dating.

A PDF of application note G10712 can be obtained from the geological section of our downloads page.


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