Someone knows how to obtain and calculate 87Rb / 87Sr by using x-ray fluorescence data ? I know that some quality parameter of the analysis can be followed such as Rb and Sr between 50 and 2000 ppm and about error of the equipment ranging at 2% or least.
For most natural samples, multiply the Rb/Sr ratio (calculated from ppm by weight) by 2.894 to obtain a very close approximation to the 87Rb/86Sr ratio. The uncertainty in the calculation is likely to be less than issues related to uncertainties in the XRF data or the fact that you will have measured the Rb/Sr and 87Sr/86Sr on different fractions of sample. The previous answers are correct that XRF does not measure isotope abundances however the conversion factor above is derived because the ratio of 87Rb/85Rb is constant (0.3855) and the isotope ratios of strontium (84Sr, 86Sr, 87Sr, 88Sr) are reasonably well constrained in natural samples.
XRF is not able to tell the difference between isotopes, so any measurement of Rb or Sr will only be a measurement of all Rb or Sr atoms. These elements are adjacent in the periodic table, but with a detector with good energy resolution it should be possible to separate the peaks. The good news is that these elements are easy to excite with XRF and are at the peak sensitivity of EDXRF detectors so the signal will be good and LOD's in the 10 ppm range would be possible, depending on the matrix of the sample.
I guess that the purpose of the question is using the ratio for rock dating, so the use average abundance values would erase exactly the information we are seeking.
As already mentioned, XRF (either WD or ED) do not provide isotopic information, but elemental only. Thus, methods based on mass spectrometry should be considered for this application.
I've heard about that was possible obtain a roughly approximation by using XRF whole-rock geochemical data of elemental Rb and Sr.
For most natural samples, multiply the Rb/Sr ratio (calculated from ppm by weight) by 2.894 to obtain a very close approximation to the 87Rb/86Sr ratio. The uncertainty in the calculation is likely to be less than issues related to uncertainties in the XRF data or the fact that you will have measured the Rb/Sr and 87Sr/86Sr on different fractions of sample. The previous answers are correct that XRF does not measure isotope abundances however the conversion factor above is derived because the ratio of 87Rb/85Rb is constant (0.3855) and the isotope ratios of strontium (84Sr, 86Sr, 87Sr, 88Sr) are reasonably well constrained in natural samples.
You can calculate the 87Rb/86Sr ratio by the concentration of the total Rb and Sr just as Thomas H. Fleming said, but you should use it only for 87Sr/86Sr0 estimation (so as to represent in a epsilon Nd-Sr/Sr diagram).
Find attached a spreadsheet that might be useful
Hi, to calculate approximately the 87Rb/86Sr isotopic ratio you need elemental content of Rb and Sr (from XRF or ICP-MS) and the 87Sr/86Sr from TIMS or ICP-MS-MC.
The rationale of the calculation is found in the attached document.
Cheers,
Michele
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