Pitfalls in Using Isotopic Distributions for Structural Interpretation by Mass Spectrometry

This brief cautionary note reports a failure in a common and useful assumption, namely, that the isotopes of the elements occur in their natural abundance ratios in commercially sourced organic compounds. Some commercial sources of tris(pentafluorophenyl)borane, B(C6F5)(3), show severely depleted B-10, while materials from other suppliers display natural isotopic abundances. The depletion varies from lot-to-lot, and it was confirmed by inductively coupled plasma (ICP) mass spectrometry. The isotope B-10 is used in the nuclear power industry, as a neutron absorber in the power control rods. It is speculated that the residual B-11 generated when preparing B-10-enriched boron carbide for control rod use, provides B-11-rich raw material that is then used for commercial B(C6F5)(3) synthesis.