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 ¹⁰B, 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 ¹⁰B is used in the nuclear power industry, as a neutron absorber in the power control rods. It is speculated that the residual ¹¹B generated when preparing ¹⁰B-enriched boron carbide for control rod use, provides ¹¹B-rich raw material that is then used for commercial B(C₆F₅)₃ synthesis.