Stable isotope systems of major nutrients like oxygen or
carbon have been successfully used to trace the provenance of plants and food
products. The boron isotope system is of
great interest in plants because B was found to be an essential
micronutrient in plants occurring predominantly in the cell walls and acts
as a strengthening component.
Boron has two stable isotopes, 11B (~80% abundance) and 10B
(~20% abundance) and its isotope amount ratio n(11B)/n(10B) is reported as
delta (δ)-values (eqn. 1), referring to the Standard Reference Material (SRM®)
951 from the National Institute for Standards and Technology (NIST;
Gaithersburg, USA).
In aqueous solutions, B exists as uncharged trigonal boric
acid B(OH)3 in acidic media or as tetrahedral borate ion B(OH)4 - in alkaline
media. The lower vibrational energy of the trigonal coordinated boric acid
species causes a preferential incorporation of the heavier B isotope compared
to the tetrahedral species leading to
boron isotope fractionation in nature.
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