Targeted borate complex formation as followed with electrospray ionization Fourier transform ion cyclotron mass spectrometry: Monomolecular model system and polyborate formation

Fourier transform ion cyclotron resonance mass spectrometry (FTICR-MS) was applied to identify boric acid(B^o)/borate(B^-) complexes in a monomolecular model system, composed of aqueous caffeic acid and boric acid/borate solutions in various concentration ratios at pH 9.2. Using negative mode electrospray ionization as a 'soft' ionization technique, clusters of polyborate were detected beside the well-known BL^o, BL^- and BL₂^- complexes. An algorithm for the detection of boron complexes, based on their accurate mass and isotope patterns, is proposed which enabled the assignment of elemental compositions/structural formulae of boron/ligand complexes. We present experimental evidence of self-oligomerization of up to six borate units with caffeic acid, resulting in stable covalently bound polyborate-polyol complexes.