Structural analysis of diols by electrospray mass spectrometry on boric acid complexes

A method is presented to characterize diols using negative ion electrospray (ES) mass spectrometry in combination with collision-induced dissociation tandem mass spectrometry (MS/MS). The analyte diol is added to a solution c ontaining an ethylene glycol/boric acid [2:1] complex and then subjected to infusion ES, The following boric acid complexes are formed: (i) a complex with two ethylene glycol molecules, (ii) a mixed ethylene glycol/analyte co mplex, and (iii) a complex with two analyte molecules, The first complex se rves as a reference for the assessment of the extent of complex formation w ith the analyte. The ES mass spectra of acyclic vicinal diols all feature intense mixed comp lex signals, indicative of efficient complex formation. Chemical fine tunin g is achieved by MS/MS experiments. Thus, although the (2R,3R)-(-)-2,3-buta nediol and meso-2,3-butanediol stereo-isomers show the same complexation ef ficiency, MS/MS experiments reveal pronounced structure characteristic diff erences. By contrast, 1,3- and 1,4-diols are less prone to complex formatio n as they give only weak signals relative to the reference. For cyclic vici nal diols only the cis isomer produces an intense mixed complex, whose MS/M S spectrum is characteristically different from that of the trans form. The above procedure does not permit an unambiguous differentiation of acyclic polyhydroxy compounds like mannitol and sorbitol, However, structurally rel ated methyl glycosides show characteristic MS/MS spectra. Our findings indicate that the above simple procedure may be useful to prob e the presence and structure of diols and other polyols in aqueous solution s. Copyright (C) 1999 John Wiley & Sons, Ltd.