A microanalytical LC/MS method for the determination of brassinosteroi
ds as their boronates has been developed. Under reversed-phase liquid
chromatographic conditions the brassinolide boronates, formed by napht
haleneboronic acid, m-aminophenylboronic acid or dansylaminophenylboro
nic acid, have been analyzed using atmospheric pressure chemical ioniz
ation (APCI) and electrospray ionization (ESI) methods. The effects of
additives to the liquid chromatographic mobile phase on the sensitivi
ty for detection of the boronate were examined. A detection limit of 2
5 picogram (signal-to-noise ratio 3) of brassinolide as naphthalenebor
onate was attained in the selected-ion monitoring (SIM) mode with elec
trospray ionization, and that of 2.5 picogram (signal-to-noise ratio 4
) for the naphthaleneboronate derivative was attained in the multiple
reaction monitoring (MRM) mode with ESI. The LC/MS analysis of these b
oronate derivatives could be very useful for trace analysis of brassin
osteroids.