Rare sugars have gained popularity in recent years due
to their
use in antiaging treatments, their ability to sweeten with few calories,
and their ability to heal infections. Rare sugars are found in small
quantities in nature, and they exist typically as isomeric forms of
traditional sugars, rendering some challenges in their isolation,
synthesis, and characterization. In this work, we present the first
direct mass spectrometric approach for differentiating structural
isomers of sucrose that differ only by their glycosidic linkages.
The method employed a noncontact nanoelectrospray (nESI) platform
capable of analyzing minuscule volumes (5 μL) of saccharides
via the formation of halide adducts ([M+X]−; X =
Cl and Br). Tandem mass spectrometry analysis of the five structural
isomers of sucrose afforded diagnostic fragment ions that can be used
to distinguish each isomer. Detailed mechanisms showcasing the distinct
fragmentation pattern for each isomer are discussed. The method was
applied to characterize and confirm the presence of all five selected
rare sugars in raw honey complex samples. Aside from the five natural
α isomers of sucrose, the method was also suitable for differentiating
some β isomers of the same glycosidic linkages, provided the
monomeric sugar units are different. The halide adduct formation via
the noncontact nESI source was also proven to be effective for oligosaccharides
such as raffinose, β-cyclodextrin, and maltoheptaose. The results
from this study encourage the future development of methods that function
with simple operation to enable straightforward characterization of
small quantities of rare sugars.