Differentiation of underivatized monosaccharides by atmospheric pressure chemical ionization quadrupole time-of-flight mass spectrometry.

RATIONALE Differentiation of underivatized monosaccharides is essential in the structural elucidation of oligosaccharides which are closely involved in many life processes. So far, such differentiation has been usually achieved by electrospray ionization mass spectrometry (ESI-MS). As an alternative to ESI-MS, atmospheric pressure chemical ionization mass spectrometry (APCI-MS) should provide complementary results. METHODS A quadrupole time-of-flight (QTOF) mass spectrometer with accurate mass measurement ability was used with an APCI heated nebulizer ion source because we believe that a recently published article using a single quadrupole mass spectrometer assigned incorrect identities for APCI ions from hexoses. Using APCI-QTOF, the MS2 and pseudo-MS3 mass spectra of 11 underivatized monosaccharides were obtained under various collision voltages. The mass spectra were carefully interpreted after accurate mass measurement. RESULTS Differentiation of three hexoses was achieved by different MS2 spectra of their [M?+?NH4]+ and [M?-?H] ions. The MS2 spectra of the [M?+?NH4]+ ions were also used to distinguish methyl a-D-glucose and methyl beta-D-glucose, while the pseudo-MS3 spectra of the [M?+?H]+ ions were utilized to differentiate the three hexosamine and N-acetylhexosamine stereoisomers. Unique [M?+?O2] ions were observed and their distinctive fragmentation patterns were utilized to differentiate the three hexosamine stereoisomers. CONCLUSIONS Although ESI coupled with single or triple quadrupole and ion trap mass spectrometers has been widely utilized in the differentiation of monosaccharides, this report demonstrated that APCI-QTOF-MS had its own advantages in achieving the same goal. Copyright (C) 2012 John Wiley & Sons, Ltd.