RATIONALE: Ion mobility mass spectrometry (IMMS) has previously been shown to resolve small isobaric oligosaccharides, but larger alpha-oligoglucans are also abundant in biology and of industrial importance. If conformational differences between such isomers are retained in the gas phase, IMMS could be used to address questions in biology and industry. METHODS: Negative mode ESI travelling-wave IMMS was used to resolve large isobaric α-glucan ions on the basis of their different gas-phase conformations. α,ω-Dicarboxy-terminated polystyrene was used to calibrate the instrument allowing the collision cross section (CCS) of ions to be determined. RESULTS: α-1,4-Linked maltooligosaccharides with a degree of polymerisation of up to 35 could be discriminated from α-1,6-linked dextran and α-1,4/1,6-linked pullulan using IMMS. Fragmentation spectra of ions separated by IMMS could also distinguish isomers. Two conformational isomers of maltohexaose were resolvable by IMMS, likely reflecting extended and V6 helical conformations. IMMS was also able to identify a product within a mixture of maltooligosaccharides treated with the potential anti-tuberculosis drug target Mycobacterium tuberculosis GlgB branching enzyme. CONCLUSIONS: Biological samples of complex isobaric oligosaccharides can be analysed using IMMS in the negative mode providing facile analyses and high sensitivity without the need for either derivatisation or chromatographic separation.