Another notable limitation of precursor mass-based combinatorial approach is the inability to assess glycan topology or glycosidic linkages. As glycans are frequently present in multiple isomeric forms or structures, and each isomer has differing biomarker potential, additional forms of separation beyond precursor mass are needed including liquid chromatography stationary phases (such as porous graphitised carbon and hydrophilic interaction columns benchmarked here), and diagnostic product ions from MSn spectra . GlyCombo output includes scan numbers to aid in subsequent downstream MS2 annotation and structure elucidation by software such as GlycoWorkBench and spectral library searching .