Negative-Ion Mode Capillary Isoelectric Focusing Mass Spectrometry for Charge-Based Separation of Acidic Oligosaccharides

Glycosaminoglycans (GAGs) are biologically and pharmacologically important linear, anionic polysaccharides containing various repeating disaccharides sequences. The analysis of these polysaccharides generally relies on their chemical or enzymatic breakdown to disaccharide units that are separated, by chromatography or electrophoresis, and detected, by UV, fluorescence, or mass spectrometry (MS). Isoelectric focusing (IEF) is an important analytical technique with high resolving power for the separation of analytes exhibiting differences in isoelectric points. One format of IEF, the capillary isoelectric focusing (cIEF), is an attractive approach in that it can be coupled with mass spectrometry (cIEF-MS) to provide online focusing and detection of complex mixtures. In the past three decades, numerous studies have applied cIEF-MS methods to the analysis of protein and peptide mixtures by positive-ion mode mass spectrometry. However, polysaccharide chemists largely rely on negative-ion mode mass spectrometry for the analysis of highly sulfated GAGs. The current study reports a negative-ion mode cIEF-MS method using an electrokinetically pumped sheath liquid nanospray capillary electrophoresis-mass spectrometry (CE-MS) coupling technology. The feasibility of this negative-ion cIEF-MS method and its potential applications are demonstrated using chondroitin sulfate and heparan sulfate oligosaccharides mixtures.