Simple and robust determination of monosaccharides in plant fibers in complex mixtures by capillary electrophoresis and high performance liquid chromatography

• Liquid chromatography to analyze carbohydrates in plant fiber samples.
• Robust free solution capillary electrophoresis (CE) in highly alkaline buffer.
• Recovery and throughput in LC with hydrogen exchange resin and CE.
• Direct UV detection in CE due to photo-oxidation enhanced by the electric field.
• Multidimensional (heart-cutting) and NMR spectroscopy study of the photo-oxydation.

Carbohydrates partially liberated by acid hydrolysis of plant fiber can be separated by hydrophilic interaction liquid chromatography (HILIC), ligand-exchange liquid chromatography or other forms of LC with ion-exchange columns. However, the robust hydrogen-exchange columns show co-elution of galactose, xylose and mannose. Free solution capillary electrophoresis (CE) can be used without derivatization at pH 12.6 and was found to provide a higher resolution of galactose and xylose than common LC with no sample pre-treatment required, other than dilution, within 26 min. CE was able to provide resolution higher than 0.79 for all separated carbohydrates, and the RSDs of determined concentrations lower than 10% for concentrations above 1.3 g L−1. A quantitative comparison between CE and HPLC revealed that up to 22% more carbohydrates are quantified with CE. Direct UV detection in CE of mono- and disaccharides is unexpectedly possible at 270 nm. NMR analysis shows that alkaline degradation is too slow to explain this detection. This CE detection sensitivity is increased by the electric field and our CE and NMR analyses are consistent with a photo-oxidation process.