Investigation of photochemical reactions of saccharides during direct ultraviolet absorbance detection in capillary electrophoresis

• Investigation of reaction mechanisms of direct UV detection of saccharides in CE.
• Separation and detection of several photooxidation reaction products of saccharides.
• Major portion of UV absorption of different saccharides is caused by same substance.
• Main photooxidation product was identified as conjugate base of malondialdehyde.
• UV irradiation time is crucial factor as malondialdehyde can degrade further.

Direct UV detection of saccharides in capillary electrophoresis is possible due to a base-catalyzed conversion into UV absorbing substances initiated by the light of the UV detector lamp. In the present study the compounds formed during this reaction were investigated with capillary electrophoresis using an additional UV lamp for online irradiation at a certain distance before the detector resulting in a separation of the conversion products. It was found that for all investigated saccharides (glucose, ribose and sucrose) the major portion of the UV absorption in direct UV detection resulted from one and the same substance. By CE–UV, CE–MS as well as HPLC–MS measurements this substance was demonstrated to be malondialdehyde, present as its conjugate base malonenolate under alkaline conditions. Additional experiments revealed that the conversion reaction is highly sensitive to the residence time under the UV light as malondialdehyde can further degrade into non-UV absorbing products. NMR measurements showed that under alkaline conditions this degradation proceeds slowly even without UV irradiation.