To establish a new method to determine aldoses and ketoses simultaneously; the, monosaccharides were extracted with 80% ethanol after sonication for 15 min and this process was repeated three times. The extract was evaporated under vacuum Suger oximes were formed using hydroxylarnine and pyridine under heating at 90 degrees C for 30 min. After-reaction, the sample-was divided into two parts. One part was added, by pyridine and acetic anhydride for derivatization at 90 degrees C for 30 min, the other part was added by hexamethyldisiazane (HMDS) and trimethylchlorosilane (TMCS) for silylation reaction at 80 degrees C for I h. The two parts were analyzed by gas chromatography/mass spectrometry ( GC-MS) in selected ion monitoring ( SIM) mode, and inositol was used as internal standard. Results indicate that the linearity is superior in 1 similar to 4 mg/L, the detection limit of aldoses is 8. 15 similar to 22. 39 mu g/L, whereas ketoses from 2. 32 mu g/L to 3. 47 mu g/L. The average recovery rate is from 73. 0 % to 95. 7 % and the RSDs were 3. 1 % similar to 10. 0%. The free monosaccharides in Lyceum barbarum L. was determined by this method with the results of 0. 26 similar to 368. 6 mg/g.
