Simple sugars, with all of their hydroxyl groups, interact poorly with RP columns; they stay dissolved in the water.
That's right. Sugars are hydrophilic/polar substances, which doesn't react that good with RP material, which is hydrophobic! A better choice would be to usw HILIC or NP-HPLC.
Amine columns are commonly used as HILIC columns for carbohydrate analysis.
You can analyse mono, di, tri- oligossacharides to up to 20-25 glucose units in an RP-18 with only water as eluent, with a RI detetor. However they do not have interactions as said above to separate monossacarides.
as said previously by Jorge, the higher the DP (degree of polymerization) is, the better the oligo/polysaccharide will be resolved (because their hydrophobicity increase with the number of carbons). However, it is very difficult to seperate different monosaccharides (or dissacharides) which are eluted in the dead volume or with very short retention times.
Another solution exists : it consists in using cation exchange columns, from Alltech. I used such columns some years ago, and owing to the cation (Ca, K, Pb), it is possible to separate the carbohydrates of a same class (mono, di, tri saccharides respectively). Operating conditions are water as eluent, and column temperature of 90°C.
You can, actually, if you methylate them, although that has problems of its own. As pointed out by the others you would also have to specify what kind of "sugars" you are looking at. Monosaccharides behave very differently from e.g. glycans, consequently the separation methodology will be different.
In addition to the suggested methods, HPAEC (anion exchange) at high pH or GC-MS after derivatization may be employed for monosaccharides.
By HPLC, no, the reducing/non-reducing property of the sugar will not have any influence on their separation
However, methods exist for the quantification of total reducing sugars (DNS method for instance)
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