Separation Mechanism and Control of Selectivity

The basic mechanism of separation of carbohydrates is by ligand exchange chromatography but is quite similar to ion-exclusion chromatography described earlier in this chapter for weak organic and inorganic acids. The column contains fully sulfonated polystyrene polymer beads cross-linked with polydivinylbenzene. The polymers are fully hydrated and contain occluded water within the gel polymer matrix, just as in ion-exclusion polymer beads. Analytes partition between the occluded water within the bead matrix and the mobile phase. Water is most often used as the mobile phase and the detection method is most often refractive index. 

Tabic 8.5. Comparison of metal ionic form and cross-linking. Data courtesy of Transgenomic. 

Figwe 8.9. Wine analysis by high re.solution ion exclusion. Analysis Conditions Column Transgenomic ION-300 Eluent 0.005 N H2SO4 Flow rate 0.3 mL/min Temperature 60 °C Detection DrI Sample 1. citric acid. 2. tartaric acid, 3. glucose. 4. malic acid, 5. fructo.se.6. acetic acid. 7. glycerol, 8. methanol. 9. ethanol. 

In chromatography it is always necessary to adjust conditions so that retention times of the analytes fall within a convenient time period. This is usually accom-pHshed by adjusting the ionic concentration of the mobile phase, but water is commonly used as the mobile phase for carbohydrate separations, so other means 

The calcium-form columns are the most popular and are the column of choice for most separations. Some separations require resin to be in different metal ionic forms. Arabinose and mannose, for example, are quite difficult to separate except with a lead-form column. Lead columns are used for mono-, di- and tetra-sacchar-ides and for sugar alcohols. 

Hydrogen-form columns are also useful when mixtures of sugars, acids and alcohols are to be separated. Ion exclusion or ligand exchange are both appropriate names for this type of chromatography because both separation mechanisms apply. Because the column is in the hydrogen form, it is possible to use an eluent 

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