Charged Bonded Phases

Amino-bonded silica phases were among the first bonded stationary phases to be used for the separation of carbohydrates [27,38], amino acids, proteins [39], and some antibiotics [40] under HILIC conditions. 

Their advantage for carbohydrate separations over underivatized silica is an increased rate of mutarotation, which prevents the formation of double peaks 

Brand name Manufacturer Particle size (pm) Pore size (A) Surface area (m /g) 

Hypersil Gold Amino Thermo Fisher Scientific 1.9, 3, 5 175 220 

However, there are a number of challenges with amino-bonded silica columns. Amino propyl-bonded silica is more reactive than other HILIC materials that results in irreversible adsorption problems, particularly for acidic analytes [41]. They also exhibit significant bleed and extended equilibration times with certain buffer t)q)es. Valette et al. [40], for instance, observed that it takes several hundred column volumes to establish a stable baseline when changing from a citrate buffer to an acetate buffer. According to Olsen [41], care must be taken when selecting an amino-bonded silica column for HILIC applications, because the various column brands show large differences in performance. 

---

The use of bonded, silica column supports has also become a useful way to characterize cationic, water-soluble polymers. CATSEC SEC columns from Micra Scientific contain a silica support with a polymerized polyamine-bonded phase. This imparts a cationic surface charge on the packing that can be... 

The suspension of influenza virus, strain A/X-53, was separated from the constituents of alantoic fluid by preparative size exclusion chromatography on a 10 x 120 cm column. The hemo-agglutination method revealed an elevated level (93%) of viral activity. The leakage of the bonded phases can be more efficiently minimized, however, with the use of positively charged polymers. 

The column should permit the modulation of retention behavior over a very wide range of conditions. This requirement in fact means that the stationary phase is inert, that is it does not facilitate specific interactions with certain molecular functions of solute molecules with the concomitant advantage of a relatively clean and rapid adsorption-desorption kinetics. Preferably then the stationary phase has no functional groups such as fixed charges that would have strong affinity to counterionic solutes and exclude solutes of co-ionic nature. In this regard the properties of well-prepared hydrocarbonaceous bonded phases indeed approach those that we would expect from an ideal phase. 

Dispersive Interactions are more difficult to describe. Although electric In nature, they result from charge fluctuations rather than permanent electric charges on the molecule. Examples of purely dispersive interactions are the molecular forces that exist between hydrocarbon molecules. n-Heptane is not a gas due to the collective effect of all the dispersive interactions that hold the molecules together as a liquid. To retain solutes selectively, solely on the basis of dispersive interactions, the stationary phase must not contain polar or ionic substances but only hydrocarbon-type materials such as the reverse -bonded phases now so popular in LC. It follows that to allow dispersive selectivity to dominate in the stationary phase, the mobile phase... 

Carboxylic acid (CBA) Si-O-Si-C -CH2CH2COOH Polar bonded phase Medium-polarity phase that has a weak cation exchange capability useful for strong cations above pH of 4.8, most of the functional groups are negatively charged, and therefore the phase can be used for cationic compounds lowering pH to 2.8 elutes retained analytes... 

In a study presented by Jinno et al. [124], packed column capillary electrochromatography, open-tubular CEC, and microcolumn liquid chromatography using a cholesteryl silica bonded phase have been studied to compare the retention behavior for benzodiazepines. The results indicated that CEC was a promising method, as it yielded better resolution and faster analysis than microcolumn LC for benzodiazepines. Similar selectivity to HPLC was noted, except for a few solutes that were charged under the separation conditions. Columns packed with the ODS and cholesteryl phases were compared and showed totally different migration orders of the analytes. The retention on the cholesteryl silica sta-... 

In the contrast to the irreversible adsorption of amphiphilic ions on the reversed-phase surface, the liophiUc ions shows relatively weak interactions with the alkyl chains of the bonded phase. Liophilic means oil-loving. These liophilic ions are usually small inorganic ions and they possess an important ability for dispersive type interactions. They are (a) characterized by significant delocalization of the charge, (b) primarily symmetrical, (c) usually spherical in shape, and (d) absence in surfactant properties. 

---