Pure aqueous mobile phases

Two retention indices are then measured, log kgs (log k obtained with a mobile phase containing 95% of acetonitrile and 5% of buffer solution) and log ko (retention factor obtained with a pure aqueous mobile phase). The difference between these two values (log ko 9s) is correlated to the 1-octanol/water partition coefficient of the neutral form of tested compounds (Equation 5.13) ... 

Adsorption isotherms. Pure aqueous mobile phases The surfactant... 

Retention study. At surfactant concentrations below CMC, micelles do not exist and, as demonstrated by Knox (12), Deming (13) and our previous works (14-15), the degree of retention was directly related to the surface charge arising from the adsorbed surfactant With both the surfactants, the retention of neutral species (toluene and caffeine) slightly decreased. When an anionic surfactant was adsorbed, the retention of negatively charged solutes (benzoate and SOBS) fell dramatically whereas the retention of cationic solutes (BTAB and CPC) increased. The reverse occured with the cationic surfactant (14). The same kind of behavior was observed with pure aqueous mobile phases, 5-95% v/v methanol-water phases and 0.1 mol/L NaCl phases. 

Pure aqueous mobile phases are only suitable for separations on weakly hydro-phobic stationary phases hence materials containing one tenth to one hundredth of the carbon load of classical reversed phases have been developed. This is achieved by low coverages of short-chain groups such as butyl or phenyl. Proteins can then be retained when the eluent has a relatively high salt content (e.g. 1 M or more) and eluted when the salt content drops. This mild method of protein separation, which is a variant of reversed-phase chromatography, is known as hydrophobic interaction chromatography (HIC Figure 10.16). 

Applications. CART is not generally established yet, and as a consequence, not many applications for electrophoretic or similar data in the pharmaceutical held are found. Put et al. (52) apphed CART in a quantitative structure-retention relationship context on a retention data set of 83 structurally diverse drugs, in order to predict chromatographic retention. There were 266 molecular descriptors calculated and used as explanatory variables (X matrix). The considered response (y) was the retention factor of the compounds, predicted for a pure aqueous mobile phase. The total sum of squares of the response values about the mean of the node was applied as impurity measure. From all descriptors, three were selected to describe and predict the retention, and four terminal nodes were obtained (Fig. 13.11b). Arbitrarily, the drugs were then divided into hve retention classes. Each terminal node was then labeled with either one or two class names. The regression tree thus becomes a classihcation tree. From CV, it was concluded that only 9% serious misclassihcations were observed. 

Recently, there has been an increase in the number of commercially available phases which the manufacturers claim to be suitable for chromatography using purely aqueous mobile phases. These have been further classified, by the manufacturers, as polar embedded, enhanced polar selectivity, and Aqua phases. Some of the phases in the latter two classifications have been further described by the manufacturers as polar/hydrophilic endcapped phases, depending on the bonding technology employed. Unfortunately, there is a dearth of information regarding the bonding chemistry employed [2]. 

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