Hydrophilic endcapping

Octadecylsilyl stationary phases with hydrophilic endcapping have been developed for the separation of very polar analytes, which are not sufficiently retained on conventional reversed-phase columns. Among numerous other applications, they have been demonstrated to be suitable for the separation of flavonol and xanthone glycosides from mango Mangifera indica, Anacardiaceae) peels. °... 

The number and nature of unreacted surface silanols affects the character of a stationary phase. Initially free, geminol or associated silanols are minimized through a process known as endcapping, which bonds various species to the residual silanols. Hydrophilic endcaps or bulky steric endcaps that separate the hydrocarbon chains and prevent analyte interaction with the silica surface can be used. If residual silanols are left unreacted (and some always are), the analyte will be separated based on a combination of interactions with both the reverse-phase support and the highly polar silanol groups. Increased retention, changes in elution order, and tailing will result for basic compounds. 

AQ, AQUA Aqueous phases with hydrophilic endcapping for the separation of polar, organic analytes with hydrophilic eluent or pure water (polar RP phases). 

The current development in reversed-phase chromatography goes in the direction of sterically or chemically protected phases (shield), phases with hydrophilic endcapping (AQ, EPS) and hybrid phases (XTerra). 

A good example of the capability of MLC, in the separation of mixtures of drugs, is given by the screening of twelve sulfonamides in human urine and cow milk with an SDS-2-propanol mobile phase and a hydrophilic endcapped C18 column (Fig. 11.9) [30]. Using a 0.07 M SDS-6% 1-propanol eluent at pH 3.0, isocratic separation of the sulfonamides was achieved within 15 min. 

One uses six completely different columns, e.g., one of moderate hydro-phobicity but elevated polarity (e.g., SynergiFUSION RP), a monolith (e.g., Chromolith Performance), one with a long alkyl chain and hydrophilic endcapping (e.g., Develosil), one with incorporated ionic groups (Primesep A), and two high-speed columns (20-30 mm, 2-3 mm, 1.5-2 pm). 

Hydrophilic endcapping ( AQ , AQUA ) imparts a phase with only slight polar character compared to the analogous phase without hydrophilic endcapping. Such phases represent the most polar types of hydrophobic phases. 

If it is known, for example, that if the sample consists of small organic molecules, polar phases can easily be waived. On the other hand, if it is known that the sample contains components of similar polarity but different structure, classical, hydrophobic RP materials will be less useful. In this case, it is better to test a non-endcapped or any polar RP-Cjg-phase, considering several types of polar phases, e.g. Develosil (Cjq chain plus hydrophilic endcapping). If the sample contains... 

Strong polar surface SynergiPOLAR RP Short alkyl chain, embedded polar group, polar terminal phenyl group, hydrophilic endcapping... 

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]. 

---