Bonded stationary phases polar-embedded

The polar groups are, on the other hand, responsible for an induced polar selectivity. Analytes able to form hydrogen bonds like phenols are retarded more strongly with polar-embedded stationary phases than with the corresponding classical RP of an identical carbon content. This is demonstrated in Figure 2.5 for the separation of polyphenolic compounds present in red wine. The retention time of the polyphenolic compound kaempferol with the shielded phase is more than three times longer than with the corresponding RP column of an identical carbon content. The polar... 

Since the 1970s numerous HPLC methods using lEC, RP and ion-pair chromatography have been proposed. In the last years, RP chromatography has become the most used method, thanks to its simplicity, sensitivity, and compatibility with different detection techniques. The stationary phases usually used are C18 or phenyl-bonded silica-based phases. More recently, alternative stationary phases, such as polar-embedded, polar endcapped, and perfluorinated phases, have been successfully tested for folate analysis [577]. The mobile phase is usually a mixture of phosphate or acetate buffer and acetonitrile or methanol. 

Use of alkylamide phases, in which alkyl chains are attached to the silica surface via an alkylamide group, reduces interactions with free silanols, by an internal masking mechanism. Residual silanols interact by hydrogen bonding with the embedded amide groups and thus become less active toward analytes. The embedded polar amide groups lessen the hydrophobicity of these phases compared to that of C18 bonded phases prepared from the same silica. Improved peak shapes of ionizable compounds were reported and this stationary phase was successfully used under IPC conditions to analyze streptomycin and its dihydrostreptomycin derivative in food... 

The separation of chiral compounds will be discussed in Chapter 22. However, the separation of diastereomers can be accomplished using achiral stationary phases. Another alternative is the use of chiral columns for the separation of diastereomers in either the reversed-phase or normal-phase mode. The use of achiral bonded phases without chiral additives, such as phenyl and alkyl bonded phases for the separation of diastereomeric pharmaceutical compounds, is acceptable. Different selectivities can be obtained by employing stationary phases containing varying functionalities (phenyl, polar embedded moieties). The effect of aqueous mobile-phase pH, temperature, and type of organic eluent (acetonitrile versus methanol) can also play a dramatic role on the separation selectivity of diastereomeric compounds. 

In this case study, two different Cl 8 columns from different manufacturers were used. Alternatively, other stationary phase types could also be used such as a polar embedded phase and a Cl 8 phase. Some systems come also equipped with a six-column switcher and in that case, two different types of polar embedded phases, phenyl phase, pentafluorophenyl phases, two different Cl 8 phases (of different bonding density) and an alternate C8 phase could be used. 

Chiral WCOT columns suitable for the enantiomeric separation of small molecules such as esters, ketones, alkanes and alcohols have recently been introduced. One example of a chiral column has a stationary phase film consisting of a non-bonded mid-polar, 35% phenyl 65% methyl siloxane modified by embedding in the film permethylated a- or /3-cyclodextrin. Elution characteristics are modified by the cyclodextrin content DEX 110 contains 10% cyclodextrin and DEX 120, 20% [127, 128]. 

To better distinguish the contributions of polar interactions to retention, the LEER model was transformed into the so-called hydrophobic subtraction model (HSM) for RPLC, where the hydrophobic contribution to retention is compensated for by relating the solute retention to a standard nonpolar reference compound. This approach was applied to characterize more than 300 stationary phases for RPLC, including silica gel supports with bonded alkyl-, cyanopropyl-, phenylalkyl-, and fluoro-substituted stationary phases and columns with embedded or end-capping polar groups. The QSRR models can be used to characterize and compare the suitabihty of columns not only for reversed-phase, but also for NP and HILIC systems. 

In addition, on the same score plot (Fig. 6), we can observe how some of the tested supports (ABZ, DIS, NAU, ABZ) showed better batch or column reproducibility in comparison to others (AB). These results clearly demonstrate that new generation stationary phases (embedded polar group and bidentate bonding supports) are well adapted to the analysis of basic compounds, even imder these critical chromatographic conditions. 

Manufacturers have also provided a solution for the poor peak shapes that result from secondary interactions by providing silica-based stationary phases with restricted access to any residual silanols (e.g., endcapped, bidentate, hybrid silica, high-density bonding, and embedded polar group stationary phase). In addition, the performance can be improved by raising the temperature of the mobile phase. Because the mobile phase viscosity is reduced at elevated temperatures, analyte diffusion is enhanced, and the kinetic and mass transfer rates are improved, which strongly... 

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