Polar embedded phases

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. 

Figure 2.24. The concept of orthogonality as shown by retention plots of two sets of columns for a variety of different analytes. (A) Since the log k data of the two columns (C8 and C18) are well correlated for most analytes, these two columns are expected to yield similar elution profiles. (B) The selectivity differences of a C18 and a polar-embedded phase (amide) column lead to very scattered correlation of their respective retention data. Methods using a C18 and a polar-embedded column are therefore termed orthogonal and expected to yield very dissimilar profiles. Diagram courtesy of Supelco, Inc.

Figure 3.16. Chromatogram illustrating the phenomenon of phase collapse for convention C18 phases when used with highly aqueous mobile phases (chromatogram on the left-hand side). Polar-embedded phases are not prone to phase collapse and are better suited for the separation of very water-soluble analytes. Diagram courtesy of Dionex Corporation.

FIPLC columns packed with high-purity, silica-based bonded phases continue to dominate the market. Modern columns yield more symmetric peaks for basic analytes (less silanophilic activity) and have better batch-to-batch reproducibility and longer lifetimes. Improved bonding chemistries have widened the usable pFI range from 2-8 to 1.5-10 or more. Although C8- and CIS-bonded phases remain the most common, other phases have become quite popular, including phenyl, cyano, and several polar-embedded phases (e.g., amide, carbamate). 

Zorbax Bonus is more hydrophobic than Nucleosil Nautilus, and this in turn is more hydrophobic than Prontosil ACE. The most polar embedded phase with a long alkyl chain is Symmetry Shield, with carbamate as the embedded polar group. 

The most polar embedded phases of all are such phases with even shorter alkyl chains, e.g. HyPURITY ADVANCE and SynergiPOLAR RP. 

Conversely, this means that polar/embedded phases are not very selective in this case, ft should be briefly mentioned once more that methanol shows a better selectivity at a longer retention time and frequently a lower efficiency than acetonitrile. On the other hand, if strongly acidic components are dissociated, i.e. ionic, then phases with additional polar properties show a better selectivity. 

Polar groups HyPURITY Polar embedded phase ... 

Those possessing enhanced shape-selectivity (oCjyo) reduced retention (fepg), as typified by polar embedded phases, e.g., Suplex pkj, 100, HyPURITY ADVANCE, Polaris Amide Cjg, BetaMax Acidic (column nos. 100, 46, 76, and 9, respectively) - group C. 

Fig. 6. PC1-PC2 score and loading plots for polar embedded, enhanced polar selectivity and Aqua" phases A = enhanced polar selectivity B = high lipophilic retention C = polar embedded phases...

Fig. 12. Differences in stationary phase seiectivity on the analysis of degraded Ketoprofen using identical chromatographic conditions. Note the change in elution order for peaks marked (A) and (B) on the polar embedded phase - Polaris Amide Cl8.

Use of PCA and column characterization data as an aid to select appropriate stationary phase chemistries and those that are orthogonal in separation mechanism. Phases based on acidic silica would not be suitable for the analysis of basic compounds. In addition, polar embedded phases with residual amino functionalities would not be suitable for analyzing ionized acids. 

All r values are small, which refleds the minor difference between the stationary and mobile phases with respect to this interaction. The r values of the polar embedded phases (symmetry shield) are dearly higher than those of all the other columns, confirming the n-/jr-electron activity of the incorporated carbamate moieties. The nitrile phase also shows a relatively prominent r parameter, which is easily traceable from the electronic structure of the surface groups. Whilst all other parameters of the cyano phase resemble those of the C4 and Cg phases, the r parameter indicates a higher contribution of electron interactions than with any other of the tested columns. Nevertheless, the nitrile phase r parameter is the smallest in its parameter set, whereas its n parameter is 5.5 times the size. It can be concluded that the Zorbax SB 300 CN column mainly behaves as a reversed phase in the given system. 

The dipolarity/polarizability parameter s closes the list of the interaction parameters from Eq. (5). Again, the data show a negative sign, indicating more pronounced dipole or induced-dipole activity in the eluent than in the stationary phase. Principally, phases with higher carbon load show weaker dipolar interactions and thus more strongly negative s parameters. As expected, the polar embedded phases and the cyano phase exhibit smaller values. The difference between Symmetry Shield Cjg and Cg reflects the differences in the steric effects of the alkyl chains. 

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