Packings with embedded polar

This hydrogen bonding capacity expressed by the polar selectivity is not limited to an increase in retention for phenols. Packings with embedded polar groups that excel in this feature also exhibit increased retention for analytes with other functional groups. Specifically, carboxylic acids at acidic pH and with acetonitrile as the organic modifier and compounds with sulfonamide functions at acidic and neutral pH exhibit significant... 

Fully endcapped Cg and Cig phases can exhibit hydrophobic collapse in eluents that contain close to 100% water. Polar functional groups on the packing prevent this phenomenon. Thus nonendcapped packings or packings with embedded polar fimctional groups are prefer for very polar analytes that require moWe phases with no or very little organic modifiers. 

We have drawn a horizontal line in Fig. 1 to differentiate between classical bonded phases, which can all be foimd above the line, and phases with embedded polar groups, which lie below the line. Other phases with very low silanol activity can also be found below the line. An example is ODPerfect, 113, which is not based on silica. Beyond the limits of the chart shown here, one finds packings with exceptionally high silanol activities, such as the classical Zorbax Cjg or Resolve Cjg, both of which lack end-capping and are based on older silicas. 

Hypersil Elite C13, 89 or SymmetryShield RPig, 70. However, if secondary interactions play a role, different selectivities will be found. This is especially true for packings with an embedded polar group. 

Moreover, the effect of pH on a particular compound s retention needs to be determined first before exploring the retentivity and selectivity of different columns. The strategy and choice of the optimal pH for analysis was discussed in Chapter 4 and is further reinforced in the case studies within this chapter. After the optimal pH is chosen for the separation and the gradient has been optimized on a particular column and the optimal selectivity still has not been achieved between critical pairs, then a column screening can be performed. For method column screening, generally columns with 10-cm or 5-cm X 3.0-mm i.d. could be used that are packed with 3-pm particles. Implementation of a column switcher that can use six different types of stationary phases such as two types of C18 from different vendors, phenyl, two polar embedded, and pentafluorphenyl is suggested. 

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). 

The development of reversed-phase chromatographic media has intensified during the last decade. Now, many manufacturers pride themselves on their high degree of the batch to batch reproducibility, both with respect to the retention properties of the stationary phase and also in regards to the quality of the packed bed. Recent years have seen the development of a variety of polar embedded and polar endcapped reveresed-phase materials. There is in reality no limitation placed on the type of reversed-phase material that could be used for reversed-phase applications, other than commercial availability. Therefore expansion of the available reversed-phase media that enter the market place continues, as chromatographers continually seek new ways to gain selectivity. 

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