Hydrophobicity and Silanol Activity Ion Exchange

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. 

These comments demonstrate that the classification of the stationary phases is in line with expectation and is consistent with practical experience. The reproducibility of the values surely depends on the phase and the manufacturer. Therefore, one should expect some variation of the values given here. For the retention factor of a hydrophobic probe, one should expect today a relative standard deviation of around 5%. Kele [16] has demonstrated that the relative standard deviation of the batch-to-batch reproducibility achieved with Symmetry Cjg is as low as 1.3%. The silanol activity can have a standard deviation as high as 15% in 

One can see that the relative positions of different phases might change, especially for phases with a lower level of reproducibility. Therefore, one should be careful in interpreting the relative positions of the different phases on this chart 

In addition, it is important to understand that the values shown have only an absolute meaning in the context of the mobile phase composition of the test. In other mobile phases with different percentages of organic solvent or even different solvents, the positions of the different phases relative to each other will change. 

The fundamental reason for this phenomenon is the fact that the selectivity of a separation arises from a combination of the influence of the stationary phase and the influence of the mobile phase. If the composition of the mobile phase is drastically different from the test conditions, one can expect a different position of the different columns relative to each other. It remains correct that Symmetry Cj8 has a lower silanol activity than Spherisorb ODS-2, but whether a Lima Cjg(2) or a YMC-Pack Pro Cjg has a higher hydrophobicity or a lower silanol activity surely depends on the details of the measurement conditions. 

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