Phases with embedded polar

An IPC-ESI-MS/MS method allowed the simultaneous determination of neomycin and bacitracin in human and rabbit sera [79] and the analysis of aminoglycoside antibiotics in human plasma [80]. IPC was recently validated for the estimation of bulk and formulated gatifloxacin [81]. The IPC determination of norfloxacin in diverse matrices worked as a stability indicating method [82], A CI2 stationary phase with embedded polar group successfully achieved IPC baseline tetracycline separation simply by using a phosphate as the IPR [83], A practical IPC method for the quality control of fosfomycin calcium and its related substances was recently suggested [84],... 

Phases with embedded polar groups SymmetryShield RP ... 

For the chromatography of basic compounds on nonpolar stationary phases (classical reversed phase), use preferably aprotic solvents (e.g., acetonitrile) as the mobile phase and on polar stationary phases (reversed phase with embedded polar groups) preferably protic solvents (e.g., methanol). 

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. 

In a first follow-up publication [18], general investigations into this subject are described. In three later publications, 87 stationary phases based on ultra-pure (type B) silica were studied with regard to these correlations [19], then a further 87 phases based on a metal-rich (type A) silica [20], and finally 21 reversed phases with embedded polar groups or polar endcapping [21]. 

If a sorbent has the potential to interact with analytes through more than one retention mechanism, it should be possible to adjust the chromatographic conditions in order to strengthen just one of the interactions and thereby obtain different selectivity between the analytes. This is illustrated here for the case of the elution of vitamin A metabolites 1-5 from a stationary phase with embedded polar groups at different mobile phase compositions (see Fig. 2). 

Fig. 2. A graph showing the Ig k w. water content relationship when the analytes 1-5 were eluted from a phase with embedded polar groups using different concentrations of acetonitrile in the mobile phase.

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. 

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. 

Liu, X.D., Bordunov, A., Tracy, M., Slingsby, R., Avdalovic, N., and Pohl, C., Development of a polar-embedded stationary-phase with unique properties, J. Chromatogr. A, 1119, 120, 2006. 

The top two stationary phases in Figure 12 are the C g and Cg phases, which are the most frequently used phases in reversed-phase LC. Below that, three phases with an embedded polar group are shown the carbamate phases (e.g., SymmetryShield), amide phases (e.g.. Discovery RPAmide C g) and urea phases (such as Prism or Spectrum). It should... 

Wilson, N. S., Gilroy, J. J., Dolan, J. W. and Snyder, L. R., Column Selectivity in Reversed-phase Liquid Chromatography VI. Columns with Embedded or End-capped Polar Groups,/. Chromatogr. A, 1026 91—100,2004. 

FIGURE 2.8 Hydrophobic retention and selectivity with RP columns. The stationary phases are ordered according to the increasing retention of toluene in methanol-water 50-50 v-v. Dashed line Stationary phases with a silica pore diameter below lOnm. Solid line Stationary phases with a silica pore diameter >12nm. ( ) Stationary phases with polar-embedded functional groups. (( ) Stationary phase based on a wide pore silica (30 nm)). 

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. 

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