Retention factor silanophilic

If NaCl is replaced by the butyl-3-methyl imidazolium (BMIM) chloride IL, a 30% decrease in retention factor associated with a remarkable peak shape improvement is observed. In this case, the IL cation adsorbs on the C18 stationary phase more than Cl", thereby preventing detrimental attractive silanophilic interaction of the cationic additive. Charge-charge repulsion occurs, the retention factor is lower, and the peak shape is better. The analyte cation is largely retained by hydrophobic fast interactions. When BMIM BF4 IL replaces NaCl, both the cation and anion of the IL adsorb on the C18 surface and all the interactions cited above take place simultaneously and contradict each other. Global retention depends on the extent to which one interaction is stronger than the other [124],... 

In a lot of tests in the literature for the characterization of RP-phases, retention factors of often ideal analytes are used to rank the phases according their hydrophobic character. Irrespective of the analytes used, one should consider the principle question as to whether retention factors are a suitable criterion for comparison at all. The retention behavior (i.e., a measure of the hydrophobicity) is, from the practical point of view, only of secondary interest. In the case of comparison tests, the aim is to identify columns that separate any compotmds similarly well, and a measure of the separability is the selectivity. Besides the retention factor, also the third parameter, the plate number, seems to be less suitable. It depends on so many factors that one needs a very large-scale standardization procedure to relate detected differences in plate numbers exclusively to differences in the quality of the packing procedure of several columns. One should only consider the simple case that the next column is better/worse packed. It is quite labor-intensive to obtain statistically relevant results in this context. Also in the case of the declaration of asymmetry factors, a correlation is generally assumed between asymmetry and silanophilicity. This undoubtedly exists, but not exclusively. Also, if a comparison of columns is made correctly, of what use is the information that two columns are similarly well-packed and have similar silanophilic character in relation to two standard bases as model compounds ... 

Multivariate linear regression can be carried out with common statistics software and was performed in the given example with Statistica 5.0 (StatSoft). Retention factors were measured for a set of selected test solutes using a set of different columns under one fixed set of eluting conditions (acetonitrile/water, 3 7, vjv, without addition of a buffer). A buffer was avoided, since Engelhardt and coworkers reported a marked modulation of column properties by salt or buffer additives this adulterates phase characterization and makes columns look less silanophilic than they actually are [15]. 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

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