Organic modifier effect phase

T. Hanai, Quantitative in silica analysis of organic modifier effect on retention in reversed-phase liquid chromatography,/. Chramatagr. Set, 2014, 52, 75-80. 

Some advice can be formulated for the choice of organic modifier, (i) Acetonitrile as an aprotic solvent cannot interact with residual silanols, whereas the protic methanol can. Thus, when measuring retention factors, methanol is the cosolvent of choice, as it reduces the secondary interactions between the solutes and the free silanol groups, (ii) For the study of the performance of new stationary phases one should use acetonitrile, as the effects of free silanol groups are fuUy expressed [35]. (iri) Acetonitrile with its better elution capacity can be considered as the best organic modifier for Hpophilicity measurements of highly Hpophihc compounds with adequate stationary phases [36]. 

Fig. 25. Effect of percentage of acetonitrile (A) and methanol (B) on electroosmotic mobility in a packed column. (Reprinted with permission from [56]. Copyright 1997 Elsevier). Conditions capillary column 100 pm i. d., total length 33.5 cm, active length 25 cm packed with 3 pm CEC Hypersil C18, mobile phase organic modifier-water+4% 25 mmol/1 TRIS pH = 8, voltage 30 kV, temperature 20 °C, marker thiourea... 

Anions and uncharged analytes tend to spend more time in the buffered solution and as a result their movement relates to this. While these are useful generalizations, various factors contribute to the migration order of the analytes. These include the anionic or cationic nature of the surfactant, the influence of electroendosmosis, the properties of the buffer, the contributions of electrostatic versus hydrophobic interactions and the electrophoretic mobility of the native analyte. In addition, organic modifiers, e.g. methanol, acetonitrile and tetrahydrofuran are used to enhance separations and these increase the affinity of the more hydrophobic analytes for the liquid rather than the micellar phase. The effect of chirality of the analyte on its interaction with the micelles is utilized to separate enantiomers that either are already present in a sample or have been chemically produced. Such pre-capillary derivatization has been used to produce chiral amino acids for capillary electrophoresis. An alternative approach to chiral separations is the incorporation of additives such as cyclodextrins in the buffer solution. 

As the pH of the mobile phase markedly influences the retention of ionizable compounds, it can be assumed that the separation capacity of RP-HPLC for ionizable analyses can be modified by changing the pH of the mobile phase. The theory of effect of pH gradient on the performance of RP-HPLC systems has been recently elaborated. The basic equation describing the dependence of the retention of the solute on the gradient of pH or organic modifier is ... 

Figure 4.7 Effect of high organic modifier concentration on peptide retention. Data obtained using an octadecyl column and 20 mM ammonium acetate acetonitrile mobile phases. (Reproduced from C.T. Wehr and L. Correia, LC at Work LC-121, Varian. With permission from Varian Associates.)...

A very minor effect on shape selectivity has been observed for changes in mobile phase composition [109]. Shape selectivity increased slightly with an increase in percent organic modifier for water-organic mobile phase systems and increased in order of methanol acetonitrile < ethanol. Changes in the shape selectivity factor... 

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