The Organic Modifier Effect

The main difficulties in CE analysis of cationic surfactants arise from their strong adsorption to the capillary wall and their ability to form micelles at low concentrations. The addition of organic modifiers in high amounts or separation in absolutely non-aqueous media disrupt micelle formation within the sample and also effectiveness of the organic modifier to disrupt micelles of alkylbenzyl dimethyl ammonium... 

Therefore, it is not further surprising that the organic modifier type and content is an effective variable to adjust retention and to optimize enantioselectivities. Methanol and acetonitrile have been frequently found to be complementary in their selectivity profiles and these two organic modifiers are advised to be tested in a preliminary screening experiment. 

Gritti, F. and Guiochon, G., Adsorption mechanism in RFLC. Effect of the nature of the organic modifier, AnaZ. Chem., 11, 4257, 2005. 

Secondly, the pH can have an effect on the selectivity. In pharmaceutical analysis, mainly compounds with either basic or acidic properties have to be analyzed. Since the pH influences the dissociation and charge of these substances, it will affect their retention behavior. Changing the organic modifier does not lead to major selectivity differences, but it can enhance the differences induced by pH and/or stationary phase changes. Also, the column... 

In sub-FC, a detailed study of the influence of mobile phase additives on the chiral resolution of isoxazoline-based Ilb/IIIb receptor antagonists was carried out by Blackwell [145] on Chiralcel OD-H CSPs. The different mobile phase additives used were acetic acid, trifluoroacetic acid, formic acid, water, triethylamine, triethanolamine, n-hexylamine, trimethyl phosphate, and tri-w-butyl phosphate. In general, n-hexylamine and tri-/ -butyl phosphate mobile phase additives resulted in better resolution. The chiral separation of four 1,3-dioxolane derivatives on an amylose-based column has been described [151]. The effects of mobile phase composition, temperature, and pressure have been investigated. The nature of the modifier is the main parameter it has the highest impact on chiral resolution and is more important than the polarity of the mobile phase. Therefore, the organic modifier that gave the best enantiomeric separation was different for each compound. 

FIGURE 7 Effect of the organic modifiers on the chiral resolution of aromatic alcohol using Whelk-Ol CSP at different temperatures with negative values of In k indicating the reversal order of elution ( ) ethanol, ( ) 1-butanol, (A) 2-propanol, ( ) 1-propanol, (+ ) trifluoroethanol, and (x) tetrahydrofuran. (From Ref. 138.)... 

EFFECT OF METHANOL IN THE ELUENT ON RETENTION. The effect of increasing the volume of the organic modifier, methanol, in the mobile phase on the retention of methyl anthraquinone and naphthalene is given in Figure i(. Methyl anthraquinone and napthalene are more soluble in methanol than in water so increasing the volume of methanol in the mobile phase should result in the increase in the solubility of both compounds, and as a result, a decrease in the retention time. Also, the presence of methanol in the mobile phase affects retention when cyclodextrin bonded columns are used. Methanol is much more tightly bound in the cyclodextrin cavity than... 

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