Cationic surfactants mobile phase additives

For the analysis of aliphatic anionic surfactants by HPLC, other detection systems than UV or fluorescence detection have to be used because of the lack of chromophoric groups. Refractive index detection and conductivity detection provide a solution for this t5rpe of anionic smfactants but their detection limits are rather high and gradient elution is not usually possible. Another possibihty is the application of indirect photometric detection, which is based on the formation of ion pairs between UV-active cationic compoimds, such as N-methylpyridinium chloride, used as mobile phase additives and the anionic surfactants followed by UV detection [60]. Gradient elution with indirect photometric detection is possible in principle but the detection limits increase considerably [61]. 

With respect to chromatographic techniques, cationic surfactant micelles have been used as mobile phase additives in the well-known micellar liquid chromatography (MLC) [4]. They have also been employed as pseudostationary phases in micellar electrokinetic capillary chromatography (MEKC) [5]. 

One of the main problems to be solved in the analysis of cationic surfactants is the strong adsorption of the surfactant to glassware, tubing and apparatus. To avoid losses, the solvent system used should contain a substantial percentage of organic solvent. Additionally, mobile phases containing more than 20-25% methanol will help to inhibit micelle formation [46]. 

Alternatively, retention can be predicted from solute log Po/w values. The most suitable organic solvent to be used as modifier of the mobile phase should be chosen according to the polarity of the eluted compound. For SDS, a low propanol content ( 1%, v/v) is useful to separate compounds with logPo/w< — 1> such as amino acids. A larger amount of propanol ( 5-7%) is needed for compounds in the range -1 < log Po/w < 2, such as diuretics and sulfonamides. Other alcohols (<10% butanol or <6% pentanol) are required for apolar compounds with log Po/w >3, such as steroids. This rule of thumb is, however, not always valid propanol is too weak for cationic solutes, such as phenethylamines (0 < log Po/w < 1-7) or j -blockers (1 electrostatic attraction to the anionic surfactant molecules adsorbed on the stationary phase makes a stronger solvent necessary. 

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. 

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