Phases composition on retention

A study of the effects of mobile phase composition on retention and selectivity of some carboxylic acids and amino acids was performed on a commercially available teicoplanin CSP, under analytical conditions, on the profile of the adsorption isotherms of the enantiomers and on the overloaded separation [87]. 

For all these reasons, it will be understandable that LLC systems have been virtually replaced by chemically bonded phases (section 3.2.2) in current LC practice. Consequently, the various parameters of interest for the optimization of these systems will not be discussed extensively. With regard to the influence of temperature and mobile phase composition on retention and selectivity, it is suggested that the same relationships may be used for insoluble LLC stationary phases as are used for LBPC. LLC systems have been used extensively for the separation of ionic compounds by means of ion-pairing techniques. Such systems will be discussed in section 3.3.2. 

A simple but useful equation to express the mutual effects of temperature and mobile phase composition on retention has been described by Melander et al. [338] ... 

Both the solvent-interaction model (Scott and Kucera, 1979) and the solvent-competition model (Snyder, 1968, 1983) have been used to describe the effects of mobile-phase composition on retention in normal-phase liquid chromatography. The solvent interaction model on the one hand provides a convenient mathematical model for describing the relationship between retention and mobile phase composition. The solvent competition model on the other hand provides a more complete, quantitative description of the relative strengths of adsorbents and organic solvents used in normal-phase chromatography. 

Tesarova and Bosakova [58] proposed an HPLC method for the enantio-selective separation of some phenothiazine and benzodiazepine derivatives on six different chiral stationary phases (CSPs). These selected CSPs, with respect to the structure of the separated compounds, were either based on b-CD chiral selectors (underivatized (J>-CD and hydroxypropyl ether (3-CD) or on macrocyclic antibiotics (vancomycin, teicoplanin, teicoplanin aglycon and ristocetin A). Measurements were carried out in a reversed-phase separation mode. The influence of mobile phase composition on retention and enantio-selective separation was studied. Enantioselective separation of phenothiazine derivatives, including levopromazine (LPZ), promethazine and thioridazine, was relatively difficult to achieve, but it was at least partly successful with both types of CSPs used in this work (CD-based and glycopeptide-based CSP), except for levomepromazine for which only the [CCD-based CSP was suitable. 

The results indicate that there is little effect of mobile phase composition on the retention volume of the solutes employed, or the silica dispersion. It should be pointed out, however, that there are no values Included for methanol between 0% and 10% methanol where adsorption of the methanol on the reverse phase surface significantly changes the value of Its retention volume. 

To determine the effect of mobile-phase composition on the sorption behavior of TGs on reverse-phase columns, two mixtures were employed acetonitrile/ethanol (80 20) and aceto-nitrile/methanol (80 20). A very rapid analysis resulted, with excellent peak shape and adequate resolution, when ethanol was used as the secondary solvent. Substituting an equal amount of methanol for ethanol resulted in increased solute retention, poor detector response, and asymmetrical peaks. Methanol forms a monomolecular layer on octadecyl-derived silica, which may explain the increase in solute retention caused by methanol. Also, the use of methanol would... 

Roses, M. et al. Retention of ionizable compounds on HPLC 2 effect of pH, ionic strength, and mobile phase composition on the retention of weak acids. Anal Chem. 

Exact quantitative description of the effects of the mobile phase composition on the retention in normal-phase ternary and more complex mobile phases is not straight-... 

Doubling the volume fraction of one phase doubles the probability of solute interaction and, consequently, doubles its contribution to retention. There is another interesting outcome from the results of Purnell and his co-workers. Where a linear relationship existed between the retention volume and the volume fraction of the stationary phase, the linear functions of the distribution coefficients could be summed directly, but their logarithms could not. In many classical thermodynamic descriptions of the effect of the stationary-phase composition on solute retention, the stationary-phase composition is often taken into account by including an extra term in the expression for the standard free energy of distribution. The results of Purnell indicate that this is not acceptable, as the solute retention or distribution coefficient is linearly not exponentially related to the stationary-phase composition. The stationary phases of intermediate polarities can easily be constructed from... 

Gradient elution chromatography is a separation method that exploits the effect of the fluid phase composition on the retention behavior of the feed components. It is widely used, especially for analytical separations in the areas of the life sciences, in biochemistry, and in the biotechnologies e.g., separation of complex mixtures of proteins or peptides), hi its conventional implementations, SMB units are operated under isocratic conditions. The composition of the fluid phase, e.g., the organic modifier concentration, the pH, or the buffer concentration remain constant in all the sections of the SMB unit. However, it has recently been shown that SMB units can also be operated under solvent gradient mode (SG-SMB). Then, the feed and desorbent streams introduced have a different composition. The fluid phase composition is different in each section. It is chosen independently, in order to... 

It is widely recognized that sensitivity in either ESI or APCI is improved as the percentage of organic modifier is increased, due to the improved facility for desolvation. Adequate reverse-phase retention (/< = 1 to 5) is therefore important when LC-MS is performed. Temesi and Law studied the effect of LC mobile-phase composition on ESI response for a series of 35 compounds [18]. In their investigation, the signal response for positive-ion ESI was on average 10-20% higher in MeOH than ACN. 

Mobile phase composition on every RP column can be changed between 5% and 100% and allows changes in retention by a factor of 5000-10000. It is thus so much more effective than temperature that both can hardly be compared. 

Crego, A.L. Martinez, J. Marina, M.L. Influence of mobile phase composition on electroosmotic flow velocity, solute retention and column efficiency in open-tubular... 

Poly(methyl methacrylate) and polytetrahydrofuran polymers were studied at the critical point of adsorption. This critical point of adsorption occurs where the retention of a given polymer is governed strictly hy the number and types of functional groups on the polymer [858]. The authors show plots of log MW vs. retention time for various mobile phase compositions on a given column. The critical point is reached when the retention time becomes independent of the molecular weight of the polymer. For poly(methyl methacrylate) that point was reached on a silica column (RI detector) with a 73/27 methyl ethyl ketone/cyclohexane mobile phase. For polytetrahydrofuran, the silica column and a 95/5 acetone/hexane mobile phase created the critical conditions. This approach has enabled the individual blocks within the co-polymer to be studied (i.e., the portion of the polymer that can make contact with the support surface). 

Hara et al. [677] studied the effect of changing the mobile phase composition on the retention of 10 fat-soluble vitamins (e.g., Ini 5-retinol, retinal, ergocalciferol, cholecalciferol, menadione, phylloquinone). A silica column (2 —254nm or 292 nm) was used. The ethyl acetate level in hexane was systematically changed from 5% to 20% and the results were plotted in a Ink vs. percent ethyl acetate format. As expected, when the percent ethyl acetate increased, the overall retention of the analytes decreased. Interesting, however, was the fact that the changes in retention of both menadione and retinal were so different from the other analytes (e.g., a-, P-, y-, and -tocopherol) that reversals in retention order occurred. These data present an excellent basis from which to develop a new method for fat-soluble vitamins. 

Flieger J (2007) Effect of mobile phase composition on the retention of selected alkaloids in reversed-phase liquid chromatography with chaotropic salts. J Chromatogr A 1175 ... 

Figure 3.1 shows the effect of various mobile phase compositions on the selectivity factors for the separation of phenol/aniline and toluene/benzene pairs on 1.0, 2.1, 3.0, and 4.6 mm i.d. columns operating at a given velocity. The selectivity factors for the four columns are practically the same, even though retention times were not corrected to compensate for the differences in extra-column migration times. This finding is in agreement with Eq. (3.4) that shows that selectivity factors are independent of extra-column volume and column internal diameter. 

Usually goodness of fit is provided by adding new parameters in the model, but it decreases the prediction capability of the retention model and influences on the optimization results of mobile phase composition. 

Concentrations of moderator at or above that which causes the surface of a stationary phase to be completely covered can only govern the interactions that take place in the mobile phase. It follows that retention can be modified by using different mixtures of solvents as the mobile phase, or in GC by using mixed stationary phases. The theory behind solute retention by mixed stationary phases was first examined by Purnell and, at the time, his discoveries were met with considerable criticism and disbelief. Purnell et al. [5], Laub and Purnell [6] and Laub [7], examined the effect of mixed phases on solute retention and concluded that, for a wide range of binary mixtures, the corrected retention volume of a solute was linearly related to the volume fraction of either one of the two phases. This was quite an unexpected relationship, as at that time it was tentatively (although not rationally) assumed that the retention volume would be some form of the exponent of the stationary phase composition. It was also found that certain mixtures did not obey this rule and these will be discussed later. In terms of an expression for solute retention, the results of Purnell and his co-workers can be given as follows,... 

Although it is often possible to predict the effect of the solvent on retention, due to the unique interactive character of both the solvents and the enantiomers, it is virtually impossible to predict the subtle differences that control the separation ratio from present knowledge. Nevertheless, some accurate retention data, taken at different solvent compositions, can allow the retention and separation ratios to be calculated over a wide range of concentrations using the procedure outlined above. From such data the phase system and the column can be optimized to provide the separation in the minimum time, a subject that will be discussed later in the treatment of chromatography theory. 

The effect of temperature, although significant, is not nearly as great as that from the ethanol content and is greatest at low concentrations of the polar solvent. It is clear, that the solute retention is the least at high ethanol concentrations and high temperatures, which would provide shorter analysis times providing the selectivity of the phase system was not impaired. The combined effect of temperature and solvent composition on selectivity, however, is more complicated and to some extent... 

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