Influence of the mobile phase

Several equilibria intermesh in ion-exchange chromatography these are controlled by equilibrium constants  

Exception weak cation exchangers are more dissociated at high pH, resulting in improved sample interaction and a longer retention time one of these two effects will be dominant. 

General definition of an equilibrium constant K— [C][D]/[A][B] for reaction A -H B brackets represent concentrations). 

Exception weak anion exchangers are more dissociated at low pH hence elution is slower. 

Ion exchange may also be influenced by a suitable choice of counter ion. The ion exchanger prefers  

In practice, corrosive, reducing and highly UV-absorbing anions are avoided, the most common buffers being phosphate, borate, nitrate and perchlorate with sulphate, acetate and citrate occasionally being considered. 

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The influence of the mobile phase composition was also studied on the enanti-oseparation of several cycloaliphatic -substituted a-quaternary a-amino acids on a teicoplanin CSP [88,89] the study revealed two distinct enantiomeric and diastereo-meric discrimination mechanisms based on different interactions with the stationary phase. 

Avery recent study [128] deals with the comparison of two commercially available vancomycin-based CSPs with different surface coverage of the chiral selector in the enantioseparation of P-blockers and profens, by RP and POM separation modes. Higher retention and better resolution were obtained on the CSP with higher coverage of vancomycin in both the separation modes. However, in the case of pro fens, higher retention was not always accompanied by an improvement of the enantioselectivity in the RP mode. An accurate study of the influence of the mobile phase composition was also performed in both the separation modes. 

The effect of mobile phase composition, including pH and organic modifiers, was carried out on the chiral resolution of leucine derivatives on the fert-butyl carbamoylated quinine-based CSP [2], The results of these findings are given in Table 2. This table shows that the best resolution was obtained at pH 5, 2mM concentration of buffer, 60% methanol, and 80% acetonitrile concentrations, separately. In another study, the same authors [4] studied the influence of the mobile phase, pH, and temperature on the chiral resolution of leucine derivatives. The effect of temperature on the chiral resolution of leucine derivatives is shown in Figure 2. It is clear from this figure that the chiral... 

Cecchi, T., PncciareUi, R, and Passamonti, P. Ion interaction chromatography of zwitterions fractional charge approach to model the influence of the mobile phase concentration of the ion interaction reagent. Analyst 2004, 129, 1037-1042. 

The observation of wrong-way-around ESI indicates that the pH influence of the mobile phase is more complicated than initially thought (Ch. 6.3.1). In terms of method development for the LC-MS, this is actually good news (Ch. 6.6.3). 

Cecchi, T. Pucciarelli, F. Passamonti, P. Ion-Interaction Chromatography of Zwitterions. The Fractional Charge Approach to Model the Influence of the Mobile Phase Concentration of the Ion-Interaction Reagent The Analyst. 2004, 129, 1037-1046. (article B404721D available DOI 10.1039/b404721d http //www.rsc.org/is/joumals/current/ analyst/ anlpub.htm). 

There is only one recent report by Blackwell et al. on the investigation of the possible rationalization of such effects for a few chiral stationary phases [86], but the prediction of the influence of the mobile phase on the chiral recognition process clearly remains a challenge. Currently, automated screening devices are usually applied to help to find the optimal mobile phase composition. With the introduction of the new immobilized polysaccharide-based phases, this task has been made even more complicated as they tolerate a much wider range of solvent, which in turn may considerably influence the enantioselectivity of the separation. 

FIG U RE 9.4 Influence of the mobile-phase flow rates on the retention times. (Adapted from Hsieh, Y. et al., Anal. Chem., 79, 3856, 2007. With permission.)... 

The lack of suitable detectors in LC for trace and ultratrace analysis of complex matrices has catalyzed this trend. The major advantage of postcolumn reaction detectors is the absence of artifact information, and the only requirement is good reproducibility. A disadvantage is the great influence of the mobile phase on the reaction medium. 

Influence of the Mobile Phase and Iodine Solution Flow Rates on Induction Properties... 

The solvent compatibility of OmniPac PCX-100 is the prerequisite for the separation of ephedrines. Because ephedrine has two asymmetric C-atoms, two mirrored image isomeric ephedrines exist in addition to the pseudoephedrines, another antipodal pair. The two forms differ only in the configuration at the secondary alcohol group. Figure 6.70 illustrates the influence of the mobile-phase ionic strength on the separation of the two pseudoephedrines and their... 

Figure 6.70 Influence of the mobile-phase ionic strength on the separation of ephedrines on OmniPac PCX-100. Eluent (A) 80 mmol/L methanesulfonic acid/MeCN (80 20 v/v) and (B) 20 mmol/L methanesulfonic acid 50...

Temperature At low temperatures, the influence of the mobile phase recedes into the background and the individual properties of the stationary phase come to the fore. The enthalpy differences in the interachons between the individual components and the stahonary phase are larger than at higher temperatures and therefore a differenhahon (= selectivity) is often easier. 

The fundamental reason for this phenomenon is the fact that the selectivity of a separation arises from a combination of the influence of the stationary phase and the influence of the mobile phase. If the composition of the mobile phase is drastically different from the test conditions, one can expect a different position of the different columns relative to each other. It remains correct that Symmetry Cj8 has a lower silanol activity than Spherisorb ODS-2, but whether a Lima Cjg(2) or a YMC-Pack Pro Cjg has a higher hydrophobicity or a lower silanol activity surely depends on the details of the measurement conditions. 

The Influence of the Mobile Phase Expressed in LFER Parameters... 

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