Mobile phase ionic strength

Zhou and Pietrzyk [41] found that increasing the mobile-phase ionic strength not only increases the retention of AS and AES on a reversed stationary phase, but also improves the resolution since the peak widths are significantly reduced. The authors achieved baseline separation of a multicomponent alkane sulfonate and alkyl sulfate mixture from C2 to Cis using a mobile-phase gradient whereby acetonitrile concentration increases and LiOH concentration decreases. 

Note MS analysis was carried out in negative ionization mode due to the moderate acidity of glucuroconjugates. The mobile phase ionic strength was adjusted to 5 mM ammonium acetate at pH 5.0 in order to facilitate in-liquid ionization under established... 

Toxin Separations. A number of columns have been evaluated for suitability using the HPLC method. Of all columns tested to date, the Hamilton PRP-1 column (polystyrene divinylbenzene resin) has proven to be the most useful. Toxin retention on this column is controlled by 1) methanol concentration, 2) mobile phase ionic strength, 3) chain length of the ion-pair reagent, and M) mobile phase pH. 

In Equation 15.24, it is important to note that Pq is a function of p and the mobile phase ionic strength. 

FIGURE 15.11 Plots of the logarithmic retention factor of ovalbumin as a function of the reciprocal square root of the mobile phase ionic strength, l/yfl at different pH values 6.0 ( ), 7.0 ( ), and 8.0 (A). Experimental conditions Synchropak Q300 column, NaCl eluting salt under isocratic condition. Experimental data from Ref. [29]. (Reproduced with permission from Stahlberg, J. etal.. Ana/. Chem., 63, 1867, 1991.)... 

Polyelectrolyte contraction can be followed by determining the Kj as a function of mobile phase ionic strength (104). In practice, however, the mobile phase ionic strength must be sufficiently high to ensure that the chain is in a contracted state. In this way, small changes in ionic strength, which may be inadvertently introduced during mobile phase preparation, will not affect the elution behavior of the sample. Also, if the ionic... 

As reported by Zakaria and Brown (Zl), changing the mobile-phase ionic strength has little effect on the retention behavior of nucleosides and bases. The effect of mobile-phase ionic strength is much more pronounced in the case of ribonucleotides, where the compounds are ionized under the mobile-phase conditions. 

It is established in ion-pair chromatography that salts always reduce the ion-pair retention factors by shielding ion-ion interactions. The plot of the retention factor, k, of a ionic solute versus the inverse of the mobile phase ionic strength, I , is linear with a positive slope [9]. Often, an increase in ionic strength decreases the retention factor and reduces the efficiency of the system. From a practical point of view, the mobile phase salt concentration (ion-pairing agent, solute and buffer) should be maintained below the 0.02 M limit [10]. 

Table 4 Kq of Arginine as a Function of Mobile Phase Ionic Strength...

The optimum mobile phase ionic strength for analysis of most solutes is 0.1-0.6M. 

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...

Mixed-Mode WCX-1 [68] (Thermo Fisher Scientific, Waltham, MA, USA). The Acclaim Mixed-Mode WAX-1 column combines both hydrophobic and anion-exchange characteristics, which facilitates selectivity adjustment by changing mobile-phase ionic strength, pH, or organic modifier, either independently or concurrently. 

Figure 10.357 (a-c) Separation of imidazo-lium-based ionic liquids by cation-exchange chromatography as a function of mobile-phase ionic strength and organic modifier content. Separator column lonPac CS17 ... 

---