Mobile phase retention effects

Mobile Phase Paraneter Influence on Mobile Phase Properties Effect on Sai >le Retention... 

Mobile-phase parameter Influence on mobile-phase properties Effect on sample retention... 

On the other hand, optionally added co-ions of the eluent may also interfere with the ion-exchange process through competitive ion-pairing equilibria in the mobile phase. The effect of various amines added as co-ions to the polar-organic mobile phase was systematically studied by Xiong et al. [47]. While retention factors of 9-fluorenylmethoxycarbonyl (FMOC)-amino acids were indeed affected by the type of co-ion, enantioselectivities a and resolution values Rs remained nearly constant. For example, retention factors k for FMOC-Met decreased from 17.4 to 9.8 in the order... 

If the HPLC mobile phase is operated close to the pA of any solute or if an acidic or basic buffer is used in the mobile phase, the effects of temperature on retention can be dramatic and unpredicted. This can often be exploited to achieve dramatic changes in the separation factor for specific solutes. Likewise, the most predictable behavior with temperature occurs when one operates with mobile phase pH values far from the pA s of the analytes [10], Retention of bases sometimes increase as temperature is increased, presumable due to a shift from the protonated to the unprotonated form as the temperature increases. As noted by Tran et al. [26], temperature had the greatest effect on the separation of acidic compounds in low-pH mobile phases and on basic compounds in high-pH mobile phases. McCalley [27] noted anomalous changes in retention for bases due to variations in their pA s with temperature and also noted that lower flow rates were needed for optimal efficiency. 

The concentration of buffer is also a very important aspect in the optimization of the chiral resolution on these CSPs. It has been reported that an increase in buffer concentration caused a decrease in the retention and selectivity for all amino acids except for the basic amino acids. Therefore, the separation of basic amino acids is possible only with the most concentrated buffers. The buffers of concentrations in the 25-50-mM range were used for the chiral resolutions with some exceptions. In spite of this, few reports are available for the optimization of the chiral resolution by varying the ionic strength of the mobile phase. The effect of ionic strength of phosphate buffer on the chiral resolution of serine was carried out by Gubitz and Jellen [18] and the best resolution was achieved at 0.01 M concentration (Fig. 7). In another study, the concentration of ammonium acetate (0.001-0.01 M) was varied to optimize the chiral resolution of amino acids [19]. The effect of the concentration of ammonium acetate on the chiral resolution of amino... 

Nakagawa, T., Murata, H., Shibukawa, A., Murakami, K., and Tanaka, H. (1985) Liquid Chromatography with Crown Ether-containing Mobile Phases V. Effect of Hydrophobicity and Cavity Size of the Crown Ether on Retention of Amino Compounds in Reversed-phase High-performance Liquid Chromatography, J. Chromatogr. 330, 43-53. 

In general, the retention of monovalent and divalent ions shifts forward as the eluent concentration increases. On the other hand, a change in pH primarily affects the retention behavior of multivalent ions, since the valency of such ions depends on the pH value of the mobile phase. This effect is illustrated using orthophosphate as an example of the dissociation which occurs in three steps [65] ... 

Flavonol isomers, which differ only in the position of hydroxyl group on their chemical structures, showed different chromatographic behaviors. Liu et al. ° separated three flavonol isomers (3-hydroxyflavone, 6-hydro-xyflavone, and 7-hydroxyflavone) by a lab-constructed packed column SFC system with carbon dioxide modified with ethanol containing 0.5% (VAO phosphoric acid as the mobile phase. The effects of temperature, pressure, composition of mobile phase, and packed-column type on the separation were studied. It was indicated that the addition of phosphoric acid to the mobile phase enabled flavonol isomers to be eluted from the column. It was also shown that a phenyl-bonded silica column was better and the ODS column was not as effective for the isomer separation. Increasing pressure shortened the retention time of each compound, with good resolution, and higher temperature led to longer retention times, and even the loss of the bioactivities of these components. Under selected conditions, the separation of these isomers was very satisfactory, as illustrated in Fig. 2. 

Mobile Phase Solvent Effects and Small Molecule Retention Mechanisms. 188... 

A relatively new technique called capillary electrochromatography (CEC) is a hybrid of capillary electrophoresis (CE) and capillary LC. CEC is potentially more versatile than CE or HPLC because separation is based on both mobility differences (if the compounds are charged) and reversed-phase retention (effective for neutral organics and charged compounds with hydrophobic moieties). Mobile-phase transport through a typical 50-200 fim ID capillary packed with 3-/xm particles is achieved by elec-troosmotic flow (EOF) instead of a pressure gradient as in... 

The concentrations of benzoic acid, aspartame, caffeine, and saccharin in a variety of beverages are determined in this experiment. A Gig column and a mobile phase of 80% v/v acetic acid (pH = 4.2) and 20% v/v methanol are used to effect the separation. A UV detector set to 254 nm is used to measure the eluent s absorbance. The ability to adjust retention times by changing the mobile phase s pH is also explored. 

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