Effect of Aqueous Buffer on Chiral Separations

For most free amino acids and small peptides, a mixture of alcohol with water is a typical mobile phase composition in the reversed-phase mode for glycopeptide CSPs. For some bifunctional amino acids and most other compounds, however, aqueous buffer is usually necessary to enhance resolution. The types of buffers dictate the retention, efficiency and - to a lesser effect - selectivity of analytes. Tri-ethylammonium acetate and ammonium nitrate are the most effective buffer systems, while sodium citrate is also effective for the separation of profens on vancomycin CSP, and ammonium acetate is the most appropriate for LC/MS applications. 

2 Method Development and Optimization of Enantiomeric Separations Using.  

Typical normal-phase operations involved combinations of alcohols and hexane or heptane. In many cases, the addition of small amounts ( 0.1 %) of acid and/or base is necessary to improve peak efficiency and selectivity. Usually, the concentration of polar solvents such as alcohol determines the retention and selectivity (Fig. 2-18). Since flow rate has no impact on selectivity (see Fig. 2-11), the most productive flow rate was determined to be 2 mL miiT. Ethanol normally gives the best efficiency and resolution with reasonable back-pressures. It has been reported that halogenated solvents have also been used successfully on these stationary phases as well as acetonitrile, dioxane and methyl tert-butyl ether, or combinations of the these. The optimization parameters under three different mobile phase modes on glycopeptide CSPs are summarized in Table 2-7. 

New polar organie mode a. Type of aeid and base b. Aeid/base ratio e. Coneentration of aeid and base d. Flow rate 

Reversed phase a. Type of organie modifier b. Coneentration of organie modifier e. Type of aqueous buffer d. Coneentration of aqueous buffer e. pH of aqueous buffer f. Flow rate g. Temperature 

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