General Mobile Phase Considerations

Beside the counterion and its concentration, the composition of the mobile phase will depend on the column packing material used and the type of solutes to analyse. Uncharged or charged modifiers may be needed in the mobile phase in order to elute the solutes in a reasonable time and also to resolve them into their enantiomers.

Mobile Phase Composition when using DIOL Functionalized Silica as Stationary Phase

When DIOL functionalized silica particles are used as stationary phase, the mobile phase has to be a nonpolar solvent. The water concentration in the nonpolar solvent may then be of vital importance for the success of the separation and should, together with some of the chiral counterions, quinine and camphor-

sulfonic acid, be as low as possible in order to promote the chiral separations. The equilibration time needed to obtain a stable system with dry solvent is, however, very long and therefore it is convenient to work with higher water contents although not a water-saturated mobile phase. The water content does not influence the enantioselectivity when L-ZGP is used as counterion. Longer retention times are, however, obtained when more water is present in the solvent. Water-saturated injection solutions can be injected into the system without influence on either retention or stereoselectivity.

As the polarity of the mobile phase controls the retention of the solutes, modifiers like alcohols, acetonitrile or tetrahydrofuran have sometimes to be used in order to elute solutes in a reasonable time. Several of these modifiers also have an effect on the enantioselectivities of these systems. Some modifiers have been studied in a system with (+ )-10-camphor-sulfonate in dichloromethane as mobile phase. (In these cases the mobile phases were prepared from dry dichloromethane.) It was found that the stereoselec-tivity was lower with hydrogen donating modifiers than with hydrogen accepting ones (Table 7). One hypothesis is that the modifiers may interact with the hydrogen-bonding groups in the selectors. It should be noted that 1-pentanol is preferred as a modifier since the peaks obtained are more symmetrical than

Table 6 Counterion structure and enantioselective retention Solutes:

Table 6 Counterion structure and enantioselective retention Solutes:

Counterlons.

Counterlons.

Figure 7 Separation of (R)- and (S)-metoprolol. Solid phase: Hypercarb. Mobile phase: 3.4 mM Z-L-Glu-L-Pro and 5.5 mM NaOH in methanol. Column temperature: 35°C. (Reprinted from Karlsson A and Karlsson O (1997) Enantiomeric separation of amino alcohols using Z-L-Glu-L-Pro or Z-L-Glu-D-Pro as chiral counterions and Hypercarb as the solid phase. Chirality 9: 650-655, with permission from Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.)

Figure 7 Separation of (R)- and (S)-metoprolol. Solid phase: Hypercarb. Mobile phase: 3.4 mM Z-L-Glu-L-Pro and 5.5 mM NaOH in methanol. Column temperature: 35°C. (Reprinted from Karlsson A and Karlsson O (1997) Enantiomeric separation of amino alcohols using Z-L-Glu-L-Pro or Z-L-Glu-D-Pro as chiral counterions and Hypercarb as the solid phase. Chirality 9: 650-655, with permission from Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.)

Solute Counterion

L-ZGP L-ZGGP Captopril

Solute Counterion

L-ZGP L-ZGGP Captopril

k

V

k

V

k

V

1

1.8

1.32

4.9

1.05

9.0

1.07

2

2.2

1.0

3.9

1.09

7.0

1.0

3

1.2

1.06

4.0

1.44

-

-

Solid phase: PGC. Mobile phase: 7.2 mM of counterion in di-chloromethane (80 ppm H2O). Reprinted from Karlsson A and Pettersson C (1992) Chirality 4: 323 with permission from Wiley-Liss, Inc., a subsidiary of John Wiley and Sons, Inc.

Solid phase: PGC. Mobile phase: 7.2 mM of counterion in di-chloromethane (80 ppm H2O). Reprinted from Karlsson A and Pettersson C (1992) Chirality 4: 323 with permission from Wiley-Liss, Inc., a subsidiary of John Wiley and Sons, Inc.

when using, for example, tetrahydrofuran. The improved peak shape is believed to be due to deactiva-tion of strong adsorption sites on the silica surface.

Peak shapes may also be improved by adding to the mobile phase a compound with the same protolytic character as the solute enantiomers. Thus, for the separation of amines, triethylamine is often used to improve peak symmetry. Acidic compounds to be analysed with, for example, quinine, may need addition of acid to the mobile phase in order to effect elution. The added acid is believed to compete for adsorption to the solid phase and for ion pair formation with quinine.

Mobile Phase Composition when using PGC as Stationary Phase

When PGC is used as stationary phase, both nonpolar and polar mobile phases may be used. The nonpolar phases resemble those used with the DIOL stationary phase. The addition of a nonchiral hydrophobic acid may, however, be needed in order to elute some of the hydrophobic acids from the PGC. Naproxen was found to be too strongly retained using a mobile phase consisting of quinine in dichloromethane and hexane. When adding ^-naphthalenecarboxylic acid to the mobile phase, naproxen was eluted within a reasonable time and the enantiomers were separated (Figure 9). The effect of addition of an acid to the mobile phase is not easy to predict as the added acid may compete for both the distribution to limited adsorption sites on the support and also for ion pair formation with the chiral selector. Furthermore, if the counterion added to the polar mobile phase is expected to remain in an uncharged form, an acid or base has to be added in order to obtain a charged counterion. L-ZGP has a pKa of 7-9 in methanol. The amount of base added (e.g., sodium hydroxide) will determine the degree of protolysis of the L-ZGP. However, an excess of base will result in an alkaline solution where the solutes may be uncharged. The added base may thereby influence the retention and the stereoselectivity, in other words influence both k and the ratio kacWl/kcWl (Figure 3).

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment