Chiral Mobile Phase Additives

There are no fundamental differences between the techniques using chiral stationary phases and chiral

Table 3 Chiral selectors used as mobile-phase additives

Chiral selector

Enantlomers

Stationary phase

Metal complex

L- or D-Proline # Cu(II), L-Phenylalanine # Cu(II), W-Methyl-L-phenylalanine # Cu(II),

W,W-Dimethyl-L-phenylalanine # Cu(II),

(R, R)-Tartaric acid mono-1-octylamide # Cu(II)

L-Propyl-n-octylamide #Ni(II),

L-Histidine # Cu(II)

(R, R)-Tartaric acid mono-1-octylamide # Cu(II)

Uncharged additives

1,1 '-Binaphthyl derivatives of 18-crown-6

Amino acids

Dansyl amino acids

¡-Amino alchols

Amino acids

Cation exchanger, ODS, OS

ODS, OS

ODS ODS

Cyclodextrins a-Cyclodextrin ß-Cyclodextrin

-y-Cyclodextrin TM-ß-Cyclodextrin

Propranolol, pseudoephedrine, salsolinol, thalidomide, dansyl amino acids Norgesterol

Benzoin, ethyl mandelate 5-Butyl-1-methyl-5-phenylbarbituric acid, Dansyl phenylalanine

Aminoethylbenzodioxane derivatives, hexobarbital

Porous graphite carbon, CN, ODS ODS, CN Si

ODS BS

Chiral acid 10-Caphorsulfonic acid Z-Glycyl-L-proline

Amino alcohols Diol

Amino alcohols, W-alkylated-2-aminotetralines Diol

Chiral amine Alprenolol, quinine, quinidine, cinchonidine

10-Caphorsulfonic acid, W-(1-phenylethyl) phthalamic acid, O-methylmandelic acid, O-methoxy-a-trifluoromethyl-phenylacetic acid, 2-phenylacetic acid, naproxen

Diol

ODS, octadecylsilyl; OS, octylsilyl; ES, ethylsilyl; CN, cyanopropylsilyl; TM, heptakis(2,3,6-tri-O-methyl); Si, silica gels; CM, car-boxymethyl; CE, carboxyethyl; BS, butylsilyl; Z, N-benzoxycarbonyl.

mobile phase additives. This means that all chiral selectors covalently bound to supports can be used for the separation of enantiomers by addition to the mobile phase. With chiral mobile phase additive techniques, there are at least two possible mechanisms; one is that the chiral mobile phase additive and the enantiomers may form diastereomers in the mobile phase. Another is that the stationary phase may be coated with the chiral mobile phase additive, resulting in diastereomeric interactions with the enantiomeric pairs during chromatography. It is thought that both mechanisms occur depending on both the stationary and mobile phases used.

The techniques using chiral mobile phase additives can be divided into three categories: metal complexa-tion used in chiral ligand exchange chromatography, the use of various uncharged additives, and the ion-pairing techniques used for charged analytes. Table 3 shows chiral selectors used as mobile phase additives.

See also: N/Chromatography: Liquid: Derivatization; Mechanisms: Chiral; III/Chiral Separations: Cellulose and Cellulose Derived Phases; Chiral Derivatization; Cyc-lodextrins and Other Inclusion Complexation Approaches; Ligand Exchange Chromatography; Ion-Pair Chromatography; Protein Stationary Phases; Synthetic Multiple Interaction ('Pirkle') Stationary Phases. Inclusion Complexation: Liquid Chromatography.

M. Kempe, Lund University, Lund, Sweden Copyright © 2000 Academic Press

In 1949, Frank Dickey published what can be considered the first paper on a molecularly imprinted synthetic material. The work was inspired by the theories of Dickey's mentor Linus Pauling, who had suggested that the primary structures of all polypep-tides constituting the antibodies are the same and that the diversity originates from folding directed by physical contact with the antigens. Even if Pauling's hypothesis on antibodies later turned out to be incorrect, his ideas laid the foundation for the concept of molecular imprinting. Consistent with these early studies, molecular imprinting can be defined as a method in which the selectivity of a material for a chosen molecule is induced by the presence of the molecule during the preparation, assembling or rearrangement of the material.

Dickey's studies were followed by several other investigations in the same direction, but it was not

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