G. Schill

Reversed-phase ion-pair chromatography with UV-absorbing ions in the mobile phase

Reversed-phase systems have been developed that permit the detection and quantification of down to 0.1 nmole of non-UV-absorbing cations and anions using a UV detector. The samples give positive or negative peaks depending on their charge and retention relative to the UV-absorbing ionic component in the mobile phase. The relative detector response has a maximum, which can be considerably more than 100%, when the sample and the UV-absorbing mobile phase ion have about the same retention. Detection and separation studies on, e.g., sulphonates, sulphates, carboxylates, amino acids, dipeptides and alkylammonium compounds of different degrees of substitution are described.

Separation of enantiomeric amines by ion-pair chromatography

A high-performance liquid chromatographic method for the separation of optical isomers (enantiomers) of amines is described. It is based on ion-pair chromatography with a chiral counter ion in a system with an organic mobile phase and an adsorbing stationary phase. The method has been applied to enantiomers of 1-aryloxy-3-isopropylamine-2-propanol derivatives (alprenolol, metoprolol, propranolol) which are completely resolved with (+)-10-camphorsulphonate as the counter ion. Studies of the influence of the counter-ion structure and the mobile phase composition are presented.

Ion-pair chromatography of acidic drug metabolites and endogenic compounds.

Liquid-liquid chromatographic systems based on ion-pair partition with silica microparticles as the support for the stationary phase have been used for the separation of anionic compounds of biochemical and pharmacological interest. A high separating efficiency can be obtained with both aqueous and organic mobile phases and the retention is easily regulated by the nature and the concentration of the quaternary ammonium counter ion, present in the aqueous phase. The influence of the composition of the liquid phases on the selectivity and separating efficiency has been studied, as well as equilibration methods and the stability of the systems. Examples are given of separations of sulphonamides, barbiturates, glucuronic and sulphuric acid conjugates of steroidal compounds and phenols glycine conjugates of carboxylic acids (hippuric, nicotinuric and salicyluric acid) and anionic metabolites of biogenic amines (indoleacetic, benzoic, mandelic and phenylacetic acid derivatives).

Reversed-phase ion-pair chromatography with an adsorbing stationary phase and a hydrophobic quaternary ammonium ion in the mobile phase : I. Retention studies with tetrabutylammonium as cationic component

The retention behaviour of some acids (acetylsalicylic acid, naphthalene-2- sulphonic acid, pentobarbital and vinbarbital) and bases (antipyrine, caffeine, codeine, dextropropoxyphene, hydroxyzine and Transergan) have been investigated using μBondapak C18 as stationary phase and a solution of tetrabutylammonium in acetonitrile + aqueous phosphate buffer of pH 3 and 6 (25:75) as the mobile phase. The results indicate that the stationary phase contains sites with different adsorption abilities for hydrophobic ammonium ion paris. Models for the retention have been developed and equilibrium constants for the distrbution of the ion pairs to the hydrophobic stationary phase are presented.

Ion-pair chromatography of organic compounds

Ionized organic compounds can be extracted into organic phases as ion-pairs with counter ions of suitable hydrophobic character. The distribution ratio depends on the structure of the ion-pair and the concentration of the counter ion. Systems for liquid—liquid partition chromatography are easy to adapt to different types of compounds and have a high selectivity. The counter ion is selected on the basis of batch extraction procedures or chromatographic tests. A photometric detector can be used for samples of <0.1 nmole of non-absorbing compounds if a counter ion with high absorbance is used. Straight-phase systems with a high counter-ion concentration on a cellulose support have a high sample capacity and good separating efficiency (H ⩽ 0.5 mm at a mobile phase speed of ⩽ 3 mm/sec when k′ ⩾ 5). Examples are given of straight-phase separations of carboxylic acids with a quaternary ammonium ion as counter ion in the stationary phase and amines and quaternary ammonium ions with picrate or β-naphthalene sulphonate as counter ions. Aminophenols have been separated by reversed-phase systems containing bis-(2-ethylhexyl)phosphoric acid in chloroform as the stationary phase.

Reversed retention order and other stereoselective effects in the separation of amino alcohols on Chiralcel OD

Abstract The enantioselective resolution of a series of amino alcohols on Chiralcel OD was studied with respect to the effect of temperature, alcohol additive and water content in a mobile phase of hexane with added diethylamine. The chain length between the hydroxy and the amino groups in the solutes had a considerable influence on the stereoselectivity. For one of the amino alcohols a reversal of the retention order between the antipodes was obtained by varying the above parameters. The amino alcohols are retained by at least two chiral sites, one of which is highly dependent on hydrogen bonding. This bonding ability can be directly controlled by the water content of the mobile phase.

Reversed-phase ion-pair chromatography of drugs and related organic compounds

Summary Procedures are given for high-performance ion-pair chromatography of organi ammonium compounds (e.g., noradrenaline, dopamine, synephrine, ephedrine, zimelidine, imipramine, desipramine, amitriptyline and nortriptyline) with dihydrogen phosphate, bromide, cyclohexylsulphamate, dicyclohexylsulphamate or octylsulphate as counter ions and 1-pentanol as the liquid stationary phase on LiChrosorb-RP. With methylene chloride as the stationary phase, the highly hydrophilic noradrenaline and adrenaline can also be separated. A long-chain alkylammonium compound must be added to the system in the chromatography of hydrophobic amines and all types of quaternary ammonium compounds to prevent peak deformation. Both the liquid stationary phase and the support affect the selectivity.

Ion-pair chromatography in the low concentration range by use of highly absorbing counter ions

Abstract High-performance systems for ion-pair chromatography that permit the detection of down to 7 pmole (about 1 ng) of non-UV-absorbing samples have been constructed, the high detector response being obtained by using a stationary phase containing a counter ion of high absorbance. Amino acids, dipeptides and alkylamines are separated with naphthalene-2-sulphonate as counter ion. The systems have a high selectivity, and a change of the amino acid sequence if often sufficient for a complete chromatographic resolution of dipeptides. A comparison with batch extraction data showed that the retention of the cations, except the most hydrophobic, is due mainly to liquid-liquid distribution. The naphthalene-2-sulphonate systems have a very high stability under careful thermostatted conditions. Systems for the separation of anionic compounds with highly absorbing quaternary ammonium ions in the stationary phase are being investigated.

Supercritical fluid chromatography with ion-pairing modifiers Separation of enantiomeric 1,2-aminoalcohols as diastereomeric ion pairs

Abstract Ionizable organic compounds have been separated by supercritical and subcritical chromatography using ion-pairing modifiers. The mobile phase was carbon dioxide mixed with an acetonitrile solution containing a counter ion and a competing ion. The retention and separating efficiency are controlled by pressure and temperature as well as by the concentration and nature of the ionic mobile phase components. These highly versatile systems are applicable to polar solutes of widely different structures. The technique has been applied to the separation of enantiomeric 1,2-aminoalcohols using N-benzoxycarbonylglycyl- l -proline as a chiral counter ion. The maximum resolution and the separating efficiency at higher flow-rates are superior to that obtained in high-performance liquid chromatography.

Construction of systems for detection and quantitation by UV-absorbing mobile-phase ions in reversed-phase chromatography

Abstract Ions without inherent detectable properties can be detected and quantified in reversed-phase liquid—solid systems by use of a mobile phase containing one or two UV-absorbing ions. The highest sensitivity is obtained when the sample and the detectable ion have approximately equal retentions. The system can be adapted to a given sample by the choice of solid phase and mobile-phase ions. A series of systems covering samples with widely different hydrophobicities is given. Retention can be regulated with an unchanged organic modifier, but this will decrease the detection sensitivity. Background absorbance above 0.5 will also decrease the sensitivity. The quantitation of a solute is not affected by the composition of the sample, i.e., the solvent or other sample components.