Pair Chromatography

In a similar manner, carboxylic acid anions, phenolates, etc. can be separated from other organic matter by retention on a small anion exchanger in the OH form. 

After a brief rinse the retained sample anions are eluted with a solution of 1 M HC1 in methanol. Again, the ion exchange column should be small and the resin should be a macroporous polymer of 1 mequiv/g exchange capacity. 

The mechanism of what we will call ion-pair chromatography has been the subject of a considerable amount of investigation. Horvath et al. demonstrated the practicality of this approach [14]. They proposed an ion-pair mechanism and gave a number of ion-pair formation constants. 

Jonker and Huber used anionic surfactants in conjunction with a bonded-phase silica column and an organic-aqueous mobile phase for the separation of amino acids [15], A comprehensive study was made of the parameters, including the generation of gradients. 

In a more common form of ion-pair chromatography a smaller ion pairing reagent is added to the organic-aqueous mobile phase. This mobile phase is pumped through a reversed phase HPLC column until equilibrium is attained. Then a sample 

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Two mechanisms for chiral separations using chiral mobile-phase additives, analogous to models developed for ion-pair chromatography, have been... 

A solution of the acylated thiocyanatohydrin in a minimal amount of 5% potassium hydroxide in diglyme (other solvents such as methanol, ethanol or tetrahydrofuran have also been used) is stirred for 2 days at room temperature. Water is added to the reaction mixture to precipitate the product which is filtered or extracted with ether (or chloroform). The ether extract is washed several times with water, dried (Na2S04), and concentrated under vacuum. The thiirane usually can be crystallized from an appropriate solvent pair. Chromatography over alumina has been used for the purification of episulfides. 

K. Yamashita, M. Motohaslii and T. Yashiki, High-performance liquid cliromatograpliic determination of phenylpropanolamine in human plasma and urine, using column switching combined with ion-pair chromatography , J. Chromatogr. 527 103-114 (1990). 

EC = ion exchange chromatography IPC = ion pair chromatography LSC = liquid—solid chromatography... 

Separation of ionic and nonionic compounds of alkyl ether carboxylates can be done by reverse phase ion pair chromatography [241]. 

The ionic or polar substances can be seperated without any reaction on specially treated chromatographic columns and detected refractometrically. This is necessary because alkyl sulfosuccinates show only small absorption in the UV-visible region no sensitive photometric detection can be obtained. Separation problems can arise when common steel columns filled with reverse phase material (or sometimes silica gel) are used. This problem can be solved by adding a suitable counterion (e.g., tetrabutylammonium) to the mobile phase ( ion pair chromatography ). This way it is possible to get good separation performance. For an explanation of separation mechanism see Ref. 65-67. A broad review of the whole method and its possibilities in use is given in an excellent monograph [68]. 

Simple mixtures—like in alkyl sulfosuccinates—can be run using only one solvent. For more complex systems (e.g., ethoxylated fatty alcohol sulfosuccinates) a gradient technique is strongly recommended Technical mixtures of disodium laureth sulfosuccinate could be separated [68]. The separation was so effective that resolution of single homologs of ethoxylates was possible. The detection limit of this method lies at around 0.5 pg. Therefore reverse phase ion pair chromatography seems to be an excellent tool to analyze sulfosuccinates directly without the use of any kind of manipulation. 

There are very few examples of asymmetric synthesis using optically pure ions as chiral-inducing agents for the control of the configuration at the metal center. Chiral anions for such an apphcation have recently been reviewed by Lacour [19]. For example, the chiral enantiomerically pure Trisphat anion was successfully used for the stereoselective synthesis of tris-diimine-Fe(ll) complex, made configurationally stable because of the presence of a tetradentate bis(l,10-phenanthroline) ligand (Fig. 9) [29]. Excellent diastereoselectivity (>20 1) was demonstrated as a consequence of the preferred homochiral association of the anion and the iron(ll) complex and evidence for a thermodynamic control of the selectivity was obtained. The two diastereoisomers can be efficiently separated by ion-pair chromatography on silica gel plates with excellent yields. 

In addition to chromatography based on adsorption, ion pair chromatography (IP-HPLC) and capillary electrophoresis (CE) or capillary zone electrophoresis (CZE) are new methods that became popular and are sufficiently accurate for these types of investigations. Other methods involving electrochemical responses include differential pulse polarography, adsorptive and derived voltammetry, and more recently, electrochemical sensors. 

Hapten density, and also the common positions where haptens are bound, can also be estimated by cyanogen bromide or enzymatic cleavage of the protein and either MALDI-MS or separation of the components by reversed-phase ion-pair chromatography and electrospray or electrospray time-of-flight (TOF) analysis. 

Ion-pair chromatography (IPC) is a further example of the use of secondary chemical equilibria to control retention and... 

Figure 4.27 Flow chart for coluwi selection based on sample type (m - molecular weight). PLC precipitation-liquid chromatography SEC = size-exclusion chromatography lEC - ion-exchange chromatography HIC hydrophobic interaction chromatography LSC liquid-solid chromatography RPC - reversed-phase liquid chromatography BPC (polar) bonded-phase chromatography and IPC - ion-pair chromatography.

Typical ion-pairing reagents are, for cations, alkyl sulphonic acids, eg pentane, hexane, heptane or octane sulphonic acid, and for anions, tetrabutylammonium or dibutylamine ammonium salts. In ion-pair chromatography the retention of solutes can be controlled in a number of ways ... 

Steinbrech, B., Neugebauer, D., Zulauf, G. (1986). Determination of surfactants by liquid chromatography (HPLC). Reversed phase ion-pair chromatography of alkyl sulfates and alkyl sulfosuccinates. Analytische Chemie 324(2), 154—157. 

Ionic surfactants such as sodium dodecyl sulfate can also be detected by ESI. Figure 30 shows an overlay of sub ppm concentrations detected using ion pair chromatography with specific ion LC-MS detection (positive ESI at m/z 265). Gradient elution from 50.0% water containing 5 mM acetic acid triethylamine) to 50.0% 80/20 acetonitrile/water (5mM acetic acid triethylamine) was employed. 

Shea and MacCrehan [322] and Duane and Stock [323] determined hydrophilic thiols in marine sediment pore waters using ion-pair chromatography coupled to electrochemical detection. 

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