Ion pair mode

Thomlinson [78] was the first chromatographer to point out that the classical electrostatic ion-pair concept did not hold for IPRs that were usually bulky hydrophobic ions he also emphasized that in the interfacial region between the mobile and the stationary phases, the dielectric constant of the medium is far lower than that of the aqueous phase. Chaotropes that break the water structure around them and lipophilic ions that produce cages around their alkyl chains, thereby disturbing the ordinary water structnre, are both amenable to hydrophobic ion-pairing since they are both scarcely hydrated. The practical proof of such ion-pairing mode can be found in References 80 and 81 many examples of such pairing modes are reported in the literature [79-86],... 

ION-PAIR MODE methanol (acetonitrile) - water containing ca. 0.005 M alkyl sulfonate and It acid (acetic acid), pH 2 - 4. 

Hi kino et al.4 developed two HPLC methods for the analysis of aconitine and related alkaloids in crude drugs. In the first one the alkaloids were separated on an octadecyl column using tetrahydrofuran - 0.05 M phosphate buffer (11 89) as mobile phase. Mobile phases containing methanol and acetonitrile gave broader peaks and less resolution of some of the alkaloids. In the pH range 2-5, little variation in k1 was found for the alkaloids, but above pH 5 some alkaloids showed increased k1 values. Best results were obtained at pH 2.7 (Fig. 12.1), A second method was developed to minimize the risk of interference of co-eluting compounds from the plant material. The same type of column as above was used in the reversed--phase ion-pair mode, and, as pairing-ion, 0.01 M hexanesulfonate was added to the mobile phase (tetrahydrofuran - 0.05 M phosphate buffer (pH 2.7)(15 85)). 

A particular problem associated with the separation of basic compounds both in the ion-pairing mode and in other forms of HPLC is tailing due to the interaction with the silica gel. Such interaction is typically curtailed by the addition of 20-30 mM triethylamine to the mobile phase. 

Another concurrent effect in the ion-pair mode is the adsorption of ion-pairing agent on the surface of reversed-phase packing material. This causes a decrease of the available hydrophobic surface and its transformation into an ion-exchange type surface. 

Ion-pair HPLC mode is a superposition of two competitive processes ion-exchange and reversed-phase. Component retention is strongly dependent on the type of ionpairing agent, its concentration, and most of all, on the history of the used column. The virgin reversed-phase (RP) column does show the hydrophobic selectivity in the ion-pair mode. However, with time, the adsorbent surface can become covered with a dense layer of adsorbed surfactant. This may irreversibly transform the RP column into an ion-exchange one. 

A comparison of anion-exchange, reversed phase and reversed phase ion-pair modes for the separation of small oligonucleotides concluded that anion-exchange provides the best resolution although column life-time may be shorter (Haupt and Pingoud, 1983). 

Ion-pair mode methanol (acetonitrile)-water containing 0.005 mol 1 alkylsulfonate and 1% acid (e.g. acetic acid), pH 2-4. 

Except for the high molecular weight range, nearly all substances can be separated by reversed-phase (RP) HPLC. The many different separation mechanisms in RP HPLC, based on hydi ophobic, hydi ophilic and ion-pairing interactions, and size exclusion effects together with the availability of a lai ge number of high quality stationary phases, explain its great populai ity. At present approximately 90% of all HPLC separations are carried out by reversed-phase mode of HPLC, and an estimated 800 different stationai y phases for RP HPLC are manufactured worldwide. 

Figure 5. (a) The ( A, SO,) anion symmetric streching mode of polypropylene glycol)- LiCF,SO, for 0 M ratios of 2000 1 and 6 1. Solid symbols represent experimental data after subtraction of the spectrum corre-ponding to the pure polymer. Solid curves represent a three-component fit. Broken curves represent the individual fitted components, (b) Relative Raman intensities of the fitted profiles for the ( Aj, SO,) anion mode for this system, plotted against square root of the salt concentration, solvated ions ion pairs , triple ions, (c) The molar conductivity of the same system at 293 K. Adapted from A. Ferry, P. Jacobsson, L. M. Torell, Electrnchim. Acta 1995, 40, 2369 and F. M. Gray, Solid State Ionics 1990, 40/41, 637. 

Now we can proceed to assemble the positive evidence for the path (I II -> IV, Fig. 7). Once the outer sphere complex, (II), is formed, all replacements of water should occur at the same rate, k - lO- If the ion pairing constant Ka is known, or a limiting rate of anion entry corresponding to saturation of the association is observable, the rates of conversion of (II) into (IV) may be compared for various X. All should be equal to / -h20 if the activation mode is d, but they will not equal the rate of water exchange which was identified with on the D path. The reason is that species (II) has a number of solvent molecules in its... 

Ferrocyanide reduces persulphate, the reaction being second-order in a fairly saline medium (0.5 M K2S04) with /c2 = 3.2x 10 exp(—11.9 x lO /Hr) l.mole . sec. The rate is strongly influenced by the presence of potassium ions and this has been shown not to be merely an ionic strength effect" . Consideration of all possible modes of ion-pairing led to the conclusion that the two reactants are [K(Fe(CN)6] and [KS20g] . At zero ionic strength, E = 9.6 kcal.mole and AS = —34.7 eu. Kershaw and Prue have measured the specific effects of many other cations on the rate of this reaction. 

Fig. 1.7 Possible hydration modes of an ion pair (A) contact of primary hydration shells, (B) sharing of primary hydration shells, (C) direct contact of ions...

Ionic solutes can be separated by ion-exchange chromatography using microparticulate resins or bonded ion-exchangers based on microparticulate silica. Such separations are often achieved more easily by ion-suppression or ion-pairing techniques, which use bonded phase columns in the reverse phase mode. 

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