Ion-Pairing Additives

The methods discussed so far are essentially limited to isolated ion-pairs or, in the admittedly crude approximation, to cases when a multiple ion-pair spur can be considered to be a collection of single ion-pairs. Additionally, it is difficult to include an external field, as that will destroy the spherical symmetry of the problem. Stochastic treatments can incorporate both multiple ion-pairs and the effects of an external field. 

The use of RP columns, ion pairing additives, and a gradient ending with rather high concentration of organic modifier are, of course, only rough sketches of a detailed picture. 

In a similar way, the use of formic acid/ammonium formate was shown to be superior to the use of acetic acid/ammonium acetate in the RP HPLC-ESI MS/MS analysis of a number of heterocyclic aromatic amines [7]. It is clear from numerous other examples in the literature that HPLC resolution and MS sensitivity can be dramatically influenced by the correct selection of mobile phases, organic modifiers, and ion-pairing additives. 

Figure 4-46. Logarithm of the retention of dopamine and 1-benzenesulfonic acid on reversed-phase column as a function of the mobile-phase concentration of ion-pairing additives (pH 2.1). Column Hypersil-ODS, T = 25°C constant ionic strength was maintained by addition of NaH2P04 open circles, butylsulfate triangles, cyclohexylsnlfamic acid x, li-camphor-lO-suIfonic acid half-closed circles, 1-hexanesnlfonate black circles, octansulfonate. (Reprinted from reference 145, with permission.)...

Contact ion pairs are occasionally called bound ion pairs, and solvent separated ion pairs are also known as solvated ion pairs or loose ion pairs. Additionally, free ions, solvated ion pairs, and contact ion pairs can also be in equilibrium with the corresponding ionic associates, for example. 

The range of most convenient hydrophobicity of organic compounds for reversed-phase (RP) HPLC separation may be estimated approximately as —1 < log P < + 5 (log P is the logarithm of the partition coefficient of the compound being characterized in the standard solvent system 1-octanol/water). Highly hydrophUic substances with log P < —1 need a special choice of analysis conditions, e.g., introduction of ion-pair additives into the eluents. Another approach is their conversion to more hydrophobic derivatives by the modification of functional groups with active hydrogen atoms. 

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