IPR Counter Ions

In IPC, the potential difference between the stationary phase and the bulk eluent develops because of the different adsorbophilicities of the anion and cation of the IPR. The potential determining ion is the ionic species of the IPR that experiences the strongest tendency toward adsorption onto the stationary phase it may be either the anion or the cation. The potential determining ion is the actual ionpairing reagent. 

For sodium dodecanesulfonate, a classical IPR for positively charged analytes, the potential determining ion is dodecanesulfonate, while the counter ion is the sodium cation. The electrified interphase will carry a barely negative charge since 

The presence of adsorbophilic counter ions on the surface reduces the surface potential difference since C charges partially counterbalance the opposite charges of the adsorbed H ions. The surface potential has to be calculated according to the following expression  

Equation 7.1 parallels Equation 3.5 (see Chapter 3, also for symbols meaning) that holds when [LC], that is the surface concentration of the counterions C, is negligible compared to [LH], as in the case of most traditional IPRs. 

Since the amount of Na+paired to the adsorbed CIO4 is not easily measurable, we may expect that the surface excess of CIO4- is a fraction of the total adsorbed NaC104 that can be determined via the adsorption isotherm. 

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Many subsequent stoichiometric mixed mode models are based on various combinations of these ion-pair and dynamic ion exchange extreme mechanisms. The effect of the IPR counter ion [13] and the reduction of available hydrophobic surfaces... 

All non-stoichiometric models adopt the electrical double layer concept and disagree with the stoichiometric hypothesis of an electroneutral stationary phase they emphasize the higher adsorbophilicity of the IPR compared to that of its counter ion a surface excess of IPR ions generates a primary charged layer and a charged interface. Like-sign co-ions are repelled from the surface while IPR counter ions are attracted by the charged surface. 

The addition of many classes of IPRs leads to a modification of the eluent pH. Volatile amines, greatly appreciated for their volatility in IPC-MS use, increase the eluent pH. Conversely, using perfluorinated acids of different chain length as IPRs for basic analytes involves a concomitant lowering of the eluent pH that in turn provides the protonation the basic analytes need under IPC conditions. The IPR counter ion is also important when dealing with eluent pH for example, the behavior of tetrabutylam-monium hydroxide would be very different from that of the corresponding chloride. 

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