Dealing with Multiply Charged Ions

The discussion of ion formation in ESI has revealed that - beside coiipound class - the actual experimental conditions exert significant influence on the appearance of an ESI spectrum. The most influential factors are i) pH of the sprayed solution, ii) flow of sample solution, iii) flow (or pressure) of nebulizing gas, and iv) flow and temperature of the desolvation gas or the temperature of the heated desolvation capillary, respectively (also cf. Figs. 12.1 and 12.19). 

Note In ESI spectra of (synthetic) polymers, multiple charging causes the simultaneous occurrence of superimposing ion series each of them representing the molecular weight distribution of the polymer. Therefore, MALDI is generally preferred for polymer analysis as it delivers only singly charged ions. 

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The most effective technique to deal with complex spectra due to multiply charged ions is to achieve the full separation between signals corresponding to different charge states and to resolve their isotopic patterns. Beyond a molecular weight of about 2000 u this requires high-resolving mass analyzers. 

The hydrated layer has finite thickness, therefore the exchanging ions can diffuse inside this layer, although their mobility is quite low compared to that in water (n 10-11cm2s-1 V-1). As we have seen in the liquid junction, diffusion of ions with different velocities results in charge separation and formation of the potential. In this case, the potential is called the diffusion potential and it is synonymous with the junction potential discussed earlier. It can be described by the equation developed for the linear diffusion gradient, that is, by the Henderson equation (6.24). Because we are dealing with uni-univalent electrolytes, the multiplier cancels out and this diffusion potential can be written as... 

The decreased iR drop in voltammetric experiments at ultramicroelectrodes has been exploited to perform electrochemistry under conditions in which no or only a small concentration of supporting electrolyte is added and allows measurements in low-polarity solvents (e.g. hydrocarbons), without the presence of excess ions, or even in the gas phase [51]. This topic is discussed further in Chapter 2.5 (Sect. 2.5.5.6). In these cases, the transport of charge in the electrolyte is realized by small amounts of impurities [48], by ions of the substrate material itself [52], or those generated in the electrode reaction [39]. Thus, migration has to be considered as an additional mode of transport, in particular for multiply charged species [52]. A recent modeling study [53] has provided evidence that LSV should be best suited to deal with situations of high uncompensated resistance as compared to chronopotentiometry and chronoamperometry. 

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