Parameters affecting chemical speciation

The reaction of calcium and carbonate ions, including the protonation of carbonate (Equations 1.5 and 1.6)  

The oxygen content of groundwaters, or usually the geological environment, is also important. It determines the oxidation state of the different substances, that is, the redox equilibria. (Note In the previous example of hydrocarbonate, it has no effect on the oxidation state.) A simplified picture of the redox equilibria is given by the equilibrium thermodynamic potential-pH diagrams (Pourbaix 1966). 

The detailed explanations of potential-pH diagrams can be found elsewhere (Pourbaix 1966) here, only a short discussion is given. The electrochemical potentials of the reactions between the two different oxidation states are calculated 

From thermodynamic data, the redox potential of reaction 1.7 is as follows  

In groundwater, the range between lines a and b is interesting. At typical pH values of groundwaters (pH = 6-8), oxidizing (aerobic +400-500 mV), anoxic, and reducing (anaerobic below -100 mV) atmospheres are usually separated on the basis of redox potential. 

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Mass discrimination is most important for qua-drupole-based instruments where the transmission of ions is not uniform over the mass range. In particular, the transmission of light ions is reduced compared to heavier ions. Another parameter which may also affect the detection capability of ICP-MS is the plasma sampling position, which may not be optimal for all the analytes being determined simultaneously. This position is indeed important to maximize the density of analyte ions extracted from the ICP. Even the chemical form (i.e. speciation) of an element in solution may influence where it undergoes the various processes (desolvation, vapourization, etc) in the plasma, ultimately determining where the maximum ion density will be. 

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