Eluent concentrates
The role of the eluent is to transport the sample through the system and contribute to the selectivity of the separation.
The eluent is a solution of a salt in water, that also acts as a buffer, providing stable pH. The most common type of elution in IC is an isocratic elution, where the eluent has a constant concentration and composition during the entire run. Another possibility is a gradient elution, where the eluent concentration is changed during the separation.
| Parameter | Effect on retention in anion IC |
|---|---|
| Ion strength | The eluting ability increases with eluent ionic strength. The selectivity among ions of equal charge is only marginal, whereas the selectivity between ions of different charge (mono- or polyvalent) is far more sensitive to changes in ion strength. |
| pH | The retention times for anions of weak acids increase when the pH of the eluent increases in the vicinity of the pKa of the acid. This is due to the sample ion charge being controlled by the eluent pH – the more basic the eluent, the stronger the negative charge. In a suppressed system, the eluent pH is substantially higher than the pKa's of most analytes, with the excep- tion of, e.g., acetate or phosphate ions |
| Temperature | The ion exchange rate between the statio- nary and the mobile phase increases with increasing temperature. The viscosity of the eluent, and thereby the column back- pressure, decreases and can give a better separation efficiency. The temperature can also affect the column selectivity. |
| Flow rate | The ions are eluted faster with high eluent flow rates, but faster elution will decrease the separation efficiency. The flow rate is also limited by the pressure durability of the separation column. |
| Buffer salt | The eluting power of the eluent, and also to some extent its selectivity, are affected by the anion of the eluent (through its pKa value). A change of salt will normally also result in a change of the eluent pH. |
The two most common eluents for the separation of anions are based on carbonate or hydroxide as eluting anion. The carbonate eluent is an aqueous solution of carbonate and hydrogen carbonate salts, and has the advantage that the total ionic strength as well as the proportions of the monovalent (HCO3 - ) and divalent (CO3 2- ) ions can be varied to optimize the retention time and the selectivity between monovalent and multivalent sample ions.
However, the main drawback of carbonate eluent is fact that it still gives some background conductivity after the suppressor reaction, where carbonic acid (H2CO3) is formed. The carbonate eluent is usually used to perform isocratic separations.
Hydroxide eluents are commonly used for gradient elution. The advantage of using a hydroxide eluent is that it is transformed into pure water in the suppressor reaction, and consequently gives a very low background conductivity.