Application Modes of Conductivity Detection

When passing the separator column effluent into the conductivity cell without applying a suppressor system, the electric conductance j of a solution, according to Fritz and Gjerde [3], is given by 

When the cell constant and the equivalent conductances of eluent anions and cations are known, Eq. (8.26) makes it possible to calculate the conductivities of eluents that are typical for this kind of detection method. To determine the cell constant, the conductivity of a potassium chloride solution with a defined concentration is usually measured, as the equivalent conductances of potassium and chloride ions are known to be 74 and 76 S cm /val, respectively (see Table 8.1). 

When the eluent of the concentration Ce contains cations E and anions E , the background conductivity, Xe may be calculated via Eq. (8.26) as follows  

The change in conductivity associated with the elution of a solute ion is Aj = Js -Xe 

In principle, Eq. (8.29) applies to all ion chromatographic methods. It reveals that the detector signal not only depends on the solute ion concentration, but also on the equivalent conductances of eluent cations and eluent- and solute anions, as well as on their degree of dissociation. The degree of eluent- and solute ion dissociation is determined by the pH value of the mobile phase. 

When the concentration of solute ions passing the detector is denoted as Cs and their degree of dissociation as Os, the eluant concentration in the measuring cell during the elution of solute ions is given by (Ce — Cs Os). Hence, in this instance, the measured conductivity is caused by eluant and solute ions as well as by the eluant counter ions that are required to maintain electroneutrality. In 

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