Liquid Chromatography - The Electrical Conductivity Detectors 2

A more practical system shown in the lower part of the diagram consists of short lengths of stainless steel tube insulated from each other by a PTFE connecting sleeves. For convenience, the first tube (that connected to the column) is usually grounded (earthed). The resistance between the inlet tube and the center tube is continuously monitored which will constitute the resistance across the tiny gap between the tubes contained in the first PTFE sleeve. The volume of eluent in this gap can be extremely small and thus, the peak dispersion can also be made very small. The resistance of the solution situated between the tubes is inversely proportional to the electric conductivity of the solution which, in turn, is related to the ion concentration in mobile phase.

Some typical specifications for an electrical conductivity detector are as follows.

Typical Specifications for an Electrical Conductivity Detector

 

Sensitivity

5x 10-9 g/ml

Linear Dynamic Range

5 x 10-9 to 1 x 10-6 g/ml

Response Index

0.97 - 1.03

The separation of a mixture of alkali and alkaline earth cations at levels of a few parts per million, shown in figure 22 gives an example of the use of the electrical conductivity detector. The cations lithium, sodium, ammonium, potassium, magnesium and calcium were present in the original mixture at concentrations of 1, 4, 10, 10, 5 and 10 ppm respectively.

Courtesy of Dionex Inc

 

Figure 22. Determination of Alkali and Alkaline Earth Cations