Ion Chromatography - The Electrical Conductivity Detector

 

 

The Electrical Conductivity Detector

The sensor of the electrical conductivity detector is the simplest of all the detector sensors and consists of only two electrodes situated in a suitable flow cell. An example of an electrical conductivity sensor cell is shown in figure 20. The sensor consists of two electrodes sealed into a glass flow cell as depicted in the upper diagram in figure 20.

 

 

Figure 20. An Electrical Conductivity Detector Sensing Cell

 

 

In the electric circuit, the two electrodes are arranged to be the impedance component in one arm of a Wheatstone Bridge. When ions move into the sensor cell, the electrical impedance between the electrodes changes and the 'out of balance signal' from the bridge is fed to a suitable electronic circuit. The 'out of balance' signal is not inherently linearly related to the ion concentration in the cell. Thus, the electronic circuit modifies the response of the detector to provide an output that is linearly related to the ion concentration.

 

The amplifier output is then either digitized, and the binary number sent to a computer for storage and processing, or the output is passed directly to a potentiometric recorder. It is essential that an AC voltage is used across the electrodes to avoid electrode polarization which will certainly occur if a DC voltage is used. This would result in a false change in impedance due to the generation of gases at the electrode surfaces. The frequency of the AC potential that is applied across the electrodes is normally about 10 kHz A more practical arrangement is shown in the lower part of figure 20 and in its simplest form can consist of short lengths of stainless steel tube insulated from each other by PTFE connecting sleeves.

 

It is convenient and electrically propitious, to connect the first tube (that is physically connected to the column) to ground (earthed). The resistance between the first tube and the second (isolated) tube is monitored which is, in fact, the impedance of the small gap between the first and second tubes in the Teflon sleeve.

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 ion used for measurement was the sodium+.ion

The separation of a mixture of alkali and alkaline earth cations at levels of a few parts per million is shown in figure 21. The chromatogram gives a typical example of the use of the electrical conductivity detector. The cations lithium, sodium, ammonium, potassium, magnesium and calcium were present at concentrations of 1, 4, 10, 10, 5 and 10 ppm respectively in the original mixture.