Polarization Polarization has a number of meanings but, in chromatography, it usually refers to the condition of the electrodes used in an electrical conductivity detector. If two electrodes are immersed in a conducting liquid and a DC (direct current) potential is applied to the electrodes, ions are neutralized at the electrode surface and deposits are formed. These deposits may consist simply of bubbles of hydrogen and oxygen, metals if inorganic salts are present, or, with organic salts, organic acids or bases. These deposits interfere with the performance of the detector and the electrodes are said to have become ‘polarized’. This problem is solved by employing AC (alternating current) potential to the electrode which continually reverses the electrolytic effect and, thus, prevents deposits and polarization.
Author: RPW Scott
Book:Liquid Chromatography Detectors
Section:HPLC-Detectors Electrical-Conductivity
conductivity detector measures the conductivity of the mobile phase. There is usually background conductivity which must be backed-off by suitable electronic adjustments. If the mobile phase contains buffers, the detector gives a base signal that completely overwhelms that from any solute usually making detection impossible. Thus, the electrical conductivity detector a bulk property detector. and senses all ions whether they are from a solute or from the mobile phase. In order to prevent polarization of the sensing electrodes, AC voltages must be used and so it is the impedance not the resistance of the electrode system that is actually measured. From a physical chemistry stand point the conductivity of a solution is more important than its resistance. However, it is the resistance (impedance) of the electrode system that determines the current across it. The resistance (R) of any conductor varies directly as its length (L) and inversely as its cross sectional area (a).
Author: RPW Scott
Book:Liquid Chromatography
Section:HPLC Electrical-Conductivity
amplifier. The output from the amplifier is either digitized, and the binary number sent to a computer for storage, or the output is passed directly to a potentiometric recorder. The detector actually measures the electrical resistance between the electrodes which by suitable non-linear amplification, can be made to provide an output that is linearly related to solute concentration. It is essential that an AC voltage is used across the electrodes to measure the cell impedance to avoid electrode polarization. The frequency of the AC potential across the electrodes is usually around 10 kHz Figure 21. An Electrical Conductivity Detector Sensing Cell
