The Electrical Conductivity Detector

Experiments using AC potentials across the electrode system demonstrated that ionic solutions obeyed Ohms Law i.e. 

Generally, in physical chemistry the conductivity of the solution is of greater interest and is equivalent to the reciprocal of the specific resistance of a solution. The specific resistance of a solute is numerically equivalent to the potential in volts across the faces of a centimeter cube of the solution when carrying unit current. In electrical conductivity detectors it is the resistance of the solution that is actually monitored and it is the change in electrical resistance of the mobile phase in the presence of a solute that provides the output from the detector. For this reason, despite the term electrical conductivity detector, the functioning of the detector will be considered in term of resistance measurement. 

The chromatographer is so familiar with water as the medium in which the ionization of solutes takes place that he tends to overlook the possibility of other solvents. Liquid ammonia, 

Liquid ammonia and liquid sulphur dioxide appear to offer attractive possibilities for chromatographic purposes as both these substances, particular liquid sulphur dioxide, are very good solvents for organic substances. Such liquids would also lead to a unique form of supercritical chromatography (7). Furthermore, they are likely to ionize specific organic materials that otherwise do not normally ionize in water or are insoluble in 

The liquid chromatographer, at the first thought of such a system, may well be dismayed at the problems of toxicity and mechanical handling that appears to accompany the use of such materials. However, in the early days of refrigeration, entirely sealed systems were manufactured successfully for use in domestic environments employing both ammonia and liquid sulphur dioxide as the refrigerating liquid. It follows that the development of sealed LC systems incorporating such materials for use by scientists in a laboratory environment appears distinctly possible. 

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The electrical conductivity detector is probably the second most commonly used in LC. By its nature, it can only detect those substances that ionize and, consequently, is used frequently in the analysis of inorganic acids, bases and salts. It has also found particular use in the detection of those ionic materials that are frequently required in environmental studies and in biotechnology applications. The detection system is the simplest of all the detectors and consists only of two electrodes situated in a suitable detector cell. An example of an electrical conductivity detector sensing cell is shown in figure 13. 

The popularity of the UV detector, the electrical conductivity detector and the fluorescence detector motivated Schmidt and Scott (5,6) to develop a trifunctional detector that detected solutes by all three methods simultaneously in a single low volume cell. 

Chromatograms demonstrating the simultaneous use of all three detector functions are shown in figure 22. It is seen that the anthracene is clearly picked out from the mixture of aromatics by the fluorescence detector and the chloride ion, not shown at all by the UV adsorption or fluorescence detectors, clearly shown by the electrical conductivity detector. 

Perhaps the most important of all electrochemical detection schemes currently in use is the electrical conductivity detector. This detector is specifically useful for ion exchange, or ion, chromatography in which the analyte is in ionic form. Such ions elute from the column and need to be detected as peaks on the recorder trace. 

Electrical conductivity detector is commonly use. 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. The sensor consists of two electrodes sealed into a glass flow 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... 

The problem arises in choosing the most appropriate solute with which to measure the concentration sensitivity of all detectors. Obviously toluene could be used to define the sensitivity of the FDD, UV detector and the RI detector but would be useless for measuring the sensitivity of the electrical conductivity detector or the nitrogen phosphorus detector (NPD) in GC. In a similar way, if the reference solute was chosen to have high refractive index and low UV absorption and contain no phosphorus or nitrogen then the RI detector... 

Although over the years a large number of LC detectors have been developed and described, over 95% of all contemporary LC analyses are carried out using one of four detectors, the UV detector in one of its forms, the electrical conductivity detector, the fluorescence detector and the refractive index detector. In addition, some form of the UV detector probably accounts for 80% of those analyses. 

In contrast, the electrochemical detector responds only to substances that can be oxidized or reduced and thus, providing the mobile phase is free of such materials, it will only detect oxidizable or reducible substances when they are eluted. It follows that this detector is not only a solute property detector but is also a specific detector. The electrical conductivity detector is a non-specific detector and used widely in ion chromatography where it occupies a unique and almost exclusive position. In contrast, the electrochemical detector, in its... 

One Form of the Wheatstone Bridge Used with the Electrical Conductivity Detector... 

The electronic system of the electrical conductivity detector is comprised of a 1 kHz frequency generator, the output of which is fed via a suitable impedance to the detector electrodes. The voltage across the electrodes is fed to a precision rectifier to provide a DC signal that is related to the conductivity of the fluid between the sensor plates. The DC signal is then passed to a signal modifying amplifier to... 

The four most commonly used LC detectors are the UV detector, the fluorescence detector, the electrical conductivity detector and the refractive index detector. Despite there being a wide range of other detectors to choose from, these detectors appear to cover the needs of 95% of all LC applications. This is because the major use of LC as an analytical technique occurs in research service laboratories and industrial control laboratories where analytical methods have been deliberately developed to utilize the more straight forward and well established detectors that are easy and economic to operate. LC detectors are more compact than their GC counterparts and need much less ancillary support. Most operate solely on the mobile phase and need no other fluid supplies for their effective use. All LC detectors are 3-5 orders of magnitude less sensitive than their GC counterparts and thus sensor contamination is not so severe, and generally less maintenance is required. 

The electrical conductivity detector measures the conductivity of the mobile phase. Conductivity detectors are universal and nondestructive and can be used in either direct or indirect modes. The conductivity sensor is the simplest of all the detectors, consisting of only two electrodes situated in a suitable flow cell. The basis of conductivity is the forcing of ions in solution to move toward the electrode of opposite charge on the application of a potential. To prevent polarisation 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... 

The ion chromatographic methods used in water analysis usually employ electric conductivity detectors with some kind of suppressor [85]. Its ease of use, simplicity to maintain, and broad d)mamic concentration range make it popular. Solute-specific amperometric detectors have also been successfully used. The electric conductivity detectors are universal ion sensors. Less frequently specific electrometric detectors are... 

More recently (1984), Baba and Housako (7) described another bifunctional detector but this time based on the UV absorption detector combined with the electrical conductivity detector. A diagram of their detector is shown in Figure 2. The UV absorption system is very similar to that of the DuPont bifunctional detector. UV light is collimated through the cell and focussed by a second quartz lens onto a photo diode, the output from which, is processed by suitable electronic circuitry in the usual manner. 

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