The Conductivity Detector

The conductivity detector measures the conductivity of the mobile phase by determining the impedance between two electrodes situated appropriately in the column eluent. This is achieved by making the conductivity cell one arm of a Wheatstone Bridge across which voltage is applied. It follows that only one, or in some cases neither, of the electrodes can be at earth potential. As the mobile phase is conducting there will be electrical continuity from the electrodes via the mobile phase to the column. Hiere will 

In some commercial refractive index detectors the optical system is designed to cover a specific range of refractive indices. Manufacturers of such instruments usually provide a number of cell or optical systems that are interchangeable and cover the practical range of refractive indices normally met in LC. Where such instruments are used the refractive index of the mobile phase being employed should be checked and the appropriate cell system used. If the incorrect optical system is used by mistake, it will usually be found that a balance point cannot be obtained with the mobile phase in both cells or, if balance is obtained, the sensitivity of the system is extremely low. The refractive index detector although relatively simple to operate is probably the most difficult instrument to use at maximum sensitivity due to is general instability under these conditions. 

Most commercial refractometers have a very restricted linear dynamic range, sometimes less than three orders of magnitude. When using the refractive index detector for quantitative analysis it is advisable to check the linear range of the instrument. 

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Limits of detection become a problem in capillary electrophoresis because the amounts of analyte that can be loaded into a capillary are extremely small. In a 20 p.m capillary, for example, there is 0.03 P-L/cm capillary length. This is 1/100 to 1/1000 of the volume typically loaded onto polyacrylamide or agarose gels. For trace analysis, a very small number of molecules may actually exist in the capillary after loading. To detect these small amounts of components, some on-line detectors have been developed which use conductivity, laser Doppler effects, or narrowly focused lasers (qv) to detect either absorbance or duorescence (47,48). The conductivity detector claims detection limits down to lO molecules. The laser absorbance detector has been used to measure some of the components in a single human cell (see Trace AND RESIDUE ANALYSIS). 

The conductance detector is a universal detector for ionic species and is widely used in ion chromatography (see Section 7.4). 

A well-known fact of fundamental solution science is that the presence of ions in any solution gives the solution a low electrical resistance and the ability to conduct an electrical current. The absence of ions means that the solution would not be conductive. Thus, solutions of ionic compounds and acids, especially strong acids, have a low electrical resistance and are conductive. This means that if a pair of conductive surfaces are immersed into the solution and connected to an electrical power source, such as a simple battery, a current can be detected flowing in the circuit. Alternatively, if the resistance of the solution between the electrodes were measured (with an ohmmeter), it would be low. Conductivity cells based on this simple design are in common use in nonchromatography applications to determine the quality of deionized water, for example. Deionized water should have no ions dissolved in it and thus should have a very low conductivity. The conductivity detector is based on this simple apparatus. 

For many years, the concept of the conductivity detector could not work, however. Ion chromatography experiments utilize solutions of high ion concentrations as the mobile phase. Thus, changes in conductivity due to eluting ions are not detectable above the already high conductivity of the mobile phase. This was true until the invention of so-called ion suppressors. Today, conductivity detectors are used extensively in HPLC ion chromatography instruments that also include suppressors. 

As a result of these reactions in the suppressor column, the sample ions are presented to the conductivity detector as H+X, not in the highly... 

On the other hand, because of the nonspecific nature of the conductivity detector, the chromatograph peaks are identified only by their retention times. Thus, the two ions having the same or close retention times will be detected as one broad peak giving erroneous results. 

Orthophosphate in an untreated or treated sample may be determined by ion chromatography (See Chapter 1.11). A detection limit of 0.1 mg/L may be achieved with a 100-pL sample loop and a 10 mmho full-scale setting on the conductivity detector. The column and conditions for a typical wastewater analysis are listed below. Equivalent column and alternate conditions may be used. 

Name two analytes that can be detected by the conductivity detector. 

Applicability of the conductivity detector can be extended by chemical derivatization or by the use of postcolumn photochemical reactions [78]. The use of a photochemical reaction detector, also known as a photoconductivity detector, can also be very selective. Only certain organic compounds such as trinitroglycerin, chloramphenicol, and hydrochlorothiazide will undergo photolytic decomposition to produce ionic species. 

Figure 5.3 Chromatogram of choline in the presence of equal molar quantity of antibiotic. A silica gel packed column with the mobile phase of aqueous 1% phosphoric acid with direct conductivity detection. The antibiotic is not detected by the conductivity detector.

Separation capillary columns are made in fluorinated ethylene- propylene copolymer. Detection is achieved by conductivity cells and an ac. conductivity mode of detection is used for making the separations visible. The driving current is supphed by a unit enabling independent currents to be preselected for the preseparation and final analytical stages. The run of the analyser is controlled by a programmable timing and control unit. The zone lengths from the conductivity detector, evaluated electronically, can be printed on a line printer. 

Since the cation suppression device relies on an addition of hydroxide in exchange for eluent anion (typically Cl ), this method precludes transition metal determinations because the metal hydroxides would precipitate out of solution before fore entering the conductivity detector. Thus, several years went by before the development and addition of transition metal capabilities to ion chromatography. Several methods have been reported for the determination of stable anionic metal complexes by... 

Fig. 2.7 shows a separation of seven common anions monitored using the conductivity detector. Fig. 2.7(b) was obtained with the ultraviolet detector after the suppressor column. As is illustrated in Fig. 2.7, nitrite, bromide, and nitrate absorb strongly in the ultraviolet, while fluoride, phosphate and sulphate do not show appreciable absorption above 190nm. Chloride absorbs weakly in the ultraviolet region below 200nm. Note that... 

Arsenous acid is a veiy weak acid and cannot be detected at low levels by the conductivity detector. However, like sulphide, it is easily detected with the ultraviolet detector. The simultaneous determination of arsenite and arsenate is possible. 

Fig. 2.23 shows the separation achieved on a 12 anion standard by this procedure. Sulphide, cyanide, bromide, and sulphite are detected at the silver electrode while nitrite, nitrate, phosphate and sulphate produce no response. Due to the low dissociation of hydrogen sulphide and hydrogen cyanide following protonation by the suppressor column, they are not detected by the conductivity detector. 

The ion chromatographic determination of weak acid anions is complicated by ion exclusion in the suppressor column, resulting in faster elution and sharper peaks, directly proportional to the degree of exhaustion of the suppressor column [7], A lOmg L 1 nitrite standard showed a 37% increase in peak height over an 8h period when monitored with the conductivity detector while on only a minor 2% increase in peak height was observed over the same time period by using the ultraviolet detector after the separator column. 

The ultraviolet detector can also be used in some cases to resolve overlapping peaks. Determination of the nitrite peak by using the conductivity detector is complicated by both the ion exclusion effect and the incomplete resolution between the large chloride peak and the much smaller nitrite peak. 

Hydrogen sulphide acid is a veiy weak acid that does not ionise sufficiently to be detected with the conductivity detector. The ultraviolet detector, however, is able to detect sulphide at low levels. 

Other Detectors. The conductivity detector is increasing in popularity because it is usually the detector used in ion chromatographs. The special problems associated with its use were discussed in the section on IC. 

Currently, ion chromatography (IC is a specialized field of IEC. The development of suitable ion exchange columns and utilization of the conductivity detector has led to the use of IC for the analysis of inorganic ions such as metal ions, F , CL, SC>42, and P043-. References to IC applications have increased significantly each year since the mid-1970s (see Fig. 1-12). As the capabilities of HPLC expand, the sharp distinction between HPLC and IC becomes increasingly blurred. 

The conductivity detector is a bulk property detector and, as such, responds to any electrolytes present in the mobile phase e.g. buffers etc.) as well as the solutes. It follows that the mobile phase must be arranged to be either non-conducting, which in many cases is difficult if not impossible to achieve, or the mobile phase buffer electrolytes must be removed prior to the detector. This technique of buffer ion removal is commonly called ion suppression. The first type of ion... 

Organic and inorganic anions are encountered in forensic examination and qualitative and quantitative analyses are required. Since the introduction of ion chromatography, the analysis of several anions can be performed simultaneously and good sensitivity can be obtained with the conductivity detector. 

The conductivity detector measures the conductivity of a solution containing an electrolyte. When current is allowed to pass through two electrodes, there is resistance (or better impedance) to the flow of current... 

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