Conductometric Detectors

It is universal for aU anions or cations in solution (i.e., a bulk property of mobile-phase detector), nondestructive, and a concentration detector compound sensitivities differ over an order of magnitude range, useable primarily with isocratic RP-HPLC without buffer salts (unless subsequent suppression column is used) and it is a premier detector for ion chromatography (IC). 

The primary usefulness of the conductometric detector is for small anions or cations such as inorganic ions (e.g., CF, CIO4, NOJ,, Na , NH4) or small organic anions (e.g., acetate, oxalate, citrate) 

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A composite polymer membrane has also been used as an effective amperometric detector for ion exchange chromatography [42] and showed detection limits similar to those obtained with a conductivity detector. An advantage of the amperometric detector based on micro-ITIES over the conductometric detector is that selectively can be tailored by proper choice of the ionophore. For instance, the selectivity of the membrane toward ammonium in the presence of an excess of sodium could be substantially increased by introducing an ammonium-selective ionophore (such as valinomycin) in the gel membrane [42]. 

Many IC techniques are now available using single column or dual-column systems with various detection modes. Detection methods in IC are subdivided as follows [838] (i) electrochemical (conductometry, amper-ometry or potentiometry) (ii) spectroscopic (tJV/VIS, RI, AAS, AES, ICP) (iii) mass spectrometric and (iv) postcolumn reaction detection (AFS, CL). The mainstay of routine IC is still the nonspecific conductometric detector. A significant disadvantage of suppressed conductivity detection is the fact that weak to very weak acid anions (e.g. silicate, cyanide) yield poor sensitivity. IC combined with potentiometric detection techniques using ISEs allows quantification of selected analytes even in complex matrices. The main drawback... 

More recently, the description of a small-bore liquid chromatography by Manz et al. [83] on a 5 x 5 mm silicon chip revived the interest of the analytical chemical community. The chip incorporated an open tubular column of 6x2 pm cross section (column volume 1.8 nl) and a conductometric detector with a detection volume of only 1.2 pi. Although this work was an important benchmark study, the actual functioning of the chromatograph was never demonstrated with separations. 

Capillary electrophoresis Capillary electrophoresis (CE) is used to analyze sodium, potassium, calcium, and magnesium in water samples. The detection is conducted by reverse absorbance measurements. Sufficient separation of the four cations is established with an electrolyte solution of 5 mM imidazole/6.5 mM a-hydroxyisobutyric acid/2 mM 18-crown-6 ether of pEI 4.1 [42]. CE with a contactless conductometric detector is used to determine small anions and cations in water samples from different sources. 2-(N-Morpholino)ethanesulfonic acid/histidine-based (Mes/Elis) electrolytes are used for direct conductivity detection of anions and cations, while ammonium acetate is used for indirect conductivity determination of alkylammonium salts. Eor the simultaneous separation procedure, involving dual-opposite end injection, an electrolyte consisting of 20 mM Mes/EIis, 1.5 mM 18-crown-6 and 20 mM cetyltri-methylammonium bromide provides baseline separation of 13 anions and cations in less than 6 min [43]. Also CE with a capacitively coupled... 

The adoption of an eluent having a very low conductivity, which can be passed directly through the conductometric detector. Typical eluents used are benzoate, phthalate, or other aromatic acid salts, with low limiting equivalent conductances (leading to direct detection) or potassium hydroxide eluent, with high conduc-... 

Conductometric detector A detector for charged species finds use in ion chromatography. 

General and Selective Detectors Detectors can be classified either as general or selective. A general detector will respond to all or most of the ions that pass through the detector cell. A conductometric detector is classified as a general detector because... 

Electrochemical detectors are sometimes divided into the groups of potenliomctric. amperometric, and conductometric detectors, that is, according to the three parameters of electric measurements. Potentiometric detectors measure voltage, amperometric measure current, and conductometric measure resistance. 

Conductometric detectors respond to all ions, but the other detectors respond only to certain electroactive ions. In this book, electrochemical detection will refer to amperometric, or potentiometric detectors, but will not refer to conductometric detectors. 

Voltammetric, conductometric and coulomeiric detectors arc also available commercially. Conductometric detectors are discussed further in Section 28r-3. 

Figure 13.13 Three conductometric detector designs. (Scott, used with permission.)...

Miniature amperometric or conductometric detectors can be introduced into the capillary, by plating the appropriate electrodes in the inside of the capillary walls. It is often necessary to employ a porous glass or graphite joint between the capillary containing the detector electrodes and the end of the capillary in the detector end electrode reservoir, in order to isolate the detector from the high voltage used to power the electrophoretic separation. 

P. Kubafi, B. Karlberg, P. Kubafi and V. Kubdfl, Application of a contactless conductometric detector for the simultaneous determination of small anions and cations by capillary electrophoresis with dual-opposite end injection, J. Chromatogr. A, 964, 227-241, 2002. 

Figure 13.15 Three conductometric detector designs. (From Scott, R.P.W., Chromatographic Detectors, Marcel Dekker, Inc., New York, 1996. With permission.)...

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