Conductimetric detectors

Valproic acid has been determined in human serum using capillary electrophoresis and indirect laser induced fluorescence detection [26], The extract is injected at 75 mbar for 0.05 min onto a capillary column (74.4 cm x 50 pm i.d., effective length 56.2 cm). The optimized buffer 2.5 mM borate/phosphate of pH 8.4 with 6 pL fluorescein to generate the background signal. Separation was carried out at 30 kV and indirect fluorescence detection was achieved at 488/529 nm. A linear calibration was found in the range 4.5 144 pg/mL (0 = 0.9947) and detection and quantitation limits were 0.9 and 3.0 pg/mL. Polonski et al. [27] described a capillary isotache-phoresis method for sodium valproate in blood. The sample was injected into a column of an EKI 02 instrument for separation. The instrument incorporated a conductimetric detector. The mobile phase was 0.01 M histidine containing 0.1% methylhydroxycellulose at pH 5.5. The detection limit was 2 pg/mL. 

Also, conductimetric detection has been used, by applying two electrodes into the capillary. In this detection mode, the separation of the high voltage part of the capillary (under dc) from the detection zone (under ac) can be accomplished also by electronic filtering. A capillary electropherograph, with a conductimetric detector, is commercially available. 

The analysis of the reaction products was performed using High Performance Ionic Chromatography (HPIC, Dionex 4500i). It works with a ternary gradient of elution and includes an ion-exchange column (AS5A, Dionex) and a conductimetric detector followed by a refractive index detector. 

Various types of detector have been employed for the detection of polymers separated by liquid adsorption chromatography, including conductimetric detectors for polyoxyethylenes [115] and UV detectors for ethyl methacrylate-butyl methacrylate copolymers [116]. Hancock and Synovec [117] carried out a rapid characterisation of linear and star branched polymers, particularly PS by a gradient detection method. This method measures average molecular weight in methylene dichloride solutions of the polymers and gives more specific... 

The greatest disadvantage of conductimetric detectors is their absolute lack of selectivity, which must frequently be achieved by using a previous pretreatment such as ionic chromatography, capillary electrophoresis or gas diffusion, among others [20,24]. 

Several papers have reported use of a weak acid/crown ether eluent in ion-exclusion/cation exchange chromatography with conductimetric detection on a weakly acidic cation-exchange column to effect the simultaneous determination of both cations and anions. Resolution was significantly improved when 18-crown-6 was present in the eluent. Detector response was positive for anions and negative for cations. [76-80] A sample chromatogram is shown in Figure 9. 

Similar to other dynamic approaches, a variety of optical (spectroscopic and non-spectroscopic), electroanalytical (amperometric, potentiometric, conductimetric) and thermochemical detectors can be coupled to continuous US-assisted leachers to... 

Figure 18.6 Direct and membrane conductimetric detection methods for measuring TOC. Radiation of a high output UV lamp (185 and 254 nm) produce, in water, by different pathways, hydroxyl free radicals (OH), which oxidize the organic compounds to carbon dioxide and water from oxygen dissolved in the water. The difference between types (a) and (b) detectors is that the direct detector is subjected to interference from ionic contamination, acids, bases, and halogenated organics. In the membrane based conductimetric method, the membrane is a protective barrier to interfering ions, enabling the analysis of COj only (reproduced courtesy of GE Analytical Instruments for illustrations).

Detectors to be used in FIA should idealiy be endowed with a number of attributes such as low flow-cell volume and noise, flow-rate-independent signal, fast and linear response over a wide concentration range and high sensitivity. FIA methodology utilizes a variety of analytical detection techniques such as optical (spectroscopic and non-spectroscopic), electric (amperometric, po-tentiometrlc, conductimetric, coulometric) and thermochemical. 

Other detection methods which have been employed with a measure of success are electrochemieal (this requires the detector electrodes to be positioned very precisely on opposite sides of the capillary wall), ampero-metric (useful for electroactive species), conductimetric and mass spectrometry. The latter, though presenting particular difficulties in interfacing which have not been fully resolved is sensitive gives structural information and offers the potential for characterising complex biomolecules. 

The highly conductive anions in a low background conductance of H2CO3 are detected at high sensitivity by the conductivity detector. The nonspecific nature of the conductimetric detection allows several ions to be sequentially determined in the same sample. The conductimetric detection is highly specific and relatively free from interferences. 

Recently, electrochemical detection methods, namely, conductimetry, amperometry, and potentio-metry, have also become accessible. All three variants of electrochemical detection are intrinsically simpler than the optical methods, and their success depends highly on the electrode materials and designs used. Conductivity detection relies on measurement of the differences between the conductivities of the analyte and the separation electrolyte this provides a direct relationship between migration times and response factor, and makes this detector universal. On the contrary, amperometric detection is restricted to electroactive species and potentiometric detection is not possible for certain small ions with multiple charges. Conductimetric detection works better for inorganic compounds since the higher mobility of... 

IC sometimes employs methods of detection other than conductivity. For transition metal cations, UV-vis detection is used after postseparator reaction with a chelating agent. Electrochemical detectors such as pulsed ampero-metric detectors have also been employed, especially for organic ions. We focus here mainly on conductimetric detection since the vast majority of published work with macrocycles uses this method. ... 

Crown ethers have undergone limited investigation as components in ion chromatographic detection systems. One example is the work of Jane and Shih, who coated a piezoelectric quartz crystal with dibenzo-16-crown-5-oxydodecanoic acid. The detector was used for cation and anion detection after separation on a diaza-18-crown-6-based separator column with nonionic eluents. The frequency response of this detector for both cations and anions, due to cation complexation and anion association with the resulting complex, was as reproducible and sensitive as standard conductimetric detection, but peak broadening resulted from a relatively large cell volume. 

Ion detector ANALY CHEM Device for detection of presence or concentration of liquid solution ions, such as with a pff meter or by conductimetric techniques. T,an di.tek-tcr)... 

Also using amperometric detection, Chen, et al. developed a three-dimensionally adjustable amperometric detector for MCE and applied to separate aromatic amines and nitroaromatic pollutants. Amperometry, and related pulse amperometric detection modes, are very attractive because provide enhanced selectivity related to conductimetric detection. 

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