Electrochemical methods detection limits

Goodwin et al. [82] determined trace sulphides in turbid waters by a gas dialysis-ion chromatographic method. The sulphide is converted to hydrogen sulphide which is then isolated from the sample matrix by diffusion through a gas dialysis membrane and trapped in a dilute sodium hydroxide solution. A 200pL portion of this solution is injected into an ion chromatograph for determination with an electrochemical detector. Detection limits were >1.9ng/mL... 

A simple electrochemical flow-through cell with powder carbon as cathodic material was used and optimized. The influence of the generation current, concentration of the catholyte, carrier stream, flow rate of the sample and interferences by other metals on the generation of hydrogen arsenide were studied. This system requires only a small sample volume and is very easily automatized. The electrochemical HG technique combined with AAS is a well-established method for achieving the required high sensitivity and low detection limits. 

The HPLC-based measurement of 8-OH-dGua is a highly sensitive method, largely because of the use of electrochemical detection, introduced by Floyd et al. (1986). Floyd et al. (1986) quote a detection limit of 20 fmol, or one 8-OH-dGua per 10 nucleosides. Shigenaga et al. (1990) quote 5-50 fmol on 40-100 / g samples of DNA. 

Table 8.76 shows the main characteristics of voltammetry. Trace-element analysis by electrochemical methods is attractive due to the low limits of detection that can be achieved at relatively low cost. The advantage of using standard addition as a means of calibration and quantification is that matrix effects in the sample are taken into consideration. Analytical responses in voltammetry sometimes lack the predictability of techniques such as optical spectrometry, mostly because interactions at electrode/solution interfaces can be extremely complex. The role of the electrolyte and additional solutions in voltammetry are crucial. Many determinations are pH dependent, and the electrolyte can increase both the conductivity and selectivity of the solution. Voltammetry offers some advantages over atomic absorption. It allows the determination of an element under different oxidation states (e.g. Fe2+/Fe3+). 

Cyclic voltammetry, square-wave voltammetry, and controlled potential electrolysis were used to study the electrochemical oxidation behavior of niclosamide at a glassy carbon electrode. The number of electrons transferred, the wave characteristics, the diffusion coefficient and reversibility of the reactions were investigated. Following optimization of voltammetric parameters, pH, and reproducibility, a linear calibration curve over the range 1 x 10 6 to 1 x 10 4 mol/dm3 niclosamide was achieved. The detection limit was found to be 8 x 10 7 mol/dm3. This voltammetric method was applied for the determination of niclosamide in tablets [33]. 

Alemu et al. [35] developed a very sensitive and selective procedure for the determination of niclosamide based on square-wave voltammetry at a glassy carbon electrode. Cyclic voltammetry was used to investigate the electrochemical reduction of niclosamide at a glassy carbon electrode. Niclosamide was first irreversibly reduced from N02 to NHOH at —0.659 V in aqueous buffer solution of pH 8.5. Following optimization of the voltammetric parameters, pH and reproducibility, a linear calibration curve over the range 5 x 10 x to 1 x 10-6 mol/dm3 was achieved, with a detection limit of 2.05 x 10-8 mol/dm3 niclosamide. The results of the analysis suggested that the proposed method has promise for the routine determination of niclosamide in the products examined [35]. 

Couto et al. [11] developed a flow injection system with potentiometric detection for determination of TC, OTC, and CTC in pharmaceutical products. A homogeneous crystalline CuS/Ag2S double membrane tubular electrode was used to monitor the Cu(II) decrease due to its complexation with OTC. The system allows OTC determination within a 49.1 1.9 x 103 ppm and a precision better than 0.4%. A flow injection method for the assay of OTC, TC, and CTC in pharmaceutical formulations was also developed by Wangfuengkanagul et al. [12] using electrochemical detection at anodized boron-doped diamond thin-film electrode. The detection limit was found to be 10 nM (signal-to-noise ratio = 3). 

To date, a few methods have been proposed for direct determination of trace iodide in seawater. The first involved the use of neutron activation analysis (NAA) [86], where iodide in seawater was concentrated by strongly basic anion-exchange column, eluted by sodium nitrate, and precipitated as palladium iodide. The second involved the use of automated electrochemical procedures [90] iodide was electrochemically oxidised to iodine and was concentrated on a carbon wool electrode. After removal of interference ions, the iodine was eluted with ascorbic acid and was determined by a polished Ag3SI electrode. The third method involved the use of cathodic stripping square wave voltammetry [92] (See Sect. 2.16.3). Iodine reacts with mercury in a one-electron process, and the sensitivity is increased remarkably by the addition of Triton X. The three methods have detection limits of 0.7 (250 ml seawater), 0.1 (50 ml), and 0.02 pg/l (10 ml), respectively, and could be applied to almost all the samples. However, NAA is not generally employed. The second electrochemical method uses an automated system but is a special apparatus just for determination of iodide. The first and third methods are time-consuming. 

In contrast, the coupling of electrochemical and spectroscopic techniques, e.g., electrodeposition of a metal followed by detection by atomic absorption spectrometry, has received limited attention. Wire filaments, graphite rods, pyrolytic graphite tubes, and hanging drop mercury electrodes have been tested [383-394] for electrochemical preconcentration of the analyte to be determined by atomic absorption spectroscopy. However, these ex situ preconcentration methods are often characterised by unavoidable irreproducibility, contaminations arising from handling of the support, and detection limits unsuitable for lead detection at sub-ppb levels. 

In virtually all of the simple immersion and two electrode experiments carried out so far, in-diffused H has been detected at the 1016/cm3 level or less. There has been no demonstration that large densities (> 1018/cm3) of defects can be passivated by these methods, and where plasma and electrochemical treatments have been directly compared, the former have been found to be more effective (Tavendale et al., 1986). In contrast to plasma techniques, the electrolyte boiling point limits the temperature range of electrochemical methods, although several hundred degrees Celsius can be utilized for electrolytes like H3P04. 

Fluorescence detection can be up to four orders of magnitude more sensitive than UV absorbance, especially where laser induced excitation is used, mass detection limits being as low as 10-20—10 21 mole. Pre- and post-column derivatization methods are being developed to extend the applicability of fluorescence detection to non-fluorescent substances. Several types of electrochemical and mass spectrometric detector have also been designed. Detector characteristics are summarized in Table 4.21. 

A prerequisite for a precise and accurate titration is the reproducible identification of an end point which either coincides with the stoichiometric point of the reaction or bears a fixed and measurable relation to it. An end point may be located either by monitoring a property of the titrand which is removed when the stoichiometric point is passed, or a property which can be readily observed when a small excess of the titrant has been added. The most common processes observed in end-point detection are change of colour change of electrical cell potential change of electrical conductivity precipitation or flocculation. (Electrochemical methods are discussed in Chapter 6 precipitation indicators find only limited use.)... 

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