Detection limit voltammetry

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

The methods of investigation of metal species in natural waters must possess by well dividing ability and high sensitivity and selectivity to determination of several metal forms. The catalytic including chemiluminescent (CL) techniques and anodic stripping voltammetry (ASV) are the most useful to determination of trace metals and their forms. The methods considered ai e characterized by a low detection limits. Moreover, they allow detection of the most toxic form of metals, that is, metal free ions and labile complexes. 

PSS-SG composite film was tested for sorption of heme proteins hemoglobin (Hb) and myoglobin (Mb). The peroxidaze activity of adsorbed proteins were studied and evaluated by optical and voltammetric methods. Mb-PSS-SG film on PG electrode was shown to be perspective for detection of dissolved oxygen and hydrogen peroxide by voltammetry with linear calibration in the range 2-30 p.M, and detection limit -1.5 p.M. Obtained composite films can be modified by different types of biological active compounds which is important for the development of sensitive elements of biosensors. 

The presence of redox catalysts in the electrode coatings is not essential in the c s cited alx)ve because the entrapped redox species are of sufficient quantity to provide redox conductivity. However, the presence of an additional redox catalyst may be useful to support redox conductivity or when specific chemical redox catalysis is used. An excellent example of the latter is an analytical electrode for the low level detection of alkylating agents using a vitamin 8,2 epoxy polymer on basal plane pyrolytic graphite The preconcentration step involves irreversible oxidative addition of R-X to the Co complex (see Scheme 8, Sect. 4.4). The detection by reductive voltammetry, in a two electron step, releases R that can be protonated in the medium. Simultaneously the original Co complex is restored and the electrode can be re-used. Reproducible relations between preconcentration times as well as R-X concentrations in the test solutions and voltammetric peak currents were established. The detection limit for methyl iodide is in the submicromolar range. 

Under optimum conditions LPS voltammetry is an order of magnitude more sensitive than polarography (i.e., the detection limit is about 10 M). As in classical polarography, somewhat higher sensitivity and selectivity can be attained when using a differential version (i.e., when recording, as a function of potential, not the current but its derivative with respect to potential). 

Anodic stripping voltammetry (ASV) has been used extensively for the determination of heavy metals in samples of biological origin, such as lead in blood. ASV has the lowest detection limit of the commonly used electroanalytical techniques. Analyte concentrations as low as 10 M have been determined. Figure 16 illustrates ASV for the determination of Pb at a mercury electrode. The technique consists of two steps. The potential of the electrode is first held at a negative value for several minutes to concentrate some of the Pb " from the solution into the mercury electrode as Pb. The electrode process is... 

As with in vivo voltammetry, a variety of electrochemical techniques have been used for the stripping step. Because of its simplicity, linear sweep voltammetry has enjoyed widespread use however, the detection limit of this technique is limited by charging current. Differential pulse has become popular because it discriminates against the charging current to provide considerably lower detection limits. 

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]. 

Detection of damage caused to DNA by niclosamide in schistosomiasis was investigated using an electrochemical DNA-biosensor. It showed for the first time clear evidence of interaction of niclosamide with DNA and suggested that niclosamide toxicity can be caused by this interaction, after reductive activation. The electrochemical reduction and oxidation of niclosamide involved the use of cyclic, differential, and square-wave voltammetry, at a glassy carbon electrode. It enabled the detection limit of 8 x 10-7 M [34]. 

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]. 

Wangfuengkanagul and Chailapakul [9] described the electroanalysis of ( -penicillamine at a boron-doped diamond thin film (BDD) electrode using cyclic voltammetry. The BDD electrode exhibited a well-resolved and irreversible oxidation voltammogram, and provided a linear dynamic range from 0.5 to 10 mM with a detection limit of 25 pM in voltammetric measurement. In addition, penicillamine has been studied by hydrodynamic voltammetry and flow injection analysis with amperometric detection using the BDD electrode. 

Further improvement of the low detection limit was achieved using stripping voltammetry based on facilitated heparin adsorption and desorption [66], Stripping voltammetry yielded a detection limit of 0.13 U mL 1 in sheep blood plasma, which is lower than therapeutic heparin concentrations (>0.2 U mL-1). A linear response function in the range of 0.2-6 U mL 1 was observed. The authors also found that blood polypeptides and lipids with a mass above 25 000 significantly interfered with heparin detection, perhaps by hindrance of a charge transfer reaction at the interface. 

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. 

Luther et al. [92] have described a procedure for the direct determination of iodide in seawater. By use of a cathodic stripping square-wave voltammetry, it is possible to determine low and sub-nanomolar levels of iodide in seawater, freshwater, and brackish water. Precision is typically 5% (la). The minimum detection limit is 0.1 - 0.2 nM (12 parts per trillion) at 180 sec deposition time. Data obtained on Atlantic Ocean samples show similar trends to previously reported iodine speciation data. This method is more sensitive than previous methods by 1-2 orders of magnitude. Triton X-100 added to the sample enhances the mercury electrode s sensitivity to iodine. 

A deviation from linearity is observed in the calibration curve at higher lead concentrations. The estimated value for the original sample was found to be 0.51 xg/l, with confidence limits at the 95% confidence level of 0.036 pg/1, compared with a value of 0.65 0.08 ig/l obtained by anodic scanning voltammetry. This value is well within the normal range reported in the literature for the natural lead content of unpolluted seawater. A detection limit of 0.03 ng ml"1 was obtained. 

Van den Berg [510] carried out direct determinations of molybdenum in seawater by adsorption voltammetry. The method is based on complex formation of molybdenum (VI) with 8-hydroxyquinoline (oxine) on a hanging mercury drop electrode. The reduction current of adsorbed complexions was measured by differential pulse adsorption voltammetry. The effects of variation of pH and oxine concentration and of the adsorption potential were examined. The method was accurate up to 300 nmol/1. The detection limit was 0.1 nmol/1. 

Donat and Bruland [217] determined low levels of nickel and cobalt in seawater by a voltammetric technique, and the nioxime complexes of the two elements were concentrated on a hanging mercury drop electrode. The current resulting from the reduction of Co (II) and Ni (II) was measured by differential pulse cathodic stripping voltammetry. Detection limits are 6 pM (cobalt) and 0.45 nM (nickel). 

Platinum was determined in seawater by adsorptive cathodic stripping voltammetry in a method described by Van den Berg and Jacinto [531]. The formazone complex is formed with formaldehyde, hydrazine, and sulfuric acid in the seawater sample. The complex is adsorbed for 20 minutes at -0.925 V on the hanging mercury drop electrode. The detection limit is 0.04 pM platinum. 

Van den Berg and Huang [292] determined uranium (VI) in seawater by cathodic stripping voltammetry at pH 6.8 of uranium (Vl)-catechol ions. A hanging mercury drop electrode was used. The detection limit was 0.3 nmol/1 after... 

Cyclohexane-1,2-dione dioxime (nioxime) complexes of cobalt (II) and nickel (II) were concentrated from 10 ml seawater samples onto a hanging mercury drop electrode by controlled adsorption. Cobalt (II) and nickel (II) reduction currents were measured by differential pulse cathodic stripping voltammetry. Detection limits for cobalt and nickel were 6 pM and 0.45 mM, respectively. The results of detailed studies for optimising the analytical parameters, namely nioxime and buffer concentrations, pH, and adsorption potential are discussed. 

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

Pongratz and Hunmann [19], using differential pulse anodic scanning voltammetry, found low levels of methyl cadmium compounds in the Atlantic Ocean. Levels in the South Atlantic were approximately 700 pg/1, and those in the North Atlantic were below the detection limit of the method, i.e., below 470 pg/1. It is believed that these compounds were formed as a result of biomethylation of inorganic cadmium. 

Another method is explored with gene expression-based biosensors [19] for Cd measurement. A Cd-responsive promoter from E. coli is fused to a promoterless lacZ gene and monitored with an electrochemical assay of /J-galac-tosidase activity. The expected detection limit is about 0.1 mg L 1 with a response of a few minutes. As for stripping voltammetry, no real tests have been carried out for wastewater. However, biosensors can be considered as a promising technique for wastewater monitoring. 

Because Nafion film can preconcentrate organic cations at a substrate electrode surface, Nafion-coated electrodes can be used as sensitive and selective sensors for electroactive organic cations. The film-entrapped ion is then either oxidized or reduced and the resulting current is recorded and related to the concentration of ions in the aqueous solution (qualitatively similar to anodic stripping voltammetry). It was shown that ion-exchange voltammetric determination of the dication methyl viologen at a Nafion-film-coated electrode results in an improvement of three orders of magnitude in detection limit relative to an uncoated electrode.46... 

Cyclic voltammetry is generally considered to be of limited use in ultratrace electrochemical analysis. This is because the high double layercharging currents observed at a macroelectrode make the signal-to-back-ground ratio low. The voltammograms in Eig. 9B clearly show that at the NEEs, cyclic voltammetry can be a very powerful electroanalytical technique. There is, however, a caveat. Because the NEEs are more sensitive to electron transfer kinetics, the enhancement in detection limit that is, in principle, possible could be lost for couples with low values of the heterogeneous rate constant. This is because one effect of slow electron transfer kinetics at the NEE is to lower the measured Faradaic currents (e.g.. Fig. 8). 

Square-wave voltammetry affords several advantages to the electroanalyst. First, we note that /difference is larger than either /forward or /reverse, so the height of the voltammetric peak is usually quite easy to read, thereby increasing the accuracy. A detection limit of about 10 mol dm is attainable under optimized conditions. 

Alternative voltammetric methods that improve the sensitivity of voltammetry as an electroanalytical tool are normal pulse voltammetry (with a lower detection limit of 10 mol dm ), differential pulse voltammetry (with a detection limit of 10 -10 mol dm ) and square-wave pulse voltammetry (with a detection limit which is perhaps as low as 10 mol dm ). 

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