Amperometric detection selectivity

SCX column was used to separate oxalate and urate.140 In this separation, differential pulse and DC amperometric detection were compared. Differential pulse detection was found to allow better selectivity in detection. Anion exchange on Diaion CA08 was used to separate 20 carboxylic acids in the analysis of white wine, as shown in Figure 10.141 Because many carboxylic acids have a relatively weak absorbance, detection is difficult. The colorimetric detection scheme shown in the figure may be useful in some applications. 

Controlled potential methods have been successfully applied to ion-selective electrodes. The term voltammetric ion-selective electrode (VISE) was suggested by Cammann [60], Senda and coworkers called electrodes placed under constant potential conditions amperometric ion-selective electrodes (AISE) [61, 62], Similarly to controlled current methods potentiostatic techniques help to overcome two major drawbacks of classic potentiometry. First, ISEs have a logarithmic response function, which makes them less sensitive to the small change in activity of the detected analyte. Second, an increased charge of the detected ions leads to the reduction of the response slope and, therefore, to the loss of sensitivity, especially in the case of large polyionic molecules. Due to the underlying response mechanism voltammetric ISEs yield a linear response function that is not as sensitive to the charge of the ion. 

S. Sawada, H. Torii, T. Osakai, and T. Kimoto, Pulse amperometric detection of lithium in artificial serum using a flow injection system with a liquid/liquid-type ion-selective electrode. Anal. Chem. 70, 4286-4290 (1998). 

Hydroxylamine, JV-methylhydroxylamine and V,V-dimethylhydroxylamine were determined by ion chromatography. Amperometric detection using a GCE showed best sensitivity and selectivity, with injections of nanomole amounts620. 

Because of its advantages (high sensitivity and selectivity, low cost and miniaturization) amperometric detection has been frequently used in flow injection analysis (FIA) and RP-HPLC. However, it has been established that the peak area (detector response) considerably depends on the flow rate. A general approach has been proposed to predict the effect of flow rate on the peak area in FIA and RP-HPLC. The general form of the correlation describing the flow in a parallel plate cell with short rectangular electrodes is... 

Electrochemical detectors, which are based on the electrochemical oxidation or reduction of the analyte, can be applied to the analysis of selected compounds such as phenols. It is physically simple, but is very sensitive for catecholamines. However, the adsorption of reacted molecules on the surface of the electrodes can reduce the conductivity. To overcome this problem a pulsed voltage is applied, which cleans the electrode surface between measurements. This pulsed amperometric detection is also sensitive for carbohydrates. 

Lunte, C.E., Wheeler, J.F., and Heineman, W.R., Determination of selected phenolic acids in beer extract by liquid chromatography with voltametric-amperometric detection. Analyst 113,95,1988. 

Fluorescence detection, because of the limited number of molecules that fluoresce under specific excitation and emission wavelengths, is a reasonable alternative if the analyte fluoresces. Likewise, amperometric detection can provide greater selectivity and very good sensitivity if the analyte is readily electrochemically oxidized or reduced. Brunt (37) recently reviewed a wide variety of electrochemical detectors for HPLC. Bulk-property detectors (i.e., conductometric and capacitance detectors) and solute-property detectors (i.e., amperometric, coulo-metric, polarographic, and potentiometric detectors) were discussed. Many flow-cell designs were diagrammed, and commercial systems were discussed. 

Amperometric detection is based on applying a constant potential to the working electrode and measuring the resulting current. The selection of the optimum detection potential relies on the construction of HDYs. Since the detection potential can shift slightly depending on the electrode material and the separation voltage, HDYs must be recorded under the same separation conditions rather than those of the final sample analysis (Fig. 34.7). 

The selectivity of amperometric detection has been useful in simplifying the sample pretreatment steps in the determination of a number of drug products [82-86]. A method requiring no sample preparation using an amperometric detector and UV detector in series was developed for lido-caine hydrochloride injectable solutions [87]. The drug epinephrine is quantified with the amperometric detector, whereas lidocaine and methyl para-ben are detected by ultraviolet light. Disodium EDTA had to be added to the mobile phase to eliminate a peak response from iron leached from the stainless steel. 

Enzyme-selective electrodes (Fig. 17.11) have been made as a membrane containing immobilized enzymes placed over a pH electrode or over a gas electrode such as an ammonia electrode for potentiometric detection, or over an oxygen electrode for amperometric detection. The products of the reaction of enzyme with substrate are detected by the electrode. 

Methods utilizing newer techniques including ITMS, CE, and GC/MS/MS have recently been developed. ITMS analysis by Curcuruto et al. ° and Traldi et al. readily demonstrated the presence of morphine in hair. Free zone CE with amperometric detection was sensitive and highly selective. A novel procedure by Polettini et al. describes the treatment of methanol-washed samples with a silylating solution, followed by injection into GC. The silylating reagent was prepared by mixing MSTFA with dithioerythritol and ammonium iodide. Analysis of heroin, 6-AM, acetylcodeine. 

The potentiometric determination of cyanide using ion selective electrodes has become yet another very popular technique, because it is convenient, rapid, and sensitive method of analysis (Frant et al, 1972). Microdiffusion of biological samples containing cyanide is recommended prior to potentiometric determination. The use of a cyanide ion-selective electrode in combination with the Conway microdiffusion method for the measurement of cyanide concentrations in human red blood cells and plasma was reported with remarkable recovery of cyanide (Yagi et al, 1990). Ion chromatographic determination of sulfide and cyanide in real matrices by using pulsed amperometric detection on a silver electrode was reported by Giuriati et al. (2004). 

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