Amperometric detection sensitivity

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

Electrochemical (amperometric) detection is used especially for ftHPLC and offers high sensitivity and... 

Voltammetry has been adapted to HPLC (when the mobile phase is conducting), and CE as a detection technique for electroactive compounds. In this usage, the voltammetric cell has been miniaturised (to about 1 p.L) in order not to dilute the analytes after separation. This method of amperometric detection in the pulsed mode is very sensitive. However, this device makes it possible to detect few analytically important molecules besides phenols, aromatic amines and thiols. 

To date, electrochemical (amperometric) detection of NO is the only available technique sensitive enough to detect relevant concentrations of NO in real time and in vivo and suffers minimally from potential interfering species such as nitrite, nitrate, dopamine, ascorbate, and L-arginine. Also, because electrodes can be made on the micro- and nano-scale these techniques also have the benefit of being able to measure NO concentrations in living systems without any significant effects from electrode insertion. 

The introduction of g.l.c. for oligosaccharide analysis constituted a major breakthrough in the field. In addition to strategies for accurate and sensitive quantitation of monosaccharide type (19,20), chiral procedures may be adopted for enantiomeric (d and l) determination (21). The sensitivity of h.p.a.e. chromatography with pulsed amperometric detection now provides an alternative to g.l.c. for oligosaccharide compositional analysis (22). 

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. 

Hilmi and Luong [25] employed a gold working electrode, formed by electroless deposition onto the chip capillary outlet, for highly sensitive amperometric detection of nitroaromatic explosives [with a detection limit of 24 ppb trinitrotoluene (TNT)]. Analysis of a mixture of four explosives (TNT, 2,4-DNT,... 

A thin film of manganese oxides deposited over a glassy earbon electrode dramatically lowers the overpotential for oxidation of various hydrazines ad hydrogen peroxide, thereby facilitating their amperometric detection in flow systems. Sensors based on this principle are highly sensitive and provide... 

At strong alkaline conditions mono- and oligosaccharides are weak anions. So it is possible to separate them on alkaline-resistant HPLC columns. Whereas spectrophotometric monitoring is not possible, pulsed amperometric detection allows sensitive identification in the nanomolar range. 

Amperometric detection is a very sensitive technique. In principle, voltammetric detectors can be used for all compounds which have functional groups which are easily reduced or oxidized. Apart from a few cations (Fe , Co ), it is chiefly anions such as cyanide, sulfide and nitrite which can be determined in the ion analysis sector. The most important applications lie however in the analysis of sugars by anion chromatography and in clinical analysis using a form of amperometric detection know as Pulsed Amperometric Detection (PAD). 

Both ISP-MS in the SIM mode and pulsed amperometric detection were found to be suitable for the determination of aminoglycoside antibiotics in bovine tissues (124). Various stationary and mobile phases, and several ion-pairing reagents, were examined for efficient LC separation and optimum LC-MS sensitivity. 

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. 

Two analytical methods for priority pollutants specified by the USEPA (38) use HPLC separation and fluorescence or electrochemical detection. Method 605, 40 CFR Part 136, determines benzidine and 3,3-dichlorobenzidine by amperometric detection at +0.80 V, versus a silver/silver chloride reference electrode, at a glassy carbon electrode. Separation is achieved with a 1 1 (v/v) mixture of acetonitrile and a pH 4.7 acetate buffer (1 M) under isocratic conditions on an ethyl-bonded reversed-phase column. Lower limits of detection are reported to be 0.05 /xg/L for benzidine and 0.1 /xg/L for 3,3-dichlorobenzidine. Method 610, 40 CFR Part 136, determines 16 PAHs by either GC or HPLC. The HPLC method is required when all 16 PAHs need to be individually determined. The GC method, which uses a packed column, cannot adequately individually resolve all 16 PAHs. The method specifies gradient elution of the PAHs from a reversed-phase analytical column and fluorescence detection with an excitation wavelength of 280 nm and an emission wavelength of 389 nm for all but three PAHs naphthalene, acenaphthylene, and acenaphthene. As a result of weak fluorescence, these three PAHs are detected with greater sensitivity by UV-absorption detection at 254 nm. Thus, the method requires that fluores-... 

Fluorescence detection is sensitive to naturally fluorescent analytes or to fluorescent derivatives. Amperometric detection is sensitive to analytes that can be oxidized or reduced at an electrode (Figure 26-29). Conductivity detection with ion-exchange suppression of the background electrolyte (as in Figure 26-4) can detect small analyte ions at 1-10 ng/mL. Electrospray mass spectrometry (Figure 22-18) provides low detection limits and gives qualitative information about analytes.33... 

First examples of the amperometric detection of H202 accomplished in such a range were based on the use of an enzyme, namely horseradish peroxidase (HRP), a prototypical hemeprotein peroxidase, which catalyses the reduction of H202 and due to its peculiar structure, allows direct electron transfer between its active site and the electrode surface at low applied potential [14 17]. This approach, although it shows good sensitivity and accuracy, suffers from some important shortcomings such as low stability and the limited binding of HRP to solid surfaces. 

Amperometric detection is the preferred method for the analysis of nitroaromatic explosives on microchip devices since it offers up to three orders of magnitude higher sensitivity than indirect LIF and it has a great potential for miniaturization and integration on microchip platform. Presence of nitrogroup allows its cathodic reduction to form al-kylhydroxyamines. The reduction mechanism of polynitroaromatic compounds is complex and depends on the number of nitro groups,... 

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