Amperometry advantages

It is now clear that electrochemical measurements can often have significant advantages over the classical spectroscopic approaches. Amperometry can be more specific therefore, lower detection limits are often feasible. Because electrochemical detectors do not require optical carriers, they can be much less expensive than UV absorption or fluorescence detectors. This is especially true when one considers that electrochemical detectors are inherently tunable without the need for such things as monochrometers or filters. On the other hand, there can be significant problems with reliability, and, more often than not, there is a lack of acceptance by chemists weaned on Beer s law. Amperometric methods in biochemistry are just beginning to be commercialized, and it is now almost certain that they will come into widespread use. 

Another recent development is the advent of pulse amperometry in which the potential is repeatedly pulsed between two (or more) values. The current at each potential or the difference between these two currents ( differential pulse amperometry ) can be used to advantage for a number of applications. Similar advantages can result from the simultaneous monitoring of two (or more) electrodes poised at different potentials. In the remainder of this chapter it will be shown how the basic concepts of amperometry can be applied to various liquid chromatography detectors. There is not one universal electrochemical detector for liquid chromatography, but, rather, a family of different devices that have advantages for particular applications. Electrochemical detection has also been employed with flow injection analysis (where there is no chromatographic separation), in capillary electrophoresis, and in continuous-flow sensors. 

For monitoring catalytic (enzymatic) products, various techniques, such as spectrophotometry [32], potentiometry [33,34], coulometry [35,36] and amperometry [37,38], have been proposed. An advantage of these sensors is their high selectivity. However, time and thermal instability of the enzyme, the need of a substrate use and indirect determination of urea (logarithmic dependence of a signal upon concentration while measuring pH) cause difficulties in the use and storage of sensors. 

The stop-flow method provides a very low detection limit and the advantage that by using amperometry each sensor response is measured from its own baseline so requires no blank subtraction. At around 60 min, the assay time is longer than by batch mode and the instrumentation lends itself to off-site rather than on-site monitoring. 

A first interest of MS when used in combination with microfabricated structures, or at the outlet of microfluidic devices, is the match in the volume of liquid handled. A typical MS analysis requires less than 1 pL of liquid, for ESI-MS as well as for MALDI-MS techniques. When working with a continuous flow of liquid and ESI-MS, the MS performance is even more enhanced for flow rates down to 50-100 nL min-1 the lower the flow rate, the better the MS analysis. This flow-rate range corresponds to flow-rate values observed in microfluidic devices. Consequently, the technique of MS is easily scalable and exhibits an enhanced response when the sample size is decreased. This is not the case for instance for other detection techniques, such as UV absorbance or amperometry these two techniques require large detection area or volume, which is the opposite of the quest of microfluidics. This first advantage of MS compared to other technique goes together with its high sensitivity. 

Potentiometric and refractive index detection are not affected by volume but are relatively insensitive in the nanolitre to picolitre range compared to amperometric detection (micro surface area) and fluorimetric detection (micro amount of material). At 1 pL, limits of detection are similar for potentiometry, amperometry and fluorescence. On-chip LC is very compatible with mass spectrometry due to the low volumes and flow rates required. Battery-operated ion trap MS has been reported but miniaturisation of MS offers no sensitivity or selectivity advantages. Electrospray ionisation (ESI) has been successfully integrated into a chip format allowing for many ESI nozzles on one chip. Arrays make pattern recognition possible. 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

The advantage of amperometric measurements is that the faradaic currents are observed, at fixed electrode potentials. In these circumstances, capacitative currents no longer contribute to the overall cell current, and much lower detection limits are obtainable compared to linear sweep voltammetry. However, some of the newer variants of amperometry do involve pulsing the electrode potential to the active region measurements in these cases need to be made carefully to produce optimum signal to noise ratios. 

The advantage of integrated amperometry lies in the coulometric compensation of the charges resulting from the formation and subsequent reduction of the metal oxide. Thus, baseline drifts and baseline disturbances caused by small variations in the mobile-phase composition are eliminated. Moreover, the whole system is less sensitive to variations in pH, which influences the potentials for... 

Electrochemical Impedance Spectroscopy (EIS) is a powerful technique for the characterization of electrochemical systems. The fundamental approach of all impedance methods is to apply a small-amplitude sinusoidal excitation signal to the system xmder investigation and measure the response (current or voltage or another signal of interest). An advantage of EIS compared to amperometry or potentiometry is that labels are no longer necessary, thus simplifying sensor preparation. However, the use of labels (like enzymes or nanoparticles) increases a lot the sensitivity of the method [23-26]. 

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