Amperometry limits

Several electroactive substances 0.1-1.0 mol L 1 KC1 6.27-17.5 Amperometry Limiting current proportional to the two-thirds power of Dm. [74]... 

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. 

SECM-induced transfer [SECMIT Fig. 2(b)] can be used to characterize reversible phase transfer processes at a wide variety of interfaces. The basic idea is to perturb the process, initially at equilibrium, through local amperometry at the UME. Hitherto, diffusion-limited electrolysis has mainly been used in conjunction with metal tips, but ion transfer voltammetric probes (discussed briefly in Section III, and in detail in Chapter 15) can also be used. The application of a potential to the tip, sufficient to deplete the... 

A graphite rotating disk electrode maintained at 0.5 V is used to monitor the reaction of Ru(NHj)5 as it is being oxidized by Oj to RulNKj) . The limiting current is proportional to [RufNHj) ] and there is no interference by O2 or the product. The electrode is rotated at 3600 rpm to ensure rapid mixing of reactants within seconds, since reaction times are 20-30 s. See Ref. 333. Square-wave amperometry has been linked to stopped-flow to measure reaction half-lives as short as 5 ms. 

The analytic principles that have been applied to accumulate air quality data are colorimetry, amperometry, chemiluminescence, and ultraviolet absorption. Calorimetric and amperometric continuous analyzers that use wet chemical techniques (reagent solutions) have been in use as ambient-air monitors for many years. Chemiluminescent analyzers, which measure the amount of chemiluminescence produced when ozone reacts with a gas or solid, were developed to provide a specific and sensitive analysis for ozone and have also been field-tested. Ultraviolet-absorption analyzers are based on a physical detection principle, the absorption of ultraviolet radiation by a substance. They do not use chemical reagents, gases, or solids in their operation and have only recently been field-tested. Ultraviolet-absorption analyzers are ideal as transfer standards, but, as discussed earlier, they have limitations as air monitors, because aerosols, mercury vapor, and some hydrocarbons could, interfere with the accuracy of ozone measurements made in polluted air. 

In 1C, the election-detection mode is the one based on conductivity measurements of solutions in which the ionic load of the eluent is low, either due to the use of eluents of low specific conductivity, or due to the chemical suppression of the eluent conductivity achieved by proper devices (see further). Nevertheless, there are applications in which this kind of detection is not applicable, e.g., for species with low specific conductivity or for species (metals) that can precipitate during the classical detection with suppression. Among the techniques that can be used as an alternative to conductometric detection, spectrophotometry, amperometry, and spectroscopy (atomic absorption, AA, atomic emission, AE) or spectrometry (inductively coupled plasma-mass spectrometry, ICP-MS, and MS) are those most widely used. Hence, the wide number of techniques available, together with the improvement of stationary phase technology, makes it possible to widen the spectrum of substances analyzable by 1C and to achieve extremely low detection limits. 

The limiting current is called the diffusion current because it is governed by the rate at which analyte can diffuse to the electrode. The proportionality of diffusion current to bulk-solute concentration is the basis for quantitative analysis by amperometry and, in the next section, voltammetry. 

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. 

The resolution of the column provides much of the selectivity in LCEC therefore, the practical limitations of amperometry are circumvented to a large extent. Nevertheless, amperometry is more often than not used to improve the selectivity of an LC method. Compounds that oxidize or reduce at low potentials can be detected with great selectivity. 

PB-modified screen-printed electrodes were first tested in batch amperometry for H202 detection. The sensors showed a good linearity in the range between 0.1 and lOOpmoll-1 with a detection limit of 0.1 pmol 1 The regression equation of the linear part of the curve was y = 22.90a —0.013, where y represents the current in pA and x the H202 concentration in mmoll the R2 value was 0.9980. The sensitivity was 324 pA mmoll 1cm 2 with an RSD% of 5% (for all the concentrations tested by six different electrodes) (see Table 24.1). 

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 sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

Amperometric sensors — A class of electrochemical sensors based on amperometry. A - diffusion-limited current is measured which is proportional to the concentration of an electrochemically active analyte. Preferred technique for - biosensors with or without immobilized enzymes (biocatalytic sensors). The diffusion layer thickness must be kept constant, either by continuous stirring or by means of an external diffusion barrier. Alternatively, micro electrodes can be... 

Biamperometry — Whereas in amperometry the -> current is limited by the electrode process proceeding at one indicator electrode (and the -> counter electrode has no effect), in biamperometry the current flowing between two indicator electrodes is measured, i.e., both electrodes can limit the overall current. This approach is useful in following some -> titrations, and it may lead to zero current (dead-stop) at the equivalence point (dead-stop titration). Example iodine in an iodide solution is titrated with As(III). Two platinum electrodes with a potential difference of around 100 mV prompt iodine to be reduced on one electrode and iodide being oxidized at the other. The two processes maintain an almost constant current until the endpoint when iodine is exhausted. 

Amperometry applies a constant potential to the microelectrode that will oxidize the analyte at the electrode surface (Fig. 8a). The current is limited solely by the mass transport rate to the electrode. Measurements can be made with extremely high temporal resolution, typically 500 Hz, because time resolution is not limited by the electrochemistry at the electrode. However, little analyte selectivity occurs with amperometry because a change in the concentration of any molecule electroactive at the applied potential will alter the measured signal. 

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