Detectors amperometry

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

Schematic diagrams of flow cell detectors for HPLC using (a) UVA/is absorption spectrophotometry and (b) amperometry.

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. 

Detectors Most of the detectors used in HPLC also find use in capillary electrophoresis. Among the more common detectors are those based on the absorption of UV/Vis radiation, fluorescence, conductivity, amperometry, and mass spectrometry. Whenever possible, detection is done on-column before the solutes elute from the capillary tube and additional band broadening occurs. 

HPAEC analyses were carried out to determine the oligomeric products released from various pectic substrates after depolymerization by the PL isoenzymes. Action pattern analyses for the concerted action of PL isoenzymes utilized 68% esterified pectin as substrate. One-ml reaction mixtures in a buffer system as detailed in section 2.2. comprising 0.5% (w/v) substrate and 5 U of enzyme were incubated for 30 s to 18 h, and then thermoinactivated. Samples of 750 pi were applied to a Carbopac PA-1 (Dionex) column before the carbohydrates were eluted over a period of 70 min using a gradient of 0.2 M KOH, 0.05 M K-acetate to 0.2 M KOH, 0.7 M K-acetate. Detection employed a Pulsed Electrochemical Detector (PED, Dionex) in the integrated amperometry mode according to the manufacturer s recommendations. 

LCEC is a special case of hydrodynamic chronoamperometry (measuring current as a function of time at a fixed electrode potential in a flowing or stirred solution). In order to fully understand the operation of electrochemical detectors, it is necessary to also appreciate hydrodynamic voltammetry. Hydrodynamic voltammetry, from which amperometry is derived, is a steady-state technique in which the electrode potential is scanned while the solution is stirred and the current is plotted as a function of the potential. Idealized hydrodynamic voltammograms (HDVs) for the case of electrolyte solution (mobile phase) alone and with an oxidizable species added are shown in Fig. 9. The HDV of a compound begins at a potential where the compound is not electroactive and therefore no faradaic current occurs, goes through a region... 

Specifications for modem detectors in HPLC are given by Hanai [538] and comprise spectroscopic detectors (UV, F, FUR, Raman, RID, ICP, AAS, AES), electrochemical detectors (polarography, coulometry, (pulsed) amperometry, conductivity), mass spectromet-ric and other devices (FID, ECD, ELSD, ESR, NMR). None of these detectors meets all the requirement criteria of Table 4.40. The four most commonly used HPLC detectors are UV (80%), electrochemical, fluorescence and refractive index detectors. As these detectors are several orders of magnitude less sensitive than their GC counterparts, sensor contamination is not so severe, and... 

We have already briefly described a popular application of amperometry in Chapter 13. This was the electrochemical detector used in HPLC methods. In this application, the eluting mobile phase flows across the working electrode embedded in the wall of the detector flow cell. With a constant potential applied to the electrode (one sufficient to cause oxidation or reduction of mixture components), a current is detected when a mixture component elutes. This current translates into the chromatography peak... 

Kissinger, P. T., "Amperometrie and Coulometric Detectors for High-Performance Liquid Chromatography , Anal. Chem., 1977, 49, 447A-456A. 

Figure 26-30 shows the principle of indirect detection,M which applies to fluorescence, absorbance, amperometry, conductivity, and other forms of detection. A substance with a steady background signal is added to the background electrolyte. In the analyte band, analyte molecules displace the chromophoric substance, so the detector signal decreases when analyte passes by. Figure 26-31 shows an impressive separation of Cl isotopes with indirect... 

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. 

We wish only to remind readers that there are three main methods of electrochemical re-vealment conductivity, direct current (d.c.) amperometry, and integrated amperometry (pulsed amperometry is a form of integrated amperometry). In revealment by conductivity, the analytes, in ionic form, move under the effect of an electric field created inside the cell. The conductivity of the solution is proportional to the mobility of the ions in solution. Since the mobile phase is itself an electrolytical solution, in order to increase the signal/noise ratio and the response of the detector, it is very useful to have access to an ion suppressor before the revealment cell. By means of ionic exchange membranes, the suppressor replaces the counterions respectively with H+ or OH , allowing only an aqueous solution of the analytes under analysis to flow into the detector. 

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

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... 

Any type of detector with a flow-through cell can be used for FIA. Photometric detectors are most often used in FIA (15-18, 25). However, many other analyses using fluorimeters (28, 29), refractometers (24), atomic absorption (30, 31), and inductively coupled plasma emission spectrometers (32) have been described. Electrochemical detectors based on potentiometry with ion-selective electrodes (15, 33), anodic stripping voltammetry (15, 34), potentiometric stripping (35), and amperometry (36) have also been used. 

To overcome the problem of detection in CE, many workers have used inductively coupled plasma-mass spectrometry (ICP-MS) as the method of detection. " Electrochemical detection in CE includes conductivity, amperometry, and potentiometry detection. The detection limit of amperometric detectors has been reported to be up to 10 M. A special design of the conductivity cell has been described by many workers. The pulsed-amperometric and cyclic voltametry waveforms, as well as multi step waveforms, have been used as detection systems for various pollutants. Potentiometric detection in CE was first introduced in 1991 and was further developed by various workers.8-Hydroxyquino-line-5-sulfonic acid and lumogallion exhibit fluorescent properties and, hence, have been used for metal ion detection in CE by fluorescence detectors.Over-... 

Detectors that have produced the lowest detection Emits to date are based on fluorescence with precolumn derivatization, mass spectrometry, radiometry, and amperometry. Applications are described below for these detection systems employed with conventional CE instrumentation. For applications of these detection methods to CE analysis using microfabricated devices, the reader is referred to a review article.3... 

To date, amperometric detectors have only been applied to small, readily oxidized analyte species. A variety of alternate amperometric detection schemes are under development, however, including indirect amperometry. With this technique, the separation buffer contains a redox-active species that is continuously detected by the ultramicroelectrode. The analyte displaces the electroactive species, so that the electrochemical response is decreased during the elution of an analyte peak. 

Pulsed amperometry in a three-electrode detector cell is carried out in a non-flowing solution, in which solute ions are dissolved in the supporting electrolyte. A pulsed potential is applied and is increased stepwise after each pulse. The resulting current... 

This kind of amperometry is the most widely used electrochemical detection method in liquid chromatography. A constant DC potential is continuously applied to the electrodes of the detector cell. The theory of amperometry with constant working potential does not differ from the theory of hydrodynamic voltammetry, even though the applied potential remains constant. 

Fig. 6-10. Schematic representation of the detector cell for pulsed amperometry.

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