Flow analysis voltammetric techniques

Detection of phosphate by flow analysis voltammetric techniques that are based on the determination of the phosphomolybdate moiety has been described by Fo et al. [124-126]. Recent work by Udnan et al. [127] has shown that reliable and sensitive FRP measurement can be achieved by this approach, with a detection limit of 3.4 pg P (0.11 pM) being reported. When in-valve ion exchange preconcentration was applied to the determination of oligotrophic freshwaters, a detection limit of 0.18 pg P (5.8 nM) was achieved. While this approach is convenient and rapid (Table 8.2), it suffers from the lack of selectivity for orthophosphate that applies to all phosphomolybdate-based detection chemistries. 

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. 

Following this initial introductory chapter, the book is structured in three parts. The first part discusses different Electroanalytical Techniques in Batch and Continuous Systems as highly remarkable tools in the agricultural and food field. In this sense. Chapter 2 deeply explores the sweep potential electroanalytical techniques, while Chapter 3 allows the readers to obtain fundamental information on voltammetric techniques coupled to flow systems which could proportionate faster analysis, reproducible results, high sensitivity, with additional advantages such as the requirement of less sample, and the use of simpler instrumentation. 

In the literature, a series of good reviews about the use of electroanalytical methods for determination of pharmaceuticals, the utilization of chemical-modified electrodes,or even focusing on the utilization of voltammetric techniques in association with flow injection analysis demonstrate the great potentialities of the use of electroanalysis for quantification of low levels of pharmaceutical compounds. In the following sections, these aspects are addressed and selected examples are presented. A classification based on the electrode materials was chosen for giving the lecturer an outlook of the developments in the area. 

Flow injection analysis (FIA) (Ruzicka and Hansen), since 1975 In continuous flow, stopped flow or with merging zones (FIA scanning or intermittent pumping) Adapted voltammetric electrodes Membranes for Partial dialysis Membrane amperometry (Clark) Differential techniques (Donnan) Computerization, including microprocessors Special measuring requirements in plant control (to avoid voltage leakage, etc., Section 5.5)... 

Finally, the possibilities of automation of amperometric and voltammetric electroanalysis should be stressed, as well as the use of solvents other than water43. Pulse techniques are semi-automated by nature the responses can be transmitted directly to a microcomputer for immediate analysis. Fast on-line analysis in flow systems with automated calibration is one of the great advantages, which will be much exploited in the future. 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

In addition to on-line and in-line analysis, very often off-line systems are used in industry. Intermediates and products are analysed with laboratory analyzers. An example of this is the equipment constructed for the content uniformity analysis and dissolution rate determination of pharmaceutical tablets.The tablet is dissolved under well-defined conditions and the solution analysed using a flow technique with voltammetric detector (Fig. 11). 

An alternative method for electrochemical analysis Is to measure the current that passes at a test electrode as a function of the applied potential between it and a reference electrode. This technique takes advantage of the fact that many molecules can be easily oxidized or reduced. At a fixed applied potential) this Is referred to as an amperometric experiment. If the current Is measured as a function of voltage (termed a voltammogram) It provides a curve that Is characteristic of a particular molecule. The sign of the current Indicates whether the molecule can be oxidized or reduced) the potential at which current starts to flow Is Indicative of the formal potential of the compound) and the amplitude of the current Is proportional to the concentration of the particular species of Interest. Because In the voltammetric experiment one has the added capability of determining the formal potential of the compound of Interest. The use of chemically specific membranes over the electrode surface Is not as great a requirement. 

A hydrodynamic voltammogram is a current-potential curve which shows the dependence of the chromatographic peak height on the detection potential. The technique used to obtain the necessary information is voltammetric flow injection analysis. in which an aliquot of the analyte is injected into the flowing eluent prior to the detector and the peak current recorded. This is repeated many times, the detector potential being changed after each injection, until the peak current - potential plot reaches a plateau or a maximum, as shown... 

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