Versus preconcentration

FIGURE 50.33 Relative peak height of 8-galactosidase (peak 5) versus preconcentration time from data in Figure 50.32. (From Foote et al.. Anal. Chem., 77, 57, 2005. With permission.)... 

Sensibility The first strategy to lower the detection limits aims at minimizing the capacitive versus the faradic current intensities. In the second strategy, a preliminary step of electroactive species preconcentration in or on the electrode is realized [7,8]. 

A plot of the temperatures required for clouding versus surfactant concentration typically exhibits a minimum in the case of nonionic surfactants (or a maximum in the case of zwitterionics) in its coexistence curve, with the temperature and surfactant concentration at which the minimum (or maximum) occurs being referred to as the critical temperature and concentration, respectively. This type of behavior is also exhibited by other nonionic surfactants, that is, nonionic polymers, // - a I k y I s u I Any lalcoh o I s, hydroxymethyl or ethyl celluloses, dimethylalkylphosphine oxides, or, most commonly, alkyl (or aryl) polyoxyethylene ethers. Likewise, certain zwitterionic surfactant solutions can also exhibit critical behavior in which an upper rather than a lower consolute boundary is present. Previously, metal ions (in the form of metal chelate complexes) were extracted and enriched from aqueous media using such a cloud point extraction approach with nonionic surfactants. Extraction efficiencies in excess of 98% for such metal ion extraction techniques were achieved with enrichment factors in the range of 45-200. In addition to metal ion enrichments, this type of micellar cloud point extraction approach has been reported to be useful for the separation of hydrophobic from hydrophilic proteins, both originally present in an aqueous solution, and also for the preconcentration of the former type of proteins. 

Preliminary experiments with DPP and DPASV confirmed that In could be released from the HDI complex by acidification. Released In " was preconcentrated by reduction to In (Losev and Moledov, 1976) at a HMDE at a potential of —0.8 V versus SCE maintained for 5 min with a stirred solution. The stripping peak for indium occurred at —0.588 mV. Assay conditions were optimized with respect to pH to achieve maximal current response and maintain the highest degree of selectivity for the determination of indium (Doyle et al., 1982). 

In the most sensitive eiectroanaiytical methods, exclusively treated within this context, the analyte ion is electrodeposited on an electrode from an electrically conducting sample solution. Current and potential of subsequent redissolution are due to the concentration and the kind of ion to be determined. For thallium, the reversible redox couple TI /TI° at about -0.5 V versus saturated calomel electrode is used (Bellavance and Miller, 1975). Infinite tolerance towards alkali, alkaline earths and halogenides are great merits for the analysis of biological materials. Because of the preconcentration step included, thallium determination is more sensitive than atomic spectrometric methods. For thallium, the multielement capabilities of the method can hardly be used, because lead and frequently cadmium have to be masked with excess of complexants, leaving just Tl in the potential... 

Extraction can be used to separate and preconcentrate the analyte component(s), to recover valuable components from aqueous solutions (hydrometallurgy, biotechnology), and also to determine the stoichiometry of extracted complexes. In the latter case, the relations log D versus pH and log D versus logHL are considered, and the composition of the complex can be determined from the slopes of the linear parts of these relationships. 

Mediator 9-phenylanthracene (9-PA) was used to reduce 4-bromobiphenyl in CTAB solutions on Hg electrodes [4, 30]. Voltammetry showed a large preconcentration of 9-PA in a thick film of CTAB on the Hg surface. Reduction of 9-PA at —2.2 V versus SCE gave the anion radical that was stabilized by the positively charged CTAB film. The observed k for reaction of 9-PA anion radical with 4-bromobiphenyl was 10 in 0.1 M CTAB compared... 

LC with different modes of amperometric detection has frequently been used for the determination of phenols in water, with or without a preconcentration step. Without preconcentration, amperometric detection at +1150 mV versus Ag/AgCl was tested with six different RPLC columns [61]. Of the studied columns, the Spherisorb Cs gave the best chromatographic behavior of the 11 tested phenols. 

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