Amperometric detection background

Fluorescence detection is sensitive to naturally fluorescent analytes or to fluorescent derivatives. Amperometric detection is sensitive to analytes that can be oxidized or reduced at an electrode (Figure 26-29). Conductivity detection with ion-exchange suppression of the background electrolyte (as in Figure 26-4) can detect small analyte ions at 1-10 ng/mL. Electrospray mass spectrometry (Figure 22-18) provides low detection limits and gives qualitative information about analytes.33... 

For detection of DNA fragments, Fe(phen)32+ was used as the electrochemi-cally active intercalation reagent. The constant background current from free Fe(phen)32+ decreased in the presence of the DNA-Fe(phen)32+ complexes. Therefore, this is an indirect amperometric detection method. It was found that a distance of 300 pm, instead of 600 pm, between the working electrode and reference electrode has produced less electrical interference (in the form of a sloping baseline), allowing the use of a separation voltage up to 1200 V (240 V/cm) [745]. 

The kind of counter ion of an ion-pair reagent is vitally important for selecting the appropriate detection method. If suppressed conductivity detection is applied, the ion-pair reagent is used in its hydroxide form. With direct conductivity detection, salicylate is perferred as the counter ion for tetraalkylammonium cations [19,20], since these salts exhibit a lower background conductance in aqueous solution. According to Wheals [21], eluents such as cetyltrimethylammonium bromide in combination with citric acid at pH 5.5 have proved suitable for UV-, RI-, and amperometric detection, as well as for direct conductivity detection. This example is an impressive illustration of the versatility of ion-pair chromatography. 

Amperometric detection of electroactive species requires that reaction products from the oxidation or reduction of solutes do not precipitate at the electrode surface. Contaminated electrodes change their surface characteristics, thus leading to an enhanced baseline drift, increased background noise, and a constantly changing response. This behavior is particularly pronounced in the amperometric detection of carbohydrates. 

Thus, in practice, amperometric detection involving analyte reduction is generally carried out by application of negative potentials, while analyte oxidation requires the use of relatively positive applied potentials. Successful bench-scale CE/EC in the amperometric mode requires the accurate maintenance of EC potentials on the order of roughly 1 V at working electrodes placed in CE fields on the order of 5-30 kV and the measurement of EC currents typically pA in magnitude in the presence of pA-level background CE currents. 

Disadvantages of Pulsed Amperometric Detection (PAD) for Oxide-Catalyzed Detections Baseline/Background Sensitivities (B)... 

Boron-doped diamond presents another attractive material with low and stable background current and noise over a wide potential range, corrosion resistance, high thermal conductivity, and high current densities. Usually no mechanical or electrochemical pretreatment of BDD film electrode is needed. Therefore, BDD film electrodes find use also in the area of environmental analysis for organic explosive determinations. BDD-based electrochemical detector allowed, e.g., amperometric detection of 2,4,6-trinitrotoluene, 1,3-dinitrobenzene, and 2,4-dinitrotoluene over the 200-1,400 ppb range, with detection limits at the 100 ppb level. ... 

Depending on their conversion efficiency, electrochemical detectors can be divided into two categories those that electrolyze only a negligible fraction (0.1-5%) of the electroactive species passing through the detector (amperometric detectors), and those for which the conversion efficiency approaches 100% (coulo-metric detectors). Unfortunately, the increased conversion efficiency of the analyte is accompanied by a similar increase for the electrolyte (background) reactions, and no lowering of detection limits is reahzed. 

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