Detector electrode systems

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

Shielding The potential of the mobile phase in the cell may be influenced by electric field changes near the three electrode system. These changes may originate from motions of a statically charged operator or from electrically operated instruments in the vicinity of the detector. 

Due to close proximity of the electrodes in the Hewlett-Packard HP 1049A Electrochemical Detector cell, little conductance is already sufficient for proper functioning of the electrode system. 

The detector cell was a three-electrode system consisting of a flow-through nickel working electrode, a saturated calomel reference electrode (SCE), and a stainless steel outlet tubing counter electrode. The tubular-type electrode cell housing was constructed of molded Teflon, which was machined to provide the channels and to accommodate the fittings. The working electrode area was... 

The heated alkali bead emits electrons by thermionic emission which are collected at the anode and provides background current through the electrode system. When a solute that contains nitrogen or phosphorus is eluted, the partially combusted nitrogen and phosphorus materials are adsorbed on the surface of the bead. This adsorbed material reduces the work function of the surface and, as a consequence, the emission of electrons is increased, which raises the current collected at the anode. The sensitivity of the NPD is very high and about an order of magnitude less than that of the electron capture detector (ca. 10 g/ml for phosphorus and lO g/ml for nitrogen). 

The voltage at which a spark will occur between two electrodes situated in a gas will depend on the composition of the gas between the electrode tips. Lovelock [15] suggested that this could form the basis for a GC detector. The system suggested by Lovelock is shown in figure 10. 

The electrochemical detector in the form described above is extremely sensitive but suffers from a number of drawbacks. Firstly, the mobile phase must be extremely pure and in particular free of oxygen and metal ions. A more serious problem arises, however, from the adsorption of the oxidation or reduction products on the surface of the working electrode. The consequent electrode contamination requires that the electrode system must be frequently calibrated to ensure accurate quantitative analysis. Ultimately, the detector must be dissembled and cleaned, usually by a mechanical abrasion procedure. Much effort has been put into reducing this contamination problem but, although diminished, the problem has not been completely eliminated particularly in the amperometric form of operation. Due to potentially low sensing volume the detector is very suitable for use with small bore columns. 

The electronic system of the electrical conductivity detector is comprised of a 1 kHz frequency generator, the output of which is fed via a suitable impedance to the detector electrodes. The voltage across the electrodes is fed to a precision rectifier to provide a DC signal that is related to the conductivity of the fluid between the sensor plates. The DC signal is then passed to a signal modifying amplifier to... 

The electrical conductivity detector measures the conductivity of the mobile phase. Conductivity detectors are universal and nondestructive and can be used in either direct or indirect modes. The conductivity sensor is the simplest of all the detectors, consisting of only two electrodes situated in a suitable flow cell. The basis of conductivity is the forcing of ions in solution to move toward the electrode of opposite charge on the application of a potential. To prevent polarisation of the sensing electrodes, AC voltages must be used and so it is the impedance (not the resistance) of the electrode system that is actually... 

Detector cells can be made which contain only two electrodes, a working electrode and a reference electrode. A preselected potential equal to or greater than the half wave potential of interest is applied constantly across the electrodes. However, two electrode systems give a non-linear response as the voltage drops across the eluant as the current flow changes. Thus electrochemical detectors typically employ a three electrode cell. The additional electrode known as the auxiliary or counter-electrode, serves to carry any current generated in the flow cell thus enabling the reference electrode to ensure a fixed potential despite the decrease in the internal resistance of the detector cell. 

Fig. 7 Schematic diagrams of the microchip electrophoretic system with the movable contactless-conductivity detector (A) along with a detailed design of the movable electrode system top (B) and bottom (C) views as well as cross-sectional views without (D) and with (E) the PMMA separation chip, (a) Run buffer reservoir, (h) sample reservoir, (c) unused reservoir, (d) movable electrodes, (e) separation chaimel, (f) sample waste reservoir, (g) PMMA chip, (h) conductive silver epoxy, (0 PVC clamps, (/ ) copper wires, (k) aluminum foil electrodes, and (/) Plexiglas plate (Reprinted with permission from Ref. [9])...

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