The Multi–Electrode Array Detector（英文）喜欢的可以一看
]The Multi–Electrode Array DetectorThe success of the electrode array as an LC detector is probably due to the development of the porous carbon electrode. This electrode is made of porous graphitic carbon, which has a very high surface area, is mechanically robust and, more important, is permeable to the mobile phase. As a consequence, flow through electrodes can be constructed. The material ideal for electrochemical detection in a number of ways. As the surface area is greatly in excess of that required for efficient electrochemical reaction, it can be severely contaminated before it fails to function. In fact, as much as 95% of the surface can be contaminated before it requires cleaning. When the electrode becomes sufficiently contaminated to require cleaning (which, according to the manufacturers, may occur between one and three years of continual use), the contamination can be rapidly removed by flushing with nitric acid.
The porous graphitic carbon electrode facilitates the construction of electrode arrays. In use, the large surface area of the porous electrode ensures that 100% of the eluted material is reacted. Thus, the electrochemical reaction is no longer amperometric, but now coulometric. This is an important difference and makes the array system practical. The electrode system is shown diagramatically in figure 60.
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Each electrode unit has a central porous carbon electrode, on either side of which is situated a reference electrode and a auxiliary electrode. As the pressure drop across the porous electrode is relatively small, these electrode units can be connected in series forming an array. Up to 16 units can be placed in series and these arrays are commercially available. However, a sensor system that contains as many as 80 electrodes in the space of a few millimeters has also been constructed (53). A progressively greater potential is applied sequentially to the electrodes of each consecutive unit. This results in all the solutes migrating through the array until each reaches the unit that has the required potential to permit its oxidation or reduction.
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Although the voltage increases progressively from one unit to the next, reaction is not completed at one unit only. This is because reaction will not take place at a specific potential, but over a narrow range of potentials. In practice the signal is usually detected over three contiguous electrode units. For example, the first will oxidize a very small amount of the solute, the second unit will be the dominant unit and oxidize the majority of the solute, while the third unit will oxidize the small remaining quantity of solute. This produces a characteristic pattern of peaks for a particular solute. The ratio of peak height for the three contiguous electrode units that sense the substance will be different for different substances although they may be reacted at the same three electrodes. This is obviously a method of confirming the identity of the solute and is demonstrated in figure 62. The upper graph shows the reference chromatogram of two standards each showing specific retention times and a specific peak pattern as provided by the electrode array detector. The lower graph is for a similar sample and, although the retention times for the pair of solutes is very similar, the pattern given by the electrode array detector clearly shows that the second compound eluted is not the same as that of the second standard.
The electrode array detector also gives improved apparent chromatographic resolution in a similar way to that of the diode array detector. Two peaks that have not been chromatographically resolved and are eluted together can still be shown as two peaks that are resolved electrochemically and can be quantitatively estimated. Another advantage is that high oxidation potentials can be used without the high background currents and noise that usually accompany such operating conditions. The electrodes that are operating at high voltages are "buffered" by the previous electrodes operating at lower voltages which results in reduced background currents and noise.
Another example of the application of the detector to the separation of a number of neuroactive substances (54)is shown in figure 63. It is seen that for certain applications the electrochemical array detector can be extremely useful.
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Nevertheless, in order to use the detector, the solutes must be amenable to electrochemical reaction and capable of being separated using a mobile phase that will conduct an ion current[/hide]
[[i] 本帖最后由 马之虎 于 2008-8-5 18:10 编辑 [/i]] [s:63] [s:63] 看看是什么 猫眼[biggrin]
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