Potential Problems When Low and High Voltage Meet
This is especially true for the two methods I focus on, i.e. Capillary Electrophoresis (CE) and Electrochemical Detection (ECD). Each of them simply performs theoretically much better as a miniaturised system. To understand better, where the problem lays, please let me give you a brief sketch of what CE and ECD are, what they can be used for, and how this is done.
CE or other separation techniques are commonly used in analytical chemistry to separate complex samples and by this simplify measurements on one or more components of the sample. A typical example could be found in pharmaceutical industry, where separation techniques often are used to separate drugs from possible contaminants or by-products.
Main feature of a CE apparatus is the capillary, a thin tube with a typical diameter of 0.05 mm or less. This tube is filled with a salt solution, called electrolyte, that provides electrical conductivity. Each end of the tube is immersed in a small beaker that also contains this electrolyte and a high voltage electrode. For analysis, a very small amount of the sample, about 1/2000 of the volume of a raindrop, is introduced into one end of the capillary, and high voltage, some ten thousand volts, is applied to the high voltage electrodes. Upon this the electrolyte starts travelling through the capillary. The components of the sample start to travel too, each of them with a characteristic speed. Eventually the sample separates into small groups of identical components that leave, sorted by their characteristic speed, the capillary on its other end. Where they elute from the capillary one has to use some detection technique to identify and scale them.
ECD is one of those techniques. The set-up for ECD consists of a device that is called potentiostat and of three electrodes which are immersed in the beaker with electrolyte at the end of the CE-capillary. One of the electrodes, the reference electrode senses the electrical potential of the surrounding electrolyte. The electrical potential of the second electrode, the working electrode, is then set by the potentiostat to a chosen value. If the potential of the working electrode is sufficiently high, some types of compounds nearby would attach and deliver an electron to it. If the potential is sufficiently negative, the working electrode could pass an electron to some types of compounds. In both cases an electron moves, i.e. a current runs. The function of the third electrode, the counter electrode, is to close the circuit and provide a path for any current generated at the working electrode. This current is my analytical signal and is proportional to the concentration of the electron delivering or electron accepting compounds.
Compounds that can deliver or accept electrons by this way are said to
be electroactive. Among them one finds many important substance groups
like DNA, neurotransmitters, amino acids or heavy metals.
Thus my research so far was concerned with the high voltage effect of CE on ECD rather the miniaturising both systems to make a neat "Lab-on-a-chip". However, it has turned out that the relative positions of working and reference electrode along the path that the high voltage takes outside the capillary is responsible for these effects. Briefly, the disturbing effects could be eliminated simply by placing working and reference electrode very close to each other. Unfortunately, "quite close" in this context means less than the diameter of the capillary, i.e. 0.05 mm. That's now why the micromachining comes into the picture. I won't discuss here how this works. However, now I am back to the initial task, the Lab-on-a-chip, but not because of the benefits of miniaturisation as anticipated initially, but because CE and ECD simply can not be combined else than by miniaturisation.
Furthermore it appears that the high voltage field in CE can be used for some other interesting features related to ECD. To find them and to learn how to control the interactions between electric fields of some ten thousand volts used in CE and potentials of some hundred millivolts used in ECD is the topic of my further research.
|Modified 15 January 2003 * Contact Us|