Introduction

Electrophoresis is an established separation technique, frequently used for mixtures ranging from pro teins and DNA to small anions and cations. However, perhaps its greatest strength lies in its remarkable ability to separate charged macromolecules. Reports describing electrophoretic separations started to appear in the 1930s, but the most significant developments really took place in the 1940s and 50s when separations with a paper or gel support matrix were used for the separation of macromolecules. The early methods used relatively large scale apparatus, but during the later 1960s, and early 1970s, reports appeared describing separations being performed in small bore tubes filled with buffer solution. This work was extended in the early 1980s, with capillaries being a key feature of the basic methodology. This was the start of capillary electrophoresis (CE); however, it was not until the mid 1980s that great interest was shown towards a new approach to separation science. From that moment, development and commercialization came very quickly and soon there were a number of commercial instruments available for routine laboratory use.

It is not possible to cover all aspects of electrophor-esis in an article such as this; indeed there are several topics that have been omitted. Fluid logic devices and freeze-melt switching are two such examples; another important area not included is the use of parallel bundles of microcapillaries that permit multiple analyses to be performed at a high throughput.

The basic element of any CE system is the separation capillary, typically 10-100 |im internal diameter and 30-100 cm long. Each end of the capillary is located in a small reservoir, which contains buffer solution and a platinum anode or cathode; typically potentials of up to 30 kV can be applied between them. Detection is achieved by a range of in-line detection methods, such as ultraviolet absorbance and other detection methods, such as mass spectro-metry, can be interfaced to the capillary.

Separation is achieved due to the differing elec-trophoretic mobilities of the analytes in the sample, but in addition electroosmotic flow (EOF) takes place. This phenomenon gives rise to bulk flow of the solution in the capillary without the need for an external pump. For a unmodified silica capillary, the direction of flow would be from the anode to the cathode, which enables all uncharged species to be carried to the detector. This technique offers very high separation efficiencies and rapid analysis. This feature, coupled with the simplicity of the instrumentation, makes the technique ideally suited to miniaturization.

Interest in miniaturizing analytical systems in not new; indeed, the idea of a micro total analysis system (often referred to as |TAS) has been mooted for some time within the scientific community (see, for example, the paper by Martin cited in Further Reading). The ideal approach is to include sample manipulation and detection on a chip-sized device; this has given rise to the term 'lab on a chip'. Such systems employ microstructures fabricated on glass or other substrates to form integrated devices rather than attempting to construct miniaturized systems from discrete components. However, there is also consider able interest in the development of discrete components, such as micropumps. The conference proceedings from the recent Micro Total Analysis Systems '98 give some indication as to the diversity of the developments. While on-chip injection is feasible, some prior degree of preparation may still be necessary. For example, particulate matter would quickly block the channels, so pre-filtering would be required in such situations. Before examining in more detail electrophoresis on chips, it is important to consider the fabrication of such microchannel devices.

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Solar Panel Basics

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