Solid support and separation options

One of the first questions when considering what conformation a new assay is to take is how is the quantitative signal to be separated from the surrounding matrix Proteins bind tightly to plastic surfaces (6) and, if it is possible to adhere an antibody or antigen to a solid support of some kind without harming its activity, a variety of microtitre plates or microspheres are available for this purpose. If the reaction needs to be done in solution and the separation carried out afterwards, then, for technical ease of manipulation, a particle which can be added to the reaction solution, or some other means of precipitating the required activity from solution, is necessary. This section describes some of the more popular techniques. 

Microtitre plates are limited since they restrict the volume of the reaction, and thus the amount of sample that can be applied. Where this is a problem, polystyrene particles (or other solid bead) to which antibody is adsorbed, can be added to any reaction volume. Systems where analytes are absorbed to membranes (e.g. nitrocellulose) and then vacutim filtered through in sequence (e.g. the Pierce ELIFA system) are also less limited vrith respect to volume, although technically more difficult to use successfully. 

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In contrast, the mode of operation of a number of other modules in microfluidic devices, such as reactors, solid-phase extraction, and separation units require interactions of molecules in solution with functionalities attached to a solid support. Obviously, this support can itself be the channel wall but, in this case, the overall surface-to-volume ratio is very small. This ratio can be increased by filling the channel with a solid phase, and the monolithic materials represent one of the options to fill the channel with a suitable stationary phase. 

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