Page 25 - Tandem Techniques

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separation techniques that are presently available and satisfactory for use with combination systems will
first be discussed.

Separation Techniques for Tandem Systems

The separation techniques employed with combination systems are, in most cases, some form of
chromatography. There are some instances where classical electrophoresis has been used for
preliminary separation before subsequent off-line spectroscopic examination. In fact electropherograms
have been scanned directly by reflectance spectroscopy. However, a more appropriate electrophoretic
technique suitable for tandem operation is capillary electrophoresis and so the basic principles involved
in capillary electrophoretic separations will also be discussed. In the main, however, chromatography is
much the favored separation procedure chosen for tandem systems. A word of caution might be added
here. Isotachophoresis is at present receiving increased attention as it offers excellent possibilities for
the separation of biological polymers and, with further development, might become an attractive
alternative to chromatography for association with an appropriate identification technique.

It has been pointed out that to employ tandem techniques in an effective manner, the basic principles of
both the separation process involved and the identifying technique must be clearly understood.
Consequently, the essential characteristics of both chromatographic and electrophoretic separations will
be discussed. Particular emphasis will be given to those properties of the separation technique that
influences both the tandem interface and the efficient operation of the associated identifying technique.

Chromatography

Chromatography, by classical definition, is a separation process where resolution is achieved by the
distribution of the components of a mixture between two phases, a stationary phase and a mobile phase.
Those components held preferentially in the stationary phase are retained longer in the system than
those that are distributed in the mobile phase. As a consequence solutes are eluted from the system in
the order of their