LCxLC: Comprehensive Two-Dimensional Liquid Chromatography

Comprehensive two-dimensional separations are a definite trend in chromatography today. Comprehensive two-dimensional gas chromatography (GC×GC) emerged some 15 years ago (1). After a sluggish start — arguably attributable to a limited availability of instrumentation and software — this is now a widely accepted and commonly used technique for the separation of complex mixtures of volatile analytes. The roots of comprehensive two-dimensional liquid chromatography (LC×LC ) are older. Two-dimensional thin-layer chromatography can be considered a form of LC×LC, dating the technique back at least some 50 years. The contemporary version of LC×LC — based upon two columns and an elegant valve-switching concept — was first demonstrated by Erni and Frei (2) and it, too, preceded GC×GC by a considerable period of time.
The common way to perform LC×LC involves a first-dimension separation that is relatively slow, with a typical analysis time of 1 h or longer. Fractions of the effluent from this first-dimension column are collected in a loop of a switching valve. When this valve is switched, the fraction is injected onto a second column, which provides a much faster separation, typically with an analysis time of a minute or less. While one fraction is being analyzed on the second-dimension column, a new fraction is being collected in another loop connected to the same valve (or by using a second valve). One popular configuration employs a two-way 10-port switching valve, as illustrated in Figure 1. To construct an LC×LC system requires (at least) two columns and two pumping systems. Detection typically takes place after the second-dimension column (and not after the first dimension), so that "one-and-a-half" liquid chromatographs are needed to assemble one comprehensive two-dimensional one.