2. When to choose LC×LC over LC?

Two-dimensional chromatography is generally presented as an alternative to one-dimensional chromatography for complex samples. The complexity of a sample ready for injection (i.e. after any preparation) is a very vague notion that deserves to be discussed.

A first aspect of complexity is the number of constituents in the sample, but there are others, such as the presence or absence of isomers (positional isomers, structural isomers or enantiomers) that are difficult to separate with a single chromatographic method, the simultaneous presence of very highly and very lowly concentrated compounds (trace analysis), or a sample composition that varies over the course of a single study for which it is necessary to maintain the same analytical method (drug development).

Generally speaking, any sample that is difficult to analyze using conventional...