In general, comprehensive two-dimensional (2D) chromatography (C × C) methods enable the separation of the entire injected sample (first introduction system) on two columns (first and second dimension — 1D and 2D), characterized by different stationary phases. A transfer device, named modulator (second introduction system), enables the sequential and continuous injection of 1D eluate fractions onto the 2D column. Separations of each fraction on the 2D column are usually rapid — ideally, each fraction must be analysed before the next one exits the modulator — and are characterized by a fixed time frame (modulation period). Normally, a single detection system is used, to monitor the analytes leaving the 2D column. Hence, a native C × C chromatogram is formed of a sequence of rapid 2D separations, which develop gradually along an x axis. Dedicated software tools are used to transform such one-dimensional data in a planar (2D) format. The 1D and 2D separations are positioned along an x and y axis, respectively, while peaks are represented with circular/oval shapes. Peak area and colour intensity relate to analyte quantity.

The common benefits of C × C are an enhanced separation space and selectivity, along with the formation of organized elution patterns whenever homologous series of compounds are involved (e.g. triacylglycerols, carotenoids, monounsaturated fatty acids, pyrazines, etc.). A further benefit, related more specifically to thermal modulation comprehensive two-dimensional gas chromatography (GC × GC), is an enhancement of analyte signal-to-noise ratios. If mass spectrometry (MS) is used as “detector”, then an extremely powerful instrument is formed, composed of three analytical dimensions. The C × C-MS field has seen a great deal of instrumental and software development (involving both industrial and academic entities) over the past two decades, along with the publication of a plethora of application types (food, biological and microbiological, petrochemical, environmental, flavour and fragrance, pharmaceutical, etc.).

The most commonly-used C × C approach has been GC × GC, followed rather distantly by comprehensive two-dimensional liquid chromatography (LC × LC). Other technologies, such as that involving the liquid–gas chromatography combination (LC × GC), have been reported only in a handful of investigations. The first descriptions of GC × GC and LC × LC appeared in 1991 and 1990 [1, 2], respectively, while the benefits of using two consecutive orthogonal separation processes (in planar chromatography) dates far back [3]. However, though the advantages of using GC × GC and LC × LC have been fully demonstrated in a multitude of cases, both techniques are far from established and are still perceived to be used mainly by an elite class of chromatographers. Other reasons exist, such as the power of modern-day GC–MS and LC–MS, which can effectively cover many analytical requirements, and financial aspects. We imagine that such a scenario, at this point, will remain so for a long time. It is noteworthy that the use of GC × GC or LC × LC is well within the reach of anybody with a decent knowledge of chromatography basics and adequate training.

The present ABC “Comprehensive 2D Chromatography” Topical Collection confirms the currents trends in the field. In fact, among in well over twenty papers, three are based on LC × LC and the remaining ones on GC × GC. With regard to the LC × LC contributions, two are focused on the determination of food nutraceuticals (in one case, focus was also devoted to the use of “greener solvents”) and the other on dilution effects. Among the GC × GC manuscripts, there is one review paper (related to the plastic recycling process), while the remaining are research papers, describing diverse untargeted and targeted applications (plastics, foods, microorganisms, sediments, fuel, saliva, pharmaceuticals), data processing tools, theoretical/practical issues, and instrumental evolution (2D temperature-programming). Food analysis is herein the main GC × GC applicational field reported.

We would like to conclude thanking all the authors for their numerous participation to this Topical Collection, the reviewers for their precious evaluations enabling general improvement of the contributions, and the ABC editorial team for the constant support.