GCxGC is gas chromatography in two dimensions. The technical set-up is two columns connected in series, and to obtain additional information from the GCxGC it is necessary to have two columns with different polarities. After column 1 a cryogenic modulator is mounted, which in milliseconds can switch between cold (typically-150 ° C) and hot.
This causes the effluent from the first column to be re-focused, and by the subsequent heating injected onto the column 2. Column 2 is typically short, 1 m, to obtain a fast elution. The number of seconds (typically about 5 seconds) between two consecutive injections is called “modulation time” and may be adjusted depending on the analyzed components. If this setting is incorrect, “wrap-around” will typically happen, meaning that the last components are not eluted from the column 2, but comes out in the next “5-sec-window”.
Due to the re-focusing, components which elute from column 2 are considerably sharper, typically in the range of 35 – 200msec, and this puts considerable demands on both hardware (scanning speed) and software. Moreover, many peaks will elute over two or more “five-second” windows, and this also places considerable demands on the underlying algorithms to provide proper assembly of the peaks and thus quantization of the individual components.
A typical 2D chromatogram: