In early 2012, we published a review in Trends in Analytical Chemistry (ref 252), entitled ‘Multidimensional Gas Chromatography’ which presented an overview of both GC×GC and MDGC. It seems to us that MDGC has elements of GC×GC
encapsulated in the umbrella technique, and that a review of MDGC methods should necessarily include GC×GC as well. We certainly do GC×GC in a novel manner, with terms such as period of modulation, phase of modulation, and modulation ratio all specific to GC×GC (for a review of the nomenclature of GC×GC and a recent
update, refer to refs 98 and 254). This demands the 2D column to be a short, fast
elution column.
In this review, we present some updated information on different microfluidic devices in MDGC, and also a range of different schematic arrangements that have been
used in classical MDGC, and some of our own work that has experimented with different GC×GC approaches. We also include some systems that integrate aspects of GC×GC with MDGC. This includes research on a switchable method that can perform GC×GC and MDGC in a single GC system (ref 218). We are now increasingly involved in advanced methods based on this conceptual development.
Thus in 2012, we reported the first study on a new design of a system we refer to as hybrid GC×GC-MDGC instrument (Anal Chem; ref 253). The system performs GC×GC analysis using our modulation device, but in the described work we slow the modulation period to 20 s. Some might believe the with such a slow PM and hence low MR value, the system is no longer ‘comprehensive’, but since this work we have used much faster PM settings of e.g. 6 s. To now conduct the MDGC operation, at
the end of the 2D column we have a Deans switch, which can cut certain zones of
effluent to a 3D column. This is a true 3-dimensional analysis. Hence it is possible to excise a complete zone or e.g. a chemical class of compounds into the 3D column. We used this to analyse oxygenated compounds in a thermally degraded algal biofuel. Our prior study on this sample (ref 250) separately used both GC×GC to capture the whole sample composition, and a MDGC system to periodically sample narrow fractions of effluent from a polar 1D column to non-polar 2D. This column arrangement allows the oxygenated compounds to elute prior to the very large non- polar matrix components. Both of these methods are elegant approaches to high resolution chemical analysis of oil samples. Figure 8 is an example of the system design for our hybrid GC×GC-MDGC arrangement.
In this mode shown, effluent travels to FID1 via the Deans switch (DS). Modulation at the LMCS performs GC×GC analysis. As required, DS can divert flow to the 3D column, and this just requires the EPC to switch to the other channel. This is another mode of the diagram shown in Fig 8. For example during every modulation sequence of 20 s, the DS can be programmed to switch between the times of 4 s and 6 s after the modulation start, to pass just a zone of peaks to the 3D column which elute between 4 and 6 s in the GC×GC 2D plot. We can also ‘cut’ a single peak to the 3D column, or any number of components. Of course, one limitation with this is the number of time we have to program switching events, and we sometimes run out of events in the event control software.
We predict a good future for operations that incorporate such an arrangement, and other advanced modes of performing MDGC and GC×GC analysis. Clearly 2 columns is not the limit here, and three or more columns can be reliably and effectively hyphenated.