Index
GCxGC
Chromatographers almost always search for greater resolution, and this makes GCxGC a significant development for the GC analyst. We introduced cryogenic modulation in 1997, and this now appears to be the most accepted of the thermal methods. A range of alternative methods using cryogenic modulation have appeared since we demonstrated the power of this approach. Our system is called the LMCS (Longitudinally Modulated Cryogenic System), and our motto is:
We LOVE our MOVING modulator!!!
But what is GCxGC?
GCxGC, or comprehensive two-dimensional gas chromatography, is a gas chromatographic technique which offers superior resolution and separation power compared to conventional gas chromatography (GC).
But how?
Unlike GC, which consists of only one chromatographic column, GCxGC consists of two chromatographic columns, serially coupled, with a modulation mechanism at their junction. We call these two coupled columns a ‘”column-set”.
What sort of columns are used?
The two columns are composed of different stationary phases, and so solutes are separated by two, independent, or (effectively) ORTHOGONAL separation mechanisms.
For example?
A typical column-set is composed of a standard low-polarity column (1D), with typical dimensions 25 m x 0.25 mm ID x 0.25 μm film thickness (df), coupled to a much shorter, and more polar (or a column providing a separation mechanism capable of further differentiating target sample components) second column, 2D, with dimensions 1 m x 0.1 mm ID x 0.1 μm df.
Why is the second column so short?
Glad you asked! The second column is so short because it needs to be able to provide very fast analysis, ie a matter of seconds. That’s why the ID and df of 2D are also reduced compared to that of the first column (1D). And importantly, the short 2D column does provide useful separation of different polarity compounds
But why?
Well, there’s another important component in the GCxGC system, which is called a modulator. The modulator interfaces the two coupled columns, and is responsible for the quantitative transfer and compression of all solutes, or a representative fraction thereof, from 1D to 2D. Peaks eluting from 1D are trapped and then focussed, usually by thermal means, resulting in very sharp, narrow peaks, with increased height. These peaks are then rapidly released onto 2D, and to preserve the increased sensitivity resulting from the modulation process, we need to use a short, fast column.
What else does the modulator do?
The modulator actually collects eluent from 1D every few seconds (generally 2 - 9 seconds), and so an individual chromatographic peak is actually sliced into many fragments (at least 4 is good!). Each fragment is focussed and pulsed to 2D for fast analysis.
So how do I know which peaks belong to which compound?
Have a look at the Figure 1. Chromatogram A represents the conventional one dimensional chromatogram, and Chromatogram B shows this same peak sliced and modulated into 7 pulses using GCxGC. These pulses form the envelope of a gaussian shaped peak, and each peak is separated from the subsequent peak by exactly the same time - the modulation period.
Figure 1
How come the sensitivity is different in Figures A and B?
In Figure B, the peak eluting from 1D is trapped and focussed by the modulator, into a sequence of pulses. Each pulse becomes very narrow because of the focussing process, and because modulation is a mass conservative process, the peak height increases to accommodate for the reduction in peak width.
What do you mean by “mass conservative process”?
This means that if you add up the peak areas of each of the peak pulses in Figure B, it will give you the same area as the single peak shown in Figure A. This is because the modulator quantitatively transfers solute from 1D to 2D.
What if I have two compounds, which coleute in conventional GC?
Because we use 2 ‘orthogonal’ columns in GCxGC, we can resolve compounds which would coelute under conventional, single column GC (provided the 2D column allows their separation). If you look at Figure 2 you will see that unresolved peaks in Chromatogram A can now be separated because of the modulation process, and different pulses can be assigned to different compounds depending upon their retention times.
Figure 2
So, GCxGC not only gives improved separation of analytes, but also increases sensitivity compared to conventional GC?
Yes! This is seen in Figure 3.
How are the GCxGC data presented?
Generally, the data are presented in a two-dimensional plane, which plots the retention time on 1D (minutes) against the retention time on 2D (seconds), but that makes sense because two dimensions of separation are used. Figure 3 shows the equivalent chromatograms for a standard drug mixture using conventional GC (A) and GCxGC (B), and shows how the GCxGC chromatogram (B) can be presented in a two-dimensional format (C), known as a contour plot. In Figure 3C, each “spot” represents a different compound. In Figure 3A, peaks labelled “A” and “B” are actually shown to be a mixture of components (Figure 3B).
Figure 3
What kind of samples can I analyse by using GCxGC?
Any sample that you can analyse by conventional GC! GCxGC is particularly good for the analysis of complex samples, since peaks can be better resolved. Have a look at some of our Applications for examples.
What do I need to get started doing GCxGC?
A modern GC with EPC
Fast detection, at least 50 Hz (eg. An FID, TOFMS, or ECD)
Your regular GC injection method
Two capillary columns- 1D is the primary column, 2D is the fast elution column
Suitable data handling software
A keen interest in developing new applications and approaches to high resolution separation!!!