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the data in either a 2D contour plot where the intensity of the peaks is shown by the varying colours within the peak (Figure 4). Additionally the data can be visualised using a 3D plot where the peaks are more recognisable (Figure 5).
Results and discussion
The contour plot of the GCxGC-FID chromatogram of Talisker whisky on column set 1 is shown in Figure 4. As the first dimension column is non-polar, the x-axis in Figure 4 shows separation based on volatility, with decreasing volatility to the right. The y- axis is the separation by polarity on the second dimension column, with increasing polarity towards the top. On this zoomed-out contour plot, not many peaks can be seen, with the major component being ethanol which overloads both dimension columns as expected. Due to the nature of the components within whisky being more polar than non-polar it was decided to reverse the column phases.
Column set 2 was a polar 30m x 0.25mm i.d. x 0.25um VFWax-MS first dimension column and a non-polar 5m x 0.25mm x 0.15um DB5-MS second column. Therefore the separation on the first column is based on volatility and polarity and the second column only on volatility.
Figure 5 shows a zoomed in portion of the chromatogram of an injection of Talisker whisky on column set 2, with a modulation period of 1.55 seconds, shown in a 3D plot. Not only is the improvement in sensitivity apparent by using GCxGC, but the range of volatilities and polarities of the analytes and the relationships between the component classes can be easily seen.
The GCxGC-MS chromatogram can be used to locate peaks as well as identify unknowns. Figure 6 is a selected ion chromatogram of the masses 87-89 used to find the ethyl esters of decanoic acid (figure 7) and dodecanoic acid (figure 8), the match similarities are high despite the mass spectra not being background subtracted in this case. Figure 9 shows the zoomed-in chromatogram of the GCxGC-FID contour plot between these two ethyl esters with nearly 200 individual peaks found in this small area of the chromatogram. The
Figure 10: Butanedioic acid, diethyl ester at 23.726, 0.509 84% match
GCxGC-MS chromatogram can again be used to either identify individual peaks of interest through the GC Image software as shown in Figure 10 or from the original data acquisition software, AMDIS and NIST.
Conclusions
In conclusion, GCxGC-MS doesn’t have to be a technique available only to research laboratories or those with enough complex
samples to justify the purchase of a dedicated, high-cost instrument with high consumable costs. The use of capillary flow technology to upgrade an existing system or the purchase of a standard GC(-MS) with GCxGC capabilities and the possibility of splitting to a quadrupole MSD for the identification of unknowns makes GCxGC an option available to all.
Figure 9: GCxGC-FID chromatogram of Talisker whisky on column set 2
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