42
February / March 2012 Figure 6: Selected ion chromatogram 87-89 ions Figure 7: Decanoic acid, ethyl ester at 22.228, 1.017 89% match
introduced directly into any mass spectrometer, fast scanning or not. FIDs have long been the detector of choice for quantitation for many applications, with their molar response to carbon, linearity to seven orders of magnitude and reasonable sensitivity. GCxGC adds the high resolution required to separate hundreds if not thousands of peaks, reducing the necessity for analytical resolution. The retention times on both the non-polar and polar columns, along with the class position give added confirmation of the identification of the analyte. Therefore, the main advantage of using an MSD with GCxGC is in the identification of an unknown sample component. Splitting the eluent from the second dimension column when using a flow modulator between a detector like an FID and an MSD means that an MSD with a very high acquisition rate isn’t needed, the number of data points across the peak can be quite low to obtain a mass spectrum for identification rather than the higher number needed for quantitation or deconvolution. Hence a bench-top mass spectrometer, like a quadrupole, can be used which is standard in most analytical laboratories these days and again doesn’t require a huge investment in GCxGC-MS technology for an occasional sample. An instrument capable of working at higher flow rates is recommended with a high performance turbo to enable a higher proportion of the split to be directed to the MSD giving a higher sensitivity, but this is the same as a GC-MS capable of chemical ionisation. Therefore, by using a GCxGC flow modulator with a quadrupole mass spectrometer the investment required both to purchase the instrumentation and the running costs is far reduced, making GCxGC- MS more accessible for commercial laboratories whether it is to analyse samples routinely or to analyse the occasional sample using this technique.
Experimental
Whisky was analysed by GCxGC-FID/qMSD on an Agilent Technologies (Cheadle, UK) 7890 gas chromatograph with a 5975C mass spectrometer and a capillary flow technology GCxGC modulator using two column sets as shown in Figure 3. Column set 1 comprised of a non-polar 30m x 0.25mm i.d. x 0.25um DB5-MS first dimension column and a 5m x 0.25mm x 0.15um HP-Innowax second
dimension column. 1 µL of neat Talisker whisky was injected with a 10:1 split ratio.
Figure 8: Dodecanoic acid, ethyl ester at 26.955, 1.081 85% match
GC Image software (Zoex Corp., Houston, USA) was used to analyse the data from both the FID and MSD. GC Image can visualise
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