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February / March 2012
technique is particularly powerful in the separation of complex mixtures. Marriot et al., demonstrated that combining a cryogenic ‘heart-cut’ operation by effective timed trapping of particular zones of a primary separation, followed by rapid introduction to a second short column, could lead to improved separation performance and result in significantly increased response sensitivity [5].
Due to the narrow modulated peak widths, comprehensive GCxGC is best used in combination with fast acquisition time of flight (TOF) mass spectrometry [6]; an MS technique that does not have adequate sensitivity and reliability required for trace analysis of compounds such as dioxins and furans [7]. Nevertheless, Focant et al. demonstrated that the modulation process used in GCxGC had an advantage of increased signal intensity for compound such as dioxins. However, because of the inherent limitations of the TOF MS, limits of detection remained insufficient for the application.
The ability to combine the signal intensity increase observed for GCxGC analysis with a more sensitive detection system would present a significant step forward in the pursuit of the low limits of detection required for the analysis of dioxins & furans in DBS samples. Patterson et al., utilised a comprehensive GCxGC modulator on a single, short GC column coupled to a high resolution mass spectrometer (HRMS) – a technique which they termed Cryogenic Zone Compression Gas Chromatography (CZC GC-HRMS) [8,9]. They were able to significantly lower the detection limits for dioxins using this technique.
Cryogenic Zone Compression (CZC) GC operates in a comparable fashion to comprehensive GCxGC. The main difference is that entire 1st
dimension peaks are
trapped, followed by reinjection of the complete refocused peak onto the 2nd dimension column. Consequently in CZC only one single 2nd
using CZC. Unfortunately, the approach by Patterson et al [8] – basically using constant modulation with long modulation times to entirely trap peaks - comes with some drawbacks;
• Firstly, the continuous modulation of the cryo-trapping then releasing can ‘slice’ the elution of a single analyte from the first dimension into several chromatographic peaks in the second dimension. Whereas, in comprehensive GCxGC this is a wanted effect; for CZC this reduces the resulting overall signal intensity, compounds analytical error and complicates data evaluation.
• Secondly, this approach is based on narrow 1st
dimension peaks and thus short
columns are mandatory. The usage of 30 or 60 m columns - standard in
conventional dioxin/furan analysis - cannot or hardly be used.
• Thirdly, the use of constant long modulation times – which corresponds to non- tailored trap timing - can result in unselective trapping of multiple analytes or matrix peaks into the refocused chromatographic band; potentially
Figure 2: CO2Modulator (located inside the GC oven)
resulting in loss of information or the focussing of chromatographic noise along with the peak of interest.
• Fourthly, the modulation runs continuously throughout the analytical sequence – resulting in significant consumption of cryogenic gas, adding prohibitive cost and maintenance effort to the analysis.
To answer these drawbacks and address the challenge of trace analysis from small sample sizes, ‘Time Controlled’ CZC GC-HRMS was developed and evaluated in this study. Here a novel approach in selective time control of a standard cryogenic GCxGC modulator, rather than constant modulation, to securely capture selected whole analyte peaks was
dimension chromatogram
is produced, whereas in GCxGC each 1st dimension peak is modulated into several 2nd dimension chromatograms. In comparison peak intensities from several 2nd
dimension
chromatograms from an analogue GCxGC experiment are added up to form one single high peak in a corresponding CZC experiment. This explains why the peak enhancement effect as known from GCxGC is maximised in CZC (Figure 1).
Very low concentration broad conventional GC peaks, that would normally be lost to background noise, can now be detected
Figure 4: Multiple ion detection (MID) setup screen. Here showing typical tetrachlorodioxin ions and acquisition times used for CZC Figure 3: GC time events to control cryogenic jet
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