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The majority of modulators, in particular commercial modulators, until recently, involved the use of cryogens to trap and re- inject the cut. These two-stage thermal modulators use cold and hot jets to enable the modulation. An example of the loop thermal modulator from Zoex is shown in Figure 1. The cold jet runs continuously to create two cold spots on the looped column, about 3mm long, Figure 1(a). The hot jet is pulsed periodically to divert the cold jet and heat the cold spot to re-mobilise the trapped analytes and move them on to the next stage. The cold jet is usually cooled with a
liquid nitrogen or CO2 heat exchange, but room temperature compressed gas can be used for the modulation of low volatility analytes. The volume of nitrogen used can be around 12 standard litres per minute, resulting in a large consumable cost as well as high initial set-up costs for handling liquid nitrogen for a thermal modulator.
More recently, commercial flow modulators have enabled the modulation of gases to high boiling components without the use of cryogens; Figure 2 shows the Agilent capillary flow technology GCxGC modulator. In the load position, Figure 2(a), the eluent from the first column is accumulated in the collection channel at a flow rate of 0.8mL/min. The cut is then injected onto the second column by changing the flow direction from the modulation valve to flush the analytes at the second column flow rate of around 21mL/min as shown in Figure 2(b), to produce a tight sample band. The analytes are then quickly separated on the second column by this high flow rate ready for the next cut to be introduced. Hydrogen is typically used as it produces a very fast, efficient separation and is cheap considering the carrier gas flow rates used in the separation on the second dimension column.
When considering detectors for GCxGC, one with a fast response is necessary for the very narrow peaks eluted from the second dimension column. Typically, universal detectors like Flame Ionisation Detectors (FIDs) and specific detectors like Electron Capture Detectors (ECDs) and Sulphur Chemiluminescence Detectors (SCDs) are employed. To hyphenate a Mass Selective Detector (MSD) for additional information for compound identification, confirmation and quantitation, a fast-scanning Time-of-Flight is required to obtain enough data points across the peak.
Until recently, investment in a GCxGC-MS instrument with a thermal modulator and a Time-of-Flight mass spectrometer, was a large cost which is justified for a dedicated
Figure 3: GCxGC-FIDqMSD configuration showing both column sets
Figure 4: GCxGC-FID chromatogram of Talisker whisky on column set 1
Figure 5: GCxGC-FID 3D chromatogram of Talisker whisky on column set 2
instrument that is regularly used for the analysis of complex samples but not justifiable for the occasional sample analysis by GCxGC-MS. Introduction of a flow modulator, that can be upgraded on a standard gas chromatograph and uses cheap
hydrogen as the only consumable greatly reduces the investment in GCxGC technology and the instrument can justifiably continue to be used for non-GCxGC applications. The flow rate eluting from the second column is far too high to be
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