Parallel comprehensive two-dimensional gas chromatography
Introduction
With the aim of providing enhanced qualitative information compared to conventional comprehensive two-dimensional gas chromatography (GC × GC), Savareear et al. introduced two closely related multiplexed GC × GC approaches utilising contra-directional thermal modulation [1], [2]. Unlike other multi-column GC × GC methodologies [3], [4], [5], [6], these multiplexed techniques employ a single detector to generate two two-dimensional (2D) separation windows for each injection within a single detector channel. Complementary separation and enhanced qualitative information were provided due to the selectivity differences between the different multiplexed stationary phase columns. By nature of the multiplexed approaches, a fast second-dimension (2D) separation is especially important to avoid overlap of the two separation windows in the single chromatogram [1]. Therefore the investigators configured their setups by using relatively short (e.g. 0.4 m) and narrow internal diameter (e.g. 100 μm i.d.) 2D columns coupled with conventional first-dimension (1D) columns (i.e. 30 m × 250 μm i.d.) [2]. However, these 2D column dimensions might not be the best choice for GC × GC analysis [7]. These 2D columns lead to a considerably faster analysis, but at the expense of providing insufficient separation and band broadening due to overloading compared with longer and wider bore 2D columns (e.g. 1 m × 250 μm i.d.).
Flow-mismatch between the two separation dimensions is a typical GC × GC problem stemming from the coupling of columns with considerably different internal diameter 1D (regularly 250 μm) and 2D columns (often 100 μm). This flow-mismatch can be resolved by adopting a wider 2D column or by using 1D and 2D columns with the same internal diameter [5]. Flow conditions closer to optimal can be achieved in both dimensions, when columns with homologous internal diameter are used, leading to improved exploitation of the 2D stationary phase selectivity and increased 2D sample capacity [7], [8], [9]. A recent study has experimentally demonstrated that the use of homologous column diameter in both dimensions can substantially reduce the detrimental effect on underutilisation of the primary peak capacity (1U) compared to that of comparatively narrow 2D column. Employing homologous column diameters in both separation dimensions is one of the key factors enabling a near-theoretical maximum in peak capacity gain (Gn) [10].
In the present investigation we employed a simple but effective method of independently controlling carrier-gas flow in the first- and second-dimensions. Recently, Luong et al. utilised auxiliary flow control between the 1D and 2D columns for the purpose of Retention Time Locking and Back-Flushing in GC × GC [11]. Similarly, changing the carrier-gas pressure at the junction of two series-coupled capillary GC columns has been reported as a versatile approach to achieve the best possible separation of multicomponent mixtures [12], [13], [14]. By changing the junction pressure, it is able to alter the relative retention position of components, and therefore adjust the selectivity of the column ensemble leading to optimal separation. Independent control of flow conditions also greatly assists development of multi-column GC × GC approaches. To this end, we introduce a four-column multiplexed technique with two independent 1D columns each coupled to its own 2D column, and employ an additional gas supply and precise electronic pressure control at the midpoint between the two dimensions. Comparison of analyses with and without independent 2D flow control are made using otherwise matching column sets and the importance of applying flow control to adjust the separation speed in 2D is outlined. The capability of independent flow controlled 2GC × 2GC is demonstrated by the analyses of hop (Humulus lupulus L.) essential oil with two column combinations comprising non-polar × polar and polar × non-polar column sets. Important features of the proposed independent flow controlled 2GC × 2GC approach compared to multiplexed approaches introduced by Savareear et al. are discussed.
Section snippets
Chemicals and reagents
Hop (Humulus lupulus L.) essential oil was prepared by hydro-distillation of dried hop cones (Hop Products Australia, North Hobart, Australia), and was diluted (1:20, v/v) in dichloromethane (Sigma-Aldrich, Castle Hill, Australia) prior to GC analyses.
Instrumentation and experimental conditions
All analyses were performed using a Leco GC × GC-FID instrument with an LN2 Cooled Thermal Modulator (LECO Australia, Castle Hill, Australia). The chromatograph was equipped with a split/splitless injector, operated with a 20:1 split ratio and inlet
Performance and benefits of GC × GC-flow control system
The first part of this study was carried out by employing precise 2D flow control using an auxiliary PCM connected to a microchannel device at the union of the two separation columns. Close to optimal flow conditions in both dimensions can be achieved in a GC × GC column set having the same internal diameter in the first- and second-dimensions [7], [9]. Aligned with current thinking, we expected 1D head pressure to provide sufficient pneumatic control to achieve the best GC × GC results, since the
Conclusions
Appropriate flow control at the junction of 1D and 2D columns permits the possibility and simplicity of implementing GC × GC and 2GC × 2GC experiments using 250 μm homologous i.d. column combinations. The proposed integrated system is robust and has the unique ability to design two entirely independent conventional GC × GC separations for each injection, yielding two independent GC × GC chromatograms viewed in a single window leading to an appreciable gain of productivity. The analytical performance and
Acknowledgements
DDY gratefully acknowledges the provision of a Tasmania Graduate Research Scholarship. This work was supported by the Australian Research Council’s Linkage Projects scheme (LP140100160: Hop Flavoromics for Distinctive Beer).
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2019, TrAC - Trends in Analytical ChemistryCitation Excerpt :The majority of the work reported in the literature is based on a Deans' switch design of flow control. For a deeper examination of current research, the reader is encouraged to review several methods and modulators developed in the last few years but not discussed here [41,72–74,78,79,82,83,88,89,96–101]. The Livadaris group has reported two important comparisons of the performance of thermal and flow modulators [34,102].
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2018, TrAC - Trends in Analytical ChemistryCitation Excerpt :This was termed a multiplexed process [26]. Subsequently, a dual 1D column arrangement was developed [27], and further, two independent GC×GC sets were located in the contra-flow multiplexed system [28]. Although not a dual 2D column approach, as above Peroni et al. used a multi-capillary 2D column constructed using a very well controlled manufacturing process with uniform 0.075 mm I.D. and 1.1 m length; 3 such columns were operated in parallel, into a single detector [29].
Second dimension column ensemble pressure tuning in comprehensive two-dimensional gas chromatography
2018, Journal of Chromatography ACitation Excerpt :A dual 1D arrangement with varying lengths of non-polar and polar columns, with either a polar or non-polar 2D column, aimed to study how Te variation at the end of the dual 1D arrangement affects overall 2D presentation [12]. Multiplexed GC×2GC has been described using parallel dual 2D columns in a contra-directional fashion through the modulator to provide two sets of 2D data, but requiring only one detector [9,13]. Retention mechanisms in GC are largely based on the interaction between analyte and stationary phase.