GCxGC with Fluidic Modulation on Enantioselective Oil Analysis

Oct 01, 2011
Volume 24, Issue 10, pg 548–555

Properly informed column selection greatly simplifies the process of method development and provides greater flexibility in instrument settings for comprehensive two-dimensional gas chromatography (GC×GC) with fluidic modulation. In the present investigation a long (20 m) narrow-bore (0.10 mm i.d.) column was used in the first separation dimension. In this way, the separation column itself provided sufficient pressure-drop to alleviate flow disturbance in the first dimension column caused by actuating the modulator valve. The use of a narrow-bore column in the first dimension is highly beneficial because it is also more suited to lower volumetric flow-rates and therefore permits use of a longer modulation period without causing sample loop breakthrough. Enantioselective separation of spearmint essential oil was performed to demonstrate performance of a fluidic modulation system.

Environmental monitoring in remote areas is of great interest and we have been working to determine the suitability of comprehensive two-dimensional gas chromatography (GC×GC) for this task. Conventional GC×GC instrumentation, which relies on thermal modulation (1), is not ideally suited for use in remote field laboratories, thus alternative modulation approaches have been investigated. One of these approaches is based on fluidic modulation, which was introduced by Seeley and co-workers in 2006 (2). The basic architecture of this fluidic modulation approach has been used in the design of the Agilent CFT Modulator (Agilent Technologies, Santa Clara, California, USA) (3). It is worth noting that the specific dimensions of the channels in the CFT device are not necessarily identical to those described by Seeley et al., but the fundamental approach is consistent. The monolithic structure of the CFT modulator and its use of leak-free SilTite ferrules (SGE Analytical Science, Ringwood, Australia) offer several advantages over homemade fluidic modulators. Notwithstanding, many investigators have found fluidic modulation using this device to be less than satisfactory and the limited number of publications using the technology may be partially ascribed to the difficulty in determining rugged modulation conditions.There are a few key considerations to optimizing GC×GC instrument settings for fluidic modulation. Some of these considerations are unique to the fluidic modulation and others are more generally applicable to GC×GC. Starting with the general considerations, a wide body of literature indicates that it is highly desirable to maintain a suitable modulation ratio (4). There is a rule of thumb [which has sound theoretical and practical underpinnings (5)] that suggests there should be four modulation slices across each first dimension peak. This rule of thumb ensures that peak capacity of the first dimension separation is not too greatly diminished by the modulation process. It is also highly desirable to minimize the extent of wraparound (6). Thus the elution window in the second dimension should not significantly exceed the modulation period. When the modulation period is exceeded there can be detrimental effects on resolution in the second separation dimension. There is often a practical compromise of each of these criteria, but the best GC×GC results typically come from carefully optimized systems that adhere strictly to both considerations. Stationary phase selection is a key consideration for all GC×GC approaches. Such considerations are addressed in detail elsewhere (7).

Figure 1: Plan of a fluidic modulator, showing the sample loop (L), 3-way switching valve (V), connecting tubing and the connecting points of the first (Col 1) and second (Col 2) dimension columns.
Among the important specific considerations for fluidic modulation, the critical consideration is that proper pneumatic conditions are applied in order to:
  • ensure stop-flow modulation (at point X in Figure 1).
  • provide sufficient time to sweep the modulator sample loop (Figure 1)
  • prevent breakthrough from the sample loop during the modulation cycle.

In addition, a large volumetric flow ratio (column 2:column 1) has to be applied to assist peak-compression-in-time, which reduces the injection bandwidth into the second dimension column. Typically this flow ratio needs to be 10:1 or higher (8).

Figure 2: GC×GC–FID 2D chromatogram of the standard mixture of 45 FAMEs (Reference 9, with permission).
Maintaining appropriate pneumatic conditions, whilst satisfying the general considerations described above can be troublesome. An example drawn from the literature of fatty acid methyl esters analysis using fluidic modulation is shown in Figure 2 (9). In this example the instrument set-up comprised a first dimension separation column that had dimensions 30 m × 0.25 mm coupled to a second dimension separation column with dimensions 2 m × 0.25 mm. The film thickness of each column was 0.25 μm. The authors employed a column ensemble in which the first dimension column was coated with a 70% cyanopropyl polysilphenylene-siloxane stationary phase and the second dimension column was coated with a 5% polysilarylene/95% polydimethylsiloxane copolymer stationary phase. The most striking feature of this chromatogram is how the broad second dimension peaks sometimes fill the entire y-axis. The peak capacity of the second dimension is less than 1 in some places. The observed low performance may be a result of modulator breakthrough, incomplete flushing or a combination of both. These undesirable effects can be eliminated by informed column choice, along with appropriate instrument settings.

The primary argument of the present article is that properly informed column choice greatly simplifies the process of method development and gives greater flexibility in instrument settings for fluidic modulation. Previously, Harvey et al. showed that is was possible to increase the modulation period to at least 9 s without adverse chromatographic affects (8) using a homemade fluidic modulation device. In the present investigation, the principles reported in earlier studies (8,10) have been applied with a CFT fluidic modulator. Enantioselective analysis of essential oils offers a good illustrative example of the benefit of multidimensional separations.

There are two options for performing enantioselective GC×GC separations. First, it is possible to follow the path laid by the myriad heart-cut enantio-MDGC applications for essential oils analysis (11) and use an achiral first dimension column coupled to an enantioselective stationary phase column in the second dimension. However, practical implementation of this approach is obstructed by the need for rapid second-dimension separations (12). The preferred approach is to use an enantioselective stationary phase column in the first dimension and follow this separation with one performed using an achiral stationary phase in the second dimension (13). The results from enantioselective GC×GC analysis of spearmint essential oil are presented here to highlight suitability of a fluidic modulation system for this dedicated application.

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