GCxGC system

The modulator is the heart of the GCxGC system, and is positioned at the confluence of the coupled chromatography columns. The role of the modulator is to trap or isolate compounds present in a given time fraction eluting from the first-dimension column and reinject these components rapidly into the second column. This essentially yields a time-sampled chromatogram, from the first dimension ( D) to the second dimension ( D). It is critical that the modulator is capable of representatively and faithfully sampling peaks eluting from onto D. This can be achieved by either complete or partial transfer of the first-column eluent, however, both techniques are considered comprehensive. 

Fig. 15.2 A two-dimensional GC (GCxGC) system. D1 intermediate effluent monitor detector, column 1 high-capacity precolumn, column 2 high-resolution capillary or chiral column...

FIGURE 13.7 Instrumental setup of (a) a GCxGC system (Anon., GC x GC Comprehensive Two-Dimensional Gas Chromotography Form No. 209-184 R2.58-REV-1 LECO Corporation, St Joseph, MI 49085 P3, 2008. With permission.) (b) a LC x LC system based on a second-dimension column with two storage loops (T. Hyotylainen, LC x LC switching valves configuration Chromedia Amsterdam P Two Dimensional LC (LC x LC), 2008. With permission.). 

As an example, in Figure 3A, the inside of the oven of a GCxGC system equipped with a quad-jet dual stage cryogenic modulator is pictured (see later in this chapter for further details on the modulation process using this particular modulator). A GC split-splitless injector is located on the top left side, the column (30 m) is located on the right part, and the end of is visible on the... 

In a GCxGC system, the retention of an analyte is characterized by its retention times and Ir in the two columns. Ir is defined as the time elapsed between injection of the analyte and its arrival to the modulator, while Hr is the time taken for the analyte to elute between modulator and detector. In GCxGC, a coincidence of both Ir and Hr is required for a positive identification. 

Practical Aspects of Higher-Dimension GCxGC Systems 3.1 Operational approaches towards LCxGCxGC and 129... 

Most atmospheric analyses based on GCxGC are qualitative in nature. The main aim has been to identify chemical patterns of the samples, and only a few quantitative applications have been reported. Table 2 lists current applications together with technical information on the GCxGC systems. Similar GCxGC methods are used for both types of samples, however, more volatile compounds are analysed from gas phase (from ca. Ce) and thus, modulators operating with liquid nitrogen are better suited because of their more efficient trapping of the most volatile compounds (see Chapter 2). 

DTD-GCxGC-ToF MS has also been used for the analysis of organic compounds in ambient aerosol particles [27-29]. In this method, samples were collected on filters, and a piece of the filter was placed into an injector liner, which was put into the cold injector by autosampler instrumentation and thermally desorbed to the GCxGC system. The use of DTD as a sample introduction method simplifies the sample preparation, as no liquid extraction is needed. 

In situ analyses are possible, if sampling is connected in-line with the GCxGC system. At present, two systems have been reported, one for gaseous samples and the other one for particles [33-35]. 

---