Practical Two-Dimensional Gas Chromatography

In many respects, the coupling of GC columns is well suited since experimentally there are few limitations and all analytes may be considered miscible. There are, however, a very wide variety of modes in which columns may be utilized in what may be described as a two-dimensional manner. What is common to all processes is that segments or bands of eluent from a first separation are directed into a secondary column of differing stationary phase selectivity. The key differences of the method lie in the mechanisms by which the outflow from the primary column is interfaced to the secondary column or columns. 

Efforts have been made, however, to extend the range or extent of samples that can be analysed by using a two-dimensional separation when used in heart-cut mode. This has been reported to include the use of numerous parallel micro-traps to essentially store the primary column eluent fractions ready for second-column separation, and the use of parallel second-dimension columns. 

The ultimate extension of two-dimensional GC was introduced in the early 1990s (4) and involves the reanalysis of all components from the primary dimension on a secondary column. To enable this, the peak capacity of the secondary column is often very much smaller than the primary thus allowing completion of the separation in a time that may be considered insignificant as a fraction of the time required to complete the primary stage. This is discussed in much further detail in Chapter 4. 

Pigure 3.1 shows several potential on-line modes of two-dimensional GC operation. These couplings demonstrate HRGC-HRGC performed by using a single heart-cut from the primary to the secondary column, multiple heart-cuts, transferred to multiple intermediate traps, and heart-cuts transferred to a multiple parallel secondary column configuration. 

Although the ability to generate separation systems with significantly enhanced peak capacities is the most obvious practical usage of two-dimensional GC, there are several ancillary benefits which are often also achieved when analysis is performed using this approach. 

The non-intrusive manipulation of carrier gas effluent between two columns clearly has significant advantages in two-dimensional GC. In addition, a pressure-driven switch between the columns introduces no extra band broadening to an eluting peak. 

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