Comprehensive Gas Chromatography

The (GC)2 part of the abbreviation serves to indicate the multiplicative capacity of the system, and the term comprehensive reflects that the full suite of peaks from the first dimension is analysed.

The second dimension analysis time will be about 5 s or less, and the second dimension, comprising a short, high-phase ratio, narrow-bore open tubular column, might have a total peak capacity of only 10-20 peaks. C(GC)2 uses first and second dimensions

Figure 6 Peak contour representation of two-dimensional separation in comprehensive chromatography.

at the same oven temperature, allowing the second column effectively to resolve on the basis of polarity difference between the two columns. If adjusted properly, the full peak capacity on the second dimension should now be available for separation, and the total peak capacity should be the product of the first column capacity and the capacity available on the second column at any chosen operating temperature (the second column operates almost isothermally for each individual analysis).

Having a high peak capacity should not be too critical on the second column, but phase polarity or selectivity difference should be carefully chosen.

Peak compression, followed by fast second dimension analysis, results in improved sensitivity of detection; if a 5 s band of effluent from column 1 is compressed and leads to a detected peak width of 250 ms, a 20-fold sensitivity increase should result.

Technically, C(GC)2 with compression in time requires novel procedures. Two systems have been described for C(GC)2 employing band compression. One is based on a rotating elevated temperature modulator which passes closely over the junction between the two columns, incorporating a thick film accumulator section between the columns at the junction. An alternative device employs a longitudinal oscillating cryogenically cooled trap that can collect and focus solute from the first column, then pulse or remobilize the narrow band into the second column.

Given the need for very rapid analysis, rapidly recording detector systems are required.

Peak position in the two-dimension separation space will now be a complex function of volatility and polarity, determined by the individual mechanisms of the two columns chosen, and a full interpretation of the C(GC)2 method is required in this respect. Possibilities for class separation demonstrate that the method has potential for multiresidue and screening applications, and characterization of petroleum products.

See also: II/Chromatography: Gas: Column Technology; Historical Development; Theory of Gas Chromatography. III/Gas Analysis: Gas Chromatography.

C. E. R. Jones, Redhill, Surrey, UK Copyright © 2000 Academic Press

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