Light Pipes

From Table 1 it can be seen that the geometry of the gas flow cells used differs considerably, with path

Figure 1 The reference spectrum for isoprene and a spectrum recorded at the analysis of 200 |L cigarette smoke. Both absorption spectra are normalized and overlaid together with their first derivatives.--, Unknown absorbance; --, reference absorbance; —, unknown dA; . . . . , reference dA.

Figure 1 The reference spectrum for isoprene and a spectrum recorded at the analysis of 200 |L cigarette smoke. Both absorption spectra are normalized and overlaid together with their first derivatives.--, Unknown absorbance; --, reference absorbance; —, unknown dA; . . . . , reference dA.

lengths varying from 1 cm up to 12.5 cm and volumes ranging from 70 |L up to 22 mL. Optimization of the gas flow cell involves keeping a low volume for a good chromatographic resolution. At the same time, the path length and the light throughput should be maximized. These optimization factors are the same as for GC-FTIR, where an internally gold-coated gas flow cell is used. A total reflection is obtained at the gold-coated walls, leading to a high light throughput in gas flow cells about 10 cm long and with an internal diameter usually & 1 mm. Because of the use of internal reflection, these gas flow cells are called light pipes. For GC-UV it was realized, at an early stage, that an internal coating was not necessary and an ordinary glass tubing gave a high light throughput due to a total reflection of UV light on the internal glass walls.

A useful rule is that the optimum performance of a GC-UV system in terms of signal-to-noise and band dispersion is obtained from a light pipe with an effective volume equivalent to the volume of a 'typical' GC peak, described by the peak width at half the maximum amplitude. Typical light pipe volumes currently employed for GC-FTIR vary from 50 to 200 |L and these figures should also be valid for GC-UV light pipes.

Figure 3 shows a light pipe configuration that consists of a gas flow cell with a built in micro gas chromatograph together with the light pipe. In addition to the use of this internal micro GC, the cell can easily be linked to an external capillary GC by means of a heated transfer line, as shown in the figure. The 10 cm long gas chromatographic column, which can also be directly connected to a one-stage thermal

Computer

Figure 2 Block diagram for a GC-UV system. Reproduced from (1992) Capillary gas chromatography. Rev. Sci. Instrum. 63(1): 192.

Computer

Figure 2 Block diagram for a GC-UV system. Reproduced from (1992) Capillary gas chromatography. Rev. Sci. Instrum. 63(1): 192.

desorption unit or to a gas loop injector, is preferably used for fast separations of gaseous and relatively volatile molecules.

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