Page 157 - Tandem Techniques

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Light from the IR source is focused on the front of the light pipe, and is subjected to continual
reflections from the walls of the pipe as it passes through. The emitted light is focused by means of two
curved mirrors onto the cooled IR sensor. Between the pipe exit and the sensor is situated a cold baffle,
which ensures that none of the IR emitted by the hot oven that thermostats the tube falls on the sensor.
This simple device reduces the noise generated by the oven and increases the signal-to-noise ratio and
thus the sensitivity. Details of the light pipe are shown in Figure 4.6.

Figure 4.6
The Optical Arrangement of the Perkin Elmer Light Pipe Interface
Courtesy of the Perkin Elmer Corporation

The interface can be used with all types of GC columns including open tubular columns. The capillary
column is led into the interface through a heated tube right up to the light pipe. Concentric to the
column, and through the same heated tube, is fed a stream of scavenging gas that carries the solute
through the IR light pipe. This maintains the integrity of the separation at the expense of some solute
dilution and consequent slight loss of sensitivity. If the solute bands were not swept out by the
scavenging