Gas Chromatography - Tandem Techniques - Modern GC/IR Systems > Light Pipe Interfaces > Page 47


Many modern GC/IR systems employ a system of light pipes to augment the IR signal, and an example of a contemporary GC/IR tandem instrument that utilizes light pipe amplification is that manufactured by the Perkin Elmer Corporation. The optical system of the Perkin Elmer instrument, which is typical of many contemporary GC/IR tandem systems, is depicted diagramatically in figure 33. Light from an 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 it. The light leaving the pipe is focused by means of two curved mirrors onto a cooled IR sensor. A cold baffle is situated between the pipe exit and the sensor to ensures that none of the IR emitted by the hot oven that thermostats the tube can fall on the sensor.


Courtesy of the Perkin Elmer Corporation


Figure 34. The Optical Arrangement of Light Pipe Interface


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 34. This interface can be used with all types of GC columns including open tubular columns. The capillary column passes 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 gas, the solute peaks from the column would accumulate in the IR light pipe, and as a consequence, several solutes would be detected and measured simultaneously, and resolution would be lost. The light pipe itself, as designed by Perkin Elmer is 120 mm long, 1 mm I.D and coated internally with gold. The oven surrounding the light pipe, can be maintained at temperatures up to up to 350ūC, and must be carefully designed to eliminate any cold spots on the tube, which might allow solute condensation.