Gas Chromatography - Tandem Techniques - Modern GC/IR Systems > Light Pipe Interfaces > Page 46

 

Courtesy of the Perkin Elmer Corporation

 

Figure 33. The Optical System of the Perkin Elmer GC/IR Combined Instrument

Light Pipe Interfaces

 

IR spectrometers are very much less sensitive than mass spectrometers and, as sample sizes are often very limited, special techniques must be employed to obtain the maximum sensitivity. This is necessary even for modern FTIR instruments despite them being much more sensitive than the older dispersion type spectrometers. One device that increases the sensitivity of the IR instrument when used to obtain spectra from vapor samples in a gas is the 'light pipe'. Light pipes were first introduced by Wilks and Brown in 1964 (12). A light pipe consists of a tube circular or rectangular in cross-section constructed with highly reflecting internal surfaces. To achieve the necessary high reflectivity the pipe is usually gold plated. The light source is moved back, so that the focus (which is normally made to coincide with the entrance of the light pipe) is transferred to the exit of the light. This procedure is not completely efficient as internal reflections that take place at the walls of the light pipe increases the apparent path length by about 33%.

 

Courtesy of the Perkin Elmer Corporation

 

Figure 33. The Optical System of the Perkin Elmer GC/IR Combined Instrument