Gas Chromatography - Tandem Techniques - The Characteristics of Infrared Absorption > Page 23

This toroidal mirror focuses the portion of incident light onto the reference photocell, providing an output that is proportional to the strength of the incident light. The path of the fluorescent light is largely on the right hand side of the diagram. Fluorescent light from the cell is focused by an ellipsoidal mirror on to a spherical mirror at the top right-hand side of the diagram. This mirror focuses the light onto a grating situated at about center right of the figure. This grating can scan the fluorescent light and provide a fluorescent spectrum. Light from the grating passes to a photoelectric cell which monitors its intensity. The instrument is quite complex and, as a result, rather expensive; however, from the point of view of measuring fluorescence it is extremely versatile. Again the cell and all conduits must be heated to prevent any condensation of eluted vapor from building up in the sensor cell.

 

The Characteristics of Infrared Absorption

 

IR spectroscopy involves the absorption of light having a wavelength longer than the visible spectrum, i.e. between 2 and 15 micron (sometimes up to 40 micron but mostly between 2 and 15 micron). In practice IR spectra are usually displayed using wave numbers as the independent variable, as opposed to wavelength. Both forms of presentation are as shown in figure 14.

Figure 14. IR Spectra Presented in Wave Numbers and Microns

The wave number is the number of waves per centimeter taken as (1/l) and expressed in reciprocal centimeters (cm -1). Optically, the IR spectroscopic system is very similar to that used to measure UV absorption except that the materials of construction must be different. In the UV spectrometer the material of construction used for transmitting UV light is usually quartz or fused silica. The optical components that must be transparent to IR light are usually 'salt' (sodium chloride), silver chloride or other metal chlorides. As already stated, UV absorption occurs at frequencies where the radiation energy is equal to that of specific electronic transitions in the molecules of the substance. In contrast, IR adsorption occurs at frequencies where the radiation energy is equal to that of changes in vibrational and rotational energy of the molecule. A molecule can be regarded as an assembly of balls (atomic nuclei) and springs (chemical bonds). Such a system can vibrate in a very complex manner and for most compounds, a molecule containing (n) atoms will have (3n-6) modes of vibration (for a linear molecule this will be (3n-5) modes of vibration).