Gas Chromatography - Tandem Techniques - The Fluorescence Spectrometer > Page 20

The dispersed light from the grating is then allowed to fall onto a diode array. The array may contain many hundreds of diodes and the output from each diode is regularly and frequently sampled by a computer and stored on a hard disc. At the end of the chromatographic separation, the output from any diode can be selected and a chromatogram produced using the UV wavelength that was falling on that particular diode.

The dispersed light from the grating is then allowed to fall onto a diode array. The array may contain many hundreds of diodes and the output from each diode is regularly and frequently sampled by a computer and stored on a hard disc. At the end of the chromatographic separation, the output from any diode can be selected and a chromatogram produced using the UV wavelength that was falling on that particular diode.

 

Most instruments will permit the monitoring of at least one diode in real time so that the chromatogram can be followed as the separation develops. This system is ideal, in that by noting the time of a particular peak, a spectrum of the solute can be obtained by recalling from memory the output of all the diodes at that particular time without stopping the development of the separation. This gives directly the spectrum of the solute (i.e. a curve relating adsorption and wavelength). It should be understood, however, that to prevent condensation and a consequent build up of sample in the sensor cell, the cell and all conduits to and from the sensor cell must be maintained at a temperature at least 25ūC above the maximum temperature of the GC column oven. The problems of interfacing will be discussed in a subsequent chapter.

 

The Fluorescence Spectrometer

 

Fluorescence is a specific type of luminescence. When molecules are excited by electromagnetic radiation to produce luminescence, the phenomena is termed photoluminescence. If the release of electro-magnetic energy is immediate, or stops on the removal of the excitation radiation, the substance is said to be fluorescent. If, however, the release of energy is delayed, or persists after the removal of the exciting radiation, then the substance is said to be phosphorescent.

 

When light is absorbed by a molecule, a transition to a higher electronic state takes place and, as already discussed, this absorption will be highly specific for the molecules concerned; radiation of a particular wavelength or energy is only absorbed by a particular molecular structure. If electrons are raised, due to absorption of light energy, to an upper excited single state, and the excess energy is not dissipated rapidly by collision with other molecules or by other means, the electron will return to the ground state with the emission of light at a lower frequency and the substance is said to fluoresce. Some energy is always lost before emission occurs and thus, in contrast to Raman scattering, the wavelength of the fluorescent light is always greater than the incident light.