Gas Chromatography - Tandem Techniques - The Combination of the Gas Chromatograph with Other Spectroscopic Systems > The Measurement of UV and Visible Absorption > The Diode Array UV/Visible Spectrometer > Page 19


Light from the deuterium discharge lamp is collimated by two curved mirrors onto a holographic diffraction grating. The dispersed light is then focused, by means of a curved mirror, onto a plane mirror and the light of the required specific wavelength selected by adjusting the angle of the plane mirror. Light of the selected wavelength is then focused by means of a lens through the sensor cell. The exit beam from the cell is focused onto a photo-cell which gives a response that is some function of the intensity of the transmitted light. The wavelength is scanned by rotating the plane mirror. The output from the photocell, electronically modified and presented on a chart recorder or computer printer, will provide an adsorption curve relating adsorption to wavelength (or frequency). The adsorption curve is known as the adsorption spectrum and its shape will be characteristic for the substance being examined and be determined by the transitions that occur at the scanned wavelengths.


The Diode Array UV/Visible Spectrometer


As already explained, the diode array spectrometer functions in an entirely different way from the dispersive instrument. A diagram of a diode array detector is shown in figure 12. Light from a deuterium lamp is collimated by an achromatic lens system so that the total light passes through the sensor cell onto a holographic grating. In this way the sample is subjected to light of all wavelengths generated by the lamp.




Figure 12. The Diode Array Spectrometer