Page 73 - Tandem Techniques

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The dispersed light from the grating is then focused onto a diode array. The array may contain many
hundreds of diodes and the output from each diode is regularly sampled by a computer and stored on a
hard disk. The spectrum of the solute can be obtained by recalling from memory the output of each of
the diodes, i.e. a curve relating adsorption to wavelength. The only disadvantage of this type of
spectrometer is that its resolution is limited by the number of diodes in the diode array.

Although, the two systems described appear to be satisfactory, both suffer from certain disadvantages
due to second-order effects. When light having a narrow band wavelengths is passed through the cell
(as in the dispersive spectrometer) and the whole of the transmitted light allowed to fall on the sensor,
then the light received will not only be that light of the same wavelength that was transmitted through
the cell, but also any fluorescent light that the incident light induced. Thus the light measured is not
solely transmitted light of the selected wavelength but will also contain any fluorescent light that was
induced in the sample.
In a similar way, when light containing the whole range of wavelengths is passed through the cell (as in
the diode array spectrometer), then the light sensed at a selected wavelength will not only contain the
transmitted light of that wavelength but also any fluorescent light of that wavelength that was induced
in the sample by incident light of other wavelengths. It is seen that neither spectrometer system can be
certain of measuring true light absorption at a specific wavelength. In fact, true light absorption can
only be measured by selecting the incident wavelength and then passing the transmitted light to another
monochromator and then selecting the same wavelength to monitor.

An excellent text on the absorption of light and ultraviolet radiation has been written by Schenk [1].
This book is probably now out of print but is likely to be available in many libraries. A more recent
publication on the subject (1992) is that by H. H. Perkampus [2] which deals not only with the theory of
the subject but also includes many applications. UV spectroscopy is the simplest to use in tandem with
a chromatograph but, unfortunately, provides limited information about the structure of the solute being
eluted.