Page 283 - Tandem Techniques

Page 267

It is seen that solvents having the extremes of polarity, for example the most polar solvents, such as
water and the lower alcohols, and the least polar solvents (strongly dispersive), the n-alkanes, all have
relatively low cut-off wavelengths. This means that the solvents of intermediate polarity are those that
exclude the use of light of lower wavelengths in LC/UV tandem systems. It follows that the restrictions
on solvent selection are greatest when separating substances that require solvents of intermediate
polarity for satisfactory elution.

The use of the specific monitoring wavelengths 254 nm and 267 nm in the separations shown in Figure
7.9, does not restrict the choice of solvent, or the concentration of the solvents, as their cut-off
wavelengths are well below those used for monitoring. The choice of wavelengths for the separations in
Figure 7.9, is based solely on the response of the solutes, and is not influenced by the nature of the
separation.

The versatility and advantages of the liquid chromatograph/UV spectrometer combination are obvious.
Largely due to the relatively high cost of the first models, they were originally used mainly as research
instruments or for method development. However, the cost of the liquid chromatograph/UV
spectrometer combination has now been reduced significantly. Consequently, the LC/UV tandem
instrument is now considered simply as a liquid chromatograph fitted with a multi-wavelength UV
detector, for general use in LC analysis.

The performance of both types of multi-wavelength detectors are very similar, both have a sensitivity of
about 1 x 10-7g/ml and a linear dynamic range of about 1 x 10 to 5 x 10 g/ml.
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LC/Fluorescence Spectroscopy Tandem Systems
The fluorescence spectrum is not often used for identifying compounds, although it can be equally if
not more useful for this purpose than the UV spectrum. The fluorescence spectrometer, at present, is
available in two forms: the first has a monochromator that allows the excitation light to be selected and
the total fluorescent light is measured (this model is merely a