Page 70 - Tandem Techniques

Page 51

This, in fact, is observed in atomic spectroscopy (which will be discussed later) but in tandem systems,
most spectroscopic measurements are made in the presence of the mobile phase which complicates the
basic simple adsorption spectrum. In solution, a given molecule may exhibit numerous adsorption
levels that have energies very close to one another. The bands are so close that they cannot be observed
individually and, as a result, they occur under one envelope giving a broad band in the UV adsorption
spectrum. The breadth of the band may extend from 50 to 300 nm. Examples of the adsorption spectra
of two different compounds are shown in Figure 2.2.

It is seen that the aromatic structure of benzene gives a fairly complex spectrum that could easily be
used for identification purposes. In contrast, the spectrum for ethyl ethanoate is a very simple spectrum
containing no fine structure and would be of little use for solute identification. Unfortunately, there are
many other compounds, particularly those containing the ester and acid groups, that would give very
similar spectra to that of the example ester and would only be identified from their UV spectrum with
considerable difficulty. In addition, the spectra for ethyl ethanoate is generally more typical of the vast
majority of the UV spectra of organic compounds.

UV spectroscopy is the least helpful of all the spectroscopic techniques from the point of view of
structure confirmation or structure identification. It is, however, the most sensitive and the easiest to
employ in tandem systems. In addition, the UV spectrometer is relatively inexpensive and
consequently, despite its technical limitations, it is one of the more common spectroscopic techniques to
be employed in conjunction with separation instruments.

The UV spectrometer can take two basic forms, the dispersive spectrometer, and the diode array
spectrometer. There are normally two light sources in a UV/Visible Spectrometer. The source of UV is
usually a low-pressure deuterium lamp that emits light over the wavelength range of 185 nm to 380
nm, although xenon lamps that emit over a similar wavelength range are also used. The visible-light
source is usually a