Page 120 - Tandem Techniques

Page 101

An excellent discussion on general organic mass spectrometry is given in Practical Organic Mass
Spectrometry edited by Chapman [13].

Synopsis

Most tandem systems involve the association of the separation technique with some form of
spectroscopy, the most common being, ultraviolet, fluorescence, infrared, Raman, atomic, chiral,
nuclear magnetic resonance, and mass spectroscopy. All except the last depends on the absorption or
emission of electromagnetic radiation. UV spectroscopy provides information on electron transitions
and is useful for identifying certain classes of compounds, such as aromatic and unsaturated
compounds; although one of the more sensitive, it is the least useful of all the spectroscopic techniques
for tandem operation. Fluorescence spectroscopy depends on the emission of light that occurs as an
excited electron falls back to its ground state. Fluorescence spectra provide more information then UV
spectra as they usually contain more fine structure. Furthermore, a large number of fluorescence spectra
can be obtained from a single substance, by employing light selected from a range of different
wavelengths. Infrared spectroscopy is more informative than UV or fluorescence spectroscopy. IR
spectra demonstrate light absorption at different wavelengths resulting from the change in rotational
and vibrational energy of the molecule and contain considerable fine structure. Consequently, they can
be used to identify a substance from reference spectra, and can also demonstrate the presence of certain
chemical groups such as carbonyl, hydroxy, aromatic rings etc. The technique is inherently less
sensitive than UV spectroscopy but, with the use of Fourier transform techniques, the sensitivity can be
significantly enhanced. Raman spectroscopy involves the measurement of light that is emitted when the
rotational and vibration energy of a molecule is raised by the absorption of excitation light and then
falls back to a different energy level. It is a relatively insensitive technique and special precautions must
be taken to avoid fluorescent light, which can provide overwhelming background signal and noise. The
information Raman spectra provide is very similar to that of IR spectra to which, they are in fact,
complementary. Atomic emission spectrometry involves measuring either the light emitted by excited
atoms