Page 300 - Tandem Techniques

Page 284

apparatus is shown in Figure 7.20. Light from a xenon lamp was focused onto the flow cell by two
curved mirrors. Between the mirrors was an interference filter which selected the wavelength of the
excitation light. To change the excitation light, the filter had to be removed and an appropriate
replacement inserted. The fluorescent light was then focused by means of a plane and curved mirror
onto the diode array. The diode array was cooled by a Peltier cooling unit. The liquid chromatographic
system, that was used in conjunction with the apparatus, consisted of a Perkin Elmer Series 10 isocratic
pump, a Rheodyne 7510 injector with a 20 µl sample loop, and an ODS column, 15 cm long, 4.6 mm
I.D. packed with particles 10 µm in diameter.

The test sample used was a mixture of polycyclic aromatic hydrocarbons, which was eluted isocratically
with a solvent mixture of 80% acetonitrile and 20% water. The separation they obtained is shown in
Figure 7.21, and it is seen that the apparatus functions quite well, but the sensitivity compared with the
simple fluorescence detector is relatively low. It is also seen from the two-dimensional presentation in
Figure 7.21 that good spectra can be obtained from partially resolved peaks and would certainly aid in
confirming the identity of a specific solute.

A Tandem Instrument that Monitors UV Absorption, Fluorescence and Luminescence

In early 1989 Yappert et al. (7) described a tandem instrument involving a liquid chromatograph and a
spectrometric assembly that monitors UV absorption, fluorescence and luminescence simultaneously. A
diagram of the spectrometric system is shown in Figure 7.22, and it is seen that it is a fairly complicated
arrangement. Pulsed light from a broad wavelength xenon lamp passes through an IR filter to the first
half mirror. Light reflected at right angles, passes to a second half mirror, and the light that passes
through the second half mirror is focused onto a photoelectric cell. The output from this photoelectric
cell is used in a feedback system to control the intensity of the light emitted from the xenon lamp.

The light that passes directly through the first half mirror is focused onto a monochromator that selects
the wavelength of the light that is to act as