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fluorescence detector, that can permit the excitation light to be selected, but cannot provide a
fluorescence spectrum); the second has two monochromators that allow the excitation light to be
selected and also the fluorescent spectrum to be obtained. It is fairly obvious that the second model is
far more complex and, consequently, costs significantly more than the first. If both the wavelength of
the excitation light and the wavelength of the fluorescent light can be selected, the tandem system can
be arranged to provide exceedingly high sensitivities for chosen compounds. In addition monitoring a
chromatogram at both the optimum excitation wavelength and the optimum emission wavelength makes
the system very selective and, indeed, begins to approach the selectivity of the single ion monitoring
technique used in mass spectrometry.

Two of the companies that have designed and produced fluorescence spectrometers for tandem use,
with the liquid chromatograph, are the Hewlett-Packard (HP) Corporation, and the Perkin Elmer
Corporation (PE). The spectrometer developed by the Perkin Elmer corporation has already been
discussed in chapter 2, and so the Hewlett-Packard instrument will now be described.

The Hewlett-Packard Fluorescence Spectrometer Designed for Use as a LC/FS Tandem System

Fluorescence instruments usually employ either the deuterium lamp, the low pressure-mercury lamp, or
the xenon discharge lamp as the excitation source. The mercury lamp emits light at essentially only a
few discrete wavelengths at high intensity, and the deuterium lamp, although a broad wavelength light
source, has maximum emission between 200 nm and 300 nm which is rather limited. The deuterium
lamp, however, can be operated continuously. The xenon lamp emits light at high intensity over a
broader range of wavelengths, i.e., 150-600 nm. The xenon lamp, however, can only be operated
intermittently, and thus the discharge must be pulsed. The pulsing, however, has the advantage that, as
considerably less energy is dissipated in the lamp, base line drift from the thermal changes is virtually
eliminated. Nevertheless, due to its wider emission