Page 302 - Tandem Techniques

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The fluorescence emitted in the sample cell is also focused by means of another optical fiber onto a
simple photoelectric cell that provides an output equivalent to the total fluorescent light. Finally another
optical fiber, situated downstream in the sample cell and well away from the excitation light, monitors
the delayed fluorescence or, as it is more commonly known, the luminescence.

Depending on the method of measurement, the sensitivity of the system to quinine ranged from 1.2 to
40 ng/ml. As one might expect, the highest sensitivity observed was obtained from the fluorescence
channel. The amount of fluorescent light emitted by a sample, is a function of the intensity of the
excitation light as well as its wavelength. It follows that by increasing the intensity of the excitation
light, by the use of a laser light source, the sensitivity of the fluorescent spectrometer would also be
increased.

Laser-induced Fluorescence Detection Employing a LC/FL Tandem Combination

Roach and Harmony [8] developed a laser induced fluorescence instrument for the analysis of amino
acids, based on an apparatus developed earlier by Sepaniak and Yeung [9]. The modification was
carried out to significantly improve the sensitivity. The instrument was basically a laser source,
transmitting light through a flow cell, and the fluorescent light so produced was measured by a
fluorescence spectrometer arrangement.

A diagram of the apparatus developed by Sepaniak and Yeung is shown in Figure 7.23. The separation
apparatus was very simple and comprised a mobile phase supply, pump, sample valve and column. The
eluent passed to quartz capillary tube that was mounted on an adjustable stand so that it could be placed
directly in the laser light path. The volume of the quartz tube was about 20 µl but, according to the
authors, could be reduced to a few microliters if so desired. The mobile phase, subsequent to traversing
the quartz tube, passed to waste. An argon laser was used, the light from which was focused by means
of a simple lens onto the center of the quartz tube. Fluorescent light generated was collected by a quartz
fiber and first