Laser Excited Molecular Fluorescence of Solutions F. E. Lytle Purdue University. West Lafayette. IN 47907 A fluurimeter is usuallv comnosed of a broadband source. a monochromating devl'ce to'select the wavelength and bandwidth of the exciting radiation, a square quartz sample cell, a second monochromating device to isolate the emitted radiation, and a photomultiplierlamplifier combination to convert the optical signal into a measurable voltage. For a given instrument and emitter the magnitude of the resultant signal is proportional to the source photon flux density and, for dilute solutions, the sample concentration. The theoretical capability of fluorimetric methodology in the area of trace analysis is best exemplified by a calculation patterned after that presented by Wright ( I ) . A 150 W xenon arc lamp can deliver 10 pW @ 500 i 5 nm (2.5 X loL3photons s-') to a monochromator slit. With a throughput efficiency of 30%, 7.5 X 1012photons s-1 will he irradiating the sample. If each of these photons were absorbed and subsequently reemitted a t fluorescence wavelengths and if the emission monochromator were adjusted to spectrally pass 10%of the signal, a lens collection efficiency of 5%, monochromator throughput of 30% and a nhotomultinlier ouantum vield of 20% would nroduce 2.3 X i0s counts a t ;he ratemeter. Thus, an absdrptance (1-T) of 1.7 X 10W9would nroduce 4 counts s-' for a SIN of 2. For a compound with a molar absorptivity of lo5 and a 1 cm path length, this corresponds to a 7.4 X 10-l5 M solution. In principle the laser should provide a dramatic improvement in fluorimetric methodolorn since i t can irradiate the solution with orders of magnitude more photons. As an example, continuous wave lasers can easily deliver 1 W of 515 nm (-2.5 X loLsphotons s-1) radiation to the absorbing solution. The same arguments presented previously then predict an ahsorptance value of 5.1 X 10-l5 or a 2.3 X 10-20 M solution. This corresponds to 13 molecules cm-3! Such larae impn)vcments in performance art, difficult to reali7e in practice. The primary rcnswj for this is the well.knou,n blank limited nature of the measurement. That is, even with gas discharge lamp excitation, most real analyses are characterized by a samnle matrix which generates an ontical signal " - [scattered . radiation plus impurity emission) equal t o aualytes having a concentration-quantum yield product near 10-I= (2). Figure 1shows example data for various solvents having a wide range of polarity. The emission from 400-600 nm is due to impurity luminescence while that below 400 nm is due to Raman scatter. The Rayleigh scatter at 337 nm would be orders of magnitude more intense than any other source of emission and is reduced here by both a filter and the monochromator. As a result of these interferences, almost all of the laser-based schemes demonstrated as a general analytical tool have achieved their success hy either-stu
