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It is possible to determine fluorescence lifetimes from the frequency response of a fluorophore as well as from its time response. Various schemes for measuring such a response curve are illustrated and include the use of sources whose intensity is modulated at single, swept frequencies, at several frequencies simultaneously, or over a broad, continuous band of frequencies. Detection methods that are considered include a single broad-frequency-response device, one that is swept, and another that hops from one modulated frequency to another. Signal-to-noise (S/N) expressions are derived for each combination of source and detector and conclusions are drawn from comparison of the S/N expressions. From these considerations and the availability of existing sources for use in time-resolved fluorometry, the most attractive approaches employ a source modulated at several frequencies simultaneously, each of which is monitored by its own tuned detector, or a sinuosidally modulated source that is swept synchronously with a tuned detector.
time-resolved fluorescence, frequency response, signal-to-noise improvement, instrumentation, molecular luminescence
Michael Ramsey, J
Staff scientist, Division of Analytical Chemistry, Oak Ridge National Laboratory, Oak Ridge, TN
Professor, Indiana University, Bloomington, IN