Pulsed source excitation-gated detection (with or without time resolution) and cw excitation-cw detection are compared theoretically with respect to signal/noise (S/N) ratios in atomic and molecular luminescence spectrometry. The calculations indicate that in those cases where there is no loss in flux due to pulsing, pulsed source excitation with delayed gated detection results in a considerable improvement in S/N as long as the major noise sources are either not related to the source decay or, if related to the source decay or luminescence decay of impurities, the delay time between excitation and detection is greater than the corresponding source or impurity decay times. In general, pulsed source excitation-gated detection and modulated source lock-in detection do not provide significant S/N advantages over cw excitation and cw detection unless the major source of noise is related to either the detector or to non-source-induced background. Calculations of S/N ratio are given for a variety of analytically useful situations in atomic fluorescence spectrometry, molecular fluorescence spectrometry of gas phase and condensed phase species, and molecular phosphorescence spectrometry.
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