Fluorescence lifetime-enhanced spectral characterization of human serum

Frequency-domain fluorescence lifetime techniques were used for the characterization of pooled human serum, including normal serum, hyperlipid serum, and sera that had been stripped of various components. Fluorescence lifetime measurements of normal human serum revealed lifetime components primarily in the regions of 10(2) ps, 1-2 ns, 4-7 ns, and 9-10 ns. Phase-resolved fluorescence spectroscopy (PRFS), a frequency-domain technique that combines spectral and lifetime information, in measurements of phase-resolved fluorescence intensity (PRFI), provided the basis for comparison of the various sera. Measurements of PRFI vs excitation wavelength and emission wavelength yield a phase-resolved excitation-emission matrix (PREEM) at a given modulation frequency. Multifrequency measurements yield a three-way excitation-emission-frequency array. The multifrequency PREEMs of the various sera were compared with each other and with the corresponding two-way excitation-emission matrices (EEMs) that are obtained using conventional, steady-state fluorescence spectroscopy. Application of matrix-based analysis techniques to the steady-state and PRFS data arrays allowed direct comparison between the two approaches. Results demonstrate the enhanced discrimination among samples that is achieved through the additional dimension of fluorescence lifetime in PRFS.