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A new laser-equipped fluorescence microscopy system has been developed to significantly widen the analytical scope of coal characterization. The system uses a pulsed tuneable dye laser interfaced to a state-of-the-art Leitz MPV3 fluorescence microscope. The fluorescence of the coal macerals is excited with ultraviolet radiation in the range of 260 to 450 nm and analyzed between 300 and 800 nm. The temporal decay of the fluorescence induced by the pulsed laser is studied. The anode pulses from a fast photomultiplier detecting the fluorescence are digitized by a fast waveform digitizer, and the information is then processed by a desk-top computer to obtain the decay curves and the corresponding decay times. The anode pulses which contain the time signatures of the fluoresce ce are corrected for temporal instrument response by deconvolution. The decay curves can also be spectrally resolved and, with further data manipulation, time-resolved spectra can be obtained. The decay times are believed to be as characteristic of the fluorescing macerals as the excitation and emission spectra, which are also being studied more extensively using monochromatized radiation from the conventional xenon and mercury arc lamps in the wavelength ranges mentioned above. The photomultiplier is cooled to reduce noise and to improve signal acquisition, and, with the red-sensitive spectral response of the photomultiplier, the fluorescence of other macerals such as vitrinites are being investigated. The technique of pulse counting is employed for greater sensitivity in detecting weak y fluorescing macerals.
Current spectral studies involve observing statistical variations of fluorescence spectra of a given maceral and obtaining its averaged spectrum. The spectra are parameterized by such values as Q (red/green quotient), ^lgrmax and Qmax (intensity at ^lgrmax/intensity at 500 nm). Excitation and emission spectra are fed directly into the computer through an analog-to-digital converter as the corresponding monochromator scans the wavelengths. The computer signal averages each wavelength interval while sampling, and then corrects the averaged raw spectrum for spectral instrument response. This multiparameter analysis of the optical properties of coal is expected to enhance coal characterization.
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