Improved Fluorescence Rejection in Measurements of Raman Spectra of Fluorescent Compounds

Practical ways to achieve fluorescence rejection were developed by using the technique of Raman difference spectroscopy and a gated diode-array detector. In order to improve the signal-to-noise ratio, we had to circumvent the problem presented by the fact that the sensitivities of each element of the diode-array detector are not always uniform and linear—a problem which results in an apparent noise in the observed spectra. Therefore, in this study we tried to separately observe a Raman-plus-fluorescence spectrum ( S T ) and a pure fluorescence spectrum ( S F ), with a high S/N ratio, and to obtain a Raman spectrum ( S R ) as their difference. With a cw Ar + laser, S T and S F were excited at different wavelengths but recorded in the same wavelength region. This procedure was easy and quite successful for compounds, such as methyl orange and methylene blue, that have a Raman-to-fluorescence intensity ratio ( I R / I F ) of greater than 0.01. With a mode-locked and Q-switched Nd:YAG laser, S T and S F were obtained from different timings of the detector gate (6 ns), i.e., −6 to 0 ns for S T and 3 to 9 ns for S F . The temporal discrimination between Raman and fluorescence photons combined with the Raman difference technique significantly improved the 532-nm excited Raman spectrum of the tetraphenylporphyrin solution. This method is applicable to compounds with an I K / I F of smaller than 0.01. The time-correlated single photon counting method with a time-to-amplitude converter was improved, to enable it to work with a high repetition rate, and was applied to a Rhodamine 6G solution, which was also examined by the gated Raman difference technique. Advantages and problems associated with each technique and appropriate methods for a given sample will be discussed.