I am using an spectrometer where the intensity spanning from 400-700 nm of a broadband source (VIS-NIR) is found to be smaller than the transmission intensity of the light after passing through bromocresol green treated ph solution at pH value 4 placed in a cuvette.
Can anybody help me in explaining this property? How the intensity of a broadband source smaller than the transmission intensity of the broadband source when it passes through a liquid sample?
Fluorescence would probably still be a net loss in a single pass throughput absorption spectrometer even if the fluorescence quantum efficiency was unity it because the fluorescence would be unevenly distributed into 4Pi sterad. Is the spectrometer geometry such that it collects fluorescence as well as the transmitted beam? Is there any chance you have wavelengths below 400 nm being absorbed and re-emitted in the 400 - 700 nm range. Is this behavior constant across various dilutions of the dye? Have you tried a cuvette with only solvent in it to see if the broadband intensity changes in an unexpected way. Does the appearance of the spectrum change?
The material isn't known for strong fluorescence. And I agree with John, a standard transmission geometry in a spectrophotometer will not see most of this fluorescence anyway.
Definitely compare the transmission through your sample with the transmission through a reference sample, a sample prepared to be as similar to the real one as possible, but without the bromocresol green. That will give you some idea of how much of the difference is due to the dye, without getting confusing extra effects due to the presence or absence of a cuvette, which is going to change the optical path of the spectrometer somewhat. An empty sample holder or an empty cuvette would not be good reference samples, you want to match the refractive index to that of the real sample.
Any spectrometer has monochromator for select the narrow spectral beam by that You will exite your sample and study its absorption, exitation or fluorescence spectra. Due to that never broad band spectra can be observe in the place of sample (kuvette) of the similar device. Thanks to monochromator You can record even very weak fluorescent if You select right the exitation light in spectral region of absorption band of Your sample. Conclusion: spectrometer is better than any noisy scattering light is less in the place location of the studied sample (particular case -bromcresol green)
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