Hello, I have been measuring quantum yields in the lab for quite some time and have occasionally been running into some difficulty with my fluorescence spectra. The intensity (CPS) of the spectra for the same molecule will remain the same under the same conditions most of the time; then, every now and then it will be an order or two of magnitude lower. This obviously results very poor quantum yields, that are not even close to where they should be. What I am wondering is what might be going on. The solvent is always the same, slit widths the same, excitation the same (though it shouldn't matter). Could it be a bad blank? Any suggestions would be greatly appreciated! Also, I should note, when plotting the ∫ of fluorescence spectra vs UV-vis abs, the result is a near perfect linear relationship... as it should be; this makes me believe sample prep is not the culprit.