Fluorescence is a much more intense phenomenon than Raman scattering by several orders of magnitude - if the exciting laser for Raman also excites fluorescence then that can swamp the Raman signal. The way to overcome this is to use a different Raman laser frequency which does not excite fluorescence. The picture below show a sample of motor oil - on the left is the spectrum obtained with a 785nm laser handheld device. On the right is the same sample run using a handheld spectrometer with a 1064nm laser. Moving the excitation to longer wavelengths avoids exciting fluorescence and in this case the Raman spectrum is virtually fluorescence free and readily identified by the handheld's search algorithm.
