Methods and systems for determining contamination in a petroleum-based sample, including irradiating the petroleum-based sample with a light beam from a light source such that a fluorescence signal is generated, guiding, by a mirror, the fluorescence signal to a gear-less rotating diffraction grating, the gear-less rotating diffraction grating spatially separating a fluorescence wavelength from the florescence signal, detecting, by an optical detector, fluorescence wavelength, transforming the fluorescence wavelength into a spectral contour diagram, the spectral contour diagram comprising a fluorescence wavelength variation over time, and determining, the contamination in the petroleum-based sample using the spectral contour diagram.