The invention is provided for detection of Heroin and Morphine in illicit drug samples, also for their differentiation. A liquid sample from a street sample is prepared and divided into two equal aliquots, one of them being reference sample, another being main sample. Both samples are treated with Hydrochloric acid and Sodium hydroxide, but those substances are added to the samples in the different sequence. After that the measurement of SFS of the reference sample is performed. Further the presence of specific spectral pattern of Morphine in the measured SFS of the reference, sample is detected and the value of SFS intensity at the specific spectral point is fixed (a reference value). After 15 minutes for acidification of the main sample, the analogous measurement and detection of the Morphine in the main sample are performed.