Identification of Individual Drug Crystals on Paper Currency Using Raman Microspectroscopy

Abstract

Drug consumption, and the industry that supports it, have been targets of law enforcement efforts for several decades. Recently, several countries have allowed the seizure of paper currency associated with the drug trade for evidentiary purposes. Mass spectrometric methods are most commonly used to detect the presence of illicit drugs on paper currency. Unfortunately, these methods are destructive and the evidence is no longer available for re‐analysis by other laboratories. Raman microspectroscopy provides an alternative for non‐invasively analyzing drug crystals on banknotes. This study shows that it is possible to detect and identify single drug crystals in a heterogeneous mixture. Samples were composed of two drug surrogates: isoxsuprine and norephedrine, and two common excipients: benzocaine and lidocaine. It is possible to detect and identify individual crystals on both microscope slides and paper currency. Spectra acquired from crystals on United States paper currency suffer from significant background fluorescence. A quantitative study indicates that the fluorescence background from the dark green and gray areas of the dollar bill is much weaker than from the white or light green areas.