For multivariate analysis, the “fingerprint” region was selected (1800–550 cm−1) due the complex interacting vibrations that absorbs in this region which gave each compound a unique spectra . Fig. 1 shows the raw spectra for the standards of cocaine, salt and base, and its adulterants. The AH 6809 band in the range of 1800–1550 cm−1 is due the CO, CC stretching present in the structure of all studied compounds and CN stretching of the caffeine. Although they all have carbonyl groups, they are different functions in each compound and absorbs in different wavelengths. The carbonyls groups of cocaine are esters and, considering that one carbonyl group is conjugated with the aromatic ring, the infrared spectrum have two different carbonyl stretching. The carbonyl group of the adulterants are amides, therefore these carbonyl absorbs near 1620 cm−1. Still, the carbonyl band can be used to evaluate the adulteration of the drug. The presence of different organic functions that absorbs in the region between 1500 and 1700 cm−1 in studied adulterants (CN of the caffeine, NH of the lidocaine and phenacetin, for example), lead to a better assessment of adulteration in cocaine because this region of the spectra in the drug does not have any significant absorption band. Another part of the spectra that have significant differences between the samples is the region between 1000 and 1500 cm−1, where CH and CO absorbs. That differentiation occurs due to the structure of the compounds, however the base and salt cocaine have significantly differences in this region, what suggests that their can be differentiate also through this region  and .