We have evaluated the identification and differentiation abilities of three portable spectrometers (TruNarc: Raman spectrometer, Target-ID: infrared spectrometer, NIRSCAN-MKII: near-infrared spectrometer) for on-site drug screening.
Identification abilities were evaluated using samples containing methamphetamine (MA) and cocaine (COC), including case samples and mixtures of these drugs with diluents. Differentiation ability was evaluated using case samples that did not contain MA or COC and compounds having similar chemical structures to MA and α-pyrrolidinovalerophenone (α-PVP).
All three spectrometers commonly showed acceptable true-positive rates (93.2% to 96.6%) for the MA-containing case samples. In addition, their true-positive rates for the 25:75 mixtures of MA hydrochloride and diluents were within 47% to 67%. On the other hand, these spectrometers gave lower true-positive rates (76% to 88%) for the COC-containing case samples than those for the MA-containing case samples. NIRSCAN-MKII showed obviously lower true-positive rates for the 25:75 mixtures of COC hydrochloride and diluents than the other spectrometers (NIRSCAN-MKII: 10%; Target-ID: 90%; TruNarc: 52%). Moreover, all three spectrometers gave false-positives to the compounds similar to MA and α-PVP.
By implementing strict criteria for positive identification, the false positives from Target-ID and NIRSCAN-MKII were prevented but consequentially allowed an increase of false negatives. This increase of false negatives by NIRSCAN-MKII was suppressed by adding the spectra of the COC hydrochloride and diluent mixtures to the spectrum library. For TruNarc, the false positives may be caused by the mixture analysis algorithm. Excluding some spectra from the spectrum library or from the target of the mixture analysis prevented identification of these compounds; however, it inhibited the false positives with minimum increase of false negatives for MA-containing and COC-containing samples. We concluded that i) without improvements, all three spectrometers had a high risk of false positives, ii) improvement of software would lower the identification range but diminish the risk of false positives, and iii) all three spectrometers were applicable to on-site screening, specifically for MA-containing and COC-containing samples after the improvement.
