Abstract
Handheld Raman spectroscopy is an emerging technique for rapid on‐site detection of drugs of abuse. Most devices are developed for on‐scene operation with a user interface that only shows whether cocaine has been detected. Extensive validation studies are unavailable, and so are typically the insight in raw spectral data and the identification criteria. This work evaluates the performance of a commercial handheld Raman spectrometer for cocaine detection based on (i) its performance on 0–100 wt% binary cocaine mixtures, (ii) retrospective comparison of 3,168 case samples from 2015 to 2020 analyzed by both gas chromatography–mass spectrometry (GC–MS) and Raman, (iii) assessment of spectral selectivity, and (iv) comparison of the instrument's on‐screen results with combined partial least square regression (PLS‐R) and discriminant analysis (PLS‐DA) models. The limit of detection was dependent on sample composition and varied between 10 wt% and 40 wt% cocaine. Because the average cocaine content in street samples is well above this limit, a 97.5% true positive rate was observed in case samples. No cocaine false positives were reported, although 12.5% of the negative samples were initially reported as inconclusive by the built‐in software. The spectral assessment showed high selectivity for Raman peaks at 1,712 (cocaine base) and 1,716 cm−1 (cocaine HCl). Combined PLS‐R and PLS‐DA models using these features confirmed and further improved instrument performance. This study scientifically assessed the performance of a commercial Raman spectrometer, providing useful insight on its applicability for both presumptive detection and legally valid evidence of cocaine presence for law enforcement.
Highlights
Within the illicit drug market, cocaine is one of the most prevalent substances with a global estimated annual production of around2,000 metric tons.[1]
These results indicate that the handheld spectrometer performs well for average cocaine samples, but limitations may arise for samples with reduced cocaine levels as encountered for instance in smuggling scenarios
This is in line with partial least squares (PLS)-DA scores of the 10–80 wt% cocaine mixtures shown in Figure 5b where a majority of the samples were predicted with a >0.8 score indicating the presence of cocainespecific spectral features
Summary
Within the illicit drug market, cocaine is one of the most prevalent substances with a global estimated annual production of around2,000 metric tons.[1]. Many current standard methodologies for cocaine detection comprise of colorimetric spot tests based on the formation of a blue colored cobalt(II) thiocyanate complex in presence of cocaine.[2,3] Despite being fast and inexpensive, colorimetric tests have several limitations Such tests are destructive and require opening the packaging of the (yet) unknown substance. Colorimetric tests are known to yield false positive (FP) results on some common pharmaceuticals such as lidocaine, levamisole, and promethazine.[4,5] Third, individual colorimetric tests are only available for a limited set of common drugs meaning that they are only selective towards a limited number of substances This may lead to false negative (FN) results when the incorrect test is performed or a controlled substance is encountered for which no test is available.[3]
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