A calibration friendly approach to identify drugs of abuse mixtures with a portable near‐infrared analyzer

Abstract

Both the increasing number and diversity of illicit-drug seizures complicate forensic drug identification. Traditionally, colorimetric tests are performed on-site, followed by transport to a laboratory for confirmatory analysis. Higher caseloads increase laboratory workload and associated transport and chain-of-evidence assurance performed by police officers. Colorimetric tests are specific only for a small set of drugs.The rise of new psychoactive substances therefore introduces risks for erroneous results. Near-infrared (NIR)-based analyzers may overcome these encumbrances by their compound-specific spectral selectivity and broad applicability. This work introduces a portable NIR analyzer that combines a broad wavelength range (1300-2600 nm) with a chemometric model developed specifically for forensic samples. The application requires only a limited set of reference spectra for time-efficient model training. This calibration-light approach thus eliminates the need of extensive training sets including mixtures. Performance was demonstrated with 520 casework samples resulting in a 99.6% true negative and 97.6% true positive rate for cocaine. Similar results were obtained for MDMA, methamphetamine, ketamine, and heroin. Additionally, 236 samples were analyzed by scanning directly through their plastic packaging. Also here, a >97% true positive rate was obtained. This allows for non-invasive, operator-safe chemical identification of potentially potent drugs of abuse. Our results demonstrate the applicability for multiple drug-related substances. Ideally, the combination of this NIR approach with other portable techniques, such as Raman and IR spectroscopy and electrochemical tests, may eventually eliminate the need for subsequent laboratory analysis; therefore, saving tremendous resources in the overall forensic process of confirmatory illicit drug identification.

On‐site forensic analysis of colored seized materials: Detection of brown heroin and MDMA‐tablets by a portable NIR spectrometer

“…Combined with the conclusions from earlier work on cocaine, 17 the results in this study show that the calibration friendly data analysis approach based on dedicated matrix libraries and mixture detection works sufficiently for various types of drugs-of-abuse casework.…”

“…They showed that MDMA detection is possible using PLS modeling 33–35 . It is noteworthy that the reported MDMA HCl spectrum in these studies was different from the spectra recorded by our group 17,36 . These differences were attributed to different hydrated and anhydrous polymorphous forms of MDMA HCl 35,37 .…”

“…Earlier work showed that both samples were correctly identified as cocaine-containing using this library. 17 The decision to include either both MDMA HCl polymorphous forms or only the hydrated form in the matrix library could be made depending on the specific forensic setting in which the spectrometer is used. Although rare, anhydrous MDMA HCl does exist and may thus be encountered in a forensic setting.…”

“…In earlier work, our group reported the NIR spectrum of MDMA HCl in the implementation of a calibration‐friendly approach for mixture analysis in forensic samples that were predominantly white powders 17 . Only a very limited number of studies reported on NIR‐based MDMA detection.…”

“…Portable NIR spectrometers operating below ~1000 nm were generally found unsuitable for illicit drug detection in colored samples due to the major influence of colorants in this wavelength range 16,36 combined with a limited number of spectral features of MDMA 36 . Both a wider and higher wavelength range of the NIR spectrum (above 1000 and up to 2600 nm) is suggested for MDMA detection due to specific spectral features in this range 17 . Additionally, the spectral fingerprint obtained from forensic casework is a combination of signals from all constituents in the sample material.…”