Gas Chromatography-Fourier Transform Infrared Spectroscopy for Unambiguous Determination of Illicit Drugs: A Proof of Concept

Parallel coupling of gas chromatography to mass spectrometry and solid deposition Fourier transform infrared spectroscopy: an innovative approach to address challenging identifications

The request for novel hyphenated instruments and techniques, capable of affording exhaustive information and results, is a focus continuously watched out. In this context, the present work aimed at the development of an integrated system combining gas chromatographic (GC) separation with mass spectrometry (MS) and (solid deposition) Fourier transform infrared spectroscopy (FTIR) detection. An external transfer line was designed in the lab for the parallel coupling of the two detectors, in such a way to obtain complementary analytical information consisting of an MS spectrum, an IR spectrum and linear retention indices (LRI), within a single analysis. The instrument performance was demonstrated for the analysis of a commercial mixture consisting of 139 hydrocarbons, comprising linear, branched, unsaturated and aromatic compounds. A 100-m poly(dimethylsiloxane) column was employed for the separation, and the outlet flow was split 95:5 between the IR and MS detectors using two uncoated capillaries. The IR spectra were acquired from solid deposits on a zinc selenide disc (-90 °C), over a spot (detector area) of about 0.1 mm2, in the range of 4000-700 cm-1 and at a resolution of 4 cm-1. Final identification of the separated compounds by a library search was achieved by excluding incorrect results, sequentially using a three-filter approach (85% similarity against reference MS and IR library spectra and ±10 LRI unit tolerance). Based on these preliminary results, the GC-MS/sd-FTIR system is a promising tool for the characterization of complex matrix constituents, for which identification is cumbersome, by using only one detection technique.

An interlaboratory study to evaluate the utility of gas chromatography-mass spectrometry and gas chromatography-infrared spectroscopy spectral libraries in the forensic analysis of fentanyl-related substances

Synthetic opioids such as fentanyl account for over 71,000 of the approximately 107,000 overdose deaths reported in the United States in 2021. Fentanyl remains the fourth most identified drug by state and local forensic laboratories, and the second most identified drug by federal laboratories. The unambiguous identification of fentanyl-related substances (FRS) is challenging due to the absence or low abundance of a molecular ion in a typical gas chromatography-mass spectrometry (GC-MS) analysis and due to a low number of fragment ions that are similar among the many potential isomers of FRS. This study describes the utility of a previously reported gas chromatography-infrared (GC-IR) library for the identification of FRS within a blind, interlaboratory study (ILS) involving seven forensic laboratories. Twenty FRS reference materials, including those with isomer pairs in the library, were selected based on either their presence in the NIST library and/or some similarity of the mass spectra information produced. The ILS participants were requested to use the Florida International University (FIU) GC-MS and GC-IR libraries supplied by FIU to search for matches to their unknown spectra generated from in-house GC-MS and GC-IR analysis. The laboratories reported improvement in the positive identification of unknown FRS from ~75% using GC-MS alone to 100% correct identification using GC-IR analysis. One laboratory participant used solid phase IR analysis, which produced spectra incompatible with the vapor phase GC-IR library to generate a good comparison spectrum. However, this improved when searched against a solid phase IR library.

Detection & identification of hazardous narcotics and new psychoactive substances using Fourier transform infrared spectroscopy (FTIR)

According to the latest World Drug Report, released by the United Nations Office on Drugs and Crime (UNODC), drug use is up 30% over the past decade and there are more drugs, and more types of drugs, than ever. Herein we use Fourier Transform Infrared Spectroscopy (FTIR) for the rapid ID of narcotics in a range of concentrations - from pure forms (as it is likely to be smuggled & transported) to street forms, often mixed with conventional cutting agents. Using FTIR, 75% of "street sample" narcotics were rapidly identified, and the effects of cutting agents on identification (ID) were also investigated. The limit of detection of MDMA was assessed, with a correct ID shown from 25% w/v. Concentration was correlated with Hit Quality Index, showing the capability of FTIR use in concentration estimation.

Attenuated Total Reflection Fourier Transform Infrared Spectroscopy for Forensic Screening of Long-Term Alcohol Consumption from Human Nails

Fourier transform infrared (FT-IR) spectroscopy is used throughout forensic laboratories for many applications. FT-IR spectroscopy can be useful with ATR accessories in forensic analysis for several reasons. It provides excellent data quality combined with high reproducibility, with minimal user-induced variations and no sample preparation. Spectra from heterogeneous biological systems, including the integumentary system, can be associated with hundreds or thousands of biomolecules. The nail matrix of keratin possesses a complicated structure with captured circulating metabolites whose presence may vary in space and time depending on context and history. We developed a new approach by using machine-learning (ML) tools to leverage the potential and enhance the selectivity of the instrument, create classification models, and provide invaluable information saved in human nails with statistical confidence. Here, we report chemometric analysis of ATR FT-IR spectra for the classification and prediction of long-term alcohol consumption from nail clippings in 63 donors. A partial least squares discriminant analysis (PLS-DA) was used to create a classification model that was validated against an independent data set which resulted in 91% correctly classified spectra. However, when considering the prediction results at the donor level, 100% accuracy was achieved, and all donors were correctly classified. To the best of our knowledge, this proof-of-concept study demonstrates for the first time the ability of ATR FT-IR spectroscopy to discriminate donors who do not drink alcohol from those who drink alcohol on a regular basis.

Nanoelectromechanical Infrared Spectroscopy with In Situ Separation by Thermal Desorption: NEMS-IR-TD

We present a novel method for the quantitative analysis of mixtures of semivolatile chemical compounds. For the first time, thermal desorption is integrated directly with nanoelectromechanical infrared spectroscopy (NEMS-IR-TD). In this new technique, an analyte mixture is deposited via nebulization on the surface of a NEMS sensor and subsequently desorbed using heating under vacuum. The desorption process is monitored in situ via infrared spectroscopy and thermogravimetric analysis. The resulting spectro-temporal maps allow for selective identification and analysis of the mixture. In addition, the corresponding thermogravimetric data allow for analysis of the desorption dynamics of the mixture components. As a demonstration, caffeine and theobromine were selectively identified and quantified from a mixture with a detection limit of less than 6 pg (about 30 fmol). With its exceptional sensitivity, NEMS-IR-TD allows for the analysis of low abundance and complex analytes with potential applications ranging from environmental sensing to life sciences.

The Use of Infrared Spectroscopy for the Quantification of Bioactive Compounds in Food: A Review

Infrared spectroscopy (wavelengths ranging from 750-25,000 nm) offers a rapid means of assessing the chemical composition of a wide range of sample types, both for qualitative and quantitative analyses. Its use in the food industry has increased significantly over the past five decades and it is now an accepted analytical technique for the routine analysis of certain analytes. Furthermore, it is commonly used for routine screening and quality control purposes in numerous industry settings, albeit not typically for the analysis of bioactive compounds. Using the Scopus database, a systematic search of literature of the five years between 2016 and 2020 identified 45 studies using near-infrared and 17 studies using mid-infrared spectroscopy for the quantification of bioactive compounds in food products. The most common bioactive compounds assessed were polyphenols, anthocyanins, carotenoids and ascorbic acid. Numerous factors affect the accuracy of the developed model, including the analyte class and concentration, matrix type, instrument geometry, wavelength selection and spectral processing/pre-processing methods. Additionally, only a few studies were validated on independently sourced samples. Nevertheless, the results demonstrate some promise of infrared spectroscopy for the rapid estimation of a wide range of bioactive compounds in food matrices.

Portable near infrared spectroscopy for the isomeric differentiation of new psychoactive substances

Rapid and efficient identification of the precise isomeric form of new psychoactive substances (NPS) by forensic casework laboratories is a relevant challenge in the forensic field. Differences in legal status occur for ring-isomeric species of the same class, thus leading to different penalties and judicial control. Portable systems such as near-infrared (NIR) spectroscopy recently emerged as suitable techniques for the on-scene identification of common drugs of abuse such as cocaine, MDMA and amphetamine. This way, the overall forensic process becomes more efficient as relevant information on substance identity becomes available directly at the scene of crime. Currently, no NIR-based applications exist for the rapid, on-scene detection of NPS isomers. Herein, we present the differentiation of cathinone and phenethylamine-type NPS analogues based on their NIR spectrum recorded in 2 seconds on a portable 1350 - 2600 nm spectrometer. A prior developed data analysis model was found suitable for the identification of the methylmethcathinone (MMC) isomers 2-MMC, 3-MMC and 4-MMC. In 51 mixtures and 22 seized casework samples, the correct isomeric form was detected in all cases except for a few mixtures with an active ingredient content of 10 wt%. These results show the feasibility of on-site NPS detection as presumptive test performed directly at the scene of crime with a small size NIR-spectrometer. Additionally, in the illicit drug analysis laboratory the combination of NIR and GC-MS analysis might be suitable for robust identification of NPS isomers and analogues.

Expanding the Knowledge Related to Flavors and Fragrances by Means of Three-Dimensional Preparative Gas Chromatography and Molecular Spectroscopy

As universally known, gas chromatography (GC) coupled with mass spectrometry (MS) allows us to acquire spectra that can be searched in specific databases to attain qualitative information on a peak of interest. When not present in databases, structure elucidation is required before including a new component in a library: from that moment, scientists all around the world will be able to identify the new molecule with analytical confidence after GC-MS analysis. Conversely, if data are not shared in commercial databases, even if a molecule is studied and elucidated, it appears to be unknown or only identifiable on the basis of third-party data taken from the literature, which is a serious limitation. The present paper deals with a case that confirms this assumption. A component of Myrtus communis L. volatile fraction was tentatively identified based on literature data. Despite this, reliable identification could not be achieved due to the lack of a corresponding spectrum in commercial MS databases. Afterwards, the target component was isolated in a reasonable quantity and with a high degree of purity for downstream characterization by spectroscopic techniques. For this purpose, preparative (prep) GC may appear insufficient for the isolation of volatile components from highly complex samples. In this study, a prep-MDGC system was implemented for the isolation of the compound of interest from myrtle oil, consisting of three wide-bore columns of different selectivity coupled by means of Deans switch transfer devices. Based on the NMR and GC-FTIR data acquired, the unknown compound was identified as 2,2,5,5,7,7-hexamethyl-3,7-dihydro-1-benzofuran-4,6(2H,5H)-dione. Noticeably, this is a known molecule, yet its mass spectrum had never been registered into MS databases and thus was not available to the scientific community. Finally, the spectrum was included for the first time in a commercial library, namely the FFNSC 5.0 MS database. The aim of the present study was to highlight the opportunity to make analytical data quickly available in a reliable way by registering them in searchable MS databases to improve the identification means for researchers all over the world.

High information spectroscopic detection techniques for gas chromatography

Gas chromatography has always been a simple and widely used technique for the separation of volatile compounds and their quantitation. However, the common detectors used with this technique are mostly universal and do not provide any specific qualitative information. There have been some attempts to combine the separation power of GC with the qualitative capabilities of "high-information" spectroscopic techniques including infrared spectroscopy, nuclear magnetic resonance spectroscopy, molecular rotational resonance spectroscopy, and vacuum ultraviolet spectroscopy. Some of these hyphenations have proven to be quite successful while others were less so. The history of such attempts, up to the most recent studies in this area, are discussed. Most recently, the hyphenation of GC with molecular rotational resonance spectroscopy which provides promising results and is a newly developed technique is reviewed and compared to previous high-information spectroscopic detection approaches. The history, description and features of each method along with their applications and challenges are discussed.

The online coupling of liquid chromatography to Fourier transform infrared spectroscopy using a solute-deposition interface: A proof of concept

Hyphenated techniques combining chromatographic and spectroscopic methods are the gold standard to effectively handle the emerging challenges in the analysis of unknown chemical components in mixtures, and in this regard the coupling of liquid chromatography to Fourier transform infrared spectroscopy (LC-FTIR) is no exception. While earlier attempts to couple LC to IR spectroscopy relied almost entirely on offline techniques, clear motivations for implementing online LC-FTIR instrumentation emerged from the need for shorter analysis time, a higher degree of automation and sample throughput, better reproducibility, and reduced contamination. Most recent designs of LC-FTIR concepts have aimed to combine the advantages of both approaches by means of a solvent-elimination interface. The hyphenated instrumentation and method presented in this research are based on a pneumatically assisted LC-FTIR interface, relying on a small-scale self-regulating spray dryer to attain desolvation of the LC eluent stream while retaining the spatial and temporal resolution of the dissolved substrates. Focused deposition of the dried analytes occurs onto a ZnSe disc for continuous transmission mid-IR analysis at a resolution of 4 cm^-1. The optimization of the LC-FTIR technique is discussed in terms of interface parameters, limits of detection, and limits of quantification for a pair of furanocoumarin isomers differing in the position (linear or angular type) of the furan ring fused to coumarin. Finally, confident discrimination between the two closely related molecules was attained by matching the experimental FTIR spectra in a dedicated library. The quality match factors obtained were higher than 99% for both molecules. The limit of identification (LOI) was determined experimentally as the minimum amount of substance yielding a library-searchable IR spectrum (affording a quality match factor higher than 90%). Specifically, LOI of 0.6 μg and 1.25 μg was determined for psoralen and angelicin, respectively.

Analytical Characterization of 3-MeO-PCP and 3-MMC in Seized Products and Biosamples: The Role of LC-HRAM-Orbitrap-MS and Solid Deposition GC-FTIR

Among the phencyclidine (PCP) and synthetic cathinone analogs present on the street market, 3-methoxyphencyclidine (3-MeO-PCP) is one of the most popular dissociative hallucinogen drugs, while 3-methylmethcathinone (3-MMC) is a commonly encountered psychostimulant. Numerous 3-MeO-PCP- and 3-MMC-related intoxication cases have been reported worldwide. Identification of the positional isomers of MeO-PCP and MMC families are particularly challenging for clinical and forensic laboratories; this is mostly due to their difficult chromatographic separation (particularly when using liquid chromatography–LC) and similar mass spectrometric behaviors. 3-MeO-PCP and 3-MMC were identified in two powders, detained by two subjects and seized by the police, by different analytical techniques, including liquid chromatography-high-resolution accurate-mass Orbitrap mass spectrometry (LC-HRAM-Orbitrap-MS), and solid deposition gas chromatography-Fourier transform infrared spectroscopy (sd-GC-FTIR). LC-HRAM-Orbitrap-MS allowed us to assign the elemental formulae C₁₈H₂₇NO (MeO-PCP) and C₁₁H₁₅NO (MMC) through accurate mass measurement of the two MH⁺ ions, and the comparison of experimental and calculated MH⁺ isotopic patterns. However, MH⁺ collision-induced product ions spectra were not conclusive in discriminating between the positional isomers [(3-MeO-PCP vs. 4-MeO-PCP) and (3-MMC vs. 4-MMC and 2-MMC)]. Likewise, sd-GC-FTIR easily allowed us to differentiate between the MeO-PCP and MMC positional isomers unambiguously, confirming the presence of 3-MeO-PCP and 3-MMC, due to the high-quality match factor of the experimental FTIR spectra against the target FTIR spectra of MeO-PCP and MMC isomers in a dedicated library. 3-MeO-PCP (in contrast to 3-MMC) was also detected in blood and urine samples of both subjects and analyzed in the context of routine forensic casework by LC-HRAM-Orbitrap-MS following a simple deproteinization step. In addition, this untargeted approach allowed us to detect dozens of phase I and phase II 3-MeO-PCP metabolites in all biological specimens. Analysis of the extracted samples by sd-GC-FTIR revealed the presence of 3-MeO-PCP, thus confirming the intake of such specific methoxy-PCP isomer in both cases. These results highlight the effectiveness of LC-HRAM-Orbitrap-MS and sd-GC-FTIR data in attaining full structural characterization of the psychoactive drugs, even in absence of reference standards, in both non-biological and biological specimens.