Vibrational Spectroscopy and Chemometrics in Forensic Chemistry: Critical Review, Current Trends and Challenges

Abstract

The present manuscript makes an extensive review of the scientific approaches developed in the last decade involving infrared and Raman spectroscopy combined with chemometrics for solving several issues in the investigation of the most relevant forensic traces, such as questioned documents and currency, explosives, gunshot residues, illicit drugs and body fluids. In addition, current trends, main challenges and the adequate use of several chemometric techniques are discussed. Principal component analysis (PCA) was found to be the most used technique. This unsupervised approach, however, has sometimes been misunderstood as a classification technique. Discriminant analysis techniques are widely employed, leaving a range of possibilities for application of class-modeling techniques, particularly in cases of problems regarding only one target class. In addition, increasingly complex dataset structures frequently require nonlinear approaches or flexible techniques such as multivariate curve resolution-alternating least squares (MCR-ALS). Results reporting, however, still lack reliable quality parameters and sample representativeness, posing a significant challenge to the solution of forensic problems. Regarding the analytical techniques, Raman has been playing an important role, especially in the area of questioned documents and of body fluids. Portable and hyperspectral imaging infrared spectrometers have also been showing significant potential in forensic applications.

Preliminary study on the identification of synthetic cathinones in street seized samples by Raman spectroscopy and chemometrics

“…[ 9,10 ] There are modern compact and portable instruments available for field deployment and can be used in a complementary way with infrared spectroscopy. [ 11–15 ] Raman does not come without some limitations, including fluorescence from impurities or the sample itself, which cause large background signals; intense laser radiation, which causes sample heating and can destroy the sample or mask the Raman spectrum; and weak signals, which leads to low sensitivity. [ 12 ] The literature provides some important Raman spectral regions to the differentiation of 3,4‐methylenedioxy from most cathinones, [ 10,15,16 ] as resumed in Table S1.…”

“…The use of chemometric methods can provide statistical reliability to spectroscopic analysis and aid experts in their interpretations. For an extensive review on chemometric applications in forensics, refer to Silva et al [ 12 ] and Sena et al [ 18 ] Recently, three studies reported the use of Raman spectroscopy and Principal Component Analysis (PCA) to evaluate differences and similarities among NPS samples and improve chemical characterization. [ 15,19,20 ] In another study, [ 21 ] Partial Least Squares Discriminant Analysis (PLS‐DA) was employed to classify NPS infrared spectral data.…”

“…Consequently, those techniques are unsuitable for seized cathinone samples, and more flexible approaches are needed. In this sense, Multivariate Curve Resolution Alternating Least Squares (MCR‐ALS) can be a useful alternative for resolving issues related to mixture of signals in spectral data, [ 12 ] and it has shown great potential for verifying the authenticity of pharmaceuticals and medicines while accounting for content distribution and polymorphic forms. [ 21 ]…”

Forensic Analysis of Commercial Inks by Laser-Induced Breakdown Spectroscopy (LIBS)

“…These two types of techniques are non-destructive, have imaging capability, are based on instruments that are simple to use, and do not require sample preparation. Vibrational spectroscopy techniques combined with chemometric data processing are routinely used in forensic examinations [ 11 ]. These techniques have the potential to identify the chemical structure of the functional groups present in inks, colorants, and resins, but their success in classification is limited due to the similar functional groups present within many inks.…”

Application of chemometric methods for the determination of fading and age determination of blue ballpoint inks

“…The introduction of the chemometric methods opened new possibilities in fast, nondestructive and reliable analysis of writing inks. The extensive review of the scientific protocols developed in the last decade based on infrared and Raman spectroscopy combined with chemometrics for forensic studies of questioned documents were reported by Silva et al [28] Several chemometric approaches are used with the combinations of analytical techniques. [29][30][31][32][33][34] For example, the linear discriminant analysis based on the FTIR-ATR data (Fourier transform infrared/attenuated total reflectance spectroscopy) was successfully applied to the classification of blue pen ink according to types and brands.…”