Handheld NIR Spectrophotometers: Transforming Forensic Analysis with Portable Technology

On-site forensic analysis using portable NIR ©Aonin - stock.adobe.com

In recent years, the introduction of miniaturized near-infrared (NIR) spectrophotometers has sparked a revolution in forensic science, enabling law enforcement to conduct on-site analyses with unprecedented ease and accuracy. These compact devices, often referred to as handheld or portable NIR spectrophotometers, are not only affordable but also highly efficient, providing critical forensic data that can support criminal investigations and legal proceedings. A comprehensive review by Letícia P. Foli, Maria C. Hespanhol, Kaíque A.M.L. Cruz, and Celio Pasquini, from the Instituto de Química at Universidade Estadual de Campinas (UNICAMP) in Brazil, explores the potential and challenges of these devices, published in the journal Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy.

Read More: NIR in Forensic Analysis

The Rise of Portable NIR Spectrophotometers

Forensic science relies heavily on accurate, non-destructive methods to analyze evidence. Spectrophotometry, particularly vibrational spectroscopy, has been a staple in forensic analysis due to its ability to provide rapid and objective results without altering physical evidence. Among the different types of vibrational spectroscopy, NIR spectroscopy stands out for its versatility in analyzing a wide range of forensic samples. The typical NIR spectral range (750–2500 nm) captures the absorption of radiation due to molecular vibrations, offering detailed non-destructive insights into the chemical composition of substances.

The development of miniaturized NIR spectrophotometers has significantly enhanced the field's capabilities. These portable devices, which weigh as little as 100 to 200 grams, allow for the in-field analysis of forensic samples, making it feasible for law enforcement to obtain crucial data on-site. This portability, coupled with the relatively low cost of these instruments, has the potential to decentralize forensic analysis, extending its reach beyond specialized laboratories to crime scenes and remote locations.

Technological and Analytical Considerations

Despite their advantages, miniaturized NIR spectrophotometers come with certain limitations. Early models, while portable, were relatively heavy (over 1 kg) and expensive. Modern devices, however, are much lighter and more affordable, though some still come integrated with costly online software packages that can hinder widespread adoption.

A critical aspect of using these devices in forensic analysis is the accuracy of their spectral data, particularly at the extremes of the NIR range. Some compact instruments perform poorly at these extremes, leading to higher noise levels that can affect the reliability of forensic results. Additionally, the stability of the spectrometric signal—crucial for accurate measurements—can be compromised if the device is not given sufficient warm-up time, a factor often overlooked in the literature.

Another consideration is the necessity for a reference signal, typically obtained using reflectance reference materials like Spectralon or white ceramics, to ensure accurate measurements. The frequency with which this reference signal is captured is vital for maintaining data integrity, especially in varying environmental conditions. The literature suggests that obtaining a reference signal immediately before each sample measurement is the safest approach, particularly in forensic contexts where precision is paramount.

Chemometrics and Advanced Data Analysis

The review also highlights the importance of chemometrics in processing the complex data generated by NIR spectrophotometers. Algorithms such as standard normal variate (SNV), multiplicative signal correction (MSC), and others are commonly employed to enhance the reliability of forensic analyses. Moreover, extensions of machine learning (ML) and artificial intelligence (AI) are increasingly being used to handle non-linear data, although their application in forensic NIR analysis is still relatively new.

Conclusion

The advent of miniaturized NIR spectrophotometers represents a significant milestone in forensic science, offering the promise of real-time, in-field analysis that could revolutionize law enforcement practices. However, as the review by Foli and colleagues suggests, the technology is not without its challenges. Ensuring the reliability of these devices, particularly in terms of data accuracy and instrument stability, is crucial for their continued success in forensic applications. As the field progresses, further developments in both hardware and data analysis techniques are expected to enhance the effectiveness of these portable devices, making forensic analysis more accessible and reliable than ever before.

References

(1) Foli, L. P.; Hespanhol, M. C.; Cruz, K. A.; Pasquini, C. Miniaturized Near-Infrared spectrophotometers in forensic analytical science− a critical review. Spectrochim. Acta A Mol. Biomol. Spectrosc. 2024, 315, 124297. DOI: 10.1016/j.saa.2024.124297

(2) Fursman, H.; Morelato, M.; Chadwick, S.; Coppey, F.; Esseiva, P.; Roux, C.; Stojanovska, N. Development and evaluation of portable NIR technology for the identification and quantification of Australian illicit drugs. Forensic Sci. Int. 2024, 362, 112179. DOI: 10.1016/j.forsciint.2024.112179