Miniaturized Near-Infrared Spectrophotometers in Forensic Analytical Science

Be they referred to as compact, portable, or handheld, the introduction of miniaturized near-infrared (NIR) instruments has transformed the use of technology in the field of forensics. With miniaturized equipment, researchers and analysts now can perform their studies in the field, thus producing objective, easy-to-use, tailored, and accurate qualitative and quantitative forensic results in a fraction of the time and cost for comparable laboratory instruments. A recent research paper examines the main aspects and provides a comprehensive, critical review of this equipment. Spectroscopy recently had the opportunity to discuss this paper with Celio Pasquini, its corresponding author.

Your paper (1) offers a review of the current literature on compact near-infrared (NIR) instruments and their use in analytical methods to provide forensic information. What has been your experience with these devices?

My experience with compact near-infrared (NIR) instruments began in 2012. It resulted from the informal introduction of this type of NIR instrument during the 15th International Conference on Near-Infrared Spectroscopy (NIR 2011) in Cape Town, South Africa, where I participated. Since then, my research group and I have developed several practical applications of different types of compact instruments based on different technologies and compared their performances. Applications range from gases to fuels and agricultural products. The first work related to forensic issues produced by the group was published in 2015 (2), addressing the adulteration of diesel and biodiesel mixtures with vegetable oils.

In your opinion, when did the idea of utilizing portable instruments for this task begin?

The idea of utilizing portable/compact instruments was always present in the minds of all researchers working on the development of analytical methods dedicated to forensic analysis based on NIR spectroscopy. Forensic analysis requires in-situ analysis employing non-destructive sampling and fast, objective, and trustable results. For example, early in the year 2001, we developed an analytical procedure to identify counterfeit drugs based on a bench-top NIR spectrophotometer (3). Following the demonstration of the feasibility of the spectroscopic technique to detect fraud in medicines, I questioned whether the technology could be applied to in-situ inspection of suspect medicines, as required by the official controlling agencies. The advantages of non-destructive fast and direct analysis of drugs were demonstrated. However, although the in-situ inspection can be made by bench-top equipment, the mobility of these instruments is limited by their size, heavy weight, the requirement for a high-power electrical supply, and tolerance to mechanical shock—besides, the high cost of the laboratory instruments was not permissible due to the requirement to deploy many official units. I pondered if I could have a light and autonomous instrument to take to the field to perform the drug analysis. The so-called "portable" instruments at that time were heavy and constructed to embed the computer and display and were as expensive as the best bench-top NIR spectrophotometer in the market. The same issue has occurred with many researchers. Surveying the specialized literature, it is possible to find several instruments approaching the size and weight of the NIR spectrophotometers available in 1991 (4). At that time, the “portable” instruments weighed 20 pounds and had a limited near-infrared spectral range (680 – 1100 nm).

Nevertheless, it is representative of the researchers’ willingness to introduce the technology for in-field analysis. The use of a portable NIR instrument to approach a clearly forensic issue was published before the boom of compact spectrophotometers that occurred early in 2012 (5). The portable instrument was based on acousto-optic technology and could scan the NIR spectrum from 1400 to 2400 nm.

Briefly discuss the beginnings of using portable instruments to gather forensic analysis information.

The answer to this question depends on the definition of forensic analysis. The definition we adopted in the referenced paper (1) is broad and includes all analytical techniques, methods, and applications to provide objective information on any violation of state and official organizational laws. Please note that the definition does not restrict the application to specific types of samples, usually defined as having forensic interest, such as illicit drugs and other crime scene evidence. Within the broad definition, the first attempt at using portable or compact instruments was for fuel adulteration measurements (6). The paper reports on developing a portable NIR LED-based prototype to screen gasoline for adulteration. Regarding a stricter definition, the work published in 2012 by Melucci and associates previously mentioned (5), is an excellent example of the first attempts to deploy portable NIR instruments for forensics.

When you look at the timeline of the advancements of these instruments, where do you see the highlights?

Highlights in developing portable or compact NIR instruments are related to incorporating different spectrometric technologies that are commercially available at a low final cost. The first I noticed was the development of the sensor array, an optical wedge, and a built-in radiation source to produce a light device (less than 100 g) by Judson Inc., presently Viavi Solution. A similar approach was adopted by the Scio devices, although operating in a very short-wave region of the NIR spectrum. Then, Texas launched the NanoNIR, a dispersive device, commercialized at a low cost (though as an OEM) based on MEMS and a micro-mirror array with a single detector. The device allows Hadamard multiplexed data acquisition to improve the signal-to-noise of the measurements. Other companies have replicated the technology as InnoSpectra. More recently, the MEMS technology has also allowed compact instruments based on interferometric Fourier transform compact instruments (NeoSpectra) commercialized by Si-Ware. These last instruments extended the working spectral region from 1300 nm to the combination spectral region up to 2600 nm. Although the literature describes using several other compact instruments, the portability achieved by these four instruments surpassed all in terms of applications developed in the forensic area. In common, they have also advanced, allowing autonomous long-term operation based only on battery power.

With new developments often comes resistance; what sort of challenges did these advances encounter from users?

For those used to working with bench-top instruments, criticisms accompanied the advent of compact instruments, and, in principle, it appeared that the bench-top instruments could not be replaced by the so-called "little NIR toys," which became rapidly available. The main obstacle was, in fact, the need for knowledge about compact device performance and a clear definition of the niche of application of those devices in forensics. In many applications, the outstanding performance of the bench-top instruments is optional. Screening for illicit drugs and preliminary estimation of active contents in the field can be performed with less restrictive analytical figures of merit. Looking behind, it wasn't easy to accept that the tiny, low-cost devices then recently introduced could represent a breakthrough in forensic analysis. Nowadays, it is a reality, at least in the academy.

Which spectroscopic techniques do you believe are best suited for forensic analysis?

Forensic analysis is a broad field. It requires complementary analytical technologies to deal with myriads of sample types, requiring confident analysis to support legal decisions. The forensic laboratory reflects this fact, including the last generation of analytical instruments. However, spectroscopic techniques are the best choice considering the particular interest of in situ analysis during seizing operations. NIR has demonstrated its usefulness. However, other vibrational techniques, such as Raman and infrared, have also advanced in portability. The complementary information these spectroscopies can provide of forensic samples enables them for in-situ analysis, one of the key interests of forensic analysis.

While admittedly the range and quality of portable spectroscopic instruments has grown by leaps and bounds, I would presume that you believe there are still some gaps or weaknesses that need to be addressed. Can you talk about these, please?

Yes, you are right; there are still gaps. One of the main problems of all compact instruments that have currently attracted the attention of researchers is that they incorporate the radiation source into the device, preventing its replacement in the event of failure, deterioration, or burnout. If the radiation source fails, the entire instrument must be replaced. The Texas NanoNIR instrument has a detachable part containing the radiation sources. However, attempts to acquire these units were unsuccessful.

Another gap to be filled is knowledge about the long-term performance of these instruments. Articles in the literature are relevant as proof of concept. However, more reporting on the compact instruments' long-term performance is needed to develop practical forensic methods. Manufacturers also need to provide unequivocal information about the signal-to-noise ratio of the compact instrument, as accurate knowledge of this performance parameter throughout the entire spectral range accessed by the instrument is essential for the application of these devices.

The portable or compact instruments have a narrow measurement sampling window, which requires users’ attention when forensic heterogeneous samples are measured. A poorly designed sampling procedure may compromise the spectrum representativeness.

Another gap concerns integrating this type of instrument with the software necessary for its operation and data processing to express the final analytical results. All NIR-based analytical methods require the use of chemometrics (or artificial intelligence [AI], if you prefer) to effectively use the chemical information captured by the spectra. Therefore, the instrument alone is useless without a basic knowledge of multivariate data processing.

Are you aware of anything being worked on that might address these perceived weaknesses, or anything else being developed regarding portable forensic instruments that you’re excited about?

Compact instrument manufacturers are committed to filling the gaps previously mentioned in a major or minor grade. Sample-moving accessories have been developed to overcome these devices' small window sampling area. On the other hand, no solution for the radiation source replacement has been proposed so far.

Some initiatives address the gap between the instrument and the multivariate data processing. Some manufacturers have developed integrated systems with the possibility of using proprietary regression and multivariate classification models. However, one must rely on the representativeness of the manufacturer's database to employ compact instruments in situ in forensic analyses. This is difficult to achieve considering forensic samples such as illicit drugs whose composition changes between different countries and even between regions of the same country.

While developing a complete system for data acquisition and processing facilitates the use of compact instruments for end users, it increases the costs. Some companies prefer to sell the whole package (instrument plus data processing software) instead of just the instrument and essential spectral data acquisition software. Two of them initially used to sell only the instrument but turned to selling the complete system, increasing the cost to a level prohibiting many researchers from developing their applications using compact spectrophotometers. This change of direction also goes against the cost reduction expectation initially presumed to facilitate the distribution of compact instruments to many final users, as required by the forensic in-field application of the devices.

Can you please summarize the feedback that you have received from others regarding this work?

To date, I have received positive feedback from the collaborators of the Federal Police in Brazil and an anonymous reviewer of the original manuscript submitted to Spectrochimica Acta Part A, who considered the work an exemplary contribution to forensic analysis. I hope the work can motivate other researchers to use ​​compact instruments in forensic analysis, especially NIR instruments, as the subject is far from closed. Despite the excellent work reported in the literature, many studies still need to be carried out to explore the potential of these instruments in the forensic area.

What are the next steps in this research, and are you planning to be involved in improving this technology?

We are presently working on the integration of the knowledge acquired during the years we have been developing the applications of compact NIR instruments into an autonomous, user-friendly system capable of being effectively operated by an official agent in the field. The numerous academic works published on the subject have already demonstrated the advantages of using compact NIR instruments in forensics (1). However, a final stage to prove the technology's usefulness still needs to be developed. The autonomous system is being designed to incorporate multivariate models for in-field identification and quantification of illicit drugs, with cocaine and ecstasy used as examples of forensic samples demanding screening analysis. The cooperation between the academy and the Brazilian Federal Police with the financial support of CAPES (a Brazilian research support agency) and the National Institute of Advanced Analytical Science and Technology (INCTAA) supported by CNPq (another Brazilian research supporting agency) is helping to develop a sound, low-cost, distributable system to help the police to take decisions during seizing operations. We are also evaluating the performance of the compact instruments to assess their long-term stability, which is required for their effective use in forensic analysis.

In parallel, we continue evaluating compact instruments to solve forensic analytical problems associated with several other commercial products and, with the support of the Federal Police, illicit drugs.

References

1. Foli, L. P.; Hespanhol, M. C.; Cruz, K. A. M. L.; 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. Paiva, E. M.; Rohwedder, J. J. R.; Pasquini, C.; Pimentel, M. F.; Pereira, C. F. Quantification of Biodiesel and Adulteration with Vegetable Oils in Diesel/Biodiesel Blends Using Portable Near-Infrared Spectrometer. Fuel2015, 160, 57–63. DOI: 10.1016/j.fuel.2015.07.067

3. Scafi, S. H. F.; Pasquini, C.Identification of Counterfeit Drugs Using Near-Infrared Spectroscopy. Analyst 2001, 126, 2218-2224. DOI: 10.1039/b106744n

4. Lysaght, M. J.; van Zee, J. A. ; Callis, J. B. Laptop Chemistry: A Fiber‐Optic, Field‐Portable, Near‐Infrared Spectrometer. Rev. Sci. Instrum. 1991, 62, 507–515. DOI: 10.1063/1.1142095

5. Melucci, D.; Monti, D.; D’Elia, M.; Luciano, G. Rapid in situ Repeatable Analysis of Drugs in Powder Form Using Reflectance Near-Infrared Spectroscopy and Multivariate Calibration, J. Forensic Sci. 2012, 57, 86–92. DOI: 10.1111/j.1556-4029.2011.01945.x

6. da N. Gaião, E.; dos Santos, S. R. B.; dos Santos, V. B.; do Nascimento, E. C. L.; Lima, R. S.; de Araújo, M. C. U. An Inexpensive, Portable and Microcontrolled Near Infrared LED-Photometer for Screening Analysis of Gasoline. Talanta 2008, 75, 792–796. DOI: 10.1016/j.talanta.2007.12.014

Celio Pasquini is an invited professor at the Chemistry Institute at the Universidade Estadual de Campinas (UNICAMP), in Campinas, Brazil. He is also an invited professor of the Soil Department at the Federal University of Viçosa, in Viçosa, Brazil, and an independent consultant in analytical chemistry and near-infrared spectroscopy. Direct correspondence to: pasquini@unicamp.br

Photo courtesy of Celio Pasquini.