Brooke W. Kammrath is a Professor at the University of New Haven and Assistant Director of the Henry C. Lee Institute of Forensic Science.

Brooke W. Kammrath

Portable Raman spectrometers have become smaller over the last 20 years, while their performance has increased. This has been made possible by closer coupling of all the components, use of transmission gratings rather than reflection gratings, and general advances in electronics, displays, and battery technologies. An obvious question to ask is whether this trend can continue. This paper describes the technologies and evolution of these instruments, existing limitations, the current landscape of miniature Raman spectrometers, and the state of the art. Finally, the paper also looks at what emerging technologies could be applied in this area, and how those could lead to new applications.

The rationale for portable spectroscopic instrumentation is now very familiar—allowing the spectrometer to be taken to the sample, as opposed to bringing the sample to the spectrometer, thereby moving the laboratory to the point of need, the location of the sample (1). Portable instruments provide the ability to make informed decisions on the spot, and by rapidly delivering actionable results where and when needed, you change or transform the way the customer does business, whether that customer be a Hazardous Materials (HAZMAT) technician, a scrap metal dealer, someone in pharmaceutical manufacturing, or a member of the military. These capabilities are made possible by “point-and-shoot” sampling interfaces (2) and integrated data systems with appropriate databases, spectral libraries, calibrations, and algorithms (3–6). Continuing developmental drives include the desire to reduce the size, weight, and power consumption (SWaP) of these instruments, expand the available databases, improve identification algorithms (especially for mixtures), improve detection and identification limits, and, in the case of portable Raman instruments, mitigate the effects of fluorescence from real-world samples. A relevant question, then, is “How small can a Raman spectrometer become, and still be useful for field analyses?” To address that question, it’s useful to start with a brief description of the evolution of Raman spectrometers as a whole. In all the discussion, it is recognized that portable Raman instruments do not have the performance (spectral range, resolution, signal-to-noise ratio [S/N], and sampling versatility) of their laboratory counterparts, but they do provide extremely valuable screening of samples, so as to inform the next steps to be taken.

The Evolution of Laboratory Raman Instruments

Figure 1 highlights some of the significant milestones in the evolution of Raman spectrometers, starting in the early 1950s with the introduction of the Cary 81, the first fully-integrated, highly optimized, scanning (double monochromator) Raman instrument. The introduction of laser excitation in the 1960s (7,8), supplanting mercury arcs, made laboratory instruments considerably more friendly, but instruments of that period were still scanning (double and triple monochromators). It took until the 1990s for several technologies—diode lasers (9), array detectors (10), holographic optical elements and efficient laser rejection filters (11), and fiber optic probes (12)— to come together (13–16) to make what we would recognize today as a modern Raman spectrometer (17–20). Those technologies, along with developments in consumer electronics, enabled the next step—the miniaturization of Raman spectrometers, using single-stage spectrographs, and the possibility of making them portable.

FIGURE 1: The evolution of fully-integrated handheld Raman spectrometers and their key technologies. The 1990s saw the introduction of technologies that make portable Raman instruments possible, with the first fully-integrated handheld 785 nm excitation systems introduced in 2005. Since that time, the instruments have shrunk in size while improving their performance, and several handheld 1064 nm excitation instruments have been introduced.

Technologies for, and the Evolution of, Handheld Raman Instruments

The first stage in the development of a field-portable instrument is often the incremental step of shrinking a laboratory instrument down to the size of a briefcase (21,22). Some of these are described in early surveys of portable Raman instruments (23–25), weighing about 10 pounds, and occupying volumes around 60,000 cm

). A different approach is to start from scratch with miniature components, and build up an instrument. One example used a multi-mode solid-state diode laser, giving an eventual spectroscopic resolution of about 30 cm

 (26,27). Another product was based on as-small-an-enclosure design as possible, and a handle was placed on it, along with a wireless link to a laptop. The second stage, in 2005, was the introduction of the first fully-integrated, portable Raman spectrometer, engineered and optimized for its intended market. This was from Ahura Scientific, and was designed for use by first responders and hazardous materials personnel. It represented a considerable reduction in SWaP, being about 30 cm x 15 cm x 7.6 cm in size and weighing 1.8 kg. The resulting volume (~3420 cm

) is a factor of 20 less than the briefcase-sized instruments of just a few years earlier. A third thread has been development of instruments for planetary exploration (Mars rovers), where obviously SWaP are major considerations, and here the spatial heterodyne spectroscopy design has been implemented (28,29), but, to the author’s knowledge, this technique has not been applied to commercial field instruments. Finally, Fourier transform (FT) Raman instruments (30–32) have not made much impact in the portable sphere. A briefcase-sized instrument, weighing about 12.7 kg and using 1064 nm excitation, was introduced as early as 2006, and there was a recent report (33) of an unnamed handheld FT-Raman instrument used for street drug identification.

Today, the technologies for handheld and portable Raman instruments are well understood (34,35), instruments have been critically reviewed (36–38), and they are available from many vendors. As well as continuing efforts to shrink the SWaP, recent developments have focused on four main areas: expansion of spectral libraries; development and implementation of improved mixture search algorithms; fluorescence mitigation; and trace detection.

Expansion of spectral libraries has been most active in the field of narcotics, where novel “new psychoactive substances” (39,40) materials appear “on the street” with alarming frequency. The case of determining likely constituents from a single spectrum (that is, one obtained by a portable spectrometer) is a difficult challenge. Over the years, a number of different algorithms have been developed and described in the scientific literature (41–47), but individual manufacturers may not describe their particular algorithms or their implementation in detail.

Obscuration of the Raman signal by fluorescence in the case of real-world, colored, and mineral samples is obviously very well known, and in the limiting case, shot noise from the fluorescence can completely obscure the Raman signal. Many approaches have been used to mitigate fluorescence (48), including baseline correction and other mathematical techniques (49–53), using long-wavelength excitation (for example, 1064 nm), using deep-ultraviolet (UV) excitation (shorter than 250 nm), sequentially shifted excitation (54–57), time-gating (58,59), and, in some cases, combinations of these techniques (60). One portable instrument implements a version of the sequentially-shifted method (61), and dual laser packages are now commercially available (62,63). Time-gated approaches using single photon avalanche diode (SPAD) detectors are discussed in a following section; a laboratory instrument implementing this principle is commercially available (64), but not yet a portable or handheld instrument (65,66).

Most vendors of handheld Raman instruments also supply surface-enhanced Raman spectroscopy (SERS) substrates, which not only enhance some Raman signals, but can also act to quench fluorescence. SERS is a huge field in and of itself, and the reader is referred to the chapter by Hargreaves (35) for its use in portable instruments. One commercial product (67) incorporates the ability to “read” automated colorimetric swab tests (68), providing a trace detection capability.

Figure 2 shows the size and weight of typical portable Raman instruments available today, comparing them with the 2005 Ahura product. What is remarkable is that the size of these instruments has diminished, while their performance—S/N for the same collection time and resolution—has increased. The first-generation portable Raman instruments, typified by the Ahura Scientific TruDefender and TruScan, were based on a reflective Čzerny-Turner (single-stage) design with fiber-coupled components. Second-generation instruments eliminated fiber coupling, were significantly smaller, and were more tightly integrated, using free-space optical coupling. These instruments improved the S/N (with the same range and resolution) by about a factor of 5 over their predecessors. The current generation of instruments are a little larger than a pack of playing cards, with yet improved S/N, possibly as much as a factor of 10, made possible via transmission grating designs (69). Those instruments using 1064 nm excitation are significantly larger than their 785 nm counterparts, which is due to several factors, including the physically larger size of InGaAs detectors, compared to Si-based charge-coupled device (CCD) and complementary metal–oxide–semiconductor (CMOS) arrays.

FIGURE 2: Size and weight of handheld Raman instruments from 2005. The purple marker in the top right represents the first fully-integrated system. The two yellow markers on the bottom left represent spectrometers that do not have an embedded data system, but communicate to a smartphone for control, data display and work-up. The earlier briefcase-sized instruments are not included in this plot.

For a stand-alone, completely-integrated instrument with its own data system, there is a practical minimum size, defined by the need for a display and controls. Many of these instruments will be operated by personnel wearing protective gear, implying limited visibility, and wearing gloves (70). Therefore, touchscreen displays will not work; raised molded buttons are used for control. In addition, there is often a need for more versatile sampling (such as sample presentation to the spectrometer) than point-and-shoot. For an instrument in the size range of 20 x 15 x 10 cm, sampling accessories can be configured in a “snap-on” mode, for instance, for liquid vials, a right-angle (or downward-looking) adapter, a fiber-optic probe, or a modest-distance stand-off adapter. These all require a significant-sized port to which to attach (at least 1 x 1 cm).

To minimize the size of a portable Raman spectrometer further, the approach has been to use a smartphone as the data system. This enables instruments as small as 11.2 x 3.9 x 3.4 cm from one vendor, and 6.3 x 3.9 x 1.7 cm from a second vendor, for a volume of 42 cm

, weighing just 63 g. This represents a size reduction of more than 1000 over the first “portable” instruments. That latter instrument delivers 16–19 cm

 resolution over a range of 400–2300 cm

 Raman shift, and is positioned as a “SERS reader” (71). One proposed configuration resembles a smart car key, and could therefore be used as a “wearable,” which can be used for clandestine work.

A configuration where remote control of an instrument is useful, even if it is self-contained with its own data system, is when it is attached to a robot—for instance, the new generation with “doglike” motion capabilities. This configuration can be used to explore locations containing suspected explosives, chemical agents, or other toxic materials. These instruments need to be able to communicate back to a position outside the hot zone.

Emerging and Future Technologies for Miniature Raman Spectrometers

Excitation of Raman spectra in the deep-UV has potential advantages, including fluorescence avoidance, ability to use solar-blind detectors, and a large increase in Raman scattering efficiency due to its υ

 dependence. Balanced against that are the immaturity of compact solid-state sources operating at shorter than 250 m, the lifetime of components, and eye safety considerations. The first instrument of this type has just been announced (72).

The development of stand-off Raman instruments has been ongoing for at least 20 years, driven by both military and planetary exploration missions (73–75). As noted above, stand-off adapters are available for some portable Raman systems, and a dedicated instrument, operating at up to a 2 m distance, is commercially available (76).

One possibility for further size reduction, while maintaining an integrated design, is to use the camera in a cellphone, and an effort in that area has been described (77). The project was called “spectrometer on a phone” (SOAP), and was designed using commercial-off-the-shelf (COTS) components. The system measured 4.0 x 4.0 x 1.4 cm, and was coupled directly to the cell phone detector camera optics. However, it achieved roughly only ~40 cm

 resolution. Nonetheless, it indicates what is possible, bearing in mind that some applications may only require low resolution to perform a quick screening of a sample in the field.

Single Photon Counting Avalanche Diodes (SPADs)

Detector arrays are available using single photon avalanche photodiode (SPAD) technology (78). These detectors can produce a time-based record of arriving photons, on a time scale of a nanosecond or less. They are well-suited to low light level applications, like Raman spectroscopy, and a significant number of publications have investigated their development and use (79,80). A SPAD array has already been used in a commercial Raman spectrometer (62), and its use permits the synchronization of opening a gate on the detector to the pulse of a laser (81,82). Because Raman scattering is instantaneous, and fluorescence is slightly delayed (83,84), this provides a method for discriminating against fluorescence while using visible light excitation (for example, 532 nm), without any mathematical processing (85). Commercial SPAD arrays are available in, for instance, linear (320 x 1) and 2D (320 x 320) configurations (86).

In 2022, NASA awarded a research program to Physical Sciences, Inc., entitled Photonic Integrated Raman Spectrometer (PIRS), to develop an on-chip Raman spectrometer (87,88). The project abstract states that it “will use a silicon nitride photonics platform to develop a dual-stage spectrometer that enables simultaneous high-bandwidth and high-resolution spectroscopy with direct readout. The overall form factor of the spectrometer will be less than a square centimeter while retaining spectral resolution of better than 0.2 nm” and that “the technology developed within this program can find application for material characterization in landing vehicles, plume sampling craft, and satellite-based scientific sensors. Specific missions that would benefit from PIRS include Goddard’s Ocean Worlds Science Exploration and Analogs (OSEAN) and the Mars Exploration Rover.”

The question is “How is the current miniature size achieved, and can we go any smaller?” From an optical design point of view, it is a matter of throughput—AΩ, the product of the limiting area and the solid angle collected—spectral coverage and of spectroscopic resolution, combined with what is commercially available (for example, array detectors) at a reasonable cost. The reader is referred to specialized articles (89) and monographs (90) on these topics, but we can briefly outline some of the issues.

Raman spectrographs, suitable for integration into portable equipment, are available from several vendors, with a resolution of ~9 cm

 for just a spectrograph, to around 1700 cm

, including a laser and probe. But these are significantly larger than the smallest systems on the market today.

To make a smaller instrument, every item must be addressed, and compromises made with spectral resolution, Raman shift coverage, and optical throughput. In design terms, this means looking at the detector selection, the focal length and f/number of the optics, and the dispersion (lines per mm) of the grating. These issues were described in detail in the book by McCreery (18), in the context of laboratory instruments. Are there any other approaches or “tricks” that can help? In a portable, open-beam instrument, the laser intensity can’t be increased due to eye-safety regulations. “Photonic” or “plasmonic” approaches (91,92) may not help, as they rely on bright sources. Echelle spectrographs promise the combination of high resolution and broad spectra range, but are complex designs (93). Detector cooling can reduce dark current, but may not be practical in terms of power consumption and heat dissipation in a miniature device. Finally, a non-destructive readout might help for low-light applications (like astronomy [94] and Raman spectroscopy).

The critical commercial component is the detector, typically a line sensor, so it makes sense to start there and work backwards. A typical CMOS detector for spectroscopy today has 2048 pixels, with a size of 14 μm x 200 μm, with a total detector package width of 42 mm—quite large in our context—and a version with 1024 pixels (28 mm width) is the same overall package size. Both would probably provide more spectral resolution than is required for a miniature sample identification device. In very round numbers, a 512-element linear sensor would result in 4 cm

 Raman shift coverage, giving an effective spectral resolution of ~12 cm

 varies over the coverage region in a grating-based instrument). Utilizing a 512-element sensor, with 25 μm pixel pitch, brings the detector package width down to less than 2.5 cm. Using the standard equations relating dispersion, focal length, and grating groove density, if we chose 2.5 cm focal length, then we would require a grating with ~2000 lines/mm for the desired spectral coverage, well within the state of the art. Therefore, a spectrograph with 2.5 cm internal dimensions is quite achievable, and it then remains to select the appropriate input slit and to throughput-match the collection optics.

The ultimate limitation in Raman spectrometer size is getting sufficient scattered photons to the detector to collect a reasonable signal-to-noise spectrum in a reasonable time. But what is reasonable? For spectroscopic signal to noise, a factor of 100, measured as height of the strongest band to the baseline noise in a featureless region, is more than enough for qualitative (sample identification) analyses. For a point-and-shoot application, a few seconds measurement time is practical when the instrument is actually handheld, but longer measurement times, up to a minute, are acceptable where the instrument is placed on a surface. If the optical throughput of a miniature spectrograph drops so that its typical sample measurement time is unacceptable, then it would not be a marketable product. The use of the SERS effect can clearly reduce sample measurement times or enable lower detection limits.

Smaller instruments imply lower costs, and that can lead to wider deployments in existing applications, plus new applications. Just a few are mentioned here.

Stand-off Raman, and the integration of portable Raman instruments with “doglike” robots (for cases involving explosives or suspected chemical weapons) have already been mentioned. The major law enforcement applications concern drugs (95) and explosives, and we can expect growth there. A related field in the public health arena is “community drug checking” (96), providing information to health care providers about novel substances in an illicit drug market, and promoting improved collaboration between local health care providers and law enforcement. Detection of counterfeit drugs (97) is another public health issue, especially in low- and middle-income countries (LMICs), where the cost of instrumentation is a barrier to wide usage; lower cost portable Raman spectrometers would aid considerably there.

Portable Raman instruments can be used in the laboratory as safe, non-destructive, and non-invasive screening tools, both to determine the confirmatory tests to use (such as gas chromatography–mass spectrometry [GC–MS] and liquid chromatography–mass spectrometry [LC–MS]), and to eliminate the use of wet chemistry techniques. Bodily fluid identification and gunshot residue characterization are possible emerging fields (98). Bodily fluid identification may require extensive development to encompass all types of fluid, variable human populations, and interference from substrates. Finally, a possible application is the investigation of war crimes, where the scene may not be able to be secured, and evidence needs to be gathered rapidly under adverse circumstances.

Current generation, fully integrated, portable Raman spectrometers are currently available as small as around 10 cm x 10 cm x 5 cm, weighing less than 1 kg, and capable of supporting a variety of sampling attachments. It may be difficult to reduce the size still further, while maintaining the same spectral coverage, spectral resolution, and spectroscopic signal-to-noise. However, spectrometers relying on an external data system (such as a smartphone) can be smaller—significantly so if compromises are made in those three key parameters. The smallest instrument available today is positioned as a SERS reader, taking advantage of the much larger signal obtained using that technique. With smaller size and lower costs, we can expect a rapid expansion in the applications of these instruments (1).

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Articles

There is a growing desire among spectroscopists for having instruments
small enough to be taken to the sample, as opposed to bringing the sample
to the instrument. The result is that Raman spectrometers are becoming
more miniaturized. Because these instruments come at a lower cost and offer distinct advantages over traditional spectrometers, the expectation is that a rapid expansion of when these instruments are applied will come forthwith. We offer a preview of how future miniaturized Raman spectrometers might look.

Portable Raman Spectrometers: How Small Can They Get?

The majority of inmates in the United States are incarcerated for drug-related offenses. It is important that the technology used to test for controlled substances be accurate and reliable. For on-scene presumptive testing, color-based field tests are commonly employed. Numerous cases have been discovered where positive color-based field tests were later proven to be false positives via confirmatory laboratory testing. Portable infrared (IR) and Raman spectroscopy are capable of providing accurate and reliable identifications at the scene, but are not generally utilized for this purpose. This research evaluated important performance characteristics of these methods to determine which is best for conducting on-scene testing of cocaine HCl. This research concluded that although portable spectrometers require a large initial financial investment, their high performance characteristics (such as ease of use, rapid analysis, non-destructive capability, acceptable limit of detection, and minimal false positives and negatives) make them a superior tool to the color-based field tests for the on-scene presumptive analysis of cocaine HCl. Portable IR spectroscopy was determined to be better than portable Raman because of the lower limit of detection, less severe adulterant interferences, and Raman fluorescence of common drugs (such as heroin or additives).

	A detailed online investigation into field-based presumptive tests for illicit drugs revealed many cases where false-positive color tests at the scene resulted in the incarceration of innocent people. The case of Amy Albritton was detailed in a 

 article, and demonstrates the problem and its significance (1). A white "crumb" found in her vehicle was tested using the cobalt thiocyanate color test, and yielded a positive result for cocaine, although Albritton claimed innocence. Albritton was arrested, and pressured into taking a plea deal by her court-appointed defense lawyer, as he insisted that the test proved she was in possession of controlled substances and, thus, she would be better off taking the plea deal. She eventually complied, served her time in county jail, and went back to her life. It was not until months later that any confirmatory testing was done on the suspect crumb, and when it was conducted, it was concluded that the crumb was not a controlled substance. This, among other cases including those of Janet Lee (2), Antonia Carranza (3), and Karlos Casche (4), brings into question the reliability of color tests and indicates a need for finding superior methods for on-scene testing of controlled substances.

	There is a significant need for field drug testing to be accurate and reliable. This is because, in drug possession cases, outcomes are often decided before they even reach a trial through the use of plea bargains to obtain a conviction. Many US Courts accept guilty pleas based solely on the results of field tests, such as those in nine of the ten jurisdictions of North Carolina, and the cities of Atlanta, Dallas, Jacksonville, Las Vegas, Los Angeles, Newark, Philadelphia, Phoenix, Salt Lake City, San Diego, Seattle, and Tampa. Research indicates that at least 90 % of drug convictions are obtained through plea deals, with some even higher (94 and 97% in Tennessee and Kansas, respectively) (1). Due to the significant errors (such as false positives and interferents) associated with color tests, portable vibrational spectroscopy, including IR and Raman methods, may provide a superior alternative for field testing of illicit substances, especially since these methods offer a higher level of confirmation of the illicit substance.

	Portable IR and Raman spectrometers are both capable of identifying controlled substances, but have not seen widespread deployment by law enforcement for field testing, primarily because of the cost of the instrumentation. Another concern of the forensic science community is that nonscientist investigators would be executing scientific analyses at crime scenes, and may incorrectly conduct the test or misinterpret the results, or both. For field portable instrumentation to become a viable presumptive test for illicit drugs by law enforcement officers, the instruments need to not only be rugged, but simple to use by nonscientist personnel.

	Recently, the National Institute of Justice (NIJ)'s Forensic Technology Center of Excellence (FTCoE) published a landscape study of portable and handheld instrumentation for presumptive drug testing in order to provide background information for law enforcement who are interested in these technologies (5). This study is an excellent resource for information, but the authors did not perform any specific laboratory or field research for comparisons of the various technologies, and instead relied on literature searches, manufacturer specifications, and interviews with current users. There was no ultimate conclusion of this landscape study, as it was determined that portable technologies provide general benefits (versatility, objectivity, specificity, safety, chain of custody collaboration, technical support, and ability to be used for multiple applications), and challenges (high up-front cost, complexity, maintenance, limited mixture interpretation, and library dependence), and each jurisdiction has its own needs and circumstances.

	Performance characteristics were assessed and compared for each technique: the Narcotics Identification Kit (NIK) Test G for cocaine produced by Safariland, a Smiths Detection HazMatID Elite portable IR spectrometer with a diamond attenuated total reflection (ATR) sampling element, and a Smiths Detection ACE-ID portable Raman spectrometer. These instruments were used due to their availability, but the research could be applied to any company's comparable portable instrumentation.

	The specific performance characteristics that were evaluated for all methods included a limit-of-detection study, an investigation into specificity (potential false positives, false negatives, and adulterant interactions), and evaluation of the ease of use, speed of analysis, skill required of the operator, and destructive nature of the method.

	Two-component mixtures were created by using pure cocaine HCl (Sigma-Aldrich) as well as the following five adulterants: lidocaine, mannitol, caffeine, Sweet 'N Low artificial sweetener, and Enfragrow powdered baby formula. Samples with concentrations of 0.1, 0.5, 1, 5, 10, 25, and 50% by mass of cocaine HCl were prepared for experimentation. When portable IR and Raman analyses were conducted, samples of 35% and 15% were subsequently made to obtain more precise limits of detection.

	A positive result for cocaine HCl with the NIK test, as shown in the left side of Figure 1, was indicated by "blue or pink with blue speckles after breaking the first ampoule, a pink result after breaking the second ampoule and a pink layer over a blue layer after breaking the third ampoule (6)." If the proper color change did not occur (right side of Figure 1), this was noted as a negative result. A positive result for cocaine HCl with the portable IR and Raman instruments was indicated by a "hit" for cocaine HCl using the library search function on the instrument (Figures 2 and 4, respectively). If a "hit" was not provided, this was noted as a negative result.

	Before experimentation, pure samples of cocaine HCl, as well as each adulterant, were analyzed to establish controls, as well as ensure the adulterants were not false positives for cocaine HCl. It was found that lidocaine was a false positive for cocaine HCl with the color-based field test. Therefore, only the sample sets containing the other four adulterants were used for color testing experimentation.

	The limit of detection determined for the color-based field test was at a 10% concentration of cocaine HCl with all four adulterants tested. Sources have conflicting reports on the average purity of cocaine HCl found in street samples. In 2016, it was reported by the Colombian National Police that the purity of cocaine leaving their country was approximately 85%, but by the time the cocaine reached the UK, the purity was approximately 60% and dropped to about 30% at the retail level (7). The US Department of Justice released an article in February 2010 indicating that from 2006 to 2009, cocaine purity had decreased from 68.1% to 46.2%. Even if the lowest reported concentration of 30% were used as a reference, this would make the limit of detection for the color-based field test acceptable.

	An extensive literature review revealed numerous known substances that would provide a positive result with the cobalt thiocyanate test other than cocaine HCl (Table I), which is the basis for the color-based field test for cocaine. One article from the 

 listed 73 different compounds, indicating a large quantity of potential false positive results with the test (8).

	The color-based field test is a destructive method that prevents any further experimentation from being conducted on the sample used. It is simple to use, with directions provided in each box. The color changes are very obvious at high concentrations of cocaine HCl; however, at lower concentrations it can be difficult to discern the color of the bottom layer. The subjectivity of determining a positive result is a problem when there is ambiguity at low concentrations. This method would also not be viable for operators with a color-determination deficiency. A single analysis is fast, taking only a few minutes, and there is little adulterant interaction that affects the limit of detection. However, one sample may require several different tests to identify the illicit drug, thus taking more time than just a single analysis. Additionally, this test requires sampling of the specimen for testing, thus potentially exposing the user to an unknown substance. There is also no reproducible record of the testing, and the color results may degrade over time, although detailed documentation with photography is a possibility.

	The cost for the color-based field test for cocaine, crack, and free base is $25.50 for a box of ten tests. Other commercially available color tests cost between $2 and 5 dollars per test (5). As these tests are one-use only, whomever is employing the tests must buy a large quantity each year. The NIJ's landscape study estimated that a large metropolitan area has approximately 500 drug-related arrests per year, with two color tests being used for each arrest, resulting in a total cost of $30,000 per year. However, this does not take into account the number of cases where no arrest was made, due to a negative results with the color test. Further, the tests are only specific to a particular drug "group," so law enforcement personnel require separate color-based field tests for cocaine HCl than for marijuana, PCP, or opiates. These tests do have expiration dates, and cannot be stored indefinitely. Consequently, the $30,000 per year calculation is likely an underestimate of the actual cost. The perceived low cost of these tests is considered by law enforcement to be a significant benefit over portable instrumentation.

	Another factor is the potential litigation fees that could arise from someone who has been falsely convicted of drug possession. A person could file a civil suit, and those litigation fees could far outweigh the cost of investing in a more reliable method of presumptive testing at crime scenes. This does not include the non-monetary costs that come along with being falsely charged with drug possession due to a faulty test result, causing considerable personal distress.

	The limit of detection for portable IR using the on-board library search algorithm for mannitol, caffeine, and baby formula were at 25%. Below 25%, the instrument did not provide positive "hits" for cocaine HCl. For artificial sweetener, the instrument was able to detect cocaine HCl at 15%, and, for lidocaine, it consistently had limits of detection at 5%. When manual spectral analysis was conducted by an educated chemist in spectral interpretation, cocaine HCl was identified in all of the spectra below the established limit of detection for the instrument's search algorithm (Figure 3). This instrument provides a Reachback feature, where the user can electronically send a spectrum to a trained spectral interpreter for technical support at any time and day. This indicates that, even if the instrument does not provide a positive "hit," the user can receive technical support to assist with interpretation and troubleshooting. The spectra could also be sent to a forensic laboratory for interpretation, depending on the agency's protocols. Additionally, as previously mentioned, the lowest concentration of street cocaine HCl was reported to be 30%, which is higher than the limit of detection for the portable IR with all adulterants tested.

	There were no false positives or negatives identified with the portable IR spectrometer; however, there were a number of misidentifications found with the portable IR spectrometer. A 

 is any substance identified by the instrument that was wrongly identified in place of an authentic mixture component. There were also false identifications made in which an extra component was identified in addition to the known components that were not actually present in the mixture (called a "false hit"). Mistaken identifications are potentially a large point of concern, but only if these incorrectly identify or prevent the identification of a controlled substance. In this research, the majority of these false hits were extra identifications that were determined to be of innocuous substances. Ultimately, as long as the cocaine HCl is still being identified, an extra hit for something because of the adulterant being used does not necessarily affect the test results.

	IR spectroscopy is a nondestructive method. Portable IR instruments are simple to operate. Step-by-step prompts with icons are shown throughout an analysis. Although these instrument are easy to use and can be operated with little training, scientific knowledge is required for sample selection, evaluating the spectral quality, and trouble shooting. A large benefit over the color tests is the chain of custody corroboration in that when a sample is tested, the results are automatically stored and time-stamped in the instrument to be exported or reviewed later. Similar to the color tests, portable IR with ATR sampling requires direct testing of the specimen, thus potentially exposing the user to an unknown substance.

	Formal training would be required for the operation of a portable IR spectrometer. However, it is simple to operate, so the training could easily be incorporated into that already provided for officers. The analysis overall does not take more than a few minutes for each sample being run, which is comparable to the speed of the color-based field test.

	Though prices on portable IR spectrometers vary by company, model, and add-ons (such as libraries, attachments, and service), the price can range from $25,000 to $65,000. There are no consumables required for portable IR spectrometers, so this is a one-time, up-front cost. Additionally, these instruments are built to be rugged and used in high stress situations; this means that, though repair on the instruments may be necessary, they will not break easily and could be used for several years. Also, portable spectrometers have the capability to identify a large number of controlled substances. Instead of buying ten different color-based field tests and making sure a police department is stocked for an entire year with all of these options, one portable spectrometer could conduct the same analysis. Finally, an increased demand for these instruments would encourage companies to increase production, which could lead to a cost reduction, thus making it a more affordable technology.

	The limit of detection for the portable Raman spectrometer using the on-board library search algorithm was consistently 25% cocaine HCl for all adulterants. In most trials, the instrument could not detect cocaine HCl at lower percentages; however, it was able to detect as low as 5% with the artificial sweetener, and 10% with the baby formula. The ability to electronically send spectra to Reachback or to a forensic laboratory for manual spectral interpretation is also available for portable Raman spectrometers. However, for these samples, manual Raman spectral analysis did not enable lower detection limits.

	One of the advantages of Raman spectroscopy is the ability to analyze samples without making contact, analyzing through some containers made of clear glass and plastic bags. The ability to analyze substances contained within clear plastic bags was tested, and the limit of detection was unaffected, remaining at 25%. This limit of detection would be acceptable to detect the lowest reported concentration of cocaine HCl in street samples. Consequently, unlike color tests and portable IR spectroscopy, portable Raman does not require direct testing, thus reducing exposure to unknown substances.

	There were no false positives or negatives identified with the portable Raman spectrometer. As with the portable IR spectrometer, there were a number of "false hits" and misidentifications that occurred during analysis. The portable Raman analysis had significantly fewer misidentifications and false hits than the portable IR, with a likely explanation being the smaller onboard Raman spectral library, which limits misidentifications and false hits. Although none of the adulterants tested suffered from Raman fluorescence, it is important to recognize that other illicit substances (such as heroin) or adulterants may fluoresce, which would interfere with the ability for chemical identification of any of the mixture components. Dark-colored samples prohibit Raman analysis because they absorb the incident radiation, thus potentially destructively burning the sample. In this situation, an orbital-raster-scanning (ORS) Raman was used to minimize the transfer of incident radiation, but not all portable Raman systems use ORS Raman. These issues may prohibit Raman analysis for the identification of cocaine or other illicit drugs in the field.

	Similar to IR spectroscopy, Raman spectroscopy is nondestructive, requires only a few minutes for analysis, and creates a reviewable record of the analysis that provides chain-of-custody corroboration. Much like the portable IR spectrometer, it is a push-button method, with step-by-step instructions on the screen. However, there is a small spot size for analysis, and focusing the LASER can be challenging at times. Sometimes, a few attempts are necessary to get a viable spectrum of the sample. As with the portable IR spectrometer, formal training would be required for the operation of the portable Raman, which could be simply incorporated into general training that officers are given. The same caveat applies for both portable spectrometers: Despite their ease of use, there is a need for educated users for proper sampling and analysis.

	Portable Raman spectrometers are generally less expensive than most portable IR spectrometers, although there are some exceptions; thus prices range from $12,500 to $60,000. Similarly to IR spectrometers, these instruments are built to be rugged and do not require consumables, so this is a one-time, up-front cost.

	Portable spectrometers have excellent potential to be employed for the on-scene analysis of cocaine HCl. Performance characteristics were compared (Table II), and the advantages of portable spectroscopy over traditional color testing were demonstrated. The limit of detection for all three methods would be acceptable at the lowest reported purity of street cocaine HCl. The speed of analysis is comparable for all three methods, taking less than 3 min for each analysis. Portable IR and Raman spectroscopy methods are nondestructive, objective, and create a reviewable record for use as evidence in adjudications, which are significant advantages over color tests. Color tests are also more prone to false positives and negatives than tests using portable spectroscopy, thus the latter are more reliable for the on-scene analysis of cocaine HCl. Portable Raman also has the ability to identify cocaine HCl through some plastic bags, reducing the potential for contamination and risk of exposure to unknown chemicals by analysts. Color tests require less knowledge or skill from the operator, however, inserting training on the different spectrometers during general training for officers would be simple and feasible because the instruments are created to be user-friendly. Although color tests are generally thought to be less expensive than portable spectrometers, the total cost over several years is comparable, if not greater than, an instrument with only a one-time up-front cost.

	Of the two portable spectroscopic methods, the authors concluded that, although both technologies have similar performance characteristics, there are some features of portable IR spectroscopy that make it the preferred technique for on-scene cocaine HCl identification. The long and reputable history of IR spectroscopy means that there are more spectral libraries and the technology has had more time to be evaluated and perfected. Additionally, fluorescence and the inability to analyze dark samples lessens the utility of the portable Raman spectrometers, whereas a portable IR spectrometer is able to produce interpretable spectra for all specimens. Although these limitations were not a problem in the specific analyses tested in this research, they could prove to be prohibitive for identification by portable Raman of cocaine mixed with other adulterants and for the analysis of other drugs, like heroin. Portable Raman spectrometers have the advantages of being generally less expensive than portable IR spectrometers, as well as being able to analyze samples through some clear glass and plastics, thus providing greater safety for the analyst. Ultimately, both portable IR and Raman spectroscopy are viable alternatives for crime scene identification of cocaine HCl.

	(1) R. Gabrielson and T. Sanders, "How a $2 Roadside Drug Test Sends Innocent People to Jail" (July 7, 2016). Retrieved April 28, 2018, from 

https://www.nytimes.com/2016/07/10/magazine/how-a-2-roadside-drug-test-sends-innocent-people-to-jail.html

	(2) J. Kelly, "False Positives Equal False Justice" (2008). Retrieved April 28, 2018, from 

https://www.mpp.org/issues/criminal-justice/false-positives/

	(3) E. Peralta, "Thinking It Was Cocaine, N.C. Police Jail Man For Cheese And Tortilla Dough" (May 16, 2011). Retrieved April 28, 2018, from 

https://www.npr.org/sections/thetwo-way/2011/05/16/136359770/thinking-it-was-cocaine-n-c-police-jail-man-for-cheese-and-tortilla-dough?ft=1&f=1001&sc=tw&utm_source=twitterfeed&utm_medium=twitter

	(4) S. Jeong, "Man Spends Three Months In Jail After False Positive On Police Drug Test" (June 28, 2017). Retrieved April 28, 2018, from 

https://www.cnn.com/2017/06/28/us/drug-test-drywall-positive-arrest-trnd/index.html

	(5) Forensic Technology Center of Excellence. Landscape Study of Field Portable Devices for Chemical and Presumptive Drug Testing. U.S. Department of Justice, National Institute of Justice, Office of Investigative and Forensic Sciences (2018).

	(6) Safariland Group. NIK Test G- Cocaine, Crack, & Free Base (2018). Retrieved August 29, 2018, from 

http://www.safariland.com/products/forensics/field-drug-tests/nik-test-g---cocaine-crack-and-free-base-1006155.html#sm0000624lqmakerfrm2cnczoduu2

	(7) United Nations Office on Drugs and Crime. World Drug Report 2012. United Nations Publication, New York (2018).

 are with the Henry C. Lee College of Criminal Justice and Forensic Sciences at the University of New Haven, in West Haven, Connecticut. 

 is with the Wisconsin State Crime Laboratory, in Milwaukee, Wisconsin. 

 is with Smiths Detection, in Edgewood, Maryland. Direct correspondence to: 

On-scene analysis of cocaine is an essential tool for law enforcement. This study defines how portable infrared and Raman spectrometers are capable of providing accurate and reliable identification of cocaine and other drugs when and where needed most.

A Comparison of Portable Infrared Spectrometers, Portable Raman Spectrometers, and Color-Based Field Tests for the On-Scene Analysis of Cocaine

Morphologically directed Raman spectroscopy (MDRS) is a novel and reliable tool that would enable criminalists to obtain more discriminatory information from forensic samples than their current capabilities. MDRS combines automated particle imaging and Raman spectroscopy into one instrument. Particle imaging is performed to determine particle size and shape distributions of components in a blended sample. Particle size is an important physical property of particulate samples and can be used in conjunction with Raman spectroscopy in the analysis of a range of samples of forensic interest, including illicit and counterfeit drugs, soils, gunshot residue (GSR), and white powders. Although measurement of particle size distributions is routinely carried out across a wide range of industries and is often a critical parameter in the manufacture and analysis of many products and substances, it is not widely used in the forensic sciences. Raman spectroscopy is used in forensic science to determine the molecular chemistry of materials because it is rapid, reliable, allows for analysis without contacting the sample, is nondestructive, and enables detection at low concentrations. Combining these two analytical techniques into a single platform allows the individual components present within a blend or mixture to be independently characterized and compared.

	Morphologically directed Raman spectroscopy (MDRS) combines the power of automated particle imaging with Raman microspectroscopy into a single platform. Particle imaging is performed to determine particle size and shape distribution of components in a blended sample. These are important physical properties of particulate samples and may have a direct influence on a sample's performance. For example, the size and surface area of a particle can be related in a significant way to the physical, chemical, and pharmacologic properties of a drug. Particle size distributions (PSDs) are routinely measured across a wide range of industries because they are important, and sometimes critical, to the manufacture and performance of substances and products. In spite of this importance, they are not widely used as methods for classification, identification, or individualization in the forensic sciences, other than in forensic soil examinations. However, characterization of materials by their microscopic morphology is used to analyze a plethora of forensic science samples, including drugs, geological materials (that is, soil minerals, rocks, and so forth), glass, dust, gunshot residue (GSR), pollen and diatoms, hairs and fibers, cosmetics, other anthropogenic materials (such as various building materials), general unknowns, and white powders used in hoax powder attacks. Raman spectroscopy is useful for determining molecular and physical chemistry because it is fast, reliable, nondestructive, and a noncontact method. Methods based on Raman spectroscopy are also used in the forensic sciences (1) for the analysis of many types of physical evidence including illicit drugs (2–5), explosives (6–8), paint (9–14), fibers (15–17), ink (14,18), and general unknowns (7). There are also several research articles on the use of Raman spectroscopy for the analysis of other types of forensic evidence, with a significant focus on analysis of the organic components of GSR (17,19–25).

	Independently, both automated particle imaging and Raman microspectroscopy are valuable methods. Even when combined into a single instrument, analysis based upon each method's independent evaluation may be useful. However, the power of a combined analytical scheme is greater than the sum of the individual approaches. Together, the data from these two methods may provide insight about the sample including its manufacturing method, history, and quality. This type of information may be invaluable during analysis of evidence in forensic casework.

	When performing MDRS, the sample's morphological data is collected using a light microscope with an automated stage. This allows for the sorting of particles based on various physical parameters. Once this particle data is collected, these parameters are then used to automatically select particles for chemical analysis using Raman microspectroscopy. The ability to perform particle selection for chemical analysis using physical parameters removes subjectivity in the measurement. In addition, the automation of the stage and of the particle selection removes the need to expose the analyst to the sample for the prolonged periods of time that would be required if manual measurements were performed (26).

	The combination of morphological and PSD analysis with Raman spectroscopy has the potential to provide unexploited information about a plethora of samples of forensic interest. This research examined the use of MDRS for the forensic analysis of four evidence types: illicit and counterfeit drugs, soils, GSR, and white powders. It is shown that MDRS can be used for the comparison of questioned and known samples, material, and mixture identification or classification as well as potential provenance determination.

	The experiments were performed using a Morphologi G3-ID particle imaging and Raman spectroscopy system (Malvern Panalytical). Samples were automatically dispersed onto a quartz plate at a pressure of 4 bar using the integrated Sample Dispersion Unit (SDU) on the Morphologi G3-ID instrument. Automated imaging analysis was performed directly on the quartz sample dispersion plate to obtain particle morphology data. All image acquisition settings were defined in a standard operating procedure (SOP) and included brightfield illumination and the use of a 10-times objective.

	Particles with a circle equivalent diameter (CED) greater than 7.0 µm and solidity greater than 0.75 were tagged for chemical targeting by the image analysis software. Morphological data was collected for more than 150,000 individual particles for each sample. Of these, 3000 particles per sample were targeted for Raman spectroscopic analysis over the spectral range of the instrument (150–1850 cm

). Raman spectra were obtained with the coupled Kaiser optical systems RamanRxn1 Raman spectrometer using a 785-nm semiconductor laser with a power of <500 mW and an exposure time of 2 s. The sampling spot size of the instrument was 3 µm. Both the particle morphology data and the Raman spectra were analyzed using the Morphologi software. All experiments were performed in triplicate.

	Spectral analysis required the creation of a spectral reference library, which was made using standards purchased from Sigma-Aldrich. The individual particle spectra from each sample were then compared against the reference spectral library and identifications made based on a correlation search algorithm where a correlation value close to one indicates a close match to that reference, and a value close to zero indicates no match. Using these correlation values, the particles were chemically classified and PSDs of the individual component populations were generated.

	A five-component mixture of illicit drugs was prepared (equal parts by weight) using cocaine, phenobarbital, pentobarbital, D-methamphetamine, and amphetamine.

	Two suspected synthetic cathinones, marketed as "bath salts," were purchased over the internet: "Arctic Rush" and "Fast Forward."

	Three counterfeit Viagra samples were obtained via the internet, where two were from Singapore and one was from India. Sampling was performed by removing a small amount of white powder from the core of each tablet, and all analyses were performed in triplicate.

	Soil samples were collected from four sites along one road in Connecticut (27). The 60–120 µm mineral fraction was separated by sieving and washing. Then 7 mm

 of minerals was deposited onto the quartz plate using evaporative dispersion. The mineral data were compared to a library made by importing the Raman spectra of minerals from the RRUFF database (28), and mineral identification was made only when there was a correlation score greater than 0.85.

	GSR was collected from cotton targets onto a quartz plate from four test fires using the same ammunition. The muzzle-to-target distances varied for each test fire, and included distances of 3, 6, 12, and 24 in.

	Three commercially available artificial sweetener blends were purchased locally, all with dextrose as the bulking agent but containing different sweetening agents. Sweet'N Low (designated as sample A) has saccharin as its sweetening agent, Splenda (Sample B) contains sucralose, and Equal (sample C) contains aspartame. An additional generic sucralose-based sweetener (NutraTaste Gold) was also purchased locally for comparison with the Splenda. Samples of the pure sweetener components were purchased from Sigma-Aldrich for addition to the spectral reference library.

	All five of the drugs were identified using MDRS. Further, the different components had different particle morphologies (Figure 1) and PSDs, which could be used in the comparison of samples from different seizures to evaluate whether they come from a common source. The particle size and shape of a substance can be useful for determination of the manufacturing process and, therefore, for comparative source attribution. Different methods of preparation can result in particles with different crystal structures and habits. Usually, slow crystallization methods form larger crystals, and rapid crystallization methods form smaller crystals.


	Figure 1: Particle images showing the different morphologies of (a) amphetamine, (b) D-methamphetamine, and (c) cocaine.

	The two suspected synthetic cathinones were analyzed by MDRS. The first, "Arctic Rush," did not contain any synthetic cathinones, and instead was composed of L-DOPA, caffeine, and an unidentified chemical. The presence of caffeine is a common cutting agent because it is a stimulant. L-DOPA is commonly used for the treatment of patients with Parkinson's disease, but there have been cases of people abusing L-DOPA as a means of enhancing the dopamine rush, which explains why it would be found in a mixture that people take to induce euphoria. It has been shown to increase aggressive behavior when taken in conjunction with methamphetamine (29). Inositol and phenethylamine were detected in the second, "Fast Forward." Inositol is a sugar alcohol that is commonly used as a cutting agent for narcotics (30). Phenethylamine has a similar chemical structure to synthetic cathinones, but there is a difference in one of the functional groups (31). Phenethylamine has a similar effect to that of synthetic cathinone, often ending in a state of excited psychosis or death (32).

	MDRS was able to identify four components in the counterfeit Viagra pills, including the active pharmaceutical ingredient (API) sildenafil citrate, and three excipients: lactose, starch, and talc. In an authentic Viagra sample, one would expect to see microcrystalline cellulose, calcium phosphate dibasic, croscarmellose sodium, and magnesium stearate. These were not detected in the counterfeit samples. Of particular interest is that the sample from India only contained the API and starch, while the two from Singapore also contained the lactose and talc (Figure 2). This has the potential to serve as a useful feature for comparison that could be used for counterfeit source attribution.


	Figure 2: A graph showing the relative percentage count of each of the components (lactose, sildenafil citrate, starch, and talc) in the three counterfeit Viagra samples, with the one from India in blue and the two from Singapore in green and red.

	The mineral morphologies of for each of the four samples were compared, and it was seen that they were all very similar. This is expected since they were collected along the same road in Connecticut. Other research has shown that the morphology of quartz can be used to differentiate different mineral environments (33–38), which could be exploited with MDRS. The PSDs for each location also proved to be indistinguishable.


	Table I: The average percent particle counts for each mineral for the four soil sample locations. The four locations can be differentiated because of the presence of some minerals in some sites that are not seen in the others, such as diopside in location C and epidote in location A, or because of differences in mineral counts, such as the higher percentage of muscovite in location B than the other sites.

	However, the samples for each location were differentiated when comparing the percent particle counts for each mineral (Table I). Multivariate statistical analysis was performed using R software (The R Foundation), using code written by N.D.K. Petraco, PhD, and adapted for this research by B.W. Kammrath, PhD. Principal component analysis (PCA) was performed on the particle count data, and in three principal components, 87.7% of the variance of the data was captured (Figure 3). Further analysis of the three-dimensional (3D) scores plot shows that there is good separation between the four data sets, with good clustering among replicates, which indicates the mineral counts achieved by MDRS can be used for sample discrimination.


	Figure 3: Three-dimensional scores plot of the autoscaled PCA results from the particle counts of the four soil locations. The three replicates cluster well for each of the four locations. 

Can morphologically directed Raman spectroscopy obtain more discriminatory information from forensic samples than current tools?

Brooke W. Kammrath

Articles

Portable Raman Spectrometers: How Small Can They Get?

A Comparison of Portable Infrared Spectrometers, Portable Raman Spectrometers, and Color-Based Field Tests for the On-Scene Analysis of Cocaine

Morphologically Directed Raman Spectroscopic Analysis of Forensic Samples