Portable Raman Spectrometers: How Small Can They Get?

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 cm³ (2.12 ft³). 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^-1 (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 cm³) 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.

How Small are These Spectrometers Today?

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 cm³, 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^-1 resolution over a range of 400–2300 cm^-1 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

Deep-UV Excitation

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 υ⁴ 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).

Stand-Off Detection

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).

Smartphone Spectroscopy

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^-1 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).

Photonic Integrated Circuits

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.”

Design Approaches

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^-1 and Raman shift coverage of ~3000 m^-1, ranging in volume from about 300 cm³ for just a spectrograph, to around 1700 cm³, 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^-1/pixel in a spectrograph with 2000 cm^-1 Raman shift coverage, giving an effective spectral resolution of ~12 cm^-1 (recognizing that resolution in cm^-1 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.

Expanding and Emerging Applications

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.

Conclusions

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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Richard Crocombe is the Principal at Crocombe Spectroscopic Consulting, specializing in miniature and portable spectroscopic instrumentation. Brooke W. Kammrath is a Professor at the University of New Haven and Assistant Director of the Henry C. Lee Institute of Forensic Science. Pauline Leary is a CBRNE SME at Noble, specializing in miniature and portable spectroscopic instruments. Direct correspondence to: racrocombe@gmail.com.