Formaldehyde is an indoor air pollutant that is widespread across many industries. The established implication of formaldehyde in respiratory conditions and its carcinogenicity pose a risk to millions of workers in the wood and textile industries. We have developed an optical sensor based on the evanescent wave absorption of light by the tailored formaldehyde-sensitive cladding of a modified commercial plastic optical fiber. This known spectroscopic technique, in combination with the inexpensive plastic optical fibers, has proven to be a powerful tool for formaldehyde monitoring. Sensor prototypes following that principle are being tested in two Spanish plywood board production plants. The manufactured fiberoptic sensors have been designed to meet the requirements of in situ, continuous, and unattended occupational FA monitoring with a detection limit of 250 μg/m3 (0.20 ppm).

Formaldehyde is a relevant indoor air pollutant that is widespread in many industries. The implication of formaldehyde in respiratory conditions and its carcinogenic potential pose a risk to millions of workers in multiple sectors, such as the wood, textile, construction, and automotive industries (1). Reducing the risks related to formaldehyde exposure can be economically demanding in terms of equipment and personnel (2). Although many portable formaldehyde sensors are available, reliable and automated detection systems for affordable continuous monitoring are scarce (3). Spectroscopic techniques can greatly assist in the development of powerful analytical tools for industrial process analysis and safety assurance (4–6). In particular, fiberoptic-based sensors offer valuable advantages for industrial chemical monitoring in terms of reduced weight and size, portability, low cost, and immunity to electromagnetic interferences (7). Fiberoptic evanescent wave (EW) interrogation (also called 

, or EFS) can easily be combined with any conventional optical analytical technique that is most convenient for a given application, such as fluorescence, ultraviolet, visible, near- or mid-infrared absorption, and Raman scattering spectroscopies (8).

A typical fiberoptic-based absorption EFS setup consists of a light source coupled to an analyte-sensitive fiber, the distal end of which is interfaced to a detector. In this setup, the light propagates from the light source to the detector through the sensing waveguide. The waveguide then responds to changes in the desired optical parameter (whether that is intensity, wavelength, or phase) (9). The diversity of optical fiber materials increases the options for chemical-sensing purposes. Plastic optical fibers (POFs) are attractive alternatives to the conventional multimode silica optical fibers because of their increased robustness, significantly lower prices, and ease of handling. Their only main drawback is that their main features come at the expense of higher attenuation (10).

Multimode fibers are based on a core–cladding structure. Light propagates through the core by means of total internal reflections at the core–cladding interface, as long as the refractive index of the cladding is lower than that of the core. Each internal reflection generates an EW that passes into the cladding. EWs are exponentially decaying electromagnetic fields that propagate perpendicularly to the core–cladding interface into the cladding medium. The actual penetration depth (

), on the order of the propagating light wavelength, depends on the latter (λ), the incidence angle at the interface (θ), and the refractive indexes of cladding (

Absorption-based EW fiberoptic sensors measure the color variations of the cladding produced by an incoming analyte that typically undergoes a color-changing reaction with an “indicator” dye embedded therein. Interrogation of the colorimetric reaction occurs through the EW propagating inside the cladding. In this way, the optical fiber itself acts as an optical cell. The amount of analyte that penetrates the cladding from the sample can be directly correlated to the power loss of the light traveling along the fiber at specific wavelengths, namely those within the absorption spectrum of the indicator dye (12).

For the detection of formaldehyde, we capitalized on the EW absorption changes undergone by the reduced form of fuchsin, a known colorimetric reagent for this aldehyde. The presence of formaldehyde in the fiber cladding is detected through the formation of a violet product (VP) as a result of the reaction between the leuco (“colorless”) form of fuchsin and formaldehyde (Figures 1a–b). The absorption changes of the manufactured FA-sensitive optical fibers are detected with a custom-made portable instrument with dedicated software, tailored to in situ, near-real time, unattended airborne FA monitoring in industrial facilities (Figure 1c). The initial analytical results of the sensor before its deployment in a plywood board production plant are reported herein.

The sensitive optical fibers were fabricated from a multimode step-index plastic optical fiber (PFU-FB-1000, Toray Raytela, Japan) of 1000 μm external diameter (980 μm poly[methyl methacrylate] core and 10 μm perfluorinated acrylate cladding) (13). Before the sensitive coating could be applied, the original cladding of the fiber was removed by chemical dissolution. To remove the original cladding, a mixture of 200 mL of a mixture of acetone (VWR International, HPLC grade), methyl isobutyl ketone (Alfa Aesar, 99.0%) and type I purified water (Direct-Q, Merck) (3:1:1 by volume) was placed in an evaporation dish containing 1 m of the optical fiber. After magnetically stirring for 45 min, the remaining cladding was manually removed with latex gloves.

Then, 40 cm of an uncladded optical fiber was hung vertically while a drilled rubber septum containing 500 µL of a Nafion D-521 dispersion in lower alcohols (Alfa Aesar, 5% w/w, >0.92 meq g

 exchange capacity) was extended manually around the fiber with a slow constant slide movement. The re-cladded fibers were allowed to ambient dry for 24 h.

The dye immobilization involved immersing each Nafion-coated fiber for 2 h in 50 mL of an aqueous solution containing 40 mL of a 0.25 g/L basic fuchsin (Fluka Analytical), 2.0 g of sodium bisulfite (VWR Eurolab, reagent grade), and 10.8 mL of phosphoric acid (Fisher Scientific, ≥85%) (such a mixture quantitatively generates the leuco fuchsin in situ). After 24 h contact, 37.5 cm segments of the fiber were cut, end polished, and fitted into a nylon-made opaque fiber chamber, designed and manufactured in our facilities (Figure 1d).

Formaldehyde-Sensitive Optical Fiber Calibration

The response to formaldehyde of the sensitive optical fibers was evaluated in air in the 0 to 6.7 ppm range (equivalent to 0 to 8.24 mg/m3), at atmospheric pressure, controlled temperature (25 ± 1 °C), constant relative humidity (RH) (50%) and flow rate (75 mL/min). The carrier air was obtained from cylinders (99.995%) supplied by Carburos Metálicos (Spain). The formaldehyde air streams were generated by concentrated gaseous formaldehyde (2.5 to 14 mL/min) with pure air (25 to 60 mL/min) and a 100% RH air stream generated in a water sparger (25 to 60 mL/min). Figure 2 illustrates this scheme.

The concentrated formaldehyde stream was produced by passing air (2.5–10 mL/min) through a sparger at 25.0 ± 0.5 °C (thermostatic bath) containing an aqueous formaldehyde solution (0.18 M) made by dilution of commercial FA (Sigma-Aldrich, 37.2% in water) with ultrapure water. Every gas stream was set individually with electronic mass-flow controllers (Bronkhorst F–211CV) with maximum flows of 15, 50, and 300 mL/min, respectively, controlled by PiD Eng&Tech Process@v.2.2.1.0 software.

Formaldehyde measurements were carried out using a ruggedized portable device designed and manufactured at our university facilities. The optoelectronic unit of the sensing device monitors the changes in the transmitted light through the indicator-cladded optical fiber from the ratio of the detected intensities at 575 nm to 455 nm every minute. Figure 1d shows the sensitive optical fiber inside the U-shaped sample chamber with standard SMA connectors for POF (Ratioplast Optoelectronics), connected to a white LED (STS-DA1-1479) enclosed in an ABS 3D-printed housing and to a compact mini-spectrometer (FLAME-S-VIS-NIR-ES, Ocean Insight) equipped with a 25 μm entrance slit (Figure 2a). Instrument configuration, calibration curves, and collected data are stored locally in the ruggedized sensor system CPU. The latter manages the described components by a dedicated software built on LabVIEW drivers (NI), developed in our research group. The acquired data, including formaldehyde concentration readings associated to the sensor location, can be accessed and downloaded remotely at any time via a Wi-Fi network. Our software allows the instrument to operate fully unattended as long as required, because it can trigger the sample acquisition automatically once the baseline is recovered after each measurement, upon regeneration with clean air (Figure 1c).

The transmission spectra of the formaldehyde-sensitive optical fibers were recorded initially to evaluate the effect of the embedded LF on the light emerging from the distal fiber end, with respect to a pure optical fiber. The effect of the formation of the violet product on the transmitted light was subsequently investigated. Figure 2b shows the transmission spectra of the sensing fiber in the absence and presence of formaldehyde. The decrease of the detected intensity in the 540 to 640 nm spectral region of the LF-containing fiber upon diffusion of the analyte into the cladding reveals the effect of the interaction of formaldehyde with the indicator dye. LF displays weak absorption in the 350 to 500 nm spectral region, but the violet product strongly absorbs in the 450 to 650 nm range. Figure 1b shows the absorption peaks at 575 nm. Therefore, when the violet product forms, the intensity of the light emerging from the distal end of the fiber decreases with respect to that in the absence of analyte resulting from the absorption of the evanescent wave at every internal reflection of the transmitted light.

The identification of the transmission loss region is key for successful EW monitoring using indicator-cladded optical fibers. However, the analytical signal must also be corrected for small fluctuations in the light source or differences in the installation or manufacturing of the various sensitive POFs. Therefore, we monitored the signal by ratioing the 575 nm main band to the 455 nm peak (both integrated over 5 pixels). The 455 nm peak is a region where both LF and VP absorb weakly and allow the underlying blue LED emission of the white diode to shine through.

Formaldehyde-sensitive optical fibers were calibrated in the 0 to 6.7 ppm concentration range at 25 ºC and 50% RH (automatic correction for the sample temperature and humidity will be implemented at a later stage). Every 15-min formaldehyde exposure cycle was followed by 1 h regeneration time with a formaldehyde-free air stream using the same flow, temperature, and humidity conditions for signal stability. Figure 3 depicts the response curve and dose-response plot of the sensing optical fibers. The calibration curve shows a nonlinear behavior between the analytical response (

) at low formaldehyde levels. Therefore, a power function (

b) was used to fit the experimental data points (

The sensing fiber repeatability was evaluated using a single optical fiber. Each formaldehyde concentration was measured five times and the corresponding relative standard deviations (RSD) of their analytical signals were compared (Table I). Detection limits were determined to be 0.25 mg/m3 (0.20 ppm) for the 15 min sampling time, a value lower than the current short term exposure limit (STEL) established by different regulations (for example, 2 ppm in the United States or 0.6 ppm in the EU). The sensor displayed good measurement repeatability (RSD <10%), but its sensitivity needs to be improved for accurate measurements below the long term exposure limit (for example, 0.3 ppm of EU Directive 2019/983). The good stability and repeatability of the formaldehyde-sensitive optical fiber sensor, obtained during 50 h monitoring periods, demonstrate consistent performance of the sensor over full working shifts to guarantee that the occupational exposure safety limits are never reached.

Evanescent wave absorption spectroscopy is a valuable analytical technique that can provide useful solutions outside of the laboratory, with compact rugged devices and minimum cost of consumables if plastic optical fibers are used. We have demonstrated a fiberoptic evanescent wave absorption sensor for continuous monitoring of airborne formaldehyde. The optoelectronic interrogation unit can be readily adapted to gas phase monitoring of other chemicals. We showed how optical fibers offer the required versatility to develop new analytical strategies in combination with the proper spectroscopic technique. Even though future commercial exploitation would still require further optimization of the sensing fibers to enhance their limit of detection, the developed formaldehyde-sensitive optical fibers clearly have analytical features suitable for routine monitoring of this chemical in industrial environments.

This project has been funded by the EU LIFE+ program, grant no. 16/ENV/ES/000232 (LIFESENSSEI). Helpful comments and advice from M. Madroñero (Quirón Prevención) and J. Quinoy (FINSA) are gratefully acknowledged.

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, G. Rajan, Ed. (CRC Press Taylor & Francis, New York, New York, 2015), Chapter 12.

G. Orellana, M.C. Moreno, J. López, and R. Chamorro, M.A. Alba, Span. Pat. ES2424772.

is a PhD candidate at the Complutense University of Madrid (UCM) Chemical Optosensors & Applied Photochemistry Group (GSOLFA) in Madrid, Spain. 

 is a postdoctoral fellow with UCM’s GSOLFA. 

 is a full professor in the Analytical Chemistry Department at UCM. 

 is a senior occupational hygienist at Quirón Prevención SLU in Barcelona, Spain. 

is a full professor and the GSOLFA group leader at UCM. Direct correspondence to: 

Articles

Featured Articles

An inexpensive fiberoptic-based formaldehyde field sensor is described for monitoring low-levels of formaldehyde, a widespread indoor air pollutant, based on the principle of evanescent wave absorption of light. Sensor prototypes following that principle are being tested in two plywood board production plants.

Fiberoptic Formaldehyde Field Sensors for Industrial Environments: Capitalizing on Evanescent-Wave Spectroscopy

This article provides a set of tables summarizing the most prominent handheld and portable instruments widely applied for field and outside the laboratory analysis. Techniques covered in this article include X-ray fluorescence (XRF), laser-induced breakdown spectroscopy (LIBS), infrared (IR) spectroscopy, near-infrared (NIR) spectroscopy, Raman spectroscopy, ultraviolet, and visible spectroscopy.

The article summarizes this information by providing six tables describing the available handheld and portable instrumentation for atomic spectroscopy techniques, including X-ray fluorescence (XRF) and laser-induced breakdown spectroscopy (LIBS), as well as molecular spectroscopy techniques including ultraviolet (UV), visible (vis), infrared (IR), near-infrared (NIR), and Raman spectroscopy. Each table includes a generalized description of the technique, the technical design and specifications features, a summary of typical dimensions for the smaller footprint instruments, and a list of applications where the various instruments are primarily used. The specific applications surveyed include agriculture, archaeometry, art, biomedical, chemicals, environmental, food, forensics, materials, pharmaceuticals, and polymers, which makes sense because these are the main application areas that lend themselves to in situ analysis in either the field or a manufacturing plant. For each category, references are listed in the reference section. General references include some original inventions as well as general references for each specific technology or spectroscopic technique.

Recent History of Handheld and Portable Instruments

Excellent resources are available on the various topics described in this article, including a recent review on the subject of portable spectroscopy (P1), and a set of volumes describing the detailed theory, instrument design, methods, and applications of molecular spectroscopic techniques (P2).

The tables in this article describe the major attributes for portable atomic spectroscopy methods (Tables I and II) and for the predominant portable molecular spectroscopy methods (Tables III–VII).

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This article provides a convenient summary of portable and handheld spectroscopy techniques, outlining the most prominent types of compact instruments for both atomic and molecular spectroscopy.

A Brief Survey of Handheld and Portable Instruments Used in Spectroscopy

Successful adoption of graphene materials requires simplified techniques that can provide representative, atline data to ensure the quality and repeatability of high-volume manufacturing processes. A new Raman spectroscopy sampling technique has been applied to the characterization of batches of graphene product that provides a simple, atline method for obtaining key product data. The technique is simple to operate and does not require highly skilled operators. Atline analysis with a portable Raman spectrometer is used to monitor every batch of graphene product manufactured by a graphene supplier, confirming that the products are low defect, high-aspect ratio graphene platelets. The analysis of the 2D band indicates that the average graphene platelet is <5 layers thick.

Graphene has gained a large amount of attention in the scientific and engineering communities and is currently being explored extensively for its properties, such as high strength and high electrical and thermal conductivity (1,2). Graphene has been proposed as a highly suitable material for memory chips, batteries, touchscreens, composites, sensors, barrier films, and even medicine and drug delivery systems (3,4).

Currently, graphene materials are being applied to industrial markets for rubbers and composites, fire retardancy products, construction materials, and energy storage (5). Standard methods for the characterization of graphene materials support the establishment of internationally accepted nomenclature and categorization of graphene-like materials (6). Much analysis of graphene requires applying sophisticated techniques only available at research and speciality measurement laboratories. For successful adoption of graphene materials in industrial applications, there is a need for simplified techniques that can provide representative, atline data to ensure the quality and repeatability of high-volume manufacturing processes. In the study presented here, a new Raman spectroscopy sampling technique has been applied to the characterization of batches of graphene product that provides a simple, atline method for obtaining key product data.

Raman spectroscopy is a tool that allows for effective identification of the sp2-carbon lattices found in graphite, graphene, and carbon nanotubes. Raman spectroscopy provides chemical and structural information based on the vibrational and rotational modes of a molecule. The samples are irradiated with a monochromatic laser and emit inelastically scattered radiation, which is then collected by the spectrometer. Raman spectroscopy has been widely used for characterizing carbon allotropes, and the technique has been valuable for the identification and quantification of these materials.

Raman spectroscopy is recommended for the analysis of graphene materials by the National Physical Laboratory (NPL) and the British Standards Institute (BSI) (7,8). The identity of graphene in Raman spectra is determined by the presence of a disorder (D) band at 1350 cm

 (9). The intensity ratio of the D and G bands (ID/IG) measures the level of defects and the 2D band indicates platelet thickness. The presence of multilayer graphene in more than 10 layers or graphite is identified by a shoulder in the Raman spectrum at 2720 to 2740 cm

Figure 1 shows the differences within the 2D bands related to the quantity of the layers present. A band at 2720 to 2740 cm

 is clearly evident in the graphene materials with 5 and 10 layers. The spectrum of the 10-layer material is very similar to that of graphite.

Recent research on electrochemically exfoliated graphenes demonstrates the challenges in interpreting Raman data, including the need for representative sampling and the important impact of sample treatment prior to analysis (10).

Raman analysis of graphene products has traditionally been performed using confocal Raman microscopy. Although Raman microscopy is a useful and effective technique for research laboratories because it provides information on a microscopic scale, it is less useful for quality control and atline testing. The data cannot be considered representative of bulk products and a highly skilled operator is typically required. A new method has been developed that utilizes a portable Raman spectrometer with a large-spot, remote sampling accessory. This method enables rapid, representative analysis of graphene products directly at the manufacturing plant, ensuring excellent batch-to-batch quality control.

PureGraph graphene powders were provided by First Graphene Ltd. The PureGraph products are high aspect-ratio graphene nanoplatelet powders manufactured by the electrochemical exfoliation of Sri Lankan vein graphite. Samples of graphene powders were analyzed and received without any further sample preparation or manipulation. Sample powders were placed directly onto glass specimen holders. The samples were tested using a B&W Tek i-Raman Plus 532H system with a fiber-optic probe at 30 mW laser power at the sample and 60-s integration time with four averages (11). Samples were analyzed in an enclosure during data acquisition. The results are displayed with the anti-biasing and Savitzky-Golay smoothing algorithm.

Figures 2 and 3 show the spectra obtained for the graphene products and vein graphite raw materials. Spectra of the PureGraph samples show prominent D, G, and 2D bands present at 1350 cm

 respectively. An approach based on published methods was used to analyze the level of defect in the graphene structure by examining the ratio between the D and the G bands. The G band is a result of in-plane vibrations of sp2-bonded carbon atoms whereas the D band results from out-of-plane vibrations attributed to the presence of structural defects. The lower the value of the ID/IG ratio, the greater the quality (more graphene-like properties) of the graphene sample. Some major causes of defects are: the presence of vacancies as sp3-carbon, the presence of graphene oxide, the presence of amorphous carbon, and of course, the platelet edges.

The samples show a low defect ratio (ID/IG) of 0.17 for 5 (red) and 10 (green), and 0.12 for 20 (blue). The low values of the ID/IG for PureGraph are indicative of a low level of defects in the product. This is consistent with independent oxygen analysis of the PureGraph products performed by micro-elemental analysis, which shows typical bulk oxygen levels between 1 and 2% w/w.

Figure 4 shows an overlay of PureGraph samples against graphite raw material. The samples clearly show the absence of the graphitic band at 2720 to 2740 cm

. The 2D band of PureGraph products is indicative of fewer than 5 carbon layers (8). The Raman spectroscopy results indicating few-layer graphenes are consistent with microscopy analysis, supporting transmission electron microscopy images of the PureGraph 5 product are shown in Figure 5.

Atline analysis of graphene can be performed by portable Raman spectroscopy with simple equipment and no pretreatment, giving instant and representative batch data. The technique is simple to operate and does not require highly skilled operators. Atline Raman spectroscopy analysis is currently used to monitor every batch of graphene manufactured by First Graphene Ltd. Portable Raman spectroscopy confirms that the products examined in this study are low defect, high-aspect ratio graphene platelets. The analysis of the 2D band indicates that the average graphene platelet is <5 layers thick.

K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Grigorieva, and A.A. Firsov, 

C. Ahn, S.W. Fong, Y. Kim, S. Lee, A. Sood, C.M. Neumann, M. Asheghi, K.E. Goodson, E. Pop, and H.S.P. Wong, 

. firstgraphene.net/applications/ (accessed October 2020).

International Organization for Standardization (ISO), 

ISO/TS 80004-13:2017, Nanotechnologies — Vocabulary — Part 13: Graphene and related two-dimensional (2D) materials

A.J. Pollard, K.R. Paton, C.A. Clifford, E. Legge, A. Oikonomou, S. Haigh, C. Casiraghi, L. Nguyen, and D. Kelly, 

Characterisation of the Structure of Graphene

PAS 1201:2018 Properties of Graphene Flakes – Guide BSI

 (British Standards Institute, London, United Kingdom, 2018)

A.C. Ferrari, F. Bonaccorso, A. Lombardo, T. Hasan, Z. Sun, and L. Colombo, 

V. Nagyte, D.J. Kelly, A. Felten, G. Picardi, Y. Shin, A. Alieva, R.E. Worsley, K. Parvez, S. Dehm, R. Krupke, S.J. Haigh, A. Oikonomou, A.J. Pollard, and C. Casiraghi, 

 are with First Graphene Ltd. Direct correspondence to: 

Graphene exhibits special properties, such as high strength and high electrical and thermal conductivity and as such is highly desirable for key electronic components. A new Raman spectroscopy sampling technique has been applied to the characterization of batches of graphene that provides a simple, at-line method for obtaining key product data.

Atline Analysis of Commercial Graphene Products with Raman Spectroscopy

Raman spectroscopy is now a first-choice process analytical technology for several reasons. Within a short time frame, Raman spectroscopy has transitioned from an esoteric laboratory technique to a practical industrial tool in laboratory and manufacturing applications. We highlight important technology milestones in Raman spectroscopy’s history, modern approaches to sampling probes, and the role of instrument accuracy in analytical model development. Applications of Raman spectroscopy in process control and understanding its fundamentals form the basis for expanding Raman spectroscopy in new applications and integrating Raman spectroscopy into new process automation strategies.

Raman spectroscopy is an optical spectroscopy technique based on inelastically scattered light. The resulting Raman spectrum provides a “molecular fingerprint” of the sample, delivering highly specific information about chemical composition and molecular structure without sample preparation. We provide an overview of Raman spectroscopy for industrial applications.


The specificity of a Raman spectrum is powerful because it provides a chemical fingerprint of a sample, thus allowing a qualitative assessment of a material’s chemical composition and molecular structure. From the first reports of the Raman effect in 1928 until the early 1970s, technology for certain components, such as lasers and scientific cameras, enabled Raman spectroscopy to be used for academic laboratory studies in geology, biology, and chemistry fields. Although these developments in Raman spectroscopy represented a significant advance over original instrumentation, Raman spectroscopy instrumentation was still complex, inefficient, and difficult to operate. One reason for this difficulty was the inefficiency and instability of optical components such as filters and wavelength separators. Until the early 1990s, it was a challenge for Raman spectroscopists to simply measure a usable Raman spectrum. However, the academic community consensus was that the challenges of using Raman spectroscopy were worthwhile to get the composition and molecular structure information unobtainable via other spectroscopy techniques. In the 1990s, however, process and quantitative applications were limited because of instrument instability.

The standardization of optical components represented the next generation of enabling technologies for Raman spectroscopy outside the academic laboratory. Standardizing optical components and simplifying instrument design in the 1980s and 1990s resulted in the first generation of commercially available Raman instruments for microscopy and benchtop applications. Quantitative Raman spectroscopy for industrial applications became more practical during the 1980s and 1990s. Since then, additional improvements in optical components, laser stability, integrated spectrograph design, and fiber-optic sampling probes have helped make Raman spectroscopy a practical technology outside the laboratory. Today, Raman spectroscopy can be applied in various research settings. These include portable, industrial, environmental, and clinical research settings. In industrial process applications, Raman spectroscopy is a proven process analytical technology (PAT) in pharmaceutical, bioprocessing, chemical, and food applications.

As a PAT, Raman spectroscopy can be integrated into an in-process or manufacturing site in several ways.


Figure 1 shows the differences between inline, online, at-line, and offline installations. There are many factors that affect choices about the type of installation, such as process chemistry, cycle time, presence of hazardous conditions at the site, skill level of the operator, and application needs.

Several excellent in-depth discussions of Raman spectroscopy instrumentation fundamentals have been published (1–3). Briefly, instrumentation used to collect a Raman spectrum include a light source, a sampling optic, a signal collection optic (interchangeably called a 

), and a low-noise detector. The light source, typically a laser, is delivered to the sample either by free-standing optics, a microscope, or a fiber-optic sampling probe. A spectrograph based on holographic volume technology enables simultaneous collection of all wavelengths without moving parts, resulting in a fast, robust, and reliable instrument. The use of non-moving parts in the spectrograph imparts the advantage of consistently high spectral performance. This consistency allows the user to perform calibration transfer from instrument to instrument seamlessly and prevented calibration rework if there is a laser or
detector change.

Sampling from Nanoscale to Megaton Production

One benefit of Raman spectroscopy is sampling flexibility. For industrial uses, sampling is available in a microscope, sample chambers, or fiber-optic probes for immersion sampling or through sight glass. At the nano- and micrometer scale, Raman microscopy is a valuable laboratory instrument because it provides detailed chemical information at high spatial resolution (4). Commercially available Raman microscopes, first introduced in the early 1970s, enabled point spectroscopy and imaging studies in academic research laboratories in biomedical, chemical, materials, semiconductor, and pharmaceutical fields (5–8). Raman microscopy applications have continued to grow and have expanded into the nanoscale. Tip-enhanced Raman spectroscopy (TERS) incorporates a metallic atomic force microscopy (AFM) needle into a Raman microscope. With TERS, Raman spectroscopy on the nanometer scale can be achieved (9,10). Surface-enhanced Raman spectroscopy (SERS) is another enhancement approach that can increase Raman signal 102 to 106 times and provide detailed chemical information (11,12). New substrates for SERS have expanded its application from the analysis of single molecules to biomarkers. In addition to enhancement mediated through metal-substrate plasmonics, as in TERS and SERS, coherent Raman spectroscopy approaches such as coherent anti-Stokes Raman spectroscopy and stimulated Raman spectroscopy can provide rapid and high-quality signals for imaging or spectroscopy applications (13,14). These nano- and micro- scale approaches are undergoing a resurgence of interest because of the development of new plasmonic materials, the extension of the techniques to biological and clinical applications, and commercial interest. These approaches are used primarily in the laboratory across many industries. Industry applications of Raman nanoscopy and microscopy can include testing of multilayer films, understanding degradation mechanisms, quality control, and high-resolution imaging in melts, solid dispersions, or polymers.

In environments outside the laboratory at larger scales, fiber-optic probes provide a convenient and reliable sampling format. Sampling probes are useful because they can be directly immersed into a process, into a slip stream, or outside a sight glass. The use of visible or near-infrared fiber optics means that the probe can be placed in a process that is meters away from the Raman spectrograph. Therefore, the probe can stay inside the process media, and Raman spectra can be recorded automatically without needing to physically collect a sample for laboratory measurement.

Fiber-optic sampling probes are a key technology enabling process Raman spectroscopy for many reasons. One benefit of using sampling probes is that many industrial processes pose contamination and safety risks when manually collecting a sample for laboratory analysis. For example, in the chemical industry, processes may involve toxic chemicals or hazardous conditions, increasing the safety risks of manual sampling (15). In upstream bioprocessing, manual sampling increases the risk of contaminating the sterile cell culture.

Another reason that sampling probes are beneficial is because they can measure in solids, liquids, gases, or turbid media without requiring specialized adaptors or sample preconditioners. Its ability to maximize signal collection in each phase of matter requires a fundamental understanding of photon migration and the physical conditions at the probe–process interface before building specialized probes based on that understanding.

Chemical analysis of solid or turbid media requires special consideration to capture the spatial heterogeneity of the layers or bulk product. Raman spectroscopy is widely used to analyze solid heterogeneous samples because it provides detailed chemical information and requires no sample homogenization, allowing for bulk or cross-sectional analysis in a spatially heterogeneous sample. Some examples of solid or turbid materials monitored by process Raman spectroscopy include pharmaceutical tablets, slurries, graft copolymers, and processed foods. For these materials, their optical scattering properties are important to understand because they have an impact on the measured sampling volume (16).

Understanding the effects of optical scattering on Raman signal recovery guides the design and selection of a fiber-optic probe. There are applications that should harness the effects of optical scattering to obtain a representative sample or minimize the effects of optical scattering from bubbles or particulates. Figure 2 shows the principle of photon scattering through a layered solid system and how fiber geometries can be tailored to optimize collection of a surface or subsurface Raman signal.

Figure 3 shows commercially available variants of sampling probes for Raman spectroscopy using optical fibers. A commonly used approach is backscattered Raman spectroscopy, which is highlighted in Figure 2. In backscattered Raman spectroscopy, incident and Raman-scattered light share the same optical path. This geometry allows the collection of a small volume. In the layered system shown in Figure 2, a backscattered probe will collect a signal from the surface layer. This type of probe geometry can be applied toward measurement in liquids, especially to reduce potentially deleterious effects of bubbles, turbidity in the liquid, or immiscible liquids. This type of probe can also be used to measure surfaces of solids.

To obtain signals from a larger volume, collection fibers can be offset from the excitation fiber. Large volumetric Raman spectroscopy uses a wide laser beam and multiple collection fibers to obtain data from surface and subsurface layers. Large volumetric sampling is a useful approach for in-process measurements of content uniformity or processing solids and turbid media. Spatially offset Raman spectroscopy (SORS) and transmission Raman spectroscopy utilize a single-point excitation with spatially offset collection fibers. In SORS, the fibers are typically in the same plane, and in transmission Raman spectroscopy, the collection fibers are typically 180° from the excitation fiber. These probes are used in solids applications ranging from through-container raw material identification to in-process quality assurance of processes involving solids or turbid media such as blending. Together, these four variants provide sampling flexibility from early development to manufacturing, in applications from laboratory quality assurance (QA) to in-process monitoring, testing, or control.

There are many approaches to modeling spectroscopic data with either a univariate or multivariate model. Multivariate models are more commonly used, and these models can incorporate first principle bounds in the model (known as 

) or be based on correlation or covariance without any a priori knowledge (known as 

) (17). One important consideration for industrial applications is model transferability. 

 can refer to a) transfer across different instruments that measure the same process, or b) expanding the scope of an existing model to account for changes in process conditions, such as pressure, temperature, media composition, and reactor size. An excellent 2018 review on calibration transfer discusses how hardware design affects model transfer (18). Hardware built for high-accuracy with a uniform internal design, but with housing configurable to the installation environment, has a few advantages, including reproducible cross-instrument performance, compatibility with “turn-key” sampling probes, and ease of servicing. This flexible design allows the user to perform calibration transfer from instrument to instrument and prevents calibration rework in the event of subsampling, or a laser or detector change. Another important consideration is the choice of probe. Probe selection impacts representative sampling and model development, and certain probe optics may adversely affect model transferability or robustness in turbid media and solids (19,20).

Raman spectroscopy has been successfully applied to upstream biopharmaceutical development in industrial settings since 2010 (17). Whether for cell culture or fermentation bioprocesses, in batch or continuous mode, Raman spectroscopy has many proven benefits, including the ability to simultaneously measure nutrients, metabolites, and cell viability. Raman spectroscopy also allows for the cross-scale method transfer from the laboratory to a current good manufacturing practice (cGMP) environment without significant method adjustments (21–23). Recent studies have demonstrated that Raman spectroscopy-based feedback control of glucose feeding or lactate accumulation in cell cultures has benefits of improved monoclonal antibody product quality and increased titer (24–26). Feedback-based control was also achieved in highly fluorescing cell culture and fermentation applications using wavelengths closer to shortwave infrared (1–1.4 μm) (27,28). More recent work has further expanded upstream monitoring to amino acids and titer predictions (29,30). In downstream applications, Raman spectroscopy can be equally powerful and we have seen recent successful cases of Raman spectroscopy used to quantify protein concentration and to monitor chromatography-based purification and crystallization (31,32).

As performance demands on polymeric materials have increased, polymerization processes have become more complex and often require tighter control of process parameters. Enabling mega-volume processes that can consistently make a quality product safely have increased the demand for PAT with high uptime that enables real-time, closed-loop process control. An example of successful polymer analysis by Raman spectroscopy is polystyrene, an important polymeric material. Figure 4 shows a Raman spectrum of polystyrene. What should be noted about the spectrum is that the peaks correspond to known chemical moieties, and they are sharp, identifiable, and quantifiable, which makes robust process monitoring and control possible. Styrene polymerization can be easily quantified using the area or intensity ratio of the styrene vinyl bond at 1630 cm

Raman spectroscopy-derived concentration predictions closely match measurements carried out by gravimetry, but without the need for sample extraction. The work reported by Brun’s team indicates that Raman spectroscopy could be part of a control strategy to avoid process upsets and ensure quality of a polymer product (33). Specifically, these results demonstrate the utility of Raman spectroscopy for monitoring the polymerization of styrene into polystyrene. The data were able to be generated quickly (in a matter of seconds), and they corresponded well with data obtained by classical gravimetric methods that are known to give good results but are also cumbersome and time-consuming.

Csontos’s group developed an effective feedback control loop based on Raman spectroscopy for a hazardous exothermic oxidation reaction (34). They were able to determine the end point of the process reaction, following the quantification of reaction components and an unstable intermediate. This was another successful example of Raman spectroscopy giving visibility into an industrial chemical reaction allowing for real-time control of the overall process. Similarly, Hart’s group was able to develop an accurate Raman spectroscopy calibration model for end-point determination of an etherification, where the residual level of chloropyrazine starting material needed to be minimized (35). At the outset, they were aware the reaction may be scale dependent because a heterogenous base (K

) was used. They successfully demonstrated that the calibration work for reaction end-point determination done at the laboratory scale was easily transferred to the scaled-up pilot plant, with predicted results closely matching manual off-line high-performance liquid chromatography results.

Raman spectroscopy has also been used to improve our understanding of the reaction kinetics of many processes, such as mechanochemical Knoevenagel condensations (36). In one study, crystallization properties were examined for the condensation of three fluorinated benzaldehyde derivatives and malononitrile by Hart’s group using in situ
Raman spectroscopy.

As we continue to find ways of dealing with climate change, Raman spectroscopy will play an increasingly significant role in controlling and optimizing industrial processes that involve greenhouse gases like CO

. Recently, Jinadasa’s group demonstrated the ability of Raman spectroscopy to effectively monitor speciation of a CO

capture process for both the lean and rich amine streams (37). Other chemical processes involving carbon chemistry, such as the common Suzuki-coupling reactions of the C–C bond, can also benefit from Raman spectroscopy through yield optimization. In this scenario, parameters like temperature and concentration can be varied independently within reaction limits to find optimal operating conditions. In doing so, Heteni and Janagap were able to develop a robust partial least squares (PLS) model using their experimental Raman spectra, and online predictions from the model agreed well with offline gas chromatography–mass spectrometry (GC–MS) measurements of reaction yield (38).

Raman has been used to examine crystallization and the solid state form of small-molecule active pharmaceutical ingredients (API) since the 1980s (39). As in many industries, applications of Raman spectroscopy in small-molecule pharmaceuticals have surged in the past three decades as technology has improved. Because of these technological advances, Raman spectroscopy is now a practical analysis technique in the pharmaceutical laboratory and process environments. As a PAT in API manufacturing, Raman spectroscopy has demonstrated value from scientific understanding to process control. A recent paper by Nagy’s group highlights the industrial trend for using Raman spectroscopy for real-time monitoring and control for secondary processing batch steps (40). We are enthusiastic about recent reports in continuous solids monitoring and solid phase unit operations, including in situ control of crystallization and polymorphism, process-induced transformations, low-dose formulations, and tablet coating. Automated Raman spectroscopy measurements are compatible with the fast cycle times and can provide process understanding for continuous solids production approaches, including twin-screw granulation and hot melt extrusion (41,42).

The food and beverage industry works in a highly regulated environment with strict controls on quality and safety. In this respect, there are similarities between food and beverage production and biopharmaceutics and small molecule API manufacturing. Although the PAT framework has been successfully used in pharmaceutical and biopharmaceutical manufacturing since 2004, its application has lagged in the food and beverage industry until recently. A more current interpretation of PAT in food and beverage is that it represents “a silent revolution in industrial quality control in food processing” (43). As a result, there are more reports of Raman spectroscopy in food processing (44,45). One application area that is particularly benefitting from Raman spectroscopy is meat and fish processing. Meat and fish quality is based on taste, texture, and appearance. All of these quality attributes have a basis in the specimen’s chemical properties, making Raman spectroscopy well-suited to providing a fast, non-destructive, and multiattribute quality measurement (46). Table I shows an overview of Raman spectroscopy-provided laboratory measurements for meat or fish quality attributes.

Laboratory studies demonstrate that Raman spectroscopy can provide both a fast and effective analytical method for determining iodine value, polyunsaturated fatty acids, monounsaturated fatty acids, and saturated fatty acids in samples of pork back fat (47–49). Even in a highly sophisticated meat processing plant, a fair amount of byproducts are generated. One strategy to minimize by-product waste is to use it as input for creating other valuable materials such as hydrolyzed proteins, and new research is showing the promise of Raman spectroscopy-based feedback control in this application (50,51).

Raman spectroscopy is a valuable PAT for many applications across multiple industries. The molecular specificity, ability to be directly coupled to a reaction vessel, and compatibility with solids, liquids, gases, and turbid media are advantages of Raman spectroscopy. Application success and rapid return-on-investment are realized in Raman spectroscopy-based feedback control because it enables in-process corrections and ensures process and product quality. The established history of process Raman spectroscopy in molecular identification, quantification, and process monitoring form the basis for continuing to expand Raman spectroscopy to new applications. We are enthusiastic about bringing Raman spectroscopy even further into industry through the adoption of industry 4.0 principles in PAT, increased automation, and even more integration of Raman spectroscopy into process control applications.


We thank Dr. Linda Kidder and Dr. Casey Kneale for helpful discussions during manuscript preparation.

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CRC Press, London, United Kingdom, 2001).

, K.A. Bakeev, Ed. (Blackwell Publishing, Oxford, United Kingdom, 2005).

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Raman, Infrared, and Near-Infrared Chemical Imaging

, Y.O. Slobodan Šašić, Ed. (John Wiley & Sons, Inc., New York, New York, 2010), pp. 109–131.

Raman spectroscopy is a valuable process analytical technology (PAT) for many applications across multiple industries, as a result of its many advantages, such as molecular specificity, ability to be directly coupled to a reaction vessel, and compatibility with solids, liquids, gases, and turbid media.

Spectroscopy Outside The Laboratory

Fiberoptic Formaldehyde Field Sensors for Industrial Environments: Capitalizing on Evanescent-Wave Spectroscopy

A Brief Survey of Handheld and Portable Instruments Used in Spectroscopy

Atline Analysis of Commercial Graphene Products with Raman Spectroscopy

Making Industrial Raman Spectroscopy Practical

Raman Spectroscopy: Bringing Inline Analysis to Production