Sensor for Rapid In-Field Classification of Cannabis Samples Based on Near-Infrared Spectroscopy.

A rugged handheld sensor for rapid in-field classification of cannabis samples based on their THC content using ultra-compact near-infrared spectrometer technology is presented. The device is designed for use by the Austrian authorities to discriminate between legal and illegal cannabis samples directly at the place of intervention. Hence, the sensor allows direct measurement through commonly encountered transparent plastic packaging made from polypropylene or polyethylene without any sample preparation. The measurement time is below 20 s. Measured spectral data are evaluated using partial least squares discriminant analysis directly on the device's hardware, eliminating the need for internet connectivity for cloud computing. The classification result is visually indicated directly on the sensor via a colored LED. Validation of the sensor is performed on an independent data set acquired by non-expert users after a short introduction. Despite the challenging setting, the achieved classification accuracy is higher than 80%. Therefore, the handheld sensor has the potential to reduce the number of unnecessarily confiscated legal cannabis samples, which would lead to significant monetary savings for the authorities.

Diffraction-limited hyperspectral mid-infrared micro-ellipsometry.

The recent introduction of quantum cascade lasers (QCL) in infrared spectroscopic ellipsometry led to decisive improvements in measurement times and signal-to-noise ratios of this powerful analytical method. In this contribution, we present another significant enhancement leading to the first, to the best of our knowledge, diffraction-limited micro-ellisometry setup in the mid-infrared spectral range with a spatial resolution better than 13.3 µm. The fast spectral tunability of the QCL combined with phase-modulated polarization enabled simultaneous acquisition of broadband (900 cm-1-1204 cm-1) high-resolution (1 cm-1) hyperspectral Ψ, Δ-cubes in a scanning approach in reasonable time scales. The spatial resolution of the QCL micro-ellipsometer was experimentally characterized by the knife-edge method and measurements of a resolution test target. Furthermore, the hyperspectral ellipsometric investigation of a polymer multilayer cross section and the portrait window of a 200-euro bank note demonstrate the capabilities of diffraction-limited QCL micro-ellipsometry.

Real-Time Monitoring of a Sol-Gel Reaction for Polysilane Production Using Inline NIR Spectroscopy.

The sol-gel process is an effective method for the preparation of homogeneous structured nanomaterials whose physico-chemical properties strongly depend on the experimental conditions applied. The control of a three-component reaction with silanes showing multiple reaction sites revealed the need for an analytical tool that allows a rapid response to ongoing transformations in the reaction mixture. Herein, we describe the implementation of near-infrared (NIR) spectroscopy based on compact, mechanically robust, and cost-efficient micro-optomechanical system technology in the sol-gel process of three silanes with a total of nine reaction sites. The NIR-spectroscopically controlled reaction yields a long-time stable product with reproducible quality, fulfilling the demanding requirements for further use in coating processes. 1H nuclear magnetic resonance measurements are used as reference values for the calibration of a partial least squares (PLS) regression model. The precise prediction of the desired parameters from collected NIR spectroscopy data acquired during the sol-gel reaction proves the applicability of the calibrated PLS regression model. The determined shelf-life and further processing tests verify the high quality of the sol-gel and the produced highly cross-linked polysilane.

Advances in mid-infrared spectroscopy enabled by supercontinuum laser sources.

Supercontinuum sources are all-fiber pulsed laser-driven systems that provide high power spectral densities within ultra-broadband spectral ranges. The tailored process of generating broadband, bright, and spectrally flat supercontinua-through a complex interplay of linear and non-linear processes-has been recently pushed further towards longer wavelengths and has evolved enough to enter the field of mid-infrared (mid-IR) spectroscopy. In this work, we review the current state and perspectives of this technology that offers laser-like emission properties and instantaneous broadband spectral coverage comparable to thermal emitters. We aim to go beyond a literature review. Thus, we first discuss the basic principles of supercontinuum sources and then provide an experimental part focusing on the quantification and analysis of intrinsic emission properties such as typical power spectral densities, brightness levels, spectral stability, and beam quality (to the best of the authors' knowledge, the M2 factor for a mid-IR supercontinuum source is characterized for the first time). On this basis, we identify key competitive advantages of these alternative emitters for mid-IR spectroscopy over state-of-the-art technologies such as thermal sources or quantum cascade lasers. The specific features of supercontinuum radiation open up prospects of improving well-established techniques in mid-IR spectroscopy and trigger developments of novel analytical methods and instrumentation. The review concludes with a structured summary of recent advances and applications in various routine mid-IR spectroscopy scenarios that have benefited from the use of supercontinuum sources.

Mid-infrared DMD-based spectral-coding spectroscopy with a supercontinuum laser source.

We present a mid-infrared spectroscopic system based on a spectral-coding approach enabled by a modified digital micromirror device (DMD). A supercontinuum source offering a confined mid-infrared laser beam is employed to perform gas measurements with this system. The performance, flexibility, and programmability enabled by the DMD is experimentally demonstrated by gas-cell measurements (CO2, CH4, N2O, NO2 and CO). Full spectra are acquired in 14 ms at 10 nm spectral resolution and in 3.5 ms at 40 nm spectral resolution. Further, we employ the system for stand-off open-path spatially resolved CO2 measurements that fully exploit the laser emission properties - the bright and highly-collimated supercontinuum beam is scanned by a galvo mirror over a retroreflector array at a scalable remote distance. The measurement concept models a passing gas emitter under lab conditions; time and spatially resolved CO2 absorbance gas-plume images in the mid-infrared range are obtained.

Spectral-Coding-Based Compressive Single-Pixel NIR Spectroscopy in the Sub-Millisecond Regime.

In this contribution, we present a high-speed, multiplex, grating spectrometer based on a spectral coding approach that is founded on principles of compressive sensing. The spectrometer employs a single-pixel InGaAs detector to measure the signals encoded by an amplitude spatial light modulator (digital micromirror device, DMD). This approach leads to a speed advantage and multiplex sensitivity advantage atypical for standard dispersive systems. Exploiting the 18.2 kHz pattern rate of the DMD, we demonstrated 4.2 ms acquisition times for full spectra with a bandwidth of 450 nm (5250-4300 cm-1; 1.9-2.33 µm). Due to the programmability of the DMD, spectral regions of interest can be chosen freely, thus reducing acquisition times further, down to the sub-millisecond regime. The adjustable resolving power of the system accessed by means of computer simulations is discussed, quantified for different measurement modes, and verified by comparison with a state-of-the-art Fourier-transform infrared spectrometer. We show measurements of characteristic polymer absorption bands in different operation regimes of the spectrometer. The theoretical multiplex advantage of 8 was experimentally verified by comparison of the noise behavior of the spectral coding approach and a standard line-scan approach.

Dual-band infrared optical coherence tomography using a single supercontinuum source.

Recent developments and commercial availability of low-noise and bright infrared (IR) supercontinuum sources initiated intensive applied research in the last few years. Covering a significant part of near- and mid-infrared spectral ranges, supercontinuum radiation opened up unique possibilities and alternatives for the well-established imaging technique of optical coherence tomography (OCT). In this contribution, we demonstrate the development, performance, and maturity of a cost-efficient dual-band Fourier-domain IR OCT system (2 µm and 4 µm central wavelengths). The proposed OCT setup is elegantly employing a single supercontinuum source and a pyroelectric linear array. We discuss adapted application-oriented approaches to signal acquisition and post-processing when thermal detectors are applied in interferometers. In the experimental part, the efficiency of the dual-band detection is evaluated. Practical results and direct comparisons of the OCT system operating within the employed sub-bands are exhibited and discussed. Furthermore, we introduce the 2 µm OCT sub-system as an affordable alternative for art diagnosis; therefore, high resolution and sensitive measurements of the painting mock-ups are presented. Finally, potentials of the dual-band detection are demonstrated for lithography-based manufactured industrial ceramics.

Probeless non-invasive near-infrared spectroscopic bioprocess monitoring using microspectrometer technology.

Real-time measurements and adjustments of critical process parameters are essential for the precise control of fermentation processes and thus for increasing both quality and yield of the desired product. However, the measurement of some crucial process parameters such as biomass, product, and product precursor concentrations usually requires time-consuming offline laboratory analysis. In this work, we demonstrate the in-line monitoring of biomass, penicillin (PEN), and phenoxyacetic acid (POX) in a Penicilliumchrysogenum fed-batch fermentation process using low-cost microspectrometer technology operating in the near-infrared (NIR). In particular, NIR reflection spectra were taken directly through the glass wall of the bioreactor, which eliminates the need for an expensive NIR immersion probe. Furthermore, the risk of contaminations in the reactor is significantly reduced, as no direct contact with the investigated medium is required. NIR spectra were acquired using two sensor modules covering the spectral ranges 1350-1650 nm and 1550-1950 nm. Based on offline reference analytics, partial least squares (PLS) regression models were established for biomass, PEN, and POX either using data from both sensors separately or jointly. The established PLS models were tested on an independent validation fed-batch experiment. Root mean squared errors of prediction (RMSEP) were 1.61 g/L, 1.66 g/L, and 0.67 g/L for biomass, PEN, and POX, respectively, which can be considered an acceptable accuracy comparable with previously published results using standard process spectrometers with immersion probes. Altogether, the presented results underpin the potential of low-cost microspectrometer technology in real-time bioprocess monitoring applications. Graphical abstract.

Correlative infrared optical coherence tomography and hyperspectral chemical imaging.

Optical coherence tomography (OCT) is a high-resolution three-dimensional imaging technique that enables nondestructive measurements of surface and subsurface microstructures. Recent developments of OCT operating in the mid-infrared (MIR) range (around 4 µm) lifted fundamental scattering limitations and initiated applied material research in formerly inaccessible fields. The MIR spectral region, however, is also of great interest for spectroscopy and hyperspectral imaging, which allow highly selective and sensitive chemical studies of materials. In this contribution, we introduce an OCT system (dual-band, central wavelengths of 2 µm and 4 µm) combined with MIR spectroscopy that is implemented as a raster scanning chemical imaging modality. The fully integrated and cost-effective optical instrument is based on a single supercontinuum laser source (emission spectrum spanning from 1.1 µm to 4.4 µm). Capabilities of the in situ correlative measurements are experimentally demonstrated by obtaining complex multidimensional material data, comprising morphological and chemical information, from a multilayered composite ceramic-polymer specimen.

Broadband near-infrared hyperspectral single pixel imaging for chemical characterization.

We introduce a compressive sensing based approach for single pixel hyperspectral chemical imaging in a broad spectral range in the near-infrared. Fully integrated MEMS based Fabry-Pérot tunable filter spectrometers and a digital micro-mirror device were employed to achieve spectral and spatial resolution, respectively. The available spectral range from 1500 to 2200 nm covers molecular overtone vibrations enabling chemical identification. Hyperspectral images of different adhesives deposited on a textile were recorded revealing their chemical composition. Furthermore, spectrally resolved near-infrared images with compression rates up to 90% are presented. The approach of single pixel imaging illustrates a promising technology for the infrared spectral range superior to conventionally used costly focal plane arrays.

Sub-second quantum cascade laser based infrared spectroscopic ellipsometry.

Laser-based infrared spectroscopic ellipsometry (SE) is demonstrated for the first time, to the best of our knowledge, by applying a tunable quantum cascade laser (QCL) as a mid-infrared light source. The fast tunability of the employed QCL, combined with phase-modulated polarization, enabled the acquisition of broadband (900-1204 cm-1), high-resolution (1 cm-1) ellipsometry spectra in less than 1 second. A comparison to a conventional Fourier-transform spectrometer-based IR ellipsometer resulted in an improved signal-to-noise ratio (SNR) by a factor of at least 290. The ellipsometry setup was finally applied for the real-time monitoring of molecular reorientation during the stretching process of an anisotropic polypropylene film, thereby illustrating the advantage of sub-second time resolution. The developed method exceeds existing instrumentation by its fast acquisition and high SNR, which could open up a set of new applications of SE such as ellipsometric inline process monitoring and quality control.

Enhanced mid-infrared multi-bounce ATR spectroscopy for online detection of hydrogen peroxide using a supercontinuum laser.

A compact multi-bounce attenuated total reflection (ATR) probe combined with a Fabry-Pérot filter spectrometer (FPFS) has been developed for detection of hydrogen peroxide used for oxidative gas scrubbing operating in the mid-infrared (MIR) spectral region. A novel MIR supercontinuum light source is employed to enhance the quantification capabilities of the sensor and is compared to a classical thermal emitter. An improvement of a factor of 4 in noise and approximately a factor of 3 in limit of detection is shown in this study allowing fast inline detection of aqueous hydrogen peroxide solutions around 0.1%.

Diffraction limited mid-infrared reflectance microspectroscopy with a supercontinuum laser.

Chemical mapping was demonstrated with a mid-infrared (MIR) microspectroscopy setup based on a supercontinuum source (SC) emitting in the spectral range from 1.55 to 4.5 µm and a MEMS-based Fabry-Pérot filter spectrometer. Diffraction limited spatial resolution in reflection geometry was achieved. A multilayer film consisting of different polymers and mixtures thereof was measured and results were compared to those gained with a conventional FTIR microscope equipped with a thermal MIR source. Results show that compared to thermal sources, the application of the SC source results in higher signal-to-noise ratios together with better spatial resolution and faster scanning. Furthermore, diffraction limited imaging of red blood cells was demonstrated for the first time in the MIR spectral region in reflection mode. The distinctive characteristics of the MIR spectral region in conjunction with the high brightness, spatial coherence and broadband nature of supercontinuum radiation show the potential for improving infrared microscopy significantly.

Mid-infrared Fourier-domain optical coherence tomography with a pyroelectric linear array.

Optical technology in the mid-infrared wavelength range is currently a rapidly developing field initiated by the availability of novel high-power and spatially coherent sources. Non-destructive testing techniques based on these sources are very promising for industrial and medical applications. However, there are still many engineering problems due to the technical challenges and high prices of the optical elements suitable for the mid-infrared region. In this paper, we report the development and performances of the first mid-infrared Fourier-domain optical coherence tomography based on a supercontinuum source and low-cost pyroelectric detector. The system is designed to operate in the spectral region around 4 µm. Experimental results are demonstrated for detections of embedded microstructures in ceramic materials and subsurface oil paint layers.

Quantum cascade lasers (QCLs) in biomedical spectroscopy.

Quantum cascade lasers (QCL) are the first room temperature semiconductor laser source for the mid-IR spectral region, triggering substantial development for the advancement of mid-IR spectroscopy. Mid-IR spectroscopy in general provides rapid, label-free and objective analysis, particularly important in the field of biomedical analysis. Due to their unique properties, QCLs offer new possibilities for development of analytical methods to enable quantification of clinically relevant concentration levels and to support medical diagnostics. Compared to FTIR spectroscopy, novel and elaborated measurement techniques can be implemented that allow miniaturized and portable instrumentation. This review illustrates the characteristics of QCLs with a particular focus on their benefits for biomedical analysis. Recent applications of QCL-based spectroscopy for analysis of a variety of clinically relevant samples including breath, urine, blood, interstitial fluid, and biopsy samples are summarized. Further potential for technical advancements is discussed in combination with future prospects for employment of QCL-based devices in routine and point-of-care diagnostics.

Workflow for multi-analyte bioprocess monitoring demonstrated on inline NIR spectroscopy of P. chrysogenum fermentation.

Fourier transform near-infrared (FT-NIR) spectroscopy combined with multivariate analysis has been applied in bioprocesses for a couple of decades. Nevertheless the papers published in this field are case-specific and do not focus on providing the community generic workflows to conduct experiments, especially as a standard Design of Experiment (DoE) for a multi-analyte process might require overwhelming amount of measurements. In this paper, a workflow for feasibility studies and inline implementation of FT-NIR spectrometer in multi-analyte fermentation processes is presented. The workflow is applied to Penicillium crysogenum fermentation, where the similarities in chemical structures and growth trends between the key analytes together with the aeration and growing fungi make the task challenging: first, the pure analytes are measured off-line with FT-NIR and clustered using principal component analysis. To study the separability of the gained clusters, a DoE approach by spiking is applied. The multivariate modelling of the separable analytes is conducted using the off-line and inline data followed by a comparison of the properties of the different models. Finally, the model output constraints are set by means of outlier diagnostics. As a result, biomass, penicillin (PEN), phenoxyacetic acid (POX), ammonia and biomass were shown to be separable with root mean square error of predictions of 2.62 g/l, 0.34 g/l, 0.51 g/l and 18.3 mM, respectively. Graphical abstract Flowchart illustrating the workflow for feasibility studies and implementation of models for inline monitoring of Ammonia, Biomass, Phenoxyacetic acid and Penicillin.

Improved quantification of important beer quality parameters based on nonlinear calibration methods applied to FT-MIR spectra.

During the production process of beer, it is of utmost importance to guarantee a high consistency of the beer quality. For instance, the bitterness is an essential quality parameter which has to be controlled within the specifications at the beginning of the production process in the unfermented beer (wort) as well as in final products such as beer and beer mix beverages. Nowadays, analytical techniques for quality control in beer production are mainly based on manual supervision, i.e., samples are taken from the process and analyzed in the laboratory. This typically requires significant lab technicians efforts for only a small fraction of samples to be analyzed, which leads to significant costs for beer breweries and companies. Fourier transform mid-infrared (FT-MIR) spectroscopy was used in combination with nonlinear multivariate calibration techniques to overcome (i) the time consuming off-line analyses in beer production and (ii) already known limitations of standard linear chemometric methods, like partial least squares (PLS), for important quality parameters Speers et al. (J I Brewing. 2003;109(3):229-235), Zhang et al. (J I Brewing. 2012;118(4):361-367) such as bitterness, citric acid, total acids, free amino nitrogen, final attenuation, or foam stability. The calibration models are established with enhanced nonlinear techniques based (i) on a new piece-wise linear version of PLS by employing fuzzy rules for local partitioning the latent variable space and (ii) on extensions of support vector regression variants (𝜖-PLSSVR and ν-PLSSVR), for overcoming high computation times in high-dimensional problems and time-intensive and inappropriate settings of the kernel parameters. Furthermore, we introduce a new model selection scheme based on bagged ensembles in order to improve robustness and thus predictive quality of the final models. The approaches are tested on real-world calibration data sets for wort and beer mix beverages, and successfully compared to linear methods, showing a clear out-performance in most cases and being able to meet the model quality requirements defined by the experts at the beer company. Figure Workflow for calibration of non-Linear model ensembles from FT-MIR spectra in beer production .

External-Cavity Quantum Cascade Laser Spectroscopy for Mid-IR Transmission Measurements of Proteins in Aqueous Solution.

In this work, we report mid-IR transmission measurements of the protein amide I band in aqueous solution at large optical paths. A tunable external-cavity quantum cascade laser (EC-QCL) operated in pulsed mode at room temperature allowed one to apply a path length of up to 38 µm, which is four times larger than that applicable with conventional FT-IR spectrometers. To minimize temperature-induced variations caused by background absorption of the ν2-vibration of water (HOH-bending) overlapping with the amide I region, a highly stable temperature control unit with relative temperature stability within 0.005 °C was developed. An advanced data processing protocol was established to overcome fluctuations in the fine structure of the emission curve that are inherent to the employed EC-QCL due to its mechanical instabilities. To allow for wavenumber accuracy, a spectral calibration method has been elaborated to reference the acquired IR spectra to the absolute positions of the water vapor absorption bands. Employing this setup, characteristic spectral features of five well-studied proteins exhibiting different secondary structures could be measured at concentrations as low as 2.5 mg mL(-1). This concentration range could previously only be accessed by IR measurements in D2O. Mathematical evaluation of the spectral overlap and comparison of second derivative spectra confirm excellent agreement of the QCL transmission measurements with protein spectra acquired by FT-IR spectroscopy. This proves the potential of the applied setup to monitor secondary structure changes of proteins in aqueous solution at extended optical path lengths, which allow experiments in flow through configuration.

Ultrasound-enhanced attenuated total reflection mid-infrared spectroscopy in-line probe: acquisition of cell spectra in a bioreactor.

This article presents a novel method for selective acquisition of Fourier transform infrared (FT-IR) spectra of microorganisms in-line during fermentation, using Saccharomyces cerevisiae as an example. The position of the cells relative to the sensitive region of the attenuated total reflection (ATR) FT-IR probe was controlled by combing a commercially available ATR in-line probe with contact-free, gentle particle manipulation by ultrasonic standing waves. A prototype probe was successfully constructed, assembled, and tested in-line during fed-batch fermentations of S. cerevisiae. Control over the position of the cells was achieved by tuning the ultrasound frequency: 2.41 MHz was used for acquisition of spectra of the cells (pushing frequency f(p)) and 1.87 MHz, for retracting the cells from the ATR element, therefore allowing spectra of the medium to be acquired. Accumulation of storage carbohydrates (trehalose and glycogen) inside the cells was induced by a lack of a nitrogen source in the feed medium. These changes in biochemical composition were visible in the spectra of the cells recorded in-line during the application of f(p) and could be verified by reference spectra of dried cell samples recorded off-line with a FT-IR microscope. Comparison of the cell spectra with spectra of trehalose, glycogen, glucose, and mannan, i.e., the major carbohydrates present in S. cerevisiae, and principal components analysis revealed that the changes observed in the cell spectra correlated well with the bands specific for trehalose and glycogen. This proves the applicability and capability of ultrasound-enhanced in-line ATR mid-IR spectroscopy as a real-time PAT method for the in situ monitoring of cellular biochemistry during fermentation.

Remote mid-infrared photoacoustic spectroscopy with a quantum cascade laser.

We demonstrate non-contact remote photoacoustic spectroscopy in the mid-infrared region. A room-temperature-operated pulsed external-cavity quantum cascade laser is used to excite photoacoustic waves within a semitransparent sample. The ultrasonic waves are detected remotely on the opposite side of the sample using a fiber-optic Mach-Zehnder interferometer, thereby avoiding problems associated with acoustic attenuation in air. We present the theoretical background of the proposed technique and demonstrate measurements on a thin polystyrene film. The obtained absorption spectrum in the region of 1030-1230 cm(-1) is compared to a spectrum obtained by attenuated total reflection, showing reasonable agreement.