Hydrogen and C2-C6 Alkane Sensing in Complex Fuel Gas Mixtures with Fiber-Enhanced Raman Spectroscopy

Ultrasensitive Raman Gas Spectroscopy for Dinitrogen Sensing at the Parts-per-Billion Level

Sensing small changes in the concentration of dinitrogen (N2) is a difficult analytical task. As N2-sensing is crucial for nitrogen cycle research in general and studies of denitrification in particular, researchers went to great lengths to develop techniques like the gas-flow-soil-core method, which achieves a precision of 200 ppb at 20 ppm of N2. Here, we present a Raman gas spectroscopic technique based on high pressure, high laser power, and high-NA signal collection, which achieves a limit of detection (LoD) of 59 ppb N2 and a precision of 27 ppb at 10 ppm of N2. This improves the lowest LoD for N2 reported for Raman gas spectroscopy by 2 orders of magnitude. Furthermore, this constitutes an improvement in precision by 1 order of magnitude compared to the GC-MS-based gas-flow-soil-core method currently established in denitrification research. We show that the presented setup is both stable and tight enough to ensure highly sensitive, precise, and repeatable measurements of N2. As Raman gas spectroscopy is a versatile and comprehensive method, the described technique could be easily expanded to other relevant gases like nitrous oxide or to simultaneous multigas sensing. In summary, our method offers possibilities for N2-sensing and could eventually enable denitrification studies with increased sensitivity and a larger scope.

Exploring the potential of a setup for combined quantification of hydrogen in natural gas - Raman and NMR spectroscopy

An accurate measurement of the amount fraction of hydrogen in gas mixtures is mandatory for practical applications, requiring methods that are fast, continuous, robust, and cost-effective. This study compares the performance of Raman and benchtop NMR process spectroscopy for determining the hydrogen amount fraction in gas mixtures. A setup was designed to integrate both techniques, enabling measurements of the same sample. Tests were conducted with gravimetrically prepared gas mixtures of reference quality ranging from 1.20 cmol/mol to 85.83 cmol/mol of hydrogen. The results demonstrate that Raman spectroscopy provides superior performance, with a minimal root mean square error (RMSE) of 0.22 cmol/mol and excellent linearity. In contrast, benchtop NMR spectroscopy faced challenges, such as overlapping peaks and longer measurement times, resulting in a higher RMSE of 0.71 cmol/mol. Raman spectroscopy proves to be particularly well-suited for practical applications due to its high accuracy and linearity. Meanwhile, benchtop NMR spectroscopy holds potential for future enhancements through ongoing technological advances, such as higher magnetic field strengths. In summary, the results from our study indicate that Raman spectroscopy is already a serviceable method for precise hydrogen quantification, whereas benchtop NMR spectroscopy can be attributed potential for future applications.

Comprehensive multi-gas study by means of fiber-enhanced Raman spectroscopy for the investigation of nitrogen cycle processes

The extensive use of synthetic fertilizers has led to a considerable increase in reactive nitrogen input into agricultural and natural systems, resulting in negative effects in multiple ecosystems, the so-called nitrogen cascade. Since the global population relies on fertilization for food production, synthetic fertilizer use needs to be optimized by balancing crop yield and reactive nitrogen losses. Fiber-enhanced Raman spectroscopy (FERS) is introduced as a unique method for the simultaneous quantification of multiple gases to the study processes related to the nitrogen cycle. By monitoring changes in the headspace gas concentrations, processes such as denitrification, nitrification, respiration, and nitrogen fixation, as well as fertilizer addition were studied. The differences in concentration between the ambient and prepared process samples were evident in the Raman spectra, allowing for differentiation of process-specific spectra. Gas mixture concentrations were quantified within a range of low ppm to 100% for the gases N2, O2, CO2, N2O, and NH3. Compositional changes were attributed to processes of the nitrogen cycle. With help of multivariate curve resolution, it was possible to quantify N2O and CO2 simultaneously. The impact of fertilizers on N-cycle processes in soil was simulated and analyzed for identifying active processes. Thus, FERS was proven to be a suitable technique to optimize fertilizer composition and to quantify N2O and NH3 emissions, all with a single device and without further sample preparation.

Spectrally Resolved and Highly Parallelized Raman Difference Spectroscopy for the Analysis of Drug-Target Interactions between the Antimalarial Drug Chloroquine and Hematin

Malaria is a severe disease caused by cytozoic parasites of the genus Plasmodium, which infiltrate and infect red blood cells. Several drugs have been developed to combat the devastating effects of malaria. Antimalarials based on quinolines inhibit the crystallization of hematin into hemozoin within the parasite, ultimately leading to its demise. Despite the frequent use of these agents, there are unanswered questions about their mechanisms of action. In the present study, the quinoline chloroquine and its interaction with the target structure hematin was investigated using an advanced, highly parallelized Raman difference spectroscopy (RDS) setup. Simultaneous recording of the spectra of hematin and chloroquine mixtures with varying compositions enabled the observation of changes in peak heights and positions based on the altered molecular structure resulting from their interaction. A shift of (-1.12 ± 0.05) cm-1 was observed in the core-size marker band ν(CαCm)asym peak position of the 1:1 chloroquine-hematin mixture compared to pure hematin. The oxidation-state marker band ν(pyrrole half-ring)sym exhibited a shift by (+0.93 ± 0.13) cm-1. These results were supported by density functional theory (DFT) calculations, indicating a hydrogen bond between the quinolinyl moiety of chloroquine and the oxygen atom of ferric protoporphyrin IX hydroxide (Fe(III)PPIX-OH). The consequence is a reduced electron density within the porphyrin moiety and an increase in its core size. This hypothesis provided further insights into the mechanism of hemozoin inhibition, suggesting chloroquine binding to the monomeric form of hematin, thereby preventing its further crystallization to hemozoin.

Isotopomeric Peak Assignment for N2O in Cross-Labeling Experiments by Fiber-Enhanced Raman Multigas Spectroscopy

Human intervention in nature, especially fertilization, greatly increased the amount of N2O emission. While nitrogen fertilizer is used to improve nitrogen availability and thus plant growth, one negative side effect is the increased emission of N2O. Successful regulation and optimization strategies require detailed knowledge of the processes producing N2O in soil. Nitrification and denitrification, the main processes responsible for N2O emissions, can be differentiated using isotopic analysis of N2O. The interplay between these processes is complex, and studies to unravel the different contributions require isotopic cross-labeling and analytical techniques that enable tracking of the labeled compounds. Fiber-enhanced Raman spectroscopy (FERS) was exploited for sensitive quantification of N2O isotopomers alongside N2, O2, and CO2 in multigas compositions and in cross-labeling experiments. FERS enabled the selective and sensitive detection of specific molecular vibrations that could be assigned to various isotopomer peaks. The isotopomers 14N15N16O (2177 cm-1) and 15N14N16O (2202 cm-1) could be clearly distinguished, allowing site-specific measurements. Also, isotopomers containing different oxygen isotopes, such as 14N14N17O, 14N14N18O, 15N15N16O, and 15N14N18O could be identified. A cross-labeling showed the capability of FERS to disentangle the contributions of nitrification and denitrification to the total N2O fluxes while quantifying the total sample headspace composition. Overall, the presented results indicate the potential of FERS for isotopic studies of N2O, which could provide a deeper understanding of the different pathways of the nitrogen cycle.

Whole Landscape of the Origin and Evolution of Gassing in Supercapacitors at a High Voltage

Although supercapacitors with acetonitrile-based electrolytes (AN-based SCs) have realized high-voltage (3.0 V) applications by manufacturers, gas generation at high voltages is a critical issue. Also, the exact origins and evolution mechanisms of gas generation during SC aging at 3.0 V still lack a whole landscape. In this work, floating tests under realistic working conditions are conducted by 22450-type cylindrical cells with an AN-based commercial electrolyte. Comprehensive insights into the origins and evolution mechanisms of gas species at 2.7 and 3.0 V are acquired, which involves multiple side reactions related to the electrode, current collector, and electrolyte. Both experimental evidence and density functional theory calculations demonstrate that the primary reasons for gas generation are residual water and oxygen-containing functional groups, especially hydroxyl and carboxyl. More importantly, additional types of gas (such as CO2, NH3, and alkenes) can only be detected at a higher voltage of 3.0 V rather than 2.7 V after failure, suggesting that these gas species can be regarded as the failure signatures at 3.0 V. This breakthrough analysis will provide fundamental guidance for failure evaluation and designing AN-based SCs with extended lifetime at 3.0 V.

A New Gas Analysis Method Based on Single-Beam Excitation Stimulated Raman Scattering in Hollow Core Photonic Crystal Fiber Enhanced Raman Spectroscopy

We previously developed a hollow-core photonic crystal fiber (HCPCF) based Raman scattering enhancement technique for gas/human breath analysis. It enhances photon-gas molecule interactions significantly but is still based on CW laser excitation spontaneous Raman scattering, which is a low-probability phenomenon. In this work, we explored nanosecond/sub-nanosecond pulsed laser excitation in HCPCF based fiber enhanced Raman spectroscopy (FERS) and successfully induced stimulated Raman scattering (SRS) enhancement. Raman measurements of simple and complex gases were performed using the new system to assess its feasibility for gas analysis. We studied the gas Raman scattering characteristics, the relationship between Raman intensities and pump energies, and the energy threshold for the transition from spontaneous Raman scattering to SRS. H2, CO2, and propene (C3H6) were used as test gases. Our results demonstrated that a single-beam pulsed pump combined with FERS provides an effective Raman enhancement technique for gas analysis. Furthermore, an energy threshold for SRS initiation was experimentally observed. The SRS-capable FERS system, utilizing a single-beam pulsed pump, shows great potential for analyzing complex gases such as propene, which is a volatile organic compound (VOC) gas, serving as a biomarker in human breath for lung cancer and other human diseases. This work contributes to the advancement of gas analysis and opens alternative avenues for exploring novel Raman enhancement techniques.

Molecular Interactions Identified by Two-Dimensional Analysis-Detailed Insight into the Molecular Interactions of the Antimalarial Artesunate with the Target Structure β-Hematin by Means of 2D Raman Correlation Spectroscopy

A thorough understanding of the interaction of endoperoxide antimalarial agents with their biological target structures is of utmost importance for the tailored design of future efficient antimalarials. Detailed insights into molecular interactions between artesunate and β-hematin were derived with a combination of resonance Raman spectroscopy, two-dimensional correlation analysis, and density functional theory calculations. Resonance Raman spectroscopy with three distinct laser wavelengths enabled the specific excitation of different chromophore parts of β-hematin. The resonance Raman spectra of the artesunate-β-hematin complexes were thoroughly analyzed with the help of high-resolution and highly sensitive two-dimensional correlation spectroscopy. Spectral changes in the peak properties were found with increasing artesunate concentration. Changes in the low-frequency, morphology-sensitive Raman bands indicated a loss in crystallinity of the drug-target complexes. Differences in the high-wavenumber region were assigned to increased distortions of the planarity of the structure of the target molecule due to the appearance of various coexisting alkylation species. Evidence for the appearance of high-valent ferryl-oxo species could be observed with the help of differences in the peak properties of oxidation-state sensitive Raman modes. To support those findings, the relaxed ground-state structures of ten possible covalent mono- and di-meso(Cm)-alkylated hematin-dihydroartemisinyl complexes were calculated using density functional theory. A very good agreement with the experimental peak properties was achieved, and the out-of-plane displacements along the lowest-frequency normal coordinates were investigated by normal coordinate structural decomposition analysis. The strongest changes in all data were observed in vibrations with a high participation of Cm-parts of β-hematin.

Natural Gas Analysis Using Polarized Raman Spectroscopy

Raman spectroscopy is a promising tool for measuring the composition of natural gas. However, to obtain high measurement accuracy, it is necessary to take into account changes in the spectral characteristics of methane, since its spectrum overlaps the characteristic bands of other species. In this study we present a technique for natural gas analysis based on polarized Raman spectroscopy. It is shown that the use of only isotropic components of spectra simplifies the procedure for extracting concentrations and improves the measurement accuracy of components whose spectral bands are significantly overlapped in conventional Raman spectra. The presented technique will be very useful both in the field of analysis of various multicomponent gas mixtures and in the field of measuring the isotopic composition of molecules.

High-Precision Trace Hydrogen Sensing by Multipass Raman Scattering

Despite its growing importance in the energy generation and storage industry, the detection of hydrogen in trace concentrations remains challenging, as established optical absorption methods are ineffective in probing homonuclear diatomics. Besides indirect detection approaches using, e.g., chemically sensitized microdevices, Raman scattering has shown promise as an alternative direct method of unambiguous hydrogen chemical fingerprinting. We investigated the suitability of feedback-assisted multipass spontaneous Raman scattering for this task and examined the precision with which hydrogen can be sensed at concentrations below 2 parts per million. A limit of detection of 60, 30, and 20 parts per billion was obtained at a pressure of 0.2 MPa in a 10-min-long, 120-min-long, and 720-min-long measurement, respectively, with the lowest concentration probed being 75 parts per billion. Various methods of signal extraction were compared, including asymmetric multi-peak fitting, which allowed the resolution of concentration steps of 50 parts per billion, determining the ambient air hydrogen concentration with an uncertainty level of 20 parts per billion.

Investigations on the Novel Antimalarial Ferroquine in Biomimetic Solutions Using Deep UV Resonance Raman Spectroscopy and Density Functional Theory

Deep ultraviolet (DUV) resonance Raman experiments are performed, investigating the novel, promising antimalarial ferroquine (FQ). Two buffered aqueous solutions with pH values of 5.13 and 7.00 are used, simulating the acidic and neutral conditions inside a parasite's digestive vacuole and cytosol, respectively. To imitate the different polarities of the membranes and interior, the buffer's 1,4-dioxane content was increased. These experimental conditions should mimic the transport of the drug inside malaria-infected erythrocytes through parasitophorous membranes. Supporting density functional theory (DFT) calculations on the drug's micro-speciation were performed, which could be nicely assigned to shifts in the peak positions of resonantly enhanced high-wavenumber Raman signals at λ_exc = 257 nm. FQ is fully protonated in polar mixtures like the host interior and the parasite's cytoplasm or digestive vacuole (DV) and is only present as a free base in nonpolar ones, such as the host's and parasitophorous membranes. Additionally, the limit of detection (LoD) of FQ at vacuolic pH values was determined using DUV excitation wavelengths at 244 and 257 nm. By applying the resonant laser line at λ_exc = 257 nm, a minimal FQ concentration of 3.1 μM was detected, whereas the pre-resonant excitation wavelength 244 nm provides an LoD of 6.9 μM. These values were all up to one order of magnitude lower than the concentration found for the food vacuole of a parasitized erythrocyte.

Broadening of the ν2 Raman Band of CH4 by C3H8 and C4H10

Raman spectroscopy is a promising method for the analysis of natural gas. It is necessary to account for the broadening effects on spectral lines to improve measurement accuracy. In this study, the broadening coefficients for methane lines in the region of the ν₂ band perturbed by propane, n-butane, and isobutane at room temperature were measured. We estimated the measurement errors of the concentration of oxygen and carbon dioxide in the case of neglecting the broadening effects on the methane spectrum by the pressure of C₂-C₆ alkanes. The obtained data are suited for the correct simulation of the methane spectrum in the hydrocarbon-bearing gases and can be used to improve the accuracy of the analysis of natural gas by Raman spectroscopy.

Multiple Gas Detection by Cavity-Enhanced Raman Spectroscopy with Sub-ppm Sensitivity

Accurate and sensitive detection of multicomponent trace gases below the parts-per-million (ppm) level is needed in a variety of medical, industrial, and environmental applications. Raman spectroscopy can identify multiple molecules in the sample simultaneously and has excellent potential for fast diagnosis of various samples, but applications are often limited by its sensitivity. In this contribution, we report the development of a cavity-enhanced Raman spectroscopy instrument using a narrow-line width 532 nm laser locked with a high-finesse cavity through a Pound-Drever-Hall locking servo, which allows continuous measurement in a broad spectral range. An intracavity laser power of up to 1 kW was achieved with an incident laser power of about 240 mW, resulting in a significant enhancement of the Raman signal in the range of 200-5000 cm^-1 and a sub-ppm sensitivity for various molecules. The technique is applied in the detection of different samples, including ambient air, natural gas, and reference gas of sulfur hexafluoride, demonstrating its capability for the quantitative measurement of various trace components.

Ambient Hydrocarbon Detection with an Ultra-Low-Loss Cavity Raman Analyzer

The detection of ambient outdoor trace hydrocarbons was investigated with a multipass Raman analyzer. It relies on a multimode blue laser diode receiving optical feedback from a retroreflecting multipass optical cavity, effectively creating an external cavity diode laser within which spontaneous Raman scattering enhancement occurs. When implemented with ultra-low-loss mirrors, a more than 20-fold increase in signal-to-background ratio was obtained, enabling proximity detection of trace motor vehicle exhaust gases such as H₂, CO, NO, CH₄, C₂H₂, C₂H₄, and C₂H₆. In a 10-min-long measurement at double atmospheric pressure, the limits of detection obtained were near or below 100 ppb for most analytes.

Parallelized Raman Difference Spectroscopy for the Investigation of Chemical Interactions

Raman spectroscopy provides an extremely high chemical selectivity. Raman difference spectroscopy is a technique to reveal even the smallest differences that occur due to weak interactions between substances and changes in the molecular structure. To enable parallelized and highly sensitive Raman difference spectroscopy in a microtiter-array, a diffractive optical element, a lens array, and a fiber bundle were integrated into a Raman spectroscopy setup in a unique fashion. The setup was evaluated with a microtiter-array containing pyridine-water complexes, and subwavenumber changes below the spectrometer's resolution could be resolved. The spectral changes were emphasized with two-dimensional correlation analysis. Density functional theory calculation and "atoms in molecule" analysis were performed to simulate the intermolecular long-range interactions between water and pyridine molecules and to get insight into the involved noncovalent interactions, respectively. It was found that by the addition of pyridine, the energy portion of hydrogen bonds to the total complexation energy between pyridine and water reduces. These results demonstrate the unique abilities of the new setup to investigate subtle changes due to biochemically important molecular interactions and opens new avenues to perform drug binding assays and to monitor highly parallelized chemical reactions.

Raman Natural Gas Analyzer: Effects of Composition on Measurement Precision

Raman spectroscopy is a promising method for analyzing natural gas due to its high measurement speed and the potential to monitor all molecular components simultaneously. This paper discusses the features of measurements of samples whose composition varies over a wide range (0.005-100%). Analysis of the concentrations obtained during three weeks of experiments showed that their variation is within the error caused by spectral noise. This result confirms that Raman gas analyzers can operate without frequent calibrations, unlike gas chromatographs. It was found that a variation in the gas composition can change the widths of the spectral lines of methane. As a result, the measurement error of oxygen concentration can reach 200 ppm. It is also shown that neglecting the measurement of pentanes and n-hexane leads to an increase in the calculated concentrations of other alkanes and to errors in the density and heating value of natural gas.

Fiber-enhanced Raman spectroscopy for highly sensitive H2 and SO2 sensing with a hollow-core anti-resonant fiber

An innovative fiber-enhanced Raman gas sensing system with a hollow-core anti-resonant fiber is introduced. Two iris diaphragms are implemented for spatial filtering, and a reflecting mirror is attached to one fiber end that provides a highly improved Raman signal enhancement over 2.9 times than the typical bare fiber system. The analytical performance for multigas compositions is thoroughly demonstrated by recording the Raman spectra of carbon dioxide (CO₂), oxygen (O₂), nitrogen (N₂), hydrogen (H₂), and sulfur dioxide (SO₂) with limits of detection down to low-ppm levels as well as a long-term instability < 1.05%. The excellent linear relationship between Raman signal intensity (peak height) and gas concentrations indicates a promising potential for accurate quantification.