Mid-infrared Absorption Spectra Research Articles

Temperature-dependent absorption cross section measurements for propene, 1-butene, cis-/trans-2-butene, isobutene and 1,3-butadiene in the spectral region 8.4–11.7 µm

We present a comprehensive study on the temperature-dependent mid-infrared absorption spectra between 8.4 µm and 11.7 µm for a complete list of C3-C4 alkenes, namely propene, 1-butene, cis-/trans-2-butene and isobutene, as well as 1,3-butadiene, from room temperature to roughly 1000 K. The methodology deploys a rapid-tuning, broad-scan external-cavity quantum-cascade laser (EC-QCL) in a shock tube and can provide quantitative absorption information at a rate in excess of 30 cm−1/ms at spectral intervals between 0.35–0.6 cm−1. The experimental procedure was validated with room-temperature, nitrogen-diluted, static spectra measurements for propene, 1-butene, isobutene, and 1,3-butadiene, showing good agreement with existing spectroscopic databases. Spectral measurements were extended to higher temperatures with shock-heated test gas mixtures diluted by argon, at experimental conditions between 600–1000 K and 1.5–2.5 atm. The resulting spectra show strong temperature dependence and represent the first experimental collection of broadband high-temperature absorption spectra for these species. The measured high-temperature spectra are in reasonable agreement with predictions from fixed-wavelength absorption cross section correlations in the literature. The data presented are archived in the Stanford ShockGas-IR database and provide useful expansions of the existing knowledge on molecular absorption in the mid-infrared at elevated temperatures.

Synchrotron Photothermal Infrared Nanospectroscopy of Drug-Induced Phospholipidosis in Macrophages.

Synchrotron resonance-enhanced infrared atomic force microscopy (RE-AFM-IR) is a near-field photothermal vibrational nanoprobe developed at Diamond Light Source (DLS), capable of measuring mid-infrared absorption spectra with spatial resolution around 100 nm. The present study reports a first application of synchrotron RE-AFM-IR to interrogate biological soft matter at the subcellular level, in this case, on a cellular model of drug-induced phospholipidosis (DIPL). J774A-1 macrophages were exposed to amiodarone (10 μM) or medium for 24 h and chemically fixed. AFM topography maps revealed amiodarone-treated cells with enlarged cytoplasm and very thin regions corresponding to collapsed vesicles. IR maps of the whole cell were analyzed by exploiting the RE-AFM-IR overall signal, i.e., the integrated RE-AFM-IR signal amplitude versus AFM-derived cell thickness, also on lateral resolution around 100 nm. Results show that vibrational band assignment was possible, and all characteristic peaks for lipids, proteins, and DNA/RNA were identified. Both peak ratio and unsupervised chemometric analysis of RE-AFM-IR nanospectra generated from the nuclear and perinuclear regions of untreated and amiodarone-treated cells showed that the perinuclear region (i.e., cytoplasm) of amiodarone-treated cells had significantly elevated band intensities in the regions corresponding to phosphate and carbonyl groups, indicating detection of phospholipid-rich inclusion bodies typical for cells with DIPL. The results of this study are of importance to demonstrate not only the applicability of Synchrotron RE-AFM-IR to soft biological matters with subcellular spatial resolution but also that the spectral information gathered from an individual submicron sample volume enables chemometric identification of treatment and biochemical differences between mammalian cells.

Experimental photoluminescence and lifetimes at wavelengths including beyond 7 microns in Sm3+-doped selenide-chalcogenide glass fibers.

1000 ppmw Sm3+-doped Ge19.4Sb9.7Se67.9Ga3 atomic % chalcogenide bulk glass and unstructured fiber are prepared. Near- and mid-infrared absorption spectra of the bulk glass reveal Sm3+ electronic absorption bands, and extrinsic vibrational absorption bands, due to host impurities. Fiber photoluminescence, centred at 3.75 µm and 7.25 µm, is measured when pumping at either 1300 or 1470 nm. Pumping at 1470 nm enables the photoluminescent lifetime at 7.3 µm to be measured for the first time which was ∼100 µs. This is the longest to date, experimentally observed lifetime in the 6.5-9 µm wavelength-range of a lanthanide-doped chalcogenide glass fiber.

Possible cause of chemiluminescence in silicone rubber

Chemiluminescence (CL) was observed in silicone rubber (SiR), an important electrical insulating material. Fourier-transform mid-infrared absorption spectra were also measured before and after the CL measurement. When SiR was heated at 250 °C, CL clearly appeared in 20 min. It was also confirmed that no carbonyl groups were induced by the heating during the CL measurement. A possible mechanism to explain these results is that the oxidation of silicon, which induces the cross-linking of siloxane bonds and the formation of silica particles, generates the CL.

Intensity noise optimization of a mid-infrared frequency comb difference-frequency generation source

We experimentally demonstrate in a difference-frequency generation mid-infrared frequency comb source the effect of temporal overlap between pump and signal pulses on the relative intensity noise (RIN) of the idler pulse. When scanning the temporal delay between our 130 fs long signal and pump pulses, we observe a RIN minimum with a 3 dB width of 20 fs delay and a RIN increase of 20 dB in 40 fs delay at the edges of this minimum. We also demonstrate active long-term stabilization of the mid-infrared frequency comb source to the temporal overlap setting corresponding to the lowest RIN operation point by an online RIN detector and active feedback control of the pump-signal pulse delay. This active stabilization setup allows us to dramatically increase the signal-to-noise ratio of mid-infrared absorption spectra.

Quantitative measurements of broad-band mid-infrared absorption spectra of formaldehyde, acetaldehyde, and acetone at combustion-relevant temperatures near 5.7 µm

Temperature-dependent absorption cross sections of formaldehyde, acetaldehyde, and acetone are presented for their CO stretching bands near 5.7 µm. The measurement methodology employs a broad-scan, rapid-tuning EC-QCL in conjunction with shock tube facilities and can provide quantitative spectroscopic information with an average spectral resolution of 0.6 cm−1 at a tuning rate in excess of 30 cm−1/ms. The absorption spectrum of formaldehyde in the ν2 vibrational band is reported at 998 K and 3.8 atm, showing reasonable agreement with spectral simulations based on HITRAN 2016. The residuals are ascribed to the emergence of hotbands and high-J rovibrational transitions at elevated temperatures. The first high-temperature absorption cross section measurements of acetaldehyde and acetone are reported near atmospheric pressure at combustion-related temperatures up to 1000 K, with strong temperature broadening observed in the measured spectra. The results presented are archived in the Stanford ShockGas-IR database and are expected to be useful in future spectroscopic studies.

Ultrahigh resolution spectroscopy for dimethyl sulfide at the ν1- and ν8-bands by a distributed feedback interband cascade laser

The ultrahigh-resolution, air-broadened mid-infrared absorption spectrum of dimethyl sulfide (DMS) was recorded in the spectral region of 2994.17–2999.03 cm−1, where the strong ν1-/ν8-bands are located. Benefitting from the tunable laser absorption spectrometry with a distributed feedback interband cascade laser, the spectral resolution achieved 5.69 × 10−6 cm−1. The recorded spectra of DMS are very close to that in the Pacific Northwest National Laboratory (PNNL) database, however it has a much lower absorbance uncertainty of 0.38%. And the maximum relative difference of the full width at half maximum and the integrated area of spectral peaks between the recorded spectra and the PNNL data are as large as 16.7% and 27.9%, respectively, which could mainly come from the difference of the spectral resolution. The more accurate reference spectrum is hopeful to be applied to absolute measurement and remote sensing for gaseous DMS.

Quantum chemical verification of identification of three phenol-type antioxidants by far-infrared absorption spectroscopy

Three different kinds of phenol-type antioxidants were added in low-density polyethylene sheets. Far-infrared and mid-infrared absorption spectra were measured for these sheets and the sheet with no antioxidants to obtain spectra of antioxidants. Furthermore, chemical structures of the antioxidants were optimized by quantum chemical calculations and possible vibrational modes and their energies were calculated numerically. As a result, it was found that our calculations simulate absorption spectra of the antioxidants successfully and that their causal vibrations in the mid-infrared range are consistent with those reported in the literature. Furthermore, it has become clear that all the major far-infrared absorption peaks are due to skeletal vibrations. This clearly explains our previous results that the three antioxidants with very similar structures can be identified by the far-infrared absorption spectroscopy.

Interpretation of artifacts in Fourier transform infrared spectra of atmospheric pressure dielectric barrier discharges: relationship with the plasma frequency between 300 Hz and 15 kHz

This article describes the occurrence of a phenomenon that is observed while recording mid-infrared (4000–700 cm−1) absorption spectra of dielectric barrier discharges sustained at frequencies ranging from 300 Hz to 15 kHz. This phenomenon is observed as the presence of very sharp spikes in the spectrum, for which the wavenumber depends on both the high voltage frequency used to generate the discharge and the velocity of the moving mirror of the interferometer (which in turn determines the interferogram sampling frequency). While it is well known that the consumption of gas precursor within plasmas can be followed, we demonstrate that Fourier transform infrared spectroscopy also makes it possible to monitor frequencies and coupling of excitation mechanisms occurring in the plasma.

Comparison of Raman and Mid-Infrared Spectroscopy for Real-Time Monitoring of Yeast Fermentations: A Proof-of-Concept for Multi-Channel Photometric Sensors

Raman and mid-infrared (MIR) spectroscopy are useful tools for the specific detection of molecules, since both methods are based on the excitation of fundamental vibration modes. In this study, Raman and MIR spectroscopy were applied simultaneously during aerobic yeast fermentations of Saccharomyces cerevisiae. Based on the recorded Raman intensities and MIR absorption spectra, respectively, temporal concentration courses of glucose, ethanol, and biomass were determined. The chemometric methods used to evaluate the analyte concentrations were partial least squares (PLS) regression and multiple linear regression (MLR). In view of potential photometric sensors, MLR models based on two (2D) and four (4D) analyte-specific optical channels were developed. All chemometric models were tested to predict glucose concentrations between 0 and 30 g L−1, ethanol concentrations between 0 and 10 g L−1, and biomass concentrations up to 15 g L−1 in real time during diauxic growth. Root-mean-squared errors of prediction (RMSEP) of 0.68 g L−1, 0.48 g L−1, and 0.37 g L−1 for glucose, ethanol, and biomass were achieved using the MIR setup combined with a PLS model. In the case of Raman spectroscopy, the corresponding RMSEP values were 0.92 g L−1, 0.39 g L−1, and 0.29 g L−1. Nevertheless, the simple 4D MLR models could reach the performance of the more complex PLS evaluation. Consequently, the replacement of spectrometer setups by four-channel sensors were discussed. Moreover, the advantages and disadvantages of Raman and MIR setups are demonstrated with regard to process implementation.

Research on the measurement of hexafluoropropane based on a mid-infrared quantum cascade laser

Hexafluoropropane (HFC-236fa) is a kind of Halon substitute, which is expected to be applied in the aircraft fire suppression system. The spatial distribution of fire extinguishing agent in protected area is an important assessment parameter in the airworthiness certification of aircraft fire protection systems. In this research, a concentration measurement system of hexafluoropropane has been developed using a quantum cascade laser at ∼8280 nm. The measurement of hexafluoropropane based on mid-infrared absorption spectrum was reported for the first time. In experiment, the system showed good performance in the measurement range of 0–15% with a full scale error of 0.87%FS, while the fast response time within 250 ms was obtained. Moreover, this system can be also used in the sensitive detection of hexafluoropropane leakage and the monitoring of greenhouse gas hydrofluorocarbons. The detection limit of 100 part per million (ppm) can be obtained with an 85-cm-long gas cell.

Polarization-sensitive femtosecond mid-infrared spectrometer with a tunable Fabry–Perot filter and chirped-pulse upconversion

Femtosecond time-resolved mid-infrared (MIR) spectroscopy based on chirped-pulse upconversion is a promising method for observing molecular structure and vibrational dynamics. By using a polarized narrowband pump pulse in which center frequency is rapidly scanned with an electrically tunable Fabry-Perot filter, we have developed a femtosecond MIR spectrometer that is more sensitive to molecular structure. The pump energy and polarization dependences of transient MIR absorption spectra measured with the spectroscopic system revealed the appearance of a dark (infrared-inactive) mode due to a slight decrease in the molecular symmetry of a metal carbonyl complex, Mn2(CO)10, in solution. In addition, the rotational relaxation time of Mn2(CO)10 in solution was determined as approximately 20ps.

High-temperature mid-infrared absorption spectra of methanol (CH3OH) and ethanol (C2H5OH) between 930 and 1170 cm-1

A methodology was recently developed with a broad-tuning, rapid-scan external-cavity quantum-cascade-laser in conjunction with shock tube facilities to measure the high-temperature mid-infrared absorption spectra of gaseous molecules. This technique is deployed to measure the cross section profiles in the C-O stretching band for methanol (CH3OH) and ethanol (C2H5OH) between 930 and 1170 cm-1. Methanol spectra are presented from 620 to 1304K between 0.98-3.30 atm with distinctive P, Q, and R branches of the ν8 vibrational band. At elevated temperatures, the emergence of hotbands and high-J ro-vibrational transitions are clearly observed. The absorption cross sections of ethanol are measured from 296 to 1018K between 0.90-3.27 atm. The peak strength decreases with temperature, with the peak location shifting to lower wavenumbers. These measurements are compared with existing empirical models, illustrating a strong need for the development of a high-temperature spectroscopic database.

Measurement of the mid-infrared absorption spectra of ethylene (C2H4) and other molecules at high temperatures and pressures

A methodology for the measurement of mid-infrared absorption cross sections of gaseous molecules at high temperatures and pressures is presented. A rapid-tuning, broad-scan external cavity quantum cascade laser with a tuning rate in excess of 30000 cm−1s−1 has been employed in the measurement of full vibrational bands ( > 100 cm−1) in shock-heated test gases. The approach is demonstrated with measurements of the absorption cross section of ethylene (C2H4) in the 8.5 µm to 11.7 µm region for temperatures from 800 K to 1600 K and pressures from 1 to 5 atm. Decreasing peak strength with temperature is observed as well as pressure-insensitivity. The measurements are compared with existing experimental, empirical, and ab initio databases. Additionally, illustrative absorption cross section measurements of propene (C3H6), 1-butene (1-C4H8), 2-butene (2-C4H8), 1,3-butadiene (1,3-C4H6), and methanol (CH3OH) are presented near 1000 K and 2.3 atm.

Advanced Photonic Sensors Based on Interband Cascade Lasers for Real-Time Mouse Breath Analysis.

A multiparameter gas sensor based on distributed feedback interband cascade lasers emitting at 4.35 μm and ultrafast electro-spun luminescence oxygen sensors has been developed for the quantification and continuous monitoring of 13CO2/12CO2 isotopic ratio changes and oxygen in exhaled mouse breath samples. Mid-infrared absorption spectra for quantitatively monitoring the enrichment of 13CO2 levels were recorded in a miniaturized dual-channel substrate-integrated hollow waveguide using balanced ratiometric detection, whereas luminescence quenching was used for synchronously detecting exhaled oxygen levels. Allan variance analysis verified a CO2 measurement precision of 1.6‰ during a 480 s integration time. Routine online monitoring of exhaled mouse breath was performed in 14 mechanically ventilated and instrumented mice and demonstrated the feasibility of online isotope-selective exhaled breath analysis within microliters of probed gas samples using the reported combined sensor platform.

QCL spectroscopy combined with the least squares method for substance analysis

The article briefly describes distinctive features of quantum cascade lasers (QCL). It also describes an experimental set-up for acquiring mid-infrared absorption spectra using QCL. The paper demonstrates experimental results in the form of normed spectra. We tested the application of the least squares method for spectrum analysis. We used this method for substance identification and extraction of concentration data. We compare the results with more common methods of absorption spectroscopy. Eventually, we prove the feasibility of using this simple method for quantitative and qualitative analysis of experimental data acquired with QCL.

Intraspecific variation and influence of diet on the venom chemical profile of the Ectatomma brunneum Smith (Formicidae) ant evaluated by photoacoustic spectroscopy

Studies of venomous animals have shown that environmental and genetic factors contribute to determining the chemical composition of venom. It is well known that external effects cause differences in the toxicity, concentration, and prey specificity of venom. However, the influence of different factors on the chemical profile of Hymenoptera venom remains little explored. In view of this, the aim of this study was to evaluate intraspecific differences and the influence of diet on the chemical profile of Ectatomma brunneum venom using Fourier transform infrared photoacoustic spectroscopy. For the study of intraspecific variation of the venom, foragers were collected at locations with different environmental conditions, such as urban, intermediate, woodland and monoculture sites. To investigate the influence of diet on the venom, two colonies were sampled in the same area and were maintained in the laboratory under controlled diet conditions and at room temperature. The mid-infrared absorption spectra obtained were interpreted using discriminant function analysis. The results showed significant differences among the chemical profiles of the venoms of individuals from different environments and individuals exposed to a controlled diet in the laboratory, suggesting that venom composition was determined not only by genetic traits inherent to the species, but also by exogenous factors.

Ultrahigh-resolution spectroscopy for methyl mercaptan at the ν2-band by a distributed feedback interband cascade laser

The ultrahigh-resolution mid-infrared absorption spectrum of methyl mercaptan (CH3SH) was recorded and analyzed in the region of 2945–2953cm-1, where the strong ν2-band is located. Benefitting from the high-resolution of tunable laser absorption spectrometry (TLAS) with a distributed feedback interband cascade laser (DFB-ICL), 12-strong and 27-weak air-broadened absorption peaks belonging to the ν2-band of CH3SH were assigned with resolution of better than 2.24×10-5cm-1(~26fm/670kHz) and absorption cross section precision of 2.0%, at temperature of 298.15K and pressure of 1atm. For further description of the assigned transitions, the spectral line parameters and cross sections of these absorption peaks versus pressure were investigated. These ultrahigh resolution air-broadened spectra can be used as reference data for CH3SH remote sensing and absolute spectral measurement applications.

Modulation of intersubband light absorption and interband photoluminescence in double GaAs/AlGaAs quantum wells under strong lateral electric fields

The effect of a lateral electric field on the mid-infrared absorption and interband photoluminescence spectra in double tunnel-coupled GaAs/AlGaAs quantum wells is studied. The results obtained are explained by the redistribution of hot electrons between quantum wells and changes in the space charge in the structure. The hot carrier temperature is determined by analyzing the intersubband light absorption and interband photoluminescence modulation spectra under strong lateral electric fields.

Prediction of fatty acid chain length and unsaturation of milk fat by mid-infrared milk analysis.

Our objective was to develop partial least squares (PLS) models to predict fatty acid chain length and total unsaturation of milk fat directly from a mid-infrared (MIR) spectra of milk at 40°C and then determine the feasibility of using those measures as correction factors to improve the accuracy of milk fat determination. A set of 268 milks (modified milks, farm bulk tank milks, and individual cow) were analyzed for fat, true protein, and anhydrous lactose with chemical reference methods, and in addition a MIR absorption spectra was collected for each milk. Fat was extracted from another portion of each milk, the fat was saponified to produce free fatty acids, and the free fatty acids were converted to methyl esters and quantified using gas-liquid chromatography. The PLS models for predicting the average chain length (carbons per fatty acid) and unsaturation (double bonds per fatty acid) of fatty acids in the fat portion of a milk sample from a MIR milk spectra were developed and validated. The validation performance of the prediction model for chain length and unsaturation had a relative standard deviation of 0.43 and 3.3%, respectively. These measures are unique in that they are fat concentration independent characteristics of fat structure that were predicted directly with transmission MIR analysis of milk. Next, the real-time data output from the MIR spectrophotometer for fatty acid chain length and unsaturation of milk were used to correct the fat A (C=O stretch) and fat B (C-H stretch) measures to improve accuracy of fat prediction. The accuracy validation was done over a period of 5 mo with 12 sets of 10 individual farm milks that were not a part of the PLS modeling population. The correction of a traditional fat B virtual filter result (C-H stretch) for sample-to-sample variation in unsaturation reduced the Euclidean distance for predicted fat from 0.034 to 0.025. The correction of a traditional fat A virtual filter result (C=O stretch) modified with additional information on sample-to-sample variation of chain length and unsaturation gave the largest improvement (reduced Euclidean distance from 0.072 to 0.016) and the best validation accuracy (i.e., lowest Euclidean distance) of all the fat prediction methods.