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Probing Organometallic Reactions by Time-Resolved Infrared Spectroscopy in Solution and in the Solid State Using Quantum Cascade Lasers
James A. Calladine, Raphael Horvath, Andrew J. Davies, Alisdair Wriglesworth, Xue-Zhong Sun, and Michael W. George
The photochemistry and photophysics of metal carbonyl compounds (W(CO)6, Cp*Rh(CO)2 (Cp* = η5-C5Me5), and fac-[Re(CO)3(4,4'-bpy)2Br] [bpy = bipyridine]) have been examined on the nanosecond timescale using a time-resolved infrared spectrometer using an external cavity quantum cascade laser (QCL) as the infrared source. We show the photochemistry of W(CO)6 in alkane solution is easily monitored, and very sensitive measurements are possible with this approach, meaning it can monitor small transients with absorbance changes less than 10-6 ΔOD. The C–H activation of Cp*Rh(CO)(C6H12) to form Cp*Rh(CO)(C6H11)H occurs within the first few tens of nanoseconds following photolysis, and we show that kinetics obtained following deconvolution are in excellent agreement with those measured using an ultrafast laser-based spectrometer. We also show that the high flux and tunability of QCLs makes them suited for solid-state and time-resolved measurements.
Keywords: Quantum cascade laser; QCL; Photochemistry; Time-resolved; Infrared.
Deep-Ultraviolet Raman Measurements Using a Spatial Heterodyne Raman Spectrometer (SHRS)
Nirmal Lamsal, and S. Michael Angel
A deep-ultraviolet (UV) 244 nm excitation spatial heterodyne Raman spectrometer (SHRS) is demonstrated for the first time. The deep-UV SHRS has no moving parts, and even though it is small for a deep-UV Raman spectrometer, the spectral resolution is shown to be about 4 cm-1. The deep-UV SHRS also has a large input aperture and acceptance angle, and the resulting large field of view is shown to be useful to avoid laser-induced sample degradation. In this feasibility study, Raman spectra of several compounds are measured to demonstrate the spectral resolution and range of the system. A photosensitive compound is also measured to demonstrate the use of a large laser spot to minimize UV-laser-induced sample degradation.
Keywords: Raman spectroscopy; Fourier transform Raman; FT Raman; Interferometry; Deep-ultraviolet Raman; Deep-UV Raman; Spatial heterodyne Raman spectroscopy.
Standoff Detection of Highly Energetic Materials Using Laser-Induced Thermal Excitation of Infrared Emission
Nataly J. Galán-Freyle, Leonardo C. Pacheco-Londoño, Amanda M. Figueroa-Navedo, Migdalia Hidalgo-Santiago, and Samuel P. Hernandez-Rivera
A laser-mediated methodology for standoff infrared detection of threat chemicals is described in this article. Laser-induced thermal emissions (LITE) from vibrationally excited residue of highly energetic material (HEM) deposited on substrates were detected remotely. Telescope-based Fourier transform infrared (FT-IR) spectroscopy measurements were carried out on substrates containing small amounts of HEM at surface concentrations of 5– 200 μg/cm2. Target substrates of various thicknesses were heated remotely using a carbon dioxide laser, and their mid-infrared (mid-IR), thermally stimulated emission spectra were recorded after heating. The telescope was configured from reflective optical elements to minimize emission losses in the mid-IR frequencies. Spectral replicas were acquired at distances from 4 to 64 m using an FT-IR interferometer at 4 cm-1 resolution. The laser power, laser exposure times, and acquisition time of the FT-IR interferometer were adjusted to improve the detection and identification of samples. The advantages of increasing the thermal emission were easily observed in the results. The signal intensities were proportional to the thickness of the coated surface (a function of the surface concentration) as well as the laser power and laser exposure time. The limits of detection obtained for the HEM studied were 140–21 μg/cm2 at 4 m. Detection was achieved at 64 m for a surface concentration of 200 μg/cm2.
Keywords: Standoff detection; Laser-induced thermal emission; Highly energetic material; HEM; Mid-infrared emission spectroscopy; Carbon dioxide laser.
Solution and Solid Hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) Ultraviolet (UV) 229 nm Photochemistry
Katie L. Gares, Sergei V. Bykov, Thomas A. Brinzer, and Sanford A. Asher
We measured the 229 nm deep-ultraviolet resonance Raman (DUVRR) spectra of solution and solid-state hexahydro-1,3,5- trinitro-1,3,5-triazine (RDX). We also examined the photochemistry of RDX both in solution and solid states. RDX quickly photodegrades with a high-performance liquid chromatography (HPLC) measured solution quantum yield of φ~ 0.35. New spectral features form over time during the photolysis of RDX, indicating photoproduct formation. The photoproduct(s) show stable DUVRR spectra at later irradiation times that allow standoff detection. In the solution-state photolysis, nitrate is a photoproduct that can be used as a signature for detection of RDX even after photolysis. We used high-performance liquid chromatography–high-resolution mass spectrometry (HPLC-HRMS) and gas chromatography mass spectrometry (GCMS) to determine some of the major solution-state photoproducts. X-ray photoelectron spectroscopy (XPS) was also used to determine photoproducts formed during solid-state RDX photolysis.
Keywords: Ultraviolet resonance Raman spectroscopy; UVRRS; Hexahydro-1,3,5-trinitro-1,3,5-triazine photochemistry; RDX photochemistry; HPLC-HRMS; High-performance liquid- chromatography–high-resolution mass spectrometry; Standoff detection; Explosives; XPS; X-ray photoelectron spectroscopy.
Distribution of Hydroxyl Groups in Kukersite Shale Oil: Quantitative Determination by Fourier Transform Infrared (FT-IR) Spectroscopy
Zachariah Steven Baird, Vahur Oja, and Oliver Järvik
This article describes the use of Fourier transform infrared (FT-IR) spectroscopy to quantitatively measure the hydroxyl concentrations among narrow boiling shale oil cuts. Shale oil samples were from an industrial solid heat carrier retort. Reference values were measured by titration and were used to create a partial least squares regression model from FT-IR data. The model had a root mean squared error of 0.44 wt% OH. This method was then used to study the distribution of hydroxyl groups among more than 100 shale oil cuts, which showed that hydroxyl content increased with the average boiling point of the cut up to about 350 °C and then leveled off and decreased.
Keywords: Phenols; Shale oil; Fourier transform infrared spectroscopy; FT-IR spectroscopy; Partial least squares regression; PLS regression; Hydroxyl group.
A Promising Raman Spectroscopy Technique for the Investigation of trans and cis Cholesteryl Ester Isomers in Biological Samples
Michele Melchiorre, Carla Ferreri, Anna Tinti, Chryssostomos Chatgilialoglu, and
Lipid geometry is an important issue in biology and medicine. The cis–trans geometry conversion of double bonds in lipids is an endogenous process that can be mediated by sulfur-centered free radicals. Trans isomers of polyunsaturated fatty acids can be used as biological markers of free radical stress, and their presence in biological samples can be determined by synthesis and characterization of appropriate reference compounds. Fractions of plasma lipids, such as cholesteryl linoleate and arachidonate esters, are interesting targets because of their connection with membrane phospholipid turnover and their roles in cardiovascular health. In this context, Raman spectroscopy can provide a useful contribution, since Raman analysis can be performed directly on the lipid extracts without any derivatization reaction, is nondestructive, and can rapidly supply biochemical information. This study focused on the build up of Raman spectral libraries of different cis and trans isomers of cholesteryl esters to be used as references for the examination of complex biological samples and to facilitate isomer recognition. Unsaturated cholesteryl esters obtained by chemical synthesis and with different alkyl chain lengths, double bond numbers, or both, were analyzed. The potential of Raman analysis for trans isomer detection in biological samples was successfully tested on some cholesteryl ester lipid fractions from human serum. The data suggest promising applications of Raman spectroscopy in metabolomics and lipidomics.
Keywords: Cholesteryl ester; Raman spectroscopy; Free radical damage; Trans lipid; Lipid geometrical isomerism.
Fano-Like Resonances in Split Concentric Nanoshell Dimers in Designing Negative-Index Metamaterials for Biological–Chemical Sensing and Spectroscopic Purposes
Arash Ahmadivand, Mustafa Karabiyik, and Nezih Pala
In this study, we investigated numerically the plasmon response of a dimer configuration composed of a couple of split and concentric Au nanoshells in a complex orientation. We showed that an isolated composition of two concentric split nanoshells could be tailored to support strong plasmon resonant modes in the visible wavelengths. After determining the accurate geometric dimensions for the presented antisymmetric nanostructure, we designed a dimer array that shows complex behavior during exposure to different incident polarizations. We verified that the examined dimer was able to support destructive interference between dark and bright plasmon modes, which resulted in a pronounced Fano-like dip. Observation of a Fano minimum in such a simple molecular orientation of subwavelength particles opens new avenues for employing this structure in designing various practical plasmonic devices. Depositing the final dimer in a strong coupling condition on a semiconductor metasurface and measuring the effective refractive index at certain wavelengths, we demonstrate that each one of dimer units can be considered a meta-atom due to the high aspect ratio in the geometric parameters. Using this method, by extending the number of dimers periodically and illuminating the structure, we examined the isotropic, polarization-dependent, and transmission behavior of the metamaterial configuration. Using numerical methods and calculating the effective refractive indices, we computed and sketched corresponding figure of merit over the transmission window, where the maximum value obtained was 42.3 for Si and 54.6 for gallium phosphide (GaP) substrates.
Keywords: Split concentric nanoshells; Plasmon hybridization; Fano-like resonance; Metamaterial; Figure of merit; FOM.
High-Sensitivity Surface-Enhanced Raman Scattering (SERS) Substrate Based on a Gold Colloid Solution with a pH Change for Detection of Trace-Level Polycyclic Aromatic Hydrocarbons in Aqueous Solution
Xiaofeng Shi, Shu Liu, Xiaohong Han, Jun Ma, Yongchao Jiang, and Guifeng Yu
In this study, a gold colloid solution whose parameters were optimized, and without any surfactants, was developed as a surface-enhanced Raman scattering (SERS) substrate for the detection of trace-level polycyclic aromatic hydrocarbons (PAHs). A gold colloid solution with 57 nm gold particles and pH 13 was prepared to be the SERS substrate. It had impressive enhancement that was two orders of magnitude higher than that of a gold colloid solution with 57 nm gold particles and without pH change (pH 6). Even with a compact field-based Raman spectrometer, naphthalene, phenanthrene, anthracene, fluoranthene, and pyrene were detected, with limits of detection at 6.8 nm, 3.4 nm, 1.8 nm, 0.68 nm(680 pM), and 0.44 nm (440 pM), respectively. The significant enhancement was ascribed to an electromagnetic mechanism and a charge-transfer mechanism. Quantitative analyses for these five PAHs in water were also performed. The SERS intensities of PAHs were found to have good linear dependence relations with the concentrations in low concentration. This high-sensitivity, easily prepared substrate offers a promising technology for the quantitative detection of trace-level PAHs.
Keywords: Surface-enhanced Raman scattering; SERS; Polycyclic aromatic hydrocarbon; PAH; Gold colloid solution; pH.
Matrix Effects in Quantitative Assessment of Pharmaceutical Tablets Using Transmission Raman and Near-Infrared (NIR) Spectroscopy
Anders Sparén, Madeleine Hartman, Magnus Fransson, Jonas Johansson, and Olof Svensson
Raman spectroscopy can be an alternative to near-infrared spectroscopy (NIR) for nondestructive quantitative analysis of solid pharmaceutical formulations. Compared with NIR spectra, Raman spectra have much better selectivity, but subsampling was always an issue for quantitative assessment. Raman spectroscopy in transmission mode has reduced this issue, since a large volume of the sample is measured in transmission mode. The sample matrix, such as particle size of the drug substance in a tablet, may affect the Raman signal. In this work, matrix effects in transmission NIR and Raman spectroscopy were systematically investigated for a solid pharmaceutical formulation. Tablets were manufactured according to an experimental design, varying the factors particle size of the drug substance (DS), particle size of the filler, compression force, and content of drug substance. All factors were varied at two levels plus a center point, except the drug substance content, which was varied at five levels. Six tablets from each experimental point were measured with transmission NIR and Raman spectroscopy, and their concentration of DS was determined for a third of those tablets. Principal component analysis of NIR and Raman spectra showed that the drug substance content and particle size, the particle size of the filler, and the compression force affected both NIR and Raman spectra. For quantitative assessment, orthogonal partial least squares regression was applied. All factors varied in the experimental design influenced the prediction of the DS content to some extent, both for NIR and Raman spectroscopy, the particle size of the filler having the largest effect. When all matrix variations were included in the multivariate calibrations, however, good predictions of all types of tablets were obtained, both for NIR and Raman spectroscopy. The prediction error using transmission Raman spectroscopy was about 30% lower than that obtained with transmission NIR spectroscopy.
Keywords: Transmission Raman spectroscopy; Transmission near-infrared spectroscopy; Transmission NIR; Matrix effects; Particle size; Pharmaceutical tablets; Quantitative analysis.
Terahertz (6–15 THz) Spectroscopy and Numerical Modeling of Intermolecular Vibrations in Benzoic Acid and Its Derivatives
Lucia M. Lepodise, Josip Horvat, and Roger A. Lewis
Terahertz spectroscopy of benzoic acid (BA) and two of its derivatives, 2-hydroxybenzoic acid (2OH-BA) and 3-hydroxybenzoic acid (3OH-BA), has been investigated in the spectral region 200 to 500 cm-1 (6.06 to 15.15 THz). The spectra show distinct absorption features. There is agreement between some of the absorption lines observed for these compounds, despite a shift in energy, which is attributed to the differences in the molecular structures. Numerical modeling gave corresponding absorption lines, and this helped in the assignment. Temperature dependence studies revealed that most of the absorption lines are composite lines in this frequency region.
Keywords: Fourier transform infrared spectroscopy; FT-IR spectroscopy; Terahertz; THz; Vibrations; Intermolecular; Radiation.
Relative Intensity Correction of Raman Systems with National Institute of Standards and Technology Standard Reference Material 2242 in 90°-Scattering Geometry
Magnus Schlösser, Simone Rupp, Tim Brunst, and Timothy M. James
The U.S. National Institute of Standards and Technology (NIST) has certified a set of Standard Reference Materials (SRMs) that can be used to accurately determine the spectral sensitivity of Raman spectrometers. These solid-state reference sources offer benefits such as exact reproduction of Raman sampling geometry, simple implementation, and long-term stability. However, a serious drawback of these SRMs is that they are certified only in the backscattering (180°) configuration. In this study, we investigated if and how an SRM 2242 (applicable for 532 nm) can be employed in a 90°-scattering geometry Raman system. We found that the measurement procedure needs to be modified to comply with the certified uncertainty provided by the NIST. This requires a change in the SRM illumination that is possible only if we polish the side surfaces. In addition, we need to account for the polarization configuration of the Raman system by choosing the appropriate polarization of the excitation beam. On top of that, the spatial inhomogeneity of the luminescence light needs to be taken into account, as well as its behavior while traveling through the SRM bulk. Finally, we show in a round-robin test that the resulting uncertainty for the quantification of Raman spectra using the modified technique is on the order of ±1.5 percentage points.
Keywords: Standard Reference Material; SRM; Luminescence standard; Calibration; Spectral sensitivity; Quantum efficiency; Quantification; Raman; 90° Raman spectroscopy; Polarization.
Total Internal Reflection Fluorescence Spectrometry Using a Dual Optical Fiber Sample Cell
Cole A. Guyer, and Shiquan Tao
A novel total internal reflection fluorescence (TIRF) spectrometric technique using a dual optical fiber sample cell was developed. A conventional silica optical fiber was used for exciting fluorescence compounds in its evanescent wave field. A liquid core waveguide (LCW) was used to collect the fluorescence photons emitted from fluorescence compounds existing in an excitation fiber’s evanescent wave field. The collected fluorescence photons were guided through the LCW and sent to a fluorescence spectrometer for detection. The spatial separation of excitation light and fluorescence light reduces the excitation-light-related optical noise signal, which is the major factor limiting fluorescence techniques from achieving lower detection limit. Preliminary results obtained from this work indicate that the optical fiber TIRF system of this work can detect 4.6*10-18 mole rhodamine 6G (2.7*106 molecules) existing in the evanescent wave field of the excitation optical fiber.
Keywords: Total internal reflection fluorescence; TRIF; Evanescent wave excited fluorescence; Liquid core waveguide; Dual optical fiber sample cell, Rhodamine 6G.
Optical Characteristics and Chemical Composition of Dissolved Organic Matter (DOM) from Riparian Soil by Using Excitation-Emission Matrix (EEM) Fluorescence Spectroscopy and Mass Spectrometry
Yulai Wang, Changming Yang, Limin Zou, and Hengzhao Cui
Understanding the quantity and quality of soil dissolved organic matter (DOM) in riparian buffer zones is critical for explaining the biogeochemical processes of soil DOM in river ecosystems. Here, we investigated the dissolved organic carbon, fluorescent DOM (FDOM), and DOM molecules from riparian soils on Chongming Island in eastern China. Simultaneously, the soil DOM was extensively characterized in terms of the total aromaticity index (TAI) and several optical indices. The excitation (Ex)-emission (Em) matrix-parallel factor analysis results showed that two humic-like components were present (Ex/Em = 283(364)/454 nm; 337/410 nm), a fulvic-like component (Ex/Em = 241/426 nm) and a microbial degradation component (Ex/Em = 295/382 nm). The humic-like and fulvic-like substances were the main components in the riparian soil FDOM, accounting for ~90% of the FDOM. Mass spectrometry provided more detailed information for the soil DOM molecules. Six chemical fractions, amino acids, carbonyl compounds, fatty acids, lipids, proteins and sugars, were identified using liquid chromatography with quadrupole time-of flight mass spectrometry. Lipids, proteins, and carbonyl compounds were dominant in the soil DOM, accounting for >85% of the detected molecules (m/z < 1000). Significant differences were observed between the quantities of the six soil DOM chemical fractions at the different sampling locations. Discriminant molecules verified the hypothesis that the chemical soil DOM fractions varied with the land use of the adjacent watersheds. The TAI for the soil DOM could provide more reliable information regarding the biogeochemical processes of DOM. The carbonyl compounds and lipid fractions controlled this index. Overall, the optical indices and TAI values can improve our understanding of soil DOM quality; however, the optical indices did not provide quantitative evidence regarding the sources or properties of the soil DOM. The observations from this study provided detailed information regarding the soil DOM quality and the presence of specific molecules and improved our understanding of the biogeochemical processes of DOM.
Keywords: Chongming Island; Riparian buffer zone; Soil; Dissolved organic matter; DOM; Excitation–emission matrix; EEM; Liquid chromatography with quadrupole time-of-flight mass spectrometry; LC-QTOF-MS.
Optical Properties of Sodium Chloride Solution Within the Spectral Range from 300 to 2500 nm at Room Temperature
Xingcan Li, Linhua Liu, Junming Zhao, and Jianyu Tan
The optical properties of sodium chloride (NaCl) solution were experimentally determined by double optical pathlength transmission method in the spectral range from 300 to 2500 nm at the NaCl concentration range from 0 to 360 g/L. The results show that the refractive index of NaCl solution increases with NaCl concentrations and correlates nonlinearly with the concentration of NaCl solution. The absorption index of NaCl solution increases with NaCl concentrations in the visible spectral range of 300–700 nm, but varies little in the near-infrared spectral range of 700–2500 nm at room temperature. For the sake of applications, the fitted formulae of the refractive index and absorption index of NaCl solution as a function of wavelength and NaCl concentration are presented.
Keywords: Optical properties; Sodium chloride solution; Transmission measurement; Solar irradiation.