Multi-mode absorption spectroscopy, MUMAS, using wavelength modulation and cavity enhancement techniques

Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band $b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-}Multi-mode absorption spectroscopy, MUMAS, has been combined with the techniques of wavelength modulation spectroscopy, WMS, and cavity enhanced absorption spectroscopy, CEAS, to record multiple molecular transitions using a single laser and a single detector. MUMAS signals were recorded using a multi-mode diode laser of the A-band b¹\varSigma _g⁺? X³\varSigma _g^-b^{1}\varSigma _{g}^{+}\leftarrow X^{3}\varSigma _{g}^{-} of molecular oxygen at 760 nm. Direct MUMAS and WMS-MUMAS signals were recorded using a White cell for air and pure oxygen for pressures in the range 0 to 1 bar. CEAS-MUMAS signals were recorded with and without WMS in an open enhancement cavity containing laboratory air. Enhancement of the signal-to-noise ratio has been obtained demonstrating the potential for increased detection sensitivity for gas-sensing applications of MUMAS.     

Multi-mode absorption spectroscopy of oxygen for measurement of concentration, temperature and pressure

The use of multi-mode absorption spectroscopy (MUMAS) to detect multiple transitions in the A-band b¹Σ_g ⁺-X³Σ_g ^- of molecular oxygen is reported. The modelling of MUMAS signatures and the procedure for fitting such model signatures to experimental data obtained using a multi-mode diode laser is described. The technique is shown to allow accurate and precise measurement of concentration, temperature over the range 300 to 500 K and of pressure over the range 0.2 to 1 bar. Extension of the technique to other ranges of temperature and pressure and to other species is discussed. PACS 42.62.Fi; 33.20.Kf     

Microstructural and Radioluminescence Characteristics of Nd<Superscript>3+</Superscript> Doped Columbite-Type SrNb<Subscript>2</Subscript>O<Subscript>6</Subscript> Phosphor

Undoped and different concentration Nd³⁺ doped SrNb₂O₆ powders with columbite structure were synthesized by molten salt process using a mixture of strontium nitrate and niobium (V) oxide and NaCl-KCl salt mixture as a flux under relatively low calcining temperature. X-ray diffraction analysis results indicated that SrNb₂O₆ phases found to be orthorhombic columbite single phase for undoped, 0.5 and 3 mol% Nd³⁺ doping concentrations. Phase composition of the powders was examined by SEM-EDS analyses. Radioluminescence properties of Nd³⁺ doped samples from UV to near-IR spectral region were studied. The emissions increased with the doping concentration of up to 3 mol%, and then decreased due to concentration quenching effect. There is a sharp emission peak around 880 nm associated with ⁴F_5/2 → ⁴I_9/2 transition in the Nd³⁺ ion between 300 and 1100 nm. The broad emission band intensity was observed from 400 to 650 nm where the peak intensities increased by increasing Nd³⁺ doping concentration. All the measurements were taken under the room temperature.     

Multi-species laser absorption sensors for in situ monitoring of syngas composition

Tunable diode laser absorption spectroscopy sensors for detection of CO, CO₂, CH₄ and H₂O at elevated pressures in mixtures of synthesis gas (syngas: products of coal and/or biomass gasification) were developed and tested. Wavelength modulation spectroscopy (WMS) with 1f-normalized 2f detection was employed. Fiber-coupled DFB diode lasers operating at 2325, 2017, 2290 and 1352 nm were used for simultaneously measuring CO, CO₂, CH₄ and H₂O, respectively. Criteria for the selection of transitions were developed, and transitions were selected to optimize the signal and minimize interference from other species. For quantitative WMS measurements, the collision-broadening coefficients of the selected transitions were determined for collisions with possible syngas components, namely CO, CO₂, CH₄, H₂O, N₂ and H₂. Sample measurements were performed for each species in gas cells at a temperature of 25 °C up to pressures of 20 atm. To validate the sensor performance, the composition of synthetic syngas was determined by the absorption sensor and compared with the known values. A method of estimating the lower heating value and Wobbe index of the syngas mixture from these measurements was also demonstrated.     

Detection of C2H2 and HCl using mid-infrared degenerate four-wave mixing with stable beam alignment: towards practical in situ sensing of trace molecular species

A stable and convenient optical system to realize the forward phase-matching geometry for degenerate four-wave mixing (DFWM) is demonstrated in the mid-infrared spectral region by measuring DFWM signals generated in acetylene (C₂H₂) and hydrogen chloride (HCl) molecules by probing the fundamental ro-vibrational transitions. IR laser pulses tunable from 2900 cm^?1 to 3350 cm^?1 with a 0.025 cm^?1 linewidth were obtained using a laser system composed of an injection seeded Nd:YAG laser, a dye laser, and a frequency mixing unit. At room temperature and atmospheric pressure, a detection limit of 35 ppm (～ 9.5×10¹⁴ molecules/cm³) for C₂H₂ was achieved in a gas flow of a C₂H₂/N₂ mixture by scanning the P(11) line of the (010(11)⁰)–(0000⁰0⁰) band. The detection limit of the HCl molecule was measured to be 25 ppm (～6.8×10¹⁴ molecules/cm³) in the same environment by probing the R(4) line. The dependences of signal intensities on molecular concentrations and laser pulse energies were demonstrated using C₂H₂ as the target species. The variations of the signal line shapes with changes in the buffer gas pressures and laser intensities were recorded and analyzed. The experimental setup demonstrated in this work facilitates the practical implementation of in situ, sensitive molecular species sensing with species-specific, spatial and temporal resolution in the spectral region of 2.7–3.3 μm (3000–3700 in cm^?1), where various molecular species important in combustion have absorption bands.     

TDLAS-based sensors for in situ measurement of syngas composition in a pressurized,oxygen-blown,entrained flow coal gasifier

Tunable diode laser absorption spectroscopy based in situ sensors for CO (2.33 μm), CO₂ (2.02 μm), CH₄ (2.29 μm) and H₂O (1.35 μm) were deployed in a pilot-scale (1 ton/day), high-pressure (up to 18 atm), entrained flow, oxygen-blown, slagging coal gasifier at the University of Utah. Measurements of species mole fraction with 3-s time resolution were taken at the pre- and post-filtration stages of the gasifier synthesis gas (called here syngas) output flow. Although particulate scattering makes pre-filter measurements more difficult, this location avoids the time delay of flow through the filtration devices. With the measured species and known N₂ concentrations, the H₂ content was obtained via balance. The lower heating value and the Wobbe index of the gas mixture were estimated using the measured gas composition. The sensors demonstrated here show promise for monitoring and control of the gasification process.     

Enhanced ethanol sensing properties of TiO2/ZnO core–shell nanorod sensors

TiO₂-core/ZnO-shell nanorods were synthesized using a two-step process: the synthesis of TiO₂ nanorods using a hydrothermal method followed by atomic layer deposition of ZnO. The mean diameter and length of the nanorods were ～300 nm and ～2.3 μm, respectively. The cores and shells of the nanorods were monoclinic-structured single-crystal TiO₂ and wurtzite-structured single-crystal ZnO, respectively. The multiple networked TiO₂-core/ZnO-shell nanorod sensors showed responses of 132–1054 % at ethanol (C₂H₅OH) concentrations ranging from 5 to 25 ppm at 150 ^°C. These responses were 1–5 times higher than those of the pristine TiO₂ nanorod sensors at the same C₂H₅OH concentration range. The substantial improvement in the response of the pristine TiO₂ nanorods to C₂H₅OH gas by their encapsulation with ZnO may be attributed to the enhanced absorption and dehydrogenation of ethanol. In addition, the enhanced sensor response of the core–shell nanorods can be attributed partly to changes in resistance due to both the surface depletion layer of each core–shell nanorod and the potential barriers built in the junctions caused by a combination of homointerfaces and heterointerfaces.     

Polyhydroxy fullerenes

Characterization of C₆₀ polyhydroxyfullerenes (PHF) prepared in alkaline media, preparation facilitated by phase-transfer catalyst, presents challenges in determining the chemical structure resulting from the possibility of multiple isomers or analogs with greater or fewer hydroxyl groups from a single reaction mixture. This paper presents the utilization of analytical methods employed in tandem, especially X-ray photoelectron spectroscopy, nuclear magnetic resonance spectroscopy, Fourier transform infrared spectroscopy for semi-quantitative analysis on the number of hydroxyl groups present in PHF. Capillary Electrophoresis was used for purity estimation of the material. Multiple spectra and electropherograms were analyzed using a new simultaneous curve fitting method. The most accurate estimate of hydroxyl groups for C₆₀ polyhydroxy fullerenes obtained is between 16 and 18 allylic hydroxyl groups by combining analytical methods’ results with 5 % accuracy. High precision (reproducibility) of the experiments is observed. Purity of 98 % is estimated by capillary electrophoresis. The size of PHF nanoparticles or aggregates has been determined by atomic force microscopy to be 7.4–14.2 nm. According to the elemental analysis the average probable empirical formula for the most pure PHF at pH 7.1 is C₆₀O₁₇H₁₂Na₅(NaHCO₃)₃(H₂O)₁₃ and the average formula weight is 1,605.9 g/mol. This is the first thorough characterization of PHF in terms of purity.     

Mixture Design Optimization of an Analytical Procedure for Iron Extraction and Determination From Cassava Leaves by Slurry Sampling Flame Atomic Absorption Spectrometry

ABSTRACT

This paper applies statistical simplex-centroid design to mixture modeling for optimization of the liquid phase composition of cassava slurry leaves in the development of an analytical procedure for iron determination using flame atomic absorption spectrometry (FAAS). This procedure is based on a slurry formation from powdered cassava leaves and a liquid mixture composed of HNO₃, HCl, and H₂O₂ after an ultrasonication process. A quadratic model fitted to the analytical response shows the existence of an experimental region, characterized by low proportions of H₂O₂, for which higher responses are obtained. The proposed procedure allows the determination of iron in the cassava leaves with a detection limit of 1.1 µg g^?1. The precision expressed as relative standard deviation (%RSD, n = 10) was 1.5% for iron concentrations of 25 µg g^?1. The developed procedure was validated by the comparison of results obtained from the application of the digestion procedure and the analysis of certified reference materials: Apple leaves (NIST 1515). Results found were in agreement with the certified values. The proposed method was applied for the determination of iron in four samples of cassava leaves acquired in markets of Feira de Santana City, Brazil. The concentration of iron found in the cassava leaves varied from 250.8 ± 0.7 to 283.4 ± 0.7 µg g^?1.     

Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF₂ (111) was irradiated with single and multiple laser (Ti:sapphire, 800 nm, 25 fs) shots at fluences ranging from 0.25 to 1.5 J cm^?2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200 nm to 1.5 μm in diameter and 10–50 nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF₂ is irradiated with successive (in our case 20) shots at a laser fluence of 1.5 J cm^?2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.     

Electron temperature and ion density measurements in a glow discharge of an Ar–N2 mixture

A DC glow discharge produced in N₂ gas can generate several species that are important in different applications, such as the modification of surface properties of materials. A low-pressure glow discharge apparatus was used for the the analysis of the Ar–N₂ mixture at a total pressure of 2.0 Torr, a power of 20 W and 40 l/min flow rate of gases. The emission bands were measured in the wavelength range of 200–1100 nm. The principal elements are N₂, N ₂⁺ and Ar I. The electron temperature was found in the range of 1.72–2.08 eV, and the ion density was in the order of 10¹⁰ cm^?3.     

Determination of temperature and concentrations of main components in flames by fitting measured Raman spectra

The procedure of deriving flame temperature and major species concentrations by fitting measured Raman spectra in hydrocarbon flames is described. The approach simplifies the calibration procedure to determine temperature and major species concentrations from the measured Raman spectra. The calculations of the Raman spectra are performed using data online positions and cross sections from the current literature. Utilizing all spectral information for deriving temperature and major species concentrations substantially increases accuracy, while interferences can easily be detected and filtered out of the measured spectrum. Temperatures from the separate Raman spectra of N₂, H₂O, O₂, CO₂ and CO are systematically compared with each other over the span of more than 1,700 K. The agreement between them is generally better than 100 K. The developed procedure also allows us to determine the mole fractions of the major species with absolute accuracy of ±10 %.     

Simultaneous Determination of Ritodrine and Isoxsuprine Using Coupling Technique of Synchronous Fluorimetry and H-Point Standard Addition Method

Simultaneous determination of two structurally related ß₂ adrenergic receptor agonists namely, Ritodrine HCl (RTH) and Isoxsuprine HCl (ISP) was performed using coupling technique of synchronous fluorimetry and H-point standard addition method. Under optimum conditions, linear determination ranges were 1.48 – 14.80?×?10^?6 mol L^?1 and 1.54 – 15.44?×?10^?6 mol L^?1 for ISP and RTH respectively. RTH and ISP could be determined simultaneously without interference from each other when their concentration ratio varies from 5:1 to 1:5 in the mixed sample. The proposed method was applied to the determination of RTH and ISP in synthetic mixture of pharmaceutical samples, the accuracy and precision of the results were satisfactory.     

UV-enhanced room-temperature gas sensing of ZnGa2O4 nanowires functionalized with Au catalyst nanoparticles

ZnGa₂O₄ nanowires were synthesized using a thermal evaporation technique. Scanning electron microscopy, transmission electron microscopy, and X-ray diffraction revealed that the nanowires were single crystals 30–200 nm in diameter and ranged up to ～100 μm in length. The sensing properties of multiple networked ZnGa₂O₄ nanowire sensors functionalized with Au catalyst nanoparticles with diameters of a few nanometers toward NO₂ gas at room temperature under UV irradiation were examined. The sensors showed a remarkably enhanced response and far reduced response and recovery times toward NO₂ gas at room temperature under 254 nm-ultraviolet (UV) illumination. The response of ZnGa₂O₄ nanowires to NO₂ gas at room temperature increased from ～100 to ～861 % with increasing the UV intensity from 0 to 1.2 mW/cm². The significant improvement in the response of ZnGa₂O₄ nanowires to NO₂ gas by UV irradiation is attributed to the increased change in resistance due to the increase in the number of electrons participating in the reactions with NO₂ molecules by photo-generation of electron–hole pairs.     

Follow up of the glassy phase formation as silicon oxide was added to Brownmillerite phase of Portland cement clinker

Brownmillerite phase is one of the four main phases of Portland cement clinker. It was prepared as pure C₄AF¹ and C₄AF with different amount of SiO₂, (5, 10, 15, 20, 25, and 40 mol%) by addition. Pure C₄AF was prepared using CaO, Al₂O₃ and Fe₂O₃ according to the ratios 4:1:1. Each sample mixture was fired at 1,400°C for 1 h then ground and introduced again to 1,400°C for 1/2 h then quenched in air. The prepared samples were ground and measured using x-ray diffraction, scanning electron microscope, A.C. conductivity and Mössbauer spectroscopy. The results were correlated and discussed. The main finding is the formation of a glassy phase besides the C₄AF structure, in addition to the formation of the C₂S phase of cement clinker as SiO₂ addition was upgraded. The electrical conductivity results showed that the 20 mol% SiO₂ sample has the lowest (σ) value.     

New insight on nanostructure assembling of high-performance electrode materials: synthesis of surface-modified hexagonal β-Ni(OH)<Subscript>2</Subscript> nanosheets as an example

Hexagonal β-Ni(OH)₂ nanosheets with thickness of ～12 nm were synthesized by a hydrothermal method at 150 °C using nickel chloride as nickel source and morpholine as alkaline. Electrodes for application in pseudocapacitor were assembled through a traditional technique: pressing a mixture of β-Ni(OH)₂ nanosheets and acetylene black onto nickel foam. Due to the hexagonal shape of rigid β-Ni(OH)₂ nanosheet and the mediation of surface-modified glycerol during electrochemical charge–discharge cycles, a nanostructure of electrode material with facile interior pathway for the transfer of electrolyte was formed. As a result, the as-formed electrodes presented high specific capacitance of 1,917 F g^?1 at current density of 1.6 A g^?1 in 3 mol L^?1 KOH solution. At high charge and discharge current density of 31.3 A g^?1, the electrodes still remained a high specific capacitance of 1,289 F g^?1. The interesting results obtained from this investigation may provide a new insight for the synthesis of electrode materials with high electrochemical performance.     

Sr<Subscript>4</Subscript>Al<Subscript>14</Subscript>O<Subscript>25</Subscript>: Eu<Superscript>2+</Superscript>, Dy<Superscript>3+</Superscript>/silica core–shell particles synthesized via urea combustion method for carbon dioxide reduction in plants

Strontium Aluminate doped with Europium and Dysprosium is one of the most widely studied phosphors because of its high intensity and long persistence time. In this study, the unique characteristics of strontium aluminate based phosphors, specifically Sr₄Al₁₄O₂₅: Eu²⁺, Dy³⁺, was utilized as light source for plants for enhanced carbon dioxide reduction in dark field conditions. The Sr₄Al₁₄O₂₅: Eu²⁺, Dy³⁺ phosphor was synthesized using the combustion method. Stoichiometric amounts of aqueous precursors were dissolved in water, then placed in a high temperature furnace at 600 °C to obtain a foamy, amorphous precursor powder. The powders were cooled to room temperature and then grinded. After grinding, the powders were calcined for 8 h at 1300 °C. The powders were then encapsulated with silica particles using the Stöber process to prevent the oxidation of Eu²⁺ without a reducing atmosphere during calcination. The obtained coated and uncoated particles were then characterized using SEM, TEM–EDX, XRD and photoluminescence analysis to determine the effect of the core–shell structure on the luminescence properties of the phosphors. Finally, the obtained phosphor-silica core–shell particles will be attached to the surface of four different plant species commonly grown indoors using a mixture of natural oils and waxes as adhesive. The effect of the addition of phosphor as an external light source on the amount of carbon dioxide production of the plants will be monitored and compared to a control specimen without the phosphor as well as with other artificial light sources.     

Diamond anvil cell syntheses and compressibility studies of the spinel-structured gallium oxonitride

The formation conditions of cubic spinel-structured gallium oxonitride have been investigated in situ under high-pressure/high-temperature conditions using a laser-heated diamond anvil cell. As starting materials, a mixture of the end members w-GaN/β-Ga₂O₃ in a molar ratio of 3:2 and a gallium oxonitride ceramic derived during pyrolysis from the metallo-organic precursor (Ga(O^tBu)₂NMe₂)₂ were used. In the mixture of the end members, spinel-structured gallium oxonitride starts crystallizing at a pressure of 3 GPa and at a temperature of about 1300 °C. The precursor-derived ceramic with predefined bondings reacted completely to the spinel phase, without by-products, at a pressure of 0.7 GPa. For the spinel-structured gallium oxonitride we determined a bulk modulus K of 216(7) GPa using a fixed value of 4 for K′. The spinel-structured gallium oxonitride exhibits a cell volume of 552.9(5) ų at ambient pressure.     

Simulation of EPR Spectra as a Tool for Interpreting the Degradation Pathway of Hyaluronan

Electron paramagnetic resonance (EPR) spin trapping is one of the choice techniques for identifying free radicals and is often used in the study of biological systems. However, its sensitivity can result in a typical complicated EPR spectrum. The accurate simulation of these systems is essential for correct identification of the radical species, whenever more than one species contributes to the spectrum. Programs implementing the linear combination of single simulations allow the interpretation of EPR spectra without modifying experimental conditions. In this study, this approach was used to investigate the influence of the ferrous ion and the role of oxygen, as well on the formation of transient radical species, in the whole mechanism of hyaluronan degradation. Degradation was carried out under different environmental conditions (air, O₂, Ar, N₂, N₂ + CO₂) and EPR spin trapping studies were performed. The advantages of the simulation of multiple species EPR spectra were applied to the obtained results and some aspects of hyaluronan degradation mechanism were elucidated. The depolymerization reaction pathway has been defined according to two possible subsequent steps: the first is consistent with formation of an amidyl radical that induces a series of strand scissions, which stabilize at two different levels of molecular weight. The second step occurs when the molecular weight is lower than before and two different adducts are generated.     

Using a DS-DBR laser for widely tunable near-infrared cavity ring-down spectroscopy

Studies into the suitability of a novel, widely tunable telecom L-band (1,563–1,613 nm) digital supermode distributed Bragg reflector (DS-DBR) laser for cavity ring-down spectroscopy (CRDS) are presented. The spectrometer comprised of a 36.6?cm long linear cavity with ring-down times varying between 19–26 μs across the 50 nm DS-DBR wavelength range due to changes in the cavity mirror reflectivities with wavelength. The potential of such a broadband, high-resolution CRD spectrometer was illustrated by investigating several transitions of CO₂ in air, a 5 % calibrated mixture and breath samples. Allan variance measurements at a single wavelength indicated an optimal minimum detectable absorption coefficient (α _min) of 3 × 10^?10 cm^?1 over 20 s.