Saturation of sodium fluorescence in a flame irradiated with a pulsed tunable dye laser

Theoretical and experimental saturation curves and associated saturation parameters for the Na-D doublet in an H₂-O₂-Ar flame at 1 atm (T=1700 K) are compared. These parameters are found to agree within the experimental error of 25%. An explanation based on the spatial and temporal distribution of the laser intensity is given for the deviating saturation curves reported by various authors. The shape of the fluorescence pulse is shown to depend on the O₂ concentration in the flame. A lower limit for the rate constant of the doublet mixing transition is estimated from the ratio of the saturated fluorescence intensities of the Na-D components and found to be 3x10⁸s^-1. Conclusions are drawn which restrict the use of saturated atomic fluorescence intensities as a measure for the total atomic number density in the flame.     

Two-dimensional imaging of glyoxal (C2H2O2) in acetylene flames using laser-induced fluorescence

2 H₂O₂). Laser-induced fluorescence spectra from glyoxal vapor using the same excitation wavelength of 428 nm showed the same strongest lines as the signal from the flame. Glyoxal was visualized in two different modes; two-dimensional imaging and a spatial-spectral mode where spectra were obtained at different spatial positions in the flame simultaneously. For the premixed laminar rich flame it is shown that glyoxal is produced early in the flame, before the signals for C₂ and CH appear. For the turbulent non-premixed flames it is shown that glyoxal is produced in a layer on the fuel rich side of the flames. Here the fuel is premixed with ambient air. This layer is thin and has a high spatial resolution. The general trend was that the glyoxal signal appeared in regions with a lower temperature compared with the emission from C₂ and CH. The imaging of glyoxal in turbulent acetylene flames is a promising tool for achieving new insight into flame phenomena, as it gives very good structural information on the flame front. Tests so far do not indicate that the detected glyoxal is a result of photo-production. To our knowledge, this is the first detection of glyoxal in flames using laser-induced fluorescence. Received: 19 December 1996/Revised version: 26 May 1997     

Use of Zeeman Atomic Absorption Flame Spectrometry for Measurements in C2H2/Air and C2H2/N2O Flames

The Hitachi Zeeman atomic absorption spectrometer system is used to evaluate the influence of observation height and C₂H₂ flow rate upon the atomic absorption sensitivity (slope of calibration curve) and upon the atomic absorption signals for 6 elements in an C₂H₂/Air flame and for 3 elements in an C₂H₂/N₂O flame. Fuel-rich conditions result in greater absorption signals and sensitivities in all cases even though there is a significant temperature drop. Optimal observation heights for each case are evaluated. Greater linearity of analytical calibration curves occurs for fuel-rich conditions under Zeeman background correction than under no background correction. The Zeeman atomic absorption flame spectrometer should find more use in the future.     

Absolute CH concentrations in low-pressure flames measured with laser-induced fluorescence

Laser-induced fluorescence in both theA–X andB–X band systems was used to measure absolute number densities of CH radicals in 40 Torr propane/air flames at temperatures near 1600 K. The fluorescence signal was calibrated against Rayleigh scattering in N₂ and Raman scattering in H₂. In a rich flame, = 1.15, the concentration at the peak of the CH distribution was 5.8 ± 1.5 ppm, or (1.4 ± 0.4) × 10¹² cm^–3, with independent values obtained using both band systems and calibration methods in good agreement. This result compares well with a prediction of 8 ppm from a kinetic model of this flame.     

Comparison of Mineral Acids in Wet Digestion of Plant Materials for Flame and Flameless Atomic Absorption Measurement of Metals

Various mineral acids are compared in measuring metals in plant materials by flame and flameless atomic absorption spectrometry. NBS-SRM 1571, Orchard Leaves, is utilized as the plant material. The combination of H₂SO₄ and HNO₃ is found to be the most suitable for measuring Fe, Mn, Cu and Zn by flame atomic absorption spectroscopy, while HNO₃ is found to be the best for measuring Cu, Mn, and Pb by flameless Atomic Absorption spectroscopy.     

Single-shot,spatially-resolved stand-off detection of atomic hydrogen via backward lasing in flames

We report on an experimental demonstration of spatially-resolved detection of atomic hydrogen in flames using a single-ended configuration yielding 656-nm lasing in the backward direction upon 2-photon pumping with 205-nm femtosecond laser pulses. Spatial resolution is achieved by temporally-resolved detection of the backward lasing using a streak camera. The method is demonstrated in CH₄/O₂ flames; both in a setup consisting of two flames, with variable spacing between the flames, and in a single flame. Results from the two-flame experiment show that the backward lasing technique is able to determine changes in the separation between the flames as this distance was altered. By maximizing the temporal resolution of the streak camera, obtaining a highest spatial resolution of 1.65 mm, it is possible to resolve the hydrogen signal present in the two reaction zones in the single flame, where the separation between the reaction zones is ∼2 mm. The lasing signal is strong enough to allow single-shot measurements. Results obtained by backward lasing are compared with 2-photon planar laser-induced fluorescence (LIF) images recorded with detection perpendicular to the laser beam path and the results from the two methods qualitatively agree. Although further studies are needed in order to extract quantitative hydrogen concentrations, the present results indicate great potential for spatially resolved single-ended measurements, which would constitute a very valuable asset for combustion diagnostics in intractable geometries with limited optical access. It appears feasible to extend the technique to detection of any species for which resonant two-photon-excited lasing effect has been observed, such as O, N, C, CO and NH₃.     

Two-photon predissociative fluorescence of H2O by a KrF laser for concentration and temperature measurement in flames

Two-photon laser-induced predissociative fluorescence (LIPF) of H₂O is examined as a potential measurement technique of H₂O concentration and temperature in flames. Two-photons of 248 nm light from a narrowband KrF laser excite H₂O to the highly predissociative state in a hydrogen-air flame. The subsequent bound-free emission is observed from 400–500 nm in the flame at temperatures of 1000–2000 K and is found to be free of fluorescence interference from other flame species. This LIPF signal is not affected by collisional quenching due to the short lifetime of the predissociative state (2.5 ps). Broadband laser dispersion spectra, narrowband laser dispersion spectra, laser excitation spectra and probability density functions of the H₂O fluorescence are obtained in the hydrogen flame. The H₂O LIPF signal is found to be temperature sensitive and a two-line LIPF technique is needed for concentration and temperature measurement. The accuracy of a two-line LIPF technique for H₂O concentration and temperature measurement is determined.     

A thermometry technique based on atomic lineshapes using diode laser LIF in flames

We report on the development of a novel diode laser thermometry technique permitting temperature measurements in flames based on the fluorescence lineshapes of an atomic tracer species. The technique, which we term OLAF (one-line atomic fluorescence) requires only a single diode laser source for excitation, is simple to implement, and has excellent spatial resolution. Temperatures are deduced from the 5²P_1/2 → 6²S_1/2 transition of atomic indium, the lineshape of which is highly sensitive to temperature changes at typical flame conditions. A rigorous validation is performed in a reference flame with comparisons to measurements by CARS and by Na-line reversal, and to numerical simulations.     

Effect of ambient hydrogen sulfide on the physical properties of vacuum evaporated thin films of zinc sulfide

Evaporated thin films of zinc sulfide (ZnS) have been deposited in a low ambient atmosphere of hydrogen sulfide (H₂S ∼10⁻⁴ Torr). The H₂S atmosphere was obtained by a controlled thermal decomposition of thiourea [CS(NH₂)₂] inside the vacuum chamber. It has been observed that at elevated substrates temperature of about 200 °C helps eject any sulfur atoms deposited due to thermal decomposition of ZnS during evaporation. The zinc ions promptly recombine with H₂S to give better stoichiometry of the deposited films. Optical spectroscopy, X-ray diffraction patterns and scanning electron micrographs depict the better crystallites and uniformity of films deposited by this technique. These deposited films were found to be more adherent to the substrates and are pinhole free, which is a very vital factor in device fabrication.     

On Line Dissolution of ZnS For Sulfide Determination in Stabilized Water Samples with Zinc Acetate,Using Spectrophotometry by Methylene Blue Method

Abstract

Stabilization of sulfide as zinc sulfide is a strategy widely used in batch analysis of this ion in water. A FIA/spectrophotometry system to determine sulfide in stabilized water samples is proposed in this paper. The methodology is based on the formation of methylene blue by the reaction of H₂S with a mixture of Fe³⁺ solution and N,N-dimethyl-p-phenylenediamine (DMPD) in acidic medium. In the system, the sample (480 μL) containing stabilized sulfide (as ZnS slurry) is injected in a carrier stream formed by mixing acidic Fe³⁺ and DMPD. In the reactor coil of 1280 μL, the sulfide is drifted by acid used in the preparation of solutions, releasing H₂S that reacts with Fe³⁺ and DMPD forming methylene blue. The absorbance is measured at 662 nm. The performance of the FIA system was optimized in relation to chemical and flow parameters. The stability of the solutions were studied throughout the period of the experiment. The results did not show any loss of sulfide over a 7 day period. However, in complex matrices an analysis by using the standard addition method is recommended. The analysis of effluent sample through the time showed good agreement between the results obtained each day. The developed system presented the following figures of merit: detection limit of 50 μg L^?1; RSD of 6% at 0.5 μg mL^?1 (using ultrasonic homogenization) and analytical throughput of 60 samples per hour.     

The mutual effect of metal sample and turboflame in LIBS signal enhancement

The main aim of the present study is to evaluate the mutual effect of copper sample and turboflame in laser induced breakdown spectroscopy (LIBS) signal enhancement. The use of copper sample leads to a signal enhancement in CN (B²Σ⁺–X²Σ⁺) 384.2–388.4 nm molecular transition, N_742nm, N_744nm, N_746nm (a triplet generated by the fine splitting of the 2s²2p²(³P)3s–2s²2p²(³P)3p transition) and H_{α, 656.3 nm} (as a flame inductor) atomic lines analysis. Additionally, increase in copper sample temperature with flame can enhance the Cu atomic line intensities (as copper sample inductors). Moreover, in this paper, the comparison between turboflame and alcohol flame on LIBS analysis was studied. LIBS signal intensity variation in a turboflame and turboflame coupled with copper sample at different laser pulse energies indicated that the low laser pulse energy could be compensated by using a copper sample that is coupled with turboflame and improved LIBS signal enhancement. For flames analysis, the use of metal sample in LIBS method is demonstrated to be costeffective, compact, and capable of signal enhancement.     

Growth of Zinc Compound Nanocrystals from Different Electrolytes

The influence of the electrolyte composition on electrolytically synthesized zinc compound powders has been studied. It has been shown that if an electrolyte made by dissolving Na₂S₂O₃ ? 5H₂O is kept at room temperature, a mixture of zinc sulfide and hydrozincite is obtained. An electrolyte containing Na₂SO₃ or Na₂S ? 9H₂O gives a mixture of hydrozincite with zinc oxide and/or with zinc sulfide. The size of nanocrystals has been determined. It has been found that hydrozincite decomposes into zinc oxide, water, and carbon dioxide in the temperature range of 200–250°C.     

Cantilever enhanced photoacoustic spectrometry: Quantitative analysis of the trace H2S produced by SF6 decomposition

As one of the key characteristic components that result from sulfur hexafluoride (SF₆) decomposition in SF₆ gas-insulated equipment, hydrogen sulfide (H₂S) can reflect the severity of the internal insulation faults and indicate whether or not such faults involve solid insulation material effectively. The decomposition of SF₆ and its reaction with other impurities to form H₂S are simulated in this study via Materials Studio. The simulation verifies that H₂S is generated only when serious faults occur in the equipment; thus, the online monitoring of the trace H₂S is highly necessary. To achieve a high detection accuracy and avoid cross interference, the spectral line R (8) of the H₂S ν₁ + ν₂ + ν₃ co-frequency absorption band is taken as the absorption line for the gas detection by online simulation based on the HITRAN on the Web. In addition, this study develops a cantilever-enhanced photoacoustic spectrometry trace gas detection platform and conducts experimental research on the quantitative detection of trace H₂S/SF₆ and H₂S/N₂. Experimental results show that the detection sensitivity of the detection platform to trace H₂S under the background gas N₂ and SF₆ is 0.84 and 1.75 μL/L, respectively, and a strong linear relationship exists between the trace H₂S concentration and its corresponding PA signal. Moreover, based on both the theoretical simulation and experiment, the influence of temperature and pressure on the detection platform is discussed and analyzed. The results indicate that the change in the PA signal amplitude decreases with an increase in the pressure or temperature of the PA cell, and the detection platform is more sensitive to pressure.     

H2S-sensing properties of Pt-doped mesoporous indium oxide

Pt-doped mesoporous indium oxide (In₂O₃) has been successfully obtained by a simple and effective in situ nanocasting method. The resultant samples were characterized by XRD, FE-SEM, TEM, N₂ physisorption, XPS and EDX. The gas sensing properties for hydrogen sulfide (H₂S) of the Pt-doped mesoporous In₂O₃ specimens were also examined. The results exhibit those In₂O₃ specimens possess much higher response to H₂S even at low concentration of 2 ppm and a lower optimum working temperature of 150 °C. A possible mechanism was also provided to explain the improvement of the sensing properties.     

Atomic and molecular fluorescence as a stratospheric species monitor

The results of an extensive evaluation are presented assessing the potential of atomic and molecular fluorescence as a stratospheric monitor of the concentrations of any one of eighteen minor species. These include Cl, Cl₂, ClO, ClO₂, CO, H₂, HCHO, HCl, HNO₂, HNO₃, H₂S, NH₃, NO, NO₂, N₂O, O, OH and SO₂. All spectral regions from the vacuum u.v. through to the i.r. have been included. Where appropriate, detection limits (signal/noise ratio of unity) are presented for each species under various sample pressure conditions and are based on practical systems that could be constructed using current technology.The most promising systems, with typical detection limits indicated either as parts per million, billion or trillion by volume, are for CO(5ppb), NO₂(<1ppb), OH(0.2ppt) and O(50–200ppt). The fluorescence sensitivities for Cl(0.5–1 ppt), H₂(0.2 ppm at 10 torr sample pressure) and SO₂(1–10 ppb) are marginally insufficient at present for such a stratospheric application. Likewise HCHO(10 ppb) and NO(100 ppb) fluorescence detection may be of interest in other applications where sensitivity demands are not as severe. There are no promising analytical possibilities using direct fluorescence techniques for Cl₂, ClO₂, HCl, HNO₂, HNO₃, H₂S, NH₃ or N₂O. ClO fluorescence has not yet been characterized.It has been noted, for various reasons, that i.r. fluorescence techniques in general cannot be exploited in the development of sensitive analyzers. However, by far the most surprising outcome of this study has been the recognition of the analytical potential of vacuum u.v. fluorescence. For some species, under certain conditions, extremely high sensitivities are possible even with samples in air at atmospheric pressure.     

Effect of H-atom concentration on soot formation in premixed ethylene/air flames

Soot formation is a major challenge in the development of clean and efficient combustion systems based on hydrocarbon fuels. Fundamental understanding of the reaction mechanism leading to soot formation can be obtained by investigating the role of key reactive species such as atomic hydrogen taking part in soot formation pathways. In this study, two-dimensional laser induced incandescence (LII) measurements using λ?=?1064?nm laser have been used to measure soot volume fraction (f_V) in a series of rich ethylene (C₂H₄)/air flames, stabilized over a McKenna burner fitted with a flame stabilizing metal disc. Moreover, a comparison of UV (λ?=?283?nm), visible (λ?=?532?nm) and IR (λ?=?1064?nm) laser excited LII measurements of soot is discussed. Recently developed, femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique has been applied for obtaining spatially resolved H-atom concentration ([H]) profiles under the same flame conditions. The structure of the flames has also been determined using hydroxyl radical (OH) planar laser induced fluorescence (PLIF) imaging. The results indicate an inverse dependence of f_V on [H] for a range of C₂H₄/air rich flames up to an equivalence ratio, Φ?=?3.0. Although an absolute relationship between [H] and f_V cannot be easily derived owing to the multiple steps involving H and other intermediate species in soot formation pathways, the present study demonstrates the feasibility to couple [H] and f_V obtained using advanced optical techniques for soot formation studies.     

Laser-induced fluorescence determination of flame temperatures in comparison with CARS measurements

Temperature profiles in several premixed low pressure H₂/O₂/N₂ flames and in an atmospheric pressure CH₄/air flame were determined by laser-induced fluorescence (LIF) and by CARS experiments. In the LIF study, temperatures were derived from OH excitation spectra, CARS temperatures were deduced from N₂ Q-branch spectra. The present study is the first quantitative comparison of these two methods for temperature determination in flames burning at pressures up to 1 bar. The resulting temperatures showed good agreement.     

High‐temperature study of superconducting hydrogen and deuterium sulfide

Hydrogen‐rich compounds are extensively explored as candidates for a high‐temperature superconductors. Currently, the measured critical temperature of 203 K in hydrogen sulfide (H₃S) is among the highest over all‐known superconductors. In present paper, using the strong‐coupling Eliashberg theory of superconductivity, we compared in detail the thermodynamic properties of two samples containing different hydrogen isotopes H₃S and D₃S at 150 GPa. Our research indicates that it is possible to reproduce the measured values of critical temperature 203 K and 147 K for H₃S and D₃S by using a Coulomb pseudopotential of 0.123 and 0.131, respectively. However, we also discuss a scenario in which the isotope effect is independent of pressure and the Coulomb pseudopotential for D₃S is smaller than for H₃S. For both scenarios, the energy gap, specific heat, thermodynamic critical field and related dimensionless ratios are calculated and compared with other conventional superconductors. We shown that the existence of the strong‐coupling and retardation effects in the systems analysed result in significant differences between values obtained within the framework of the Eliashberg formalism and the prediction of the Bardeen‐Cooper‐Schrieffer theory.     

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.     

Coumarin-based Fluorescent Probes for H2S Detection

Although hydrogen sulfide (H₂S) has been known as a toxic gas with unpleasant rotten egg smell, the correlation between H₂S and physiological processes has attracted scientists to develop brand new methods to monitor such a gaseous molecule in vitro and in vivo. Herein, we described a couple of coumarin-based fluorescent probes (1a and 1b) that can be activated by reduction of azide to amine in the presence of H₂S. It should be emphasized that probe 1b demonstrated high selectivity and sensitivity for H₂S over other relevant reactive sulfur species in vitro, as well as identified exogenous H₂S in living cells.