Detection of trace nitric oxide concentrations using 1-D laser-induced fluorescence imaging

Spectrally resolved laser-induced fluorescence (LIF) with one-dimensional spatial imaging was investigated as a technique for detection of trace concentrations of nitric oxide (NO) in high-pressure flames. Experiments were performed in the burnt gases of premixed methane/argon/oxygen flames with seeded NO (15 to 50 ppm), pressures of 10 to 60 bar, and an equivalence ratio of 0.9. LIF signals were dispersed with a spectrometer and recorded on a 2-D intensified CCD array yielding both spectral resolution and 1-D spatial resolution. This method allows isolation of NO-LIF from interference signals due to alternative species (mainly hot O₂ and CO₂) while providing spatial resolution along the line of the excitation laser. A fast data analysis strategy was developed to enable pulse-by-pulse NO concentration measurements from these images. Statistical analyses as a function of laser energy of these single-shot data were used to determine the detection limits for NO concentration as well as the measurement precision. Extrapolating these results to pulse energies of ～?16 mJ/pulse yielded a predicted detection limit of ～?10 ppm for pressures up to 60 bar. Quantitative 1-D LIF measurements were performed in CH₄/air flames to validate capability for detection of nascent NO in flames at 10–60 bar.     

Temperature, pressure, and bath gas composition dependence of fluorescence spectra and fluorescence lifetimes of toluene and naphthalene

Time-resolved fluorescence spectra of gas-phase toluene and naphthalene were investigated upon picosecond laser excitation at 266 nm as a function of temperature (toluene 296–1,025 K, naphthalene 374–1,123 K), pressure (1–10 bar), and bath gas composition (varying concentrations of N₂, O₂, and CO₂) with a temporal resolution of 50 ps. In the investigated temperature range, the fluorescence spectra of both toluene and naphthalene show a significant red-shift, whereas the fluorescence lifetime decreases with increasing temperature, more pronounced for toluene than for naphthalene. Increasing the total pressure of either N₂ or CO₂ from atmospheric to 10 bar leads to an increase by about 20 % (naphthalene at 373 K) and a decrease by 60 % (toluene at 575 K) in fluorescence lifetimes, respectively. As expected, at atmospheric pressure collisions with O₂ shorten the fluorescence lifetime of both toluene and naphthalene significantly, e.g., by a factor of 30 and 90 when changing O₂ partial pressure at 373 K from 0 to 0.21 bar, respectively. The fluorescence model of Koban et al. (Appl Phys B 80: 777, 2005) for the dependence of the toluene quantum yield on temperature and O₂ partial pressure at atmospheric pressure describes toluene fluorescence lifetimes well within its range of validity. The model is modified to satisfactorily predict effective toluene fluorescence lifetimes in N₂ at pressures up to 10 bar. However, it still fails to predict the dependence at simultaneously elevated temperatures and pressures in air as bath gas. Similarly, an empirical model is presented for predicting (relative) fluorescence quantum yields and lifetimes of naphthalene. Although the fitting models have their shortcomings this publication presents a data set of great importance for practical LIF applications, e.g., in-cylinder mixture formation diagnostics in internal combustion engines.     

UV planar laser induced fluorescence imaging of hot carbon dioxide in a high-pressure flame

UV planar laser-induced fluorescence (PLIF) images of hot carbon dioxide (CO₂) are obtained in a laminar flame (CH₄/air) at high pressure (20 bar) with excitation wavelengths at 239.34 nm and 242.14 nm. Excitation wavelengths are chosen to minimize the contribution of nitric oxide and molecular oxygen LIF signals. Spectrally resolved single point measurements are used for correction of the remaining oxygen LIF interference. The continuum LIF signal from electronically excited CO₂ is detected in a broad (280–400 nm) emission region. The UV PLIF of hot CO₂ has the potential for application to a wide variety of diagnostic needs in high-pressure flames, combustors, and engines. PACS 42.62.Fi; 42.30.Va; 07.25+k; 39.30+w     

Experimental evaluation of strategies for quantitative laser-induced-fluorescence imaging of nitric oxide in high-pressure flames (1-60 bar)

Nitric oxide laser-induced-fluorescence (NO-LIF) 2-D imaging measurements using a new multi-spectral detection strategy are reported for high-pressure flames (1-60 bar). This work builds on previous research that identified interference LIF from O₂ and CO₂ in high-pressure flames and optimized the choice of excitation strategies as a function of application conditions. In this study, design rules are presented to optimize the LIF detection wavelengths for quantitative 2-D NO-LIF measurements over a wide range of pressures (1-60 bar) and temperatures. Simultaneous detection of LIF in multiple wavelength regions enables correction of the NO signal for interference from O₂ and CO₂ and allows simultaneous imaging of all three species. New experiments of wavelength-resolved 1-D LIF in slightly lean (? = 0.9) and slightly rich (? = 1.1) methane/air flames are used to evaluate the design rules and estimate the NO detection limits for a wide range of flame conditions. The quantitative 2-D measurements of NO in the burnt gas are compared with model calculations (using GRI-Mech 3.0) versus pressure for slightly lean and slightly rich flames. The discussions and demonstrations reported in this study provide a practical guideline for application of instantaneous 1-D or 2-D NO-LIF imaging strategies in high-pressure combustion systems.     

Characterization of ammonia two-photon laser-induced fluorescence for gas-phase diagnostics

Two-photon laser-induced fluorescence (LIF) of ammonia (NH₃) with excitation of the C′-X transition at 304.8 nm and fluorescence detection in the 565 nm C′-A band has been investigated, targeting combustion diagnostics. The impact of laser irradiance, temperature, and pressure has been studied, and simulation of NH₃-spectra, fitted to experimental data, facilitated interpretation of the results. The LIF-signal showed quadratic dependence on laser irradiance up to 2 GW/cm². Stimulated emission, resulting in loss of excited molecules, is induced above 10 GW/cm², i.e., above irradiances attainable for LIF imaging. Maximum LIF-signal was obtained for excitation at the 304.8 nm bandhead; however, lower temperature sensitivity over the range 400–700 K can be obtained probing lines around 304.9 nm. A decrease in fluorescence signal was observed with pressure up to 5 bar absolute and attributed to collisional quenching. A detection limit of 800 ppm, at signal-to-noise ratio 1.5, was identified for single-shot LIF imaging over an area of centimeter scale, whereas for single-point measurements, the technique shows potential for sub-ppm detection. Moreover, high-quality NH₃-imaging has been achieved in laminar and turbulent premixed flames. Altogether, two-photon fluorescence provides a useful tool for imaging NH₃-detection in combustion diagnostics.     

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.     

Quantitative NO-LIF imaging in high-pressure flames

Planar laser-induced fluorescence (PLIF) images of NO concentration are reported in premixed laminar flames from 1–60 bar exciting the A-X(0,0) band. The influence of O₂ interference and gas composition, the variation with local temperature, and the effect of laser and signal attenuation by UV light absorption are investigated. Despite choosing a NO excitation and detection scheme with minimum O₂-LIF contribution, this interference produces errors of up to 25% in a slightly lean 60 bar flame. The overall dependence of the inferred NO number density with temperature in the relevant (1200–2500 K) range is low (<±15%) because different effects cancel. The attenuation of laser and signal light by combustion products CO₂ and H₂O is frequently neglected, yet such absorption yields errors of up to 40% in our experiment despite the small scale (8 mm flame diameter). Understanding the dynamic range for each of these corrections provides guidance to minimize errors in single shot imaging experiments at high pressure. Received: 13 May 2002 / Published online: 8 August 2002     

Attenuation corrections for in-cylinder NO LIF measurements in a heavy-duty Diesel engine

Quantification of the nitric oxide (NO) concentration inside the cylinder of a Diesel engine by means of laser-induced fluorescence (LIF) measurements requires, amongst others, knowledge of the attenuation of the ultraviolet radiation involved. We present a number of laser diagnostic techniques to assess this attenuation, enabling a correction for laser intensity and detection efficiency of the raw NO LIF data. Methods discussed include overall laser beam transmission, bidirectional laser scattering (bidirectional LIF), spectrally resolved fluorescence imaging, and Raman scattering by N₂. A combination of techniques is necessary to obtain the complete attenuation of laser beam and NO fluorescence. The overall laser beam transmission measurements and bidirectional LIF measurements (the latter yielding spatially resolved transmission) provide evidence of a non-uniform attenuation distribution, with predominant attenuation within or near the piston bowl. Fluorescence imaging of multiple vibrational bands through a spectrograph is shown to be a powerful method for obtaining spatially resolved data on the transmission losses of fluorescence. Special attention is paid to the role of CO₂ and O₂ as UV light absorbers, and the consequences to different excitation-detection schemes for NO. PACS 82.33.Vx; 42.62.Fi; 33.20.t     

Measurements and simulation of in-cylinder UV-absorption in spark ignition and Diesel engines

Spatially and spectrally resolved in-cylinder absorption measurements were performed in spark-ignited internal combustion engines and in Diesel engines. With UV-broadband illumination it was shown that the UV attenuation occurs throughout the burned gas area with roughly homogeneous absorption cross-sections. Model calculations based on the absorption properties of CO₂ at elevated temperatures show that this species gives the main contribution to in-cylinder UV absorption. A previously suggested technique of assessing UV absorption using O₂ laser-induced fluorescence (LIF) as probe light is successfully applied to in-cylinder measurements of the light absorption inside a fired heavy-duty Diesel engine. Even in this environment, the comparison with model calculations shows that CO₂ is the main contributor to UV light absorption. Since the O₂-LIF absorption technique is based on the identical geometry used for LIF concentration measurements, the results can directly be used for correcting LIF signal data such as that obtained from NO imaging. Received: 20 April 2001 / Published online: 18 July 2001     

NO Fluorescence Sensing by Europium Tetracyclines Complexes in the Presence of H2O2

The effect on the fluorescence of the europium:tetracycline (Eu:Tc), europium:oxytetracycline (Eu:OxyTc) and europium:chlortetracycline (Eu:ClTc) complexes in approximately 2:1 ratio of nitric oxide (NO), peroxynitrite (ONOO^?), hydrogen peroxide (H₂O₂) and superoxide (O₂ ^·?) was assessed at three ROS/RNS concentrations levels, 30 °C and pH 6.00, 7.00 and 8.00. Except for the NO, an enhancement of fluorescence intensity was observed at pH 7.00 for all the europium tetracyclines complexes—the high enhancement was observed for H₂O₂. The quenching of the fluorescence of the Tc complexes, without and with the presence of other ROS/RNS species, provoked by NO constituted the bases for an analytical strategy for NO detection. The quantification capability was evaluated in a NO donor and in a standard solution. Good quantification results were obtained with the Eu:Tc (3:1) and Eu:OxyTc (4:1) complexes in the presence of H₂O₂ 200 μM with a detection limit of about 3 μM (Eu:OxyTc).     

Photo-physical properties of anisole: temperature, pressure, and bath gas composition dependence of fluorescence spectra and lifetimes

Anisole is a promising candidate for use as fluorescent tracer for gas-phase imaging diagnostics. Its high-fluorescence quantum yield (FQY) and its large Stokes shift lead to improved signal intensity (up to 100 times stronger) compared with the often used toluene. Fluorescence spectra and effective fluorescence lifetimes of gaseous anisole were investigated after picosecond laser excitation at 266 nm as a function of temperature (296–977 K) and bath gas composition (varying amounts of N₂ and O₂) at total pressures in the range of 1–10 bar to provide spectroscopic data and FQY for applications, e.g., in in-cylinder measurements in internal combustion engines. Fluorescence spectra of anisole extend from roughly 270–360 nm with a peak close to 290 nm at 296 K. The spectra show a red-shift with increasing temperature (0.03 nm/K) and O₂ partial pressure (5 nm from N₂ to air). In the investigated temperature range and in pure N₂ at 1 bar total pressure the effective fluorescence lifetime drops with increasing temperature from 13.3 ± 0.5 to 0.05 ± 0.01 ns. Increasing the total pressure of N₂ leads to a small decrease of the lifetime at temperatures above 400 K (e.g., at 525 K from 4.2 ± 0.2 ns at 1 bar to 2.7 ± 0.2 ns at 10 bar). At constant temperature and in the presence of O₂ the lifetimes decrease significantly (e.g., at 296 K from 13.3 ± 0.5 ns in N₂ to 0.40 ± 0.02 ns in air), with this trend diminishing with increasing temperature (e.g., at 675 K from 1.02 ± 0.08 ns in N₂ to 0.25 ± 0.05 ns in air). A phenomenological model that predicts fluorescence lifetimes, i.e., relative quantum yields as a function of temperature, pressure, and O₂ concentration is presented. The photophysics of anisole is discussed in comparison with other aromatics.     

Preparation and characterization of LiNi<Subscript>0.495</Subscript>M<Subscript>0.01</Subscript>Mn<Subscript>0.495</Subscript>O<Subscript>2</Subscript> (M = Zn,Co, and Y) for lithium ion batteries

Layered structured LiNi_0.5Mn_0.5O₂ and LiNi_0.495M_0.01Mn_0.495O₂ (M = Zn, Co, and Y) compounds were prepared by PVP (poly(vinyl pyrrolidone))-assisted sol-gel method, and their structural, morphological, vibrational, transport, and electrochemical properties were characterized by XRD, SEM, FTIR, Raman, AC impedance, and galvanostatic charge and discharge analysis. XRD patterns reveal that doping does not change the crystal structure of the LiNi_0.5Mn_0.5O₂ compound. SEM images show that the average size of the particle is in sub-micron ranges. The AC impedance studies shows an electrical conductivity of ～2.5 × 10^?7 S/cm for the parent compound. The introduction of Zn/Co/Y at equivalent sites increased the electrical conductivity by one order ～10^?6 S/cm. The compound LiNi_0.495Co_0.01Mn_0.495O₂ shows the highest electrical conductivity of 2.85 × 10^?6 S/cm and delivers a specific discharge capacity of 110 mAh/g at the end of the 25th cycle in the voltage window of 2.5–4.4 V for a current density of 30 mA/g.     

Coherence gating in optical heterodyne detection measurements of scattering and absorption in highly scattering media

Laser-Induced Fluorescence (LIF) from the S₁ state of acetone and 3-pentanone was studied as a function of temperature and pressure using excitation at 248 nm. Additionally, LIF of 3-pentanone was investigated using 277 and 312 nm excitation. Added gases were synthetic air, O₂, and N₂ respectively, in the range 0–50 bar. At 383 K and for excitation at 248 nm, all the chosen collision partners gave an initial enhancement in fluorescence intensity with added gas pressure. Thereafter, the signal intensity remained constant for N₂ but decreased markedly for O₂. For synthetic air, only a small decrease occurred beyond 25 bar. At longer excitation wavelengths (277 and 312 nm), the corresponding initial rise in signal with synthetic air pressure was less than that for 248 nm. The temperature dependence of the fluorescence intensity was determined in the range 383–640 K at a constant pressure of 1 bar synthetic air. For 248 nm excitation, a marked fall in the fluorescence signal was observed, whereas for 277 nm excitation the corresponding decrease was only half as strong. By contrast, exciting 3-pentanone at 312 nm, the signal intensity increased markedly in the same temperature range. These results are consistent with the observation of a red shift of the absorption spectra (9 nm) over this temperature range. Essentially, the same temperature dependence was obtained at 10 and 20 bar pressure of synthetic air. It is demonstrated that temperatures can be determined from the relative fluorescence intensities following excitation of 3-pentanone at 248 and 312 nm, respectively. This new approach could be of interest as a non-intrusive thermometry method, e.g., for the compression phase in combustion engines.     

A New Candidate Laser Dye Based 1,4-Bis[β-(2-Naphthothiazolyle)Vinyl]Benzene. Spectroscopic Behavior,Laser Parameters and Excitation Energy Transfer

A new candidate laser dye based 1,4-bis[β-(2-naphthothisolyl) vinyl] benzene (BNTVB) were prepared, and characterized in various organic solvents. The center polarity is less sensitive than electronic absorption. A red shift was noticed in the fluorescence spectra (ca. 40 nm) with increment in the solvent’s polarity, this means that BNTVB’s polarity appreciates upon excitation. The dipole moment of ground state (μ_g) and the excited singlet state dipole moment (μ_e) are determined from Kawski – Chamma and Bakshiev–Viallet equations using the disparity of Stokes shift with solvent polarity function of ε (dielectric constant) and n (refractive index) of the solvent. The result was found to be 0.019D and 5.13D for ground and exited state, in succession. DFT/TD-DFT manners were used to understand the electronic structures and geometric of BNTVB in other solvents. The experimental and theoretical results showed a good agreement. The photochemical quantum yield (Ф_c) of BNTVB was calculated in variable organic reagents such as Dioxane, CHCl₃, EtOH and MeOH at room temperature. The values of φ_c were calculated as 2.3?×?10^?4, 3.3?×?10^?3, 9.7?×?10^?5 and 6.2?×?10^?5 in Dioxane, CHCl₃, EtOH and MeOH, respectively. The dye solutions (2?×?10^?4 M) in DMF, MeOH and EtOH give laser emission in the blue-green region. The green zone is excited by nitrogen pulse 337.1 nm. The tuning range, gain coefficient (α) and cross – section emission (σ_e) of laser were also estimated. Excitation energy transfer from BNTVB to rhodamine-6G (R6G) and N,N-bis(2,6-dimethyphenyl)-3,4:9,10-perylenebis-(dicarboximide) (BDP) was also studied in EtOH to increase the laser emission output from R6G and BDP when excited by nitrogen laser. The dye-transfer power laser system (ETDL) obeys the Foster Power Transmission (FERT) mechanism with a critical transmission distance, R_o of 40 and 32 ? and k_ET equals 2.6?×?10¹³ and 1.06?×?10¹³ M^?1 s^?1 for BNTVB / R6G and BNTVB / BDP pair, respectively.     

Determination of Protoporphyrin IX Disodium Salt Based on Fluorescence Quenching of Glutathione-Capped Cadmium/Tellurium Quantum Dots

ABSTRACT

Aqueous glutathione-capped cadmium/tellurium quantum dots with a diameter of about 3 nm were synthesized. The fluorescence was quenched in the presence of protoporphyrin IX disodium salt, with the excitation wavelength at 320 nm. Under the optimal conditions, the quenched fluorescence intensity was linear in the range of 0.096–16 µg · mL^?1 with a concentration of protoporphyrin IX disodium salt, and the detection limit (3σ) was 2.8 × 10^?2 µg · mL^?1. The proposed method has been applied to the determination of protoporphyrin in serum samples with satisfactory results. The interaction mechanism was investigated.     

Flow‐Injection Chemiluminescence Study of Luminol–Hydrogen Peroxide–Carbendazim System

Abstract

A new flow‐injection chemiluminescence (CL) method is described for the determination of carbendazim. The method is based on the CL reaction of luminol and hydrogen peroxide (H₂O₂). Carbendazim can greatly enhance the chemiluminescence intensity in sodium hydroxide–sodium dihydrogen phosphate (NaOH–NaH₂PO₄) medium (pH=12.6). Under the optimum conditions, the linear range for the determination of carbendazim is 2.00×10^?8 to 2.00×10^?6 g mL^?1 with a detection limit (S/N=3) of 7.24×10^?9 g mL^?1. The relative standard deviation is 1.8% for 1.0×10^?7 g mL^?1 carbendazim (n=8). The proposed method has been applied to the determination of carbendazim in tap‐water samples. Furthermore, the possible enhanced CL mechanism is discussed by examining the CL spectra and fluorescence spectra.     

Reference electrode formation in potentiometric CO2 sensors with alkali ion conducting alumosilicate glasses as solid electrolytes

Potentiometric CO₂ gas sensors with Li conducting glasses/glass ceramics of the system Li₂O-Al₂O₃-SiO₂ (different nominal composition) as solid electrolytes have been investigated. Li₂CO₃ was used as CO₂ and O₂ sensitive auxiliary electrode. During the sensor test measurements, the CO₂ partial pressure was varied between 1×10⁻³ and 1×10⁻¹ bar at a constant O₂ partial pressure of 2.1×10⁻¹ bar whereas N₂ was used as carrier gas. Comparative measurements were accomplished with sensors comprising Na and K ion conducting glasses. A metastable reference electrode was formed at the contact zone between the Au metal electrode and the former Li glasses of definite nominal composition by crystallization processes taking place, which lead to stable, reproducible CO₂ dependent EMF signals for more than 90d. The thermodynamically expected EMF difference and the observed EMF difference agree quite well between 500 and 600 °C. At 600 °C, the drift of sensors with glasses as solid electrolytes and direct Au glass/glass ceramics contact as reference electrode amounts typically 0.32 mV/d (p(CO₂)=1×10⁻³ bar, p(O₂)=2.1×10⁻¹ bar at the measuring electrode), if a metastable multiphase equilibrium is formed. At identical partial pressures of CO₂ and O₂, the signal reproducibility of these sensors with different solid electrolyte glasses of the same nominal composition lies within 30 mV at 600 °C. Paper presented at the 3rd Euroconference on Solid State Ionics, Teulada, Sardinia, Italy, Sept. 15–22, 1996     

Investigation of toluene LIF at high pressure and high temperature in an optical engine

Toluene laser-induced fluorescence (LIF) emission spectra were acquired in an optical engine with excitation at 248 nm. Toluene was homogeneously seeded in pure nitrogen and air which were used as intake gases. Data were acquired during the compression phase without ignition leading to simultaneous increases in temperature and pressure from 20°C and 1 bar to 500°C and 23.6 bar. Compared to LIF emission spectra at high temperature and atmospheric pressure reported in the literature, the toluene-LIF emission signal shifts to longer wavelengths when temperature and pressure increase simultaneously, whereas the spectrally integrated emission intensity is slightly affected by the pressure level.     

A quantum cascade laser-based optical feedback cavity-enhanced absorption spectrometer for the simultaneous measurement of CH4 and N2O in air

Optical feedback cavity-enhanced absorption spectroscopy (OF CEAS) has been demonstrated with a thermoelectrically cooled continuous wave distributed feedback quantum cascade laser (QCL) operating at wavelengths around 7.84 μm. The QCL is coupled to an optical cavity which creates an absorption pathlength greater than 1000 m. The experimental design allows optical feedback of infra-red light, resonant within the cavity, to the QCL, which initiates self-locking at each TEM₀₀ cavity mode frequency excited. The QCL linewidth is narrowed to below the mode linewidth, greatly increasing the efficiency of injection of light into the cavity. At the frequency of each longitudinal cavity mode, the absorption coefficient of an intracavity sample is obtained from the transmission at the mode maximum, measured with a thermoelectrically cooled detector: spectral line profiles of CH₄ and N₂O in ambient air were recorded simultaneously and with a resolution of 0.01386 cm^?1. A minimum detectable absorption coefficient of 5.5×10^?8 cm^?1 was demonstrated after an averaging time of 1 s for this completely thermoelectrically cooled system. The bandwidth-normalised limit for a single cavity mode is 5.6×10^?9 cm^?1?Hz^?1/2 (1σ).     

Optical-feedback cavity-enhanced absorption spectroscopy with a quantum-cascade laser yields the lowest formaldehyde detection limit

We report on the first application of Optical Feedback-Cavity Enhanced Absorption Spectroscopy to formaldehyde trace gas analysis at mid-infrared wavelengths. A continuous-wave room-temperature, distributed-feedback quantum cascade laser emitting around 1,769 cm^?1 has been successfully coupled to an optical cavity with finesse 10,000 in an OF-CEAS spectrometer operating on the ν₂ fundamental absorption band of formaldehyde. This compact setup (easily transportable) is able to monitor H₂CO at ambient concentrations within few seconds, presently limited by the sample exchange rate. The minimum detectable absorption is 1.6 × 10^?9 cm^?1 for a single laser scan (100 ms, 100 data points), with a detectable H₂CO mixing ratio of 60 pptv at 10 Hz. The corresponding detection limit at 1 Hz is 5 × 10^?10 cm^?1, with a normalized figure of merit of 5 × 10^?11cm $^{-1}/\sqrt{\rm Hz}$ (100 data points recorded in each spectrum taken at 10 Hz rate). A preliminary Allan variance analysis shows white noise averaging down to a minimum detection limit of 5 pptv at an optimal integration time of 10 s, which is significantly better than previous results based on multi-pass or cavity-enhanced tunable QCL absorption spectroscopy.