Ultraviolet Laser-induced Fluorescence Detection Research Articles

Spectral fingerprint of stabilized •QOOH

Chemical Kinetics Carbon-centered radicals containing the hydroperoxy group, commonly denoted as •QOOH, are elusive but are among the most critical intermediate species for kinetic modeling of hydrocarbon oxidation in various atmospheric and combustion processes. Their direct experimental observation is a long-standing challenge, with only one successful previous attempt. Using a combination of infrared activation spectroscopy and ultraviolet laser–induced fluorescence detection, Hansen et al. directly characterized the vibrational structure of a •QOOH intermediate in isobutane oxidation, collisionally stabilized and isolated, and followed its dissociative evolution under infrared activation with time and energy resolution. High-level electronic structure calculations revealed an important role of heavy-atom tunneling in this process. Science , abj0412, this issue p. [679][1] [1]: /lookup/doi/10.1126/science.abj0412

Watching a hydroperoxyalkyl radical (•QOOH) dissociate.

A prototypical hydroperoxyalkyl radical (•QOOH) intermediate, transiently formed in the oxidation of volatile organic compounds, was directly observed through its infrared fingerprint and energy-dependent unimolecular decay to hydroxyl radical and cyclic ether products. Direct time-domain measurements of •QOOH unimolecular dissociation rates over a wide range of energies were found to be in accord with those predicted theoretically using state-of-the-art electronic structure characterizations of the transition state barrier region. Unimolecular decay was enhanced by substantial heavy-atom tunneling involving O-O elongation and C-C-O angle contraction along the reaction pathway. Master equation modeling yielded a fully a priori prediction of the pressure-dependent thermal unimolecular dissociation rates for the •QOOH intermediate-again increased by heavy-atom tunneling-which are required for global models of atmospheric and combustion chemistry.

Unimolecular Decay of Criegee Intermediates to OH Radical Products: Prompt and Thermal Decay Processes.

Alkene ozonolysis is a primary oxidation pathway for anthropogenic and biogenic alkenes emitted into the troposphere. It is also an important source of atmospheric hydroxyl (OH) radicals, often called the atmosphere's detergent. Alkene ozonolysis takes place through a highly exothermic reaction pathway with multiple intermediates and barriers prior to releasing the OH radical products. This Account focuses on a key reaction intermediate with a carbonyl oxide functional group (-COO), known as the Criegee intermediate, which is formed along with a carbonyl coproduct in alkene ozonolysis reactions. Under atmospheric conditions, the initially energized Criegee intermediates may promptly decay to OH products or be collisionally stabilized prior to thermal decay to OH radicals and other products. Alternatively, the stabilized Criegee intermediates may undergo bimolecular reactions with atmospheric species, including water vapor and sulfur dioxide, which can lead to nucleation and growth of aerosols. The dimethyl-substituted Criegee intermediate, (CH3)2COO, is utilized in this Account to showcase recent efforts to experimentally measure and theoretically predict the rates for prompt and thermal unimolecular decay processes of prototypical Criegee intermediates under laboratory and atmospheric conditions. The experimental laboratory studies utilize an alternative synthesis method to efficiently generate Criegee intermediates via the reaction of iodoalkyl radicals with O2. Infrared excitation is then used to prepare the (CH3)2COO Criegee intermediates at specific energies in the vicinity of the transition state barrier or significantly below the barrier for 1,4-hydrogen transfer that leads to OH products. The rate of unimolecular decay is revealed through direct time-domain measurements of the appearance of OH products utilizing ultraviolet laser-induced fluorescence detection under collision-free conditions. Complementary high-level theoretical calculations are carried out to evaluate the transition state barrier and the energy-dependent unimolecular decay rates for (CH3)2COO using Rice-Ramsperger-Kassel-Marcus (RRKM) theory, which are in excellent accord with the experimental measurements. Quantum mechanical tunneling through the barrier, incorporated through Eckart and semiclassical transition state theory models, is shown to make a significant contribution to the unimolecular decay rates at energies in the vicinity of and much below the barrier. Master equation modeling is used to extend the energy-dependent unimolecular rates to thermal decay rates of (CH3)2COO under tropospheric conditions (high pressure limit), which agree well with recent laboratory measurements [ Smith et al. J. Phys. Chem. A 2016 , 120 , 4789 and Chhantyal-Pun et al. J. Phys. Chem. A 2017 , 121 , 4 - 15 ]. Again, tunneling is shown to enhance the thermal decay rate by orders of magnitude. The experimentally validated unimolecular rates are also utilized in modeling the prompt and thermal unimolecular decay of chemically activated (CH3)2COO formed upon ozonolysis of 2,3-dimethyl-2-butene under atmospheric conditions [ Drozd et al. J. Phys. Chem. A 2017 , 121 , 6036 - 6045 ]. Future challenges lie in extension of these spectroscopic and dynamical methods to Criegee intermediates derived from more complex ozonolysis reactions involving biogenic alkenes.

New near ultraviolet laser-induced native fluorescence detection coupled to HPLC to analyse residues of oxolinic acid and flumequine: a comparison with conventional xenon flash lampNuevo método de análisis de residuos de oxolínico y flumequina utilizando la detección de la fluorescencia nativa inducida por láser acoplada en el ultravioleta cercano acoplada al HPLC: comparación con la lámpara de xenon convencional

A new high-performance liquid chromatography (HPLC) method is described for the determination of oxolinic acid and flumequine, with ultraviolet laser-induced fluorescence detection (UV-LIFD). Near-UV excitation at 325 nm was obtained by using an He/Cd laser. Data obtained using UV-LIFD and conventional fluorimetry (Xenon flash, λexc 325/ λem 365) are compared under the same chromatographic conditions, connecting in series both detectors, in terms of linearity, reproducibility and repeatability. The HPLC separation is carried out on a Synergi MAX-RP column with water–acetonitrile (2:1, v/v) adjusted at pH 2.5, with formic acid, as mobile phase and completed in less than 9 min. The detection limits of oxolinic acid and flumequine at a signal-to-noise ratio of 3 were 0.43 pg and 0.76 pg on column with UV-LIFD detection, making this method considerably more sensitive than traditional fluorescence detector (16.15 pg and 14.17 pg) having some obvious advantages.

Vacuum UV laser-induced fluorescence study of the collisional removal of Br( 2P 1/2) atoms by small molecules

This Letter reports on the application of the vacuum ultraviolet laser-induced fluorescence detection of Br( 2P 1/2) atoms at 157.48 nm to the kinetic study of collisional removal of Br( 2P 1/2) by small molecules at 295 K. Gas mixtures of a small amount of CH 3Br and an excess amount of collision partners are exposed to pulsed laser irradiation at 193 nm. Temporal decay profile of the Br∗ LIF intensity has been monitored to determine the collisional removal rate coefficients. The collision partners are H 2, CO 2, CF 4, CF 2H 2, H 2O, CH 3OH, and SF 5CF 3, and the results are compared to literature data.

N(4S) Formation following the 193.3-nm ArF Laser Irradiation of NO and NO2 and Its Application to Kinetic Studies of N(4S) Reactions with NO and NO2

Formation of the ground-state nitrogen atom, N((4)S), following 193.3-nm ArF laser irradiation of NO and NO(2) was detected directly by a technique of laser-induced fluorescence (LIF) spectroscopy at 120.07 nm. Tunable vacuum ultraviolet (VUV) laser radiation around 120.07 nm was generated by two-photon resonance four-wave sum frequency mixing in Hg vapor. Photoexcitation processes of NO and NO(2) giving rise to the N((4)S) formation are discussed on the basis of the Doppler profiles of the nascent N((4)S) atoms produced from the photolysis of NO and NO(2) and the photolysis laser-power dependence of the N((4)S) signal intensities. Using laser flash photolysis and vacuum ultraviolet laser-induced fluorescence detection, the kinetics of the reactions of N((4)S) with NO and NO(2) have been investigated at 295 +/- 2 K. The rate constants for the reactions of N((4)S) with NO and NO(2) were determined to be (3.8 +/- 0.2) x 10(-11) and (7.3 +/- 0.9) x 10(-12) cm(3) molecule(-1) s(-1), respectively, where the quoted uncertainties are 2sigma statistical uncertainty including estimated systematic error.

Determination of tryptamine derivatives in illicit synthetic drugs by capillary electrophoresis and ultraviolet laser-induced fluorescence detection

A method based on separation by capillary electrophoresis combined with UV-laser-induced fluorescence detection (Lambdaex = 266 nm) was developed for the determination of nine tryptamine derivatives of forensic interest and potential matrix constituents. The composition of the separation electrolyte was optimized with respect to the resolution of solutes of interest and to the sensitivity of fluorescence detection. Native alpha-cyclodextrin was employed as a complex forming modifier of the electrophoretic separation and fluorescence-enhancing agent. With the help of a stacking procedure, limits of detection of 0.1-6 microg/L for all analytes were obtained. The repeatability for the peak area (at a concentration of the analyte about 100 times the LOD) was less than 2.3% RSD. A second HPLC method was developed, and its analytical parameters were evaluated for an estimation of the accuracy of the CE-LIF method and for method comparison. The results of the determination of tryptamine derivatives in the samples of forensic interest obtained with the two independent methods are in good agreement.

Vacuum Ultraviolet Laser-Induced Fluorescence Detection of O(1S) Atom Produced in the 193 nm Photolysis of Ozone

The O(2p1S) atoms produced in the photolysis of O3 have been detected by a technique of laser-induced fluorescence spectroscopy with the tunable vacuum ultraviolet (VUV) laser radiation around 121.76 nm. The quantum yield value for O(1S) formation is determined to be (2.5 ± 1.1) × 10-3 in the 193 nm photolysis of O3, which is determined by comparing the VUV laser-induced fluorescence intensities of the O(3s1P1 − 2p1S0) transition with those of the H(2p2Pj ← 1s2S) transition at 121.56 nm of the H atoms generated from the photolysis of HCl at 193 nm. The O(1S) detection technique used in this study is very sensitive, and the detection limit is estimated to be 1 × 109 atoms cm-3. The contribution for the OH radical production from the reaction of H2O with O(1S) produced in the UV photolysis of O3 relative to that from the O(1D) + H2O reaction has been estimated as a function of altitude in the stratosphere, using the photolytic O(1S) quantum yield value obtained in this study and the O(1S) reaction rate coefficients reported previously. The maximum contribution of the O(1S) reaction to OH production rate is a 14% fraction of that from the O(1D) reaction at 40 km altitude at mid-latitudes, assuming the spin-forbidden dissociation process, O(1S) + O2(X3Σg-), for the formation of O(1S) in the photolysis of ozone. Importance of precise measurements of the temperature-dependent reaction rates for O(1S) has been suggested.

Tunable ultraviolet laser-induced fluorescence detection of trace plastics and dissolved organic compounds in water.

We developed a tunable (220-285-nm) UV and fixed 266-nm laser-induced fluorescence (LIF) system using a spectrometer and a cooled CCD imaging detector to measure the excitation-emission matrix spectra of various compounds in water, including quinine sulfate and plastic compound bisphenol-A. The LIF instrument was used for the fast, nonspecific determination of trace amounts of dissolved organic compounds present in natural water supplies and various brand name bottled distilled water and bottled drinking water. Plastic-related compounds that leached out of plastic utensils and containers were also detected with this instrument. The sensitivity of the system was approximately 1-2 orders of magnitude better than that for a commercial system.

Ultraviolet laser-induced fluorescence detection strategies in capillary electrophoresis: determination of naphthalene sulphonates in river water

Various UV-laser-induced fluorescence detection strategies for capillary electrophoresis (CE) are compared, i.e. two UV-laser systems (a pulsed laser providing up to 25 mW of tunable emission, applied at 280, 290 and 325 nm, and a continuous wave (cw) laser providing up to 100 mW of 257 nm emission) and different methods to collect the fluorescence emission signal and to reduce the background. Attention is focused on the determination of amino- and hydroxy-substituted naphthalene sulphonates (NS) in river water; these analytes exhibit native fluorescence upon UV excitation. Optimum results were obtained by applying only a minor portion of the available (average) laser powers, viz. 0.7 mW at 280 nm for the pulsed laser, and 5 mW for the cw laser. For emission collection, the most favourable results were obtained with a mirror-based microscope objective, which facilitates efficient spatial filtering and does not produce impurity fluorescence upon UV-laser irradiation. For standard solutions, the cw laser gave around 20-fold better detection limits (10 −9–10 −10 M) than the pulsed laser. For river water, excitation of interferences (presumably humic acids, which exhibit native fluorescence) could be much better suppressed if the pulsed laser was used with selective excitation at 280 nm. Therefore, for real-sample analysis the latter combination is to be preferred. The set-up was used for the identification and quantification (at the 1–35 μg l −1 level) of NS in a river Elbe sample.

Nitric oxide vibrational excitation from the N(4S)+O2 reaction

Measurements of the vibrational distribution of NO produced in a room temperature flowtube study of N(4S) + O2→NO(0≤v≤7) + O are reported. Ultraviolet laser induced fluorescence detection of NO(v) in levels v=0–7 was employed to study NO production under conditions where O2 vibrational quenching was insignificant. The results indicate that 42% of the NO molecules are produced in infrared-active states, 38% of them in levels ≥2. This is considerably more NO vibrational excitation than had been inferred from infrared chemiluminescence studies. Moreover, production of infrared active states of NO by the N(4S)+O2 reaction may be even more efficient than these numbers indicate owing to possible vibrational relaxation of the nascent NO(v) distribution by nitrogen atoms in the flowtube.