Laser-induced fluorescence, direct

Direct Measurement of HO, in the Troposphere. Techniques to measure tropospheric concentrations of HO have been reviewed (O Brien Hard, submitted to Advances in Chemistry, 1991) so only a summary will be given here. The most extensively researched technique for [HO ] measurement in the troposphere is based on laser-induced fluorescence (LIF) of HO. This approach has been developed in many configurations directing the laser into the free atmosphere and collecting fluorescence back scatter (LIDAR) (105,106,107) LIF of air sampled at atmospheric pressure... 

Hard et al (reference 110, 125, and submitted to/. Geophys, Res. 1991) have developed a system for the chemical conversion of HO2 to HO via the reaction HO2 + NO —> HO -I- N02. The hydroxyl radical is then measured by their low-pressure laser-induced-fluorescence instrument. Their multi-sample-channel LIF PAGE system is thus capable of simultaneous measurements of [HO ] (directly) and [H02 ] (by conversion to HO ). 

George, L.A. Development of a Direct, Low Pressure, Laser-induced Fluorescence Technique for NO2, Ambient Measurements and Urban NO, Ph.D. Thesis, Portland State University Portland, OR, 1991,1-135. 

Concentrations of OH and HO2 were determined, in situ, using Laser Induced Fluorescence (LIF) at low pressure, (FAGE technique). HO2 cannot be detected directly by LIF, and was converted to OH by titration with NO directly below the sampling nozzle. The detection limit for the FAGE instrument during SOAPEX-2, determined by calibration in the field, was 1.4 xlO5 molecule cm-3 for OH and 5.4x 105 molecule cm-3 for HO2. A description of the instrument, as set up in previous field campaigns and dur-... 

Webster and Weissburg (2001) visualized instantaneous versus time-averaged odor plumes by laser-induced fluorescence. The spatially varying plume at any particular point in time matters more to an animal than an average plume shape. The mean direction and speed of airflow may be relatively constant, but the animal may extract information from concentration differences on very small temporal and spatial scales. 

Of all the detection methods applied to the lab-on-a-chip, the most popular by far has been laser-induced fluorescence (LIF) detection. Direct LIF detection benefits... 

The detection of HIV-related proteins is one of the most challenging tasks. This is especially true because AIDS should be diagnosed as early as possible to enable an early and effective therapy of this infection. Pavski and Le (57) used the aptamer strategy to detect reverse transcriptase (RT) of the type 1 human immunodeficiency virus (HIV-1). A direct and specific ACE method was proposed using laser-induced fluorescence (ACE/LIF) as detection principle. Single-stranded DNA aptamers as probes fluorescently labeled were synthesized. The resulting aptamer is specific for HIV-1 RT, and it exhibited no cross-reactivity with RTs of the enhanced avian myeloblastosis virus (AMV), the Moloney murine leukemia virus (MMLV), or denatured HIV-1 RT. An affinity complex of RT 26-HIV-l RT was stable, with calibration curves linear up to 50 nM (6 /xg/mL) HIV-1 RT concentration. Both... 

Samples are introduced into the capillary by either electrokinetic or hydrodynamic or hydrostatic means. Electrokinetic injection is preferentially employed with packed or monolithic capillaries whereas hydrostatic injection systems are limited to open capillary columns and are primarily used in homemade instruments. Optical detection directly through the capillary at the opposite end of sample injection is the most employed detection mode, using either a photodiode array or fluorescence or a laser-induced fluorescence (LIF) detector. Less common detection modes include conductivity [1], amperometric [2], chemiluminescence [3], and mass spectrometric [4] detection. 

Clearly, direct techniques for measuring OH are needed that provide concentrations either at a point or over relatively restricted spatial scales. Two (absorption and laser-induced fluorescence) are direct, spectroscopic methods and two others (mass spectrometry and a radiocarbon method) rely on conversion of OH to another species that is measured. Each of these approaches and some of the intercomparisons that have been carried out are discussed briefly in the following sections. A good overview of these methods is found in a review by Eisele and Bradshaw (1993) and articles by Crosley (1994, 1995a, 1995b) and papers in a special issue of the Journal of the Atmospheric Sciences [52 (19), October 1, 1995]. 

A sensor based on two-photon laser-induced fluorescence has been described for detection of N0-N0r-N02 (72). NO is detected directly on the basis of its fluorescent properties. N02 is first converted to NO by photo-... 

The most commonly used method is the direct measurement of a decay rate by pulsed excitation and time resolved detection. The most straightforward example of this technique is laser induced fluorescence applied to alkali Rydberg atoms. Alkali atoms are typically contained in a glass cell, which also holds a known pressure of perturber gas. The alkali atoms are excited to the Rydberg state at time t = 0 and the time resolved fluorescence from the Rydberg atoms is detected... 

Photolysis - Pulsed Laser Induced Fluorescence (PLP-PLIF), a technique which allows us to directly measure OH reaction rates under atmospheric conditions and also permits the sub-microsecond time resolution necessary to observe fast equilibration processes. 

Gas phase kinetic studies of the reactions of hydroxyl radical are most conveniently carried out with direct monitoring of the OH radical with time using laser induced fluorescence (111. The low absorption coefficient of the aqueous hydroxyl radical ( 188nm 540 M 1 cm-1, (12)) precluded the direct measurement of this reactant species by its absorbance. Also, the absence of a readily observable product species for the reaction of OH + MSA at the wavelength range (275-575 nm) easily accessible in our experiments, has lead us to monitor the concentration of OH in solution indirectly by competition kinetics (13), measuring the absorption of the thiocyanate radical anion (ejsonm = 7600 M cm 1 (12)). 

Ultraviolet absorbance detection is the most prevalent type of detection in CE, and UV detectors operate in both the direct and indirect modes. Laser-induced fluorescence detection is often used for high-sensitivity work. Conductivity detection, suppressed conductivity detection, and mass spec-... 

Table IV lists the values of QAB for some strongly bound radicals. Although the direct experimental data are practically not available (radicals usually decompose while being desorbed), the accuracy of these calculations can be verified via the calculated activation barriers AE, which we shall discuss later. Here we only mention that for OH and NH on Pt(lll), with Q0 = 85 and QN = 116 kcal/mol (see Table I), Eq. (10c) gives Qoh = 39 and ( NH = 68 kcal/mol, respectively, in excellent agreement with the experimental ranges of 36-45 kcal/mol for OH on Pt(111) (153a,b) and 63-69 kcal/mol for NH on Pt wire (153c,d), found in laser-induced fluorescence studies by Lin and co-workers (153).

The atmospheric chemistry of trifluoroacetyl radical CF3CO is not well known. It was implicated to explain the products observed98 in photolysis of hexafluoroacetone in the presence of Br2 and Cl2, but its first direct observation came from rapid-scan infrared spectroscopic studies in a matrix99, and more recently its laser-induced fluorescence spectrum has been observed100 the band origin for the first excited state of the radical appears at 384 nm. A weak UV absorption band which onsets at 250 nm and continues to increase in intensity below 200 nm has been attributed to CF3C0101. 

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