Nitrogen dioxide, fluorescence

Oh, D., W. Sisk, A. Young, and H. Johnston, Nitrogen Dioxide Fluorescence from N205 Photolysis, J. Chem. Phys., 85, 7146-7158 (1986). 

The feasibility of an optical fiber system was demonstrated for the differential absorption analysis of the car pollutant nitrogen dioxide. It absorbs in the visible and can be "sensed" using an Ar-ion laser27. The yellow metabolite bilirubin has been monitored in blood via fiber optic spectrometry in serum28. The tip of a fiber optic cable was inserted into a injection needle so to reach the blood sample, and absorbance (and later fluorescence) was acquired of a sample contained in the cavity at the tip of the fiber or needle. 

Laser-induced electronic fluorescence. Two devices reported recently look very promising for continuous atmospheric monitoring. Sensitivities of 0.6 ppb for nitrogen dioxide and ppb for formaldehyde are claimed. Careful attention to possible interference from other species is necessary. Detection of the hydroxyl radical in air ( 10 molecules/cm ) has been claimed for this technique, but it has been pointed out that this concentration seems much too high, especially because the air had been removed fix>m the sunlight 6 s before analysis spurious effects, such as photolysis of the ozone in the air by the laser beam and two-photon absorption by water vapor, might have been responsible for the hydroxyl radical that was observed. 

Wu, C.-H., and H. Niki, Fluorescence Spectroscopic Study of Kinetics of Gas-Surface Reactions between Nitrogen Dioxide and Adsorbed Pyrene, Environ. Sci. Technol, 19, 1089-1094 (1985). 

In the example just described, excited nitrogen dioxide is certainly a reaction intermediate, although perhaps not in the usual sense. However, if we are to admit to the scope of this discussion excited species which do not react further, then we should include also all fluorescence phenomena. Fluorescence studies do indeed... 

Obenauf et al. [271] have measured the quantum yield of fluorescence per molecule of target gas consumed in the reaction of barium vapour with nitrogen dioxide and nitrous oxide, obtaining values of 0.20—0.27 for the stronger fluorescence from the nitrous oxide reaction, and 0.015 0.003 for the nitrogen dioxide reaction. Using the values for total reaction cross-section reported by Jonah et al. [270], Obenauf et al. [271] estimate the cross-sections for the chemiluminescent reactions to be < 6—7 for the nitrous oxide reaction and about 2—3 for the nitrogen dioxide reaction. 

Finally, Fig. 36 describes as an example the quenching of p-naphthylamine vapour by oxygen and nitrogen dioxide [117]. It is seen that, just as for atoms and simpler molecules, fluorescence quenching obe3 s a linear expression (the Stern-Volmer-expression). 

Goumri, A., Pauwels, J.F., Devolder, P. Rate of the OH-l-CgH -l-He reaction in the fall-off range by discharge flow and OH resonance fluorescence. Can. J. Chem. 69, 1057-1064 (1991) Graham, R.A., Johnston, H.S. Kinetics of the gas-phase reaction between ozone and nitrogen dioxide. J. Chem. Phys. 60, 4628-4629 (1974)... 

In general compounds with heteroatoms (N, O, S and P) are more amenable to fluorescence reactions" than pure hydrocarbons. Under the influence of the catalytic sorbents substances rich in Jt-electrons are formed, that conjugate to rigid reaction products that are fluorescent when appropriately excited. The formation of fluorescent derivatives is frequently encouraged by gassing with nitrogen or carbon dioxide. 

Titanium dioxide suspended in an aqueous solution and irradiated with UV light X = 365 nm) converted benzene to carbon dioxide at a significant rate (Matthews, 1986). Irradiation of benzene in an aqueous solution yields mucondialdehyde. Photolysis of benzene vapor at 1849-2000 A yields ethylene, hydrogen, methane, ethane, toluene, and a polymer resembling cuprene. Other photolysis products reported under different conditions include fulvene, acetylene, substituted trienes (Howard, 1990), phenol, 2-nitrophenol, 4-nitrophenol, 2,4-dinitrophenol, 2,6-dinitro-phenol, nitrobenzene, formic acid, and peroxyacetyl nitrate (Calvert and Pitts, 1966). Under atmospheric conditions, the gas-phase reaction with OH radicals and nitrogen oxides resulted in the formation of phenol and nitrobenzene (Atkinson, 1990). Schwarz and Wasik (1976) reported a fluorescence quantum yield of 5.3 x 10" for benzene in water. 

A fiber-optic oxygen sensor with the fluorescence decay time (rather than its intensity) as the information carrier has been described by two groups [119, 120]. In the former work, a ruthenium complex is immobilized in silicone-rubber, and quenching by oxygen is measured by either lifetime or intensity measurements. The 337-nm line of a nitrogen laser served as the excitation line, and the dye was dissolved in a silicone-rubber membrane placed in the fluorimeter. This sensing membrane is reported to be highly specific, and chlorine and sulfur dioxide were the only interferents. 

The nitrogen laser is pumped with a high-vollagc spark source that provides a momentary (1 to 5 ns) puLse of current through the gas. I he excitation creates a population inversion that decays very quickly by spontaneous emission because the lifetime of the excited stale is quite short relative to the lifetime of the lower level. The result is a short (a few nanoseconds) pulse of intense (up to I MW) radiaiion at. 337,1 nm. This output is used for exciting fluorescence in a variety of molecules and for pumping dye lasers. I he carbon dioxide gas laser is used to produce monochromatic Infrared radiation at 10.6 pm. 

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