Xenon arc lamps

5 Xenon Arc Lamp, The xenon arc lamp emits a continuum spectrum which can be used for the determination of a number of elements. A xenon arc lamp of the Eimac -type has been found to be best suited for AFS measurement. However, it is unstable when used periodically and its intensity decreases significantly at wavelengths below 210 nm. Thus, it is not suitable for the determination of arsenic or selenium. Detection limits obtained by a xenon arc lamp are about the same as those for flame AAS. 

An ideal atomizer should have good atomization features and low background emission. In addition, the atomizer should not contain fluorescence quenching factors and the distance for atoms passing through the excitation zone should be as long as possible. It is quite difficult to unite all these requirements in one atomizer. Commonly used atomizers in AFS are flames, plasmas, electrothermal atomizers, and various special techniques. 

1 Flames. Various hydrogen flames have very small background emission and they do not contain fluorescence quenching particles. However, their atomization power is poor and several interferences may appear in these flames. They can be used for the determination of easily volatilized elements. 

Annular premix burners are better than direct injection burners, because the scatter of the light from the particles is smaller with premix burners. Scatter will be reduced especially when concentrated salt solutions are analysed. 

2 Plasma. In 1976 Montaser and Fassel introduced an AFS instrument in which EDLs were used for excitation and an ICP for atomization. Advantages of the plasma torch are good atomization efficiency and lack of chemical interferences. Interference due to light scatter is also minimal with a plasma atomizer. In principle, it is also possible to analyse solid samples by AFS by using a plasma atomizer. 

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The S/N of any light intensity measurement varies as tire square root of tire intensity (number of photons) produced by tire source during tire time of tire measurement. The intensities typical of xenon arc lamps are sufficient for measurements of reasonable S/N on time scales longer tlian about a microsecond. However, a cw lamp will... 

Xenon arc lamp Xenon compounds Xenon dichloride Xenon difluoride... 

Fluorometry and Phosphorimetry. Modem spectrofluorometers can record both fluorescence and excitation spectra. Excitation is furnished by a broad-band xenon arc lamp foUowed by a grating monochromator. The selected excitation frequency, is focused on the sample the emission is coUected at usuaUy 90° from the probe beam and passed through a second monochromator to a photomultiplier detector. Scan control of both monochromators yields either the fluorescence spectmm, ie, emission intensity as a function of wavelength X for a fixed X, or the excitation spectmm, ie, emission intensity at a fixed X as a function of X. Fluorescence and phosphorescence can be distinguished from the temporal decay of the emission. 

Broad-band irradiation from a xenon arc lamp by Felix and Sealy produced semiquinone radicals from catecholamines. They were detected by ESR spectra of their metal complexes. Semiquinones gave rise to o-quinones and... 

Chignell etal. intensely irradiated p-aminobenzoic acid with a xenon arc lamp and showed the formation of several free radicals detected by spin traps in conjunction with ESR spectroscopy. It was suggested that such radicals could cause lipid peroxidation or react via one of the excited forms of oxygen [42, 43],... 

Motton el al. made a spin-trapping study of the radicals induced by a xenon arc lamp from promazine and chlorpromazine. The results suggested that the radical from photodechlorination of chlorpromazine was sufficiently more reactive than the promazine radical to amount for the order of magnitude difference in the potencies of their phototoxicities [233],... 

Fluorescence Measurement Fluorescence spectra were measured on a Spex Fluorolog 212 spectrofluorometer equipped with a 450 W xenon arc lamp and a Spex DM1B data acquisition station. Spectra were recorded in the front-face illumination mode using 343 nm as the excitation wavelength. Single scans were performed using a slit width of 1.0 mm. PDA fluorescence emission spectra were recorded from 360 to 600 nm, with the monomer and excimer fluorescence measured at 376.5 and 485 nm, respectively. Monomer and excimer peak heights were used in the calculation of the ratio of excimer to monomer emission intensities (Ie/Im). Excitation spectra were recorded from 300 nm to 360 nm and monitored at 376.5 and 500 nm for the monomer and excimer excitation, respectively. 

Fluorescence Instrumentation and Measurements. Fluorescence spectra of the FS samples were obtained on a steady state spectrofluorometer of modular construction with a 1000 W xenon arc lamp and tandem quarter meter excitation monochromator and quarter meter analysis monochromator. The diffraction gratings In the excitation monochromators have blaze angles that allow maximum light transmission at a wavelength of 240 nm. Uncorrected spectra were taken under front-face Illumination with exciting light at 260 nm. Monomer fluorescence was measured at 280 nm and exclmer fluorescence was measured at 330 nm, where there Is no overlap of exclmer and monomer bands. 

Fluorescence spectra of the novolac samples were measured on a Spex Fluorolog 212 spectrofluorometer with a 450 W xenon arc lamp and a Spex DM1B data station. Spectra were taken with front-face Illumination using a 343 or 348 nm excitation wavelength for solutions or films, respectively, which are near the maximum transmission region of this spectrometer. Spectra were corrected using a rhodamlne B reference. Monomer fluorescence was measured at 374 or 378 nm and exclmer fluorescence was measured at 470 nm. Monomer and exclmer peak heights were used In calculations of Ie/Im. The 1 monomer peak of pyrene was used to reduce overlap with the exclmer emission. 

Accelerated exposure equipment may also be used to test for weatherfastness in plastic materials [106], The natural destructive agents inherent in weather are approximated by filtering the radiation emitted by the xenon arc lamp and by spraying the sample with water under standardized conditions [106], Test programs are designed to relate to actual outdoor exposure to rain and humidity. In a standard program, a 3 minute wet cycle typically alternates with a 17 minute dry period. Weatherfastness tests are carried out and evaluated like lightfastness tests the black panel temperature and other parameters are the same in both procedures. 

Following the wavelength selection by the monochromator, the beam passes on to the sample compartment where the sample solution, held in the cuvette, is positioned in its path. The sample compartment is an enclosure with a lid that can be opened and closed in order to insert and remove the cuvette. When the lid is closed, the compartment should be relatively free of stray light, although this is not a requirement if a xenon arc lamp is used as the source. This is because of the high intensity of the xenon arc lamp. The cuvette is held snugly in a spring-loaded holder. 

Figure 8. Rate of dissolution of 5iiM Y-FeOOH suspended in pH 3.0, 0.01M NaCl containing various concentrations of mercaptoacetic acid under dark and light conditions. Light source unfiltered 150W Xenon arc lamp. (Reproduced from Ref. 49.)...

Figure 10.7 shows a nanosecond kinetic flash photolysis apparatus. The absorbance of the sample is monitored, using a photomultiplier, by the change in the transmittance of the sample to the xenon arc lamp continuous light source. 

Fluorometers designed for research purposes(31) are typically equipped with a xenon arc lamp, monochromators, one or more photomultiplier tubes, cuvet holders, and a computer interface. Some research level fluorometers, such as the Perkin-Elmer LS50, have optional microtiter plate reading accessories with fiber optic bundles. This is convenient since 96-well microtiter plates are commonly used for immunoassay development, and many commercial immunoassays are based on the use of microtiter plates. Fluorometers designed for commercial immunoassay purposes are generally dedicated instruments with few, if any, data acquisition and reduction parameters that can be manipulated by the user. 

The photolyzing light pulse is produced by a dye laser and enters the sample at about 10° to the axis of the sample beam. The observation beam originates from a 75-W xenon arc lamp. The apparatus is supplied by OLIS, Athens, Georgia USA. Reproduced with permission from C. A. Sawicki and R. J. Morris, Flash Photolysis of Hemoglobin, in Methods in Enzymology (E. Antonini, L. R. Bernardi, E. Chiancone eds.), 76, 667 (1981). 

Photolytic. Atkinson (1985) reported a rate constant of 2.59 x 10 " cmVmolecule-sec at 298 K. Based on an atmospheric OH concentration of 1.0 x 10 molecule/cm , the reported half-life of allyl alcohol is 0.35 d. The reaction of allyl alcohol results in the OH addition to the C=C bond (Grosjean, 1997). In a similar study, Orlando et al. (2001) studied the reaction of allyl alcohol with OH radicals at 298 K. Photolysis was conducted using a xenon-arc lamp within the range of 240-400 nm in synthetic air at 700 mmHg. A rate constant of 4.5 x 10 " cm /molecule-sec was reported. Products identified were formaldehyde, glycolaldehyde, and acrolein. 

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