Nonresonance fluorescence

A. excitation could greatly reduce photolytic interference, nonresonant fluorescence, and scattering backgrounds. The method can be calibrated by fundamental calculation or by ozone photolysis by a shortwave UV source. The successful 2-A determination of NO provides confidence in the fundamental principles of this technique and shows its adaptability for nonsimul-taneous measurements of NO, N02, and HN03. 

Cavalli P. and Rossi G. (1984) Laser excited atomic and ionic nonresonance fluorescence detection limits for several elements in an argon inductively coupled plasma, Spcctrochim Acta, Part B 39 115—117. 

Although flames are convenient sources of MOH molecules, they suffer from serious drawbacks for spectroscopic and dynamical studies. The high temperature ( 2000 K) of flames causes numerous vibrational and rotational levels to be populated resulting in very dense spectra. The high pressure (1 atm) broadens the rotational lines (>0.1 cm ) and increases the overlap of the lines. In addition, resonant laser-induced fluorescence is difficult to detect because of quenching and the overwhelming presence of nonresonant fluorescence caused by rapid collisional energy transfer. The luminescence of the flame itself also interferes with measurements. 

Nonresonance Fluorescence. Nonresonance fluorescence occurs when the exciting wavelength and the wavelength of the emitted fluorescence line are different. There are two basic types direct-line fluorescence and stepwise-line fluorescence. 

All types of nonresonance fluorescence, particularly direct-line fluorescence, can be analytically useful sometimes it is more intense than resonance fluorescence, and it offers the advantage that scattering of the exciting radiation can be eliminated from the fluorescence spectrum by removing it with a filter or a monochromator. Self-absorption problems (absorption of the emitted radiation by the sample atoms) can also be avoided by measuring fluorescence at a nonresonance line that is not also absorbed. 

The high radiant intensities provided by lasers make possible the practical use of nonresonance fluorescence processes for analytical purposes. Nonresonance fluorescence also reduces scattered radiation effects to practically zero, making line scanning generally unnecessary. Detection limits for nonresonance fluorescence include (see footnote 8) aluminum (3961 A) at 3 X 10 jug/ml, cobalt (3575 A) at 5 x 10 / g/ml, indium (4105 A) at 2 X 10" iUg/ml, and nickel (3610 A) at 2 jug/ml. 

In a nonresonance fluorescence transition, the photons involved in absorption and fluorescence processes have different wavelengths (Figure IB). The particular transition shown in Figure IB is called Stokes direct-line fluorescence, which is frequently used for AFS with laser excitation. Nonresonance transitions have the advantage that a wavelength selection device can be used to distinguish between fluorescence and scattered source radiation. 

Figure 1 AFS transitions (A) resonance fluorescence and (B) nonresonance fluorescence. Radiative transitions are shown by solid lines nonradiative transitions by dashed lines.

In atomic fluorescence spectrometry (AFS), the analyte is introduced into an atomizer (flame, plasma, glow discharge, furnace) and excited by monochromatic radiation emitted by a primary source. The latter can be a continuous source (xenon lamp) or a line source (hollow cathode lamp, electrodeless discharge lamp, or tuned laser). Subsequently, the fluorescence radiation, which may be of the same wavelength (resonance fluorescence) or of longer wavelength (nonresonance fluorescence), is measured. 

Figure 3 Two fluorescence schemes. (A) Resonance fluorescence excitation and detection at the same wavelength. (B) Nonresonance fluorescence excitation and detection at different wavelengths, Adet > Aexc-...

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