Atomic fluorescence continuous sources

In AFS, the analyte is introduced into an atomiser (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 (HCL, EDL, or tuned laser). Subsequently, the fluorescence radiation is measured. In the past, AFS has been used for elemental analysis. It has better sensitivity than many atomic absorption techniques, and offers a substantially longer linear range. However, despite these advantages, it has not gained the widespread usage of atomic absorption or emission techniques. The problem in AFS has been to obtain a... 

Line sources are mostly used in atomic absorption and fluorescence because the energy which can be generated within the bandwidth of the absorption line is much higher than for any continuous source. In the ideal source only the resonance lines of the elements to be analyzed would be excited and these would be narrow, intense, highly stable and capable of selective modulation. Developments in sources have been directed toward this ideal and increasing the range of elements. These improvements will be of greatest benefit in trace-element analysis. 

In the early work on atomic fluorescence, conventional hollow-cathode lamps often served as escitalion sources. To enhance the output inicnsity without destroying the lamp, it was necessary to operate the lamp with short pulses of current that were greater than the lamp could tolerate for continuous operation. The detector was gated to observe the fluorescence signal only during pulses of source radiation. 

Perhaps the most widely used sources for atomic fluorescence have been the EDLs (Section 9B-1), which usually produce radiant jnlensiljes greater than those of hollow-cathode lamps by an order of magnitude or two. EDLs have been operated in both the continuous and pulsed modes. Unfortunately, this type of lamp is not available for many elements. 

Atomic fluorescence has certain intrinsic advantages over atomic absorption, particularly for elements which have simple emission spectra, such as cadmium and zinc. For these and several other elements, detection limits far below those of atomic absorption have been reported. In addition, the use of a continuous source is more attractive with atomic fluorescence than it is with atomic absorption. In principle, it should be possible to use scanning atomic fluorescence to produce a qualitative and... 

Atomic fluorescence was studied as early as 1902 by Wood, and Nichols and Howes looked at fluorescence in flames, but neither of these reports dealt with analytical applications of atomic fluorescence. Alkemade discussed resonance fluorescence in flames, considering modes of excitation. Winefordner and Vickers then investigated the possibilities of using atomic fluorescence as a practical analytical technique. They used metal vapor discharge tubes as sources and were able to obtain sensitivities of better than 1 jug/ml for mercury, zinc, cadmium, and thallium in an acetylene-oxygen flame. Later, Veillon et were able to demonstrate the usefulness of a continuous source and added another dozen elements to the list of possibilities. 

Continuous spectral emission sources also are useful in atomic fluorescence if they have sufficient intensity. The most commonly used continuous source has been the xenon arc lamp, the 450-W xenon lamp being especially useful. These lamps emit a continuum in the visible and near ultraviolet. Their intensities, however, decrease rapidly below about 2500 A,... 

Metalloid compounds are usually determined by flowing-stream techniques hyphenated with hydride generation (HG)-atomic absorption or atomic fluorescence spectrometry. The continuous operation mode inherent to flow injection is specially suited for the latter detection technique as the tetrahyd-roborate reagent is a potential source of hydrogen for supporting the flame. Analyte preconcentration is frequently needed to detect the typical levels of metalloid species found in water matrices. In this context, cold trap collection of generated hydrides, sorbent extraction microcolumn methods, sorption... 

Atomic fluorescence has the advantage, compared to AAS, that with a continuous source, several elements may be determined simultaneously. There are, however, problems due to scattered radiation and quenching, but detection limits are lower than for AAS. 

It appears as if one of the level K electrons of the atom disappears into the nucleus. The void created induces X-ray fluorescence from the nucleus Y. There are several known radionucleides of this type, which have sufficiently long periods that they can be used as different energy sources (see Table I3.l). These are typically used in portable instruments. The activity of these isotopic sources is generally in the order of a few mCi and they can yield a flux of 106 to 108 photons/s/steradian. Because these sources require a permit for use as well as permanent protection because they emit continuously, their use is rapidly diminishing. 

CIO and BrO abundances are detected simultaneously and continuously as the airstream passes through the instrument. They are not detected directly but are chemically converted to Cl and Br atoms by reaction with reagent nitric oxide gas that is added to the airstream inside the instrument. The Cl and Br atoms are then detected directly with resonance fluorescence in the 2D5/2 -> 2P3/2 transitions in the vacuum ultraviolet region of the spectrum. In resonance fluorescence, the emissions from the light sources are resonantly scattered off of the Cl and Br atoms in the airstream and are detected by a photomultiplier tube set at right angles to both the light source and the flow tube. The chemical conversion reactions... 

Applications of these tunable VUV sources continue to be mostly in the detection of atoms and small molecules by laser-induced fluorescence in molecular beam scattering studies. Of particular importance has been the improved intensities available from Mg vapor, so that it will be possible, for example, to study the internal energy distributions in CO molecules following scattering from surfaces. This capability for both very sensitive and state-selective detection of small molecules will lead to important advances in our understanding of molecular interactions at surfaces. 

Although weak fluorescence in the P— P lines was observed, from a practical standpoint only the P— P lines were found to be intense enough for resonance fluorescence work at low atom concentrations. The fluorescence count rates using the whole of the fully allowed P— P transition of F were typically 2 counts s" at [F] = 1 X 10 cm . These data set a lower concentration limit of 1 X 10 cm" for the smallest detectable concentration of F P atoms. Similar lower limits for O, Br, and I atoms are appreciably less, and are continually being improved by better attention to collimation and detection. Because of the low count rates observed in the F-atom resonance fluorescence studies, it is a better approach to use resonance absorption with a non-reversed line source (see above). 

The most widespread source of X-rays is the X-ray tube. In an X-ray tube, electrons emitted from the cathode are accelerated by an electrical field and bombard the metal target (anode). Target atoms, excited by electron impact, and electrons losing their kinetic energy when decelerating in the anode substance, emit X-rays. The primary radiation of the X-ray tube consists of two parts - characteristic (line) and continuous radiation. As a result of the primary radiation impinging on a substance its atoms emit characteristic fluorescence (secondary) radiation. 

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