Radiation sources atomic absorption spectrometry

In atomic absorption spectrometry (AA) the sample is vaporized and the element of interest atomized at high temperatures. The element concentration is determined based on the attenuation or absorption by the analyte atoms, of a characteristic wavelength emitted from a light source. The light source is typically a hollow cathode lamp containing the element to be measured. Separate lamps are needed for each element. The detector is usually a photomultiplier tube. A monochromator is used to separate the element line and the light source is modulated to reduce the amount of unwanted radiation reaching the detector. 

Atomic Fluorescence Spectrometry. A spectroscopic technique related to some of the types mentioned above is atomic fluorescence spectrometry (AFS). Like atomic absorption spectrometry (AAS), AFS requires a light source separate from that of the heated flame cell. This can be provided, as in AAS, by individual (or multielement lamps), or by a continuum source such as xenon arc or by suitable lasers or combination of lasers and dyes. The laser is still pretty much in its infancy but it is likely that future development will cause the laser, and consequently the many spectroscopic instruments to which it can be adapted to, to become increasingly popular. Complete freedom of wavelength selection still remains a problem. Unlike AAS the light source in AFS is not in direct line with the optical path, and therefore, the radiation emitted is a result of excitation by the lamp or laser source. 

Graphite Furnace Atomic Absorption Spectrometry GF A AS is based on absorption by free atoms, of the resonance lines characteristic of a given element, emitted by the radiation source [6, 72]. 

Atomic fluorescence spectrometry (AFS) is the newest of the optical atomic spectroscopic methods. As in atomic absorption, an external source is used to excite the element of interest. Instead of measuring the attenuation of the source, however, the radiation emitted as a result of absorption is measured, often at right angles to avoid measuring the source radiation. 

In atomic absorption spectrometry we need a primary source delivering monochromatic radiation of which the wavelength agrees with that of a resonance line of the elment to be determined. The spectral width must be narrow with respect to the absorption profile of the analyte line. From this point of view atomic absorption... 

Laser Fluorescence Noise Sources. Finally, let us examine a technique with very complex noise characteristics, laser excited flame atomic fluorescence spectrometry (LEAFS). In this technique, not only are we dealing with a radiation source as well as an atomic vapor cell, as In atomic absorption, but the source Is pulsed with pulse widths of nanoseconds to microseconds, so that we must deal with very large Incident source photon fluxes which may result in optical saturation, and very small average signals from the atomic vapor cell at the detection limit [22]. Detection schemes involve gated amplifiers, which are synchronized to the laser pulse incident on the flame and which average the analyte fluorescence pulses [23]. 

The basic concepts of atomic absorption spectrometry were published first by Walsh in 1955, this can be regarded as the actual birth year of the technique. At the same time Alkemade and Milatz designed an atomic absorption spectrometer in which flames were employed both as a radiation source and an atomizer. The commercial manufacture of atomic absorption instruments, however, did not start until ten years later. Since then the development of atomic absorption spectrometry has been very fast, and atomic absorption (AA) instruments very quickly became common. The inventions of dinitrogen oxide as oxidant and electrothermal atomization methods have both significantly expanded the utilization field of atomic absorption spectrometry. These techniques increased the number of measurable elements and lowered detection limits. Todays graphite furnace technique is based on the studies of King at the beginning of the twentieth century. 

The most important radiation sources in atomic absorption spectrometry are the hollow cathode lamps and electrodeless discharge lamps. Other sources which have been used are lasers, flames, analytical plasmas, and normal continuum sources like deuterium and xenon arc lamps. 

In atomic absorption spectrometry, no ordinary monochromator can give such a narrow band of radiation as the width of the peak of the line of atomic absorption. In these conditions the Beer Law is not followed and the sensitivity of the method is reduced. Walsh demonstrated that a hollow-cathode, made of the same material as the analyte, emits narrower lines than the corresponding lines of atomic absorption of the atoms of the analyte in flame, this being the base of the instruments of atomic absorption. The main disadvantage is the need for a different lamp source for each element to be analysed, but no alternative to this procedure improves the results obtained with individual lamps. 

The instrumental requirements of atomic absorption spectrometry will be discussed in the following section. In Fig. 12.3, the essential components of an atomic absorption spectrometer are depicted schematically a suitable radiation source, an... 

Spectrophotometric techniques have been the basis of many coal analysis methods. One of the most widely used techniques for analysis of trace elements is atomic absorption spectrometry, in which the standards and samples are aspirated into a flame. A hollow cathode lamp provides a source of radiation that is characteristic of the element of interest and the absorption of characteristic energy by the atoms of a particular element. X-ray fluorescence is also employed as a quantitative technique for trace element determination and depends on election of orbital electrons from atoms of the element when the sample is irradiated by an x-ray source. 

Figure 3 Principle of construction of atomic absorption spectrometers. (A) Single-beam spectrometer with electrically modulated lamp radiation (B) double-beam spectrometer with reflection and splitting of the primary radiation by a rotating, partially mirrored quartz disk (chopper). 1 - radiation source, 2 -sample cell (atomizer), 3 - monochromator, 4 - detector, 5 -electronics and readout (by permission of Wiley-VCH from Welz B and Sperling M (1999) Atomic Absorption Spectrometry, 3rd, completely revised edition. Weinheim Wiley-VCH).

See also Atomic Absorption Spectrometry Principles and Instrumentation Interferences and Background Correction Flame Electrothermal. Atomic Emission Spectrometry Principles and Instrumentation Flame Photometry. Elemental Speciation Practicalities and Instrumentation. Laser-Based Techniques. Optical Spectroscopy Radiation Sources Detection Devices. 

Principle of atomic absorption spectrometry. 1, primary radiation source 2, atomizer 3, sample 4, combustion gases 5, optical dispersive system 6, detector 7, data acquisition and processing and 8, data editing. (From Ebdon, L. and Evans, E.H., An Introduction to Analytical Atomic Syectrometry, John Wiley Sons, West Sussex, 1998,206. With permission.)... 

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