Single atomic absorption

Atomic absorption spectrophotometers (Figure 10.37) are designed using either the single-beam or double-beam optics described earlier for molecular absorption spectrophotometers (see Figures 10.25 and 10.26). There are, however, several important differences that are considered in this section. 

Flame Sources Atomization and excitation in flame atomic emission is accomplished using the same nebulization and spray chamber assembly used in atomic absorption (see Figure 10.38). The burner head consists of single or multiple slots or a Meker-style burner. Older atomic emission instruments often used a total consumption burner in which the sample is drawn through a capillary tube and injected directly into the flame. 

The above stm study also discovered a facile transport of surface gold atoms in the presence of the Hquid phase, suggesting that the two-step mechanism does not provide a complete picture of the surface reactions, and that adsorption/desorption processes may have an important role in the formation of the final equiHbrium stmcture of the monolayer. Support for the importance of a desorption process comes from atomic absorption studies showing the existence of gold in the alkanethiol solution. The stm studies suggest that this gold comes from terraces, where single-a tomic deep pits are formed (281—283). 

Atomic absorption spectroscopy is more suited to samples where the number of metals is small, because it is essentially a single-element technique. The conventional air—acetylene flame is used for most metals however, elements that form refractory compounds, eg, Al, Si, V, etc, require the hotter nitrous oxide—acetylene flame. The use of a graphite furnace provides detection limits much lower than either of the flames. A cold-vapor-generation technique combined with atomic absorption is considered the most suitable method for mercury analysis (34). 

A large number of commercial instruments are now available and are based either on a single- or double-beam design. Important instrumental features of a modem atomic absorption instrument include the following facilities. 

Jeschke, W.D. Stelter, W. (1976). Measurement of longitudinal ion profiles in single roots of Hordeum and Atriplex by use of flameless atomic absorption spectroscopy. Planta, 128,107-12. 

Variation In Lead Concentration Along Single Hairs as Measured by Non-Flame Atomic Absorption Spectrophotometry". Nature (1972), 238, 162-163. 

The Cr203 content of each catalyst was determined by atomic absorption spectroscopy (Varian/Spectr AA-20 plus) on acid-digested samples. Total surface areas were determined by a single point BET method (nitrogen adsorption-desorption at 77.5 K) using a mixture of 29.7% N2 in helium. Samples were wet-loaded into the flow tube and dried at 423 K in a hydrogen flow for 15 minutes and then for another 30 minutes at 513 K before cooling in helium. 

Many of the published methods for the determination of metals in seawater are concerned with the determination of a single element. Single-element methods are discussed firstly in Sects. 5.2-5.73. However, much of the published work is concerned not only with the determination of a single element but with the determination of groups of elements (Sect. 5.74). This is particularly so in the case of techniques such as graphite furnace atomic absorption spectrometry, Zeeman background-corrected atomic absorption spectrometry, and inductively coupled plasma spectrometry. This also applies to other techniques, such as voltammetry, polarography, neutron activation analysis, X-ray fluroescence spectroscopy, and isotope dilution techniques. 

The concentrates were subsequently analysed for arsenic using Varian-Techtron AAS atomic absorption spectrophotometer fitted with a Perkin-Elmer HGA 72 carbon furnace, linked to a zinc reductor column for the generation of arsine (Fig. 5.3). A continuous stream of argon was allowed to flow with the column connected into the inert gas line between the HGA 72 control unit and the inlet to the furnace. Calcium sulfate (10-20 mesh) was used as an adsorbent to prevent water vapour entering the carbon furnace. The carbon tube was of 10 mm id and had a single centrally located inlet hole. 

The basic instrumentation used for spectrometric measurements has already been described in the previous chapter (p. 277). Methods of excitation, monochromators and detectors used in atomic emission and absorption techniques are included in Table 8.1. Sources of radiation physically separated from the sample are required for atomic absorption, atomic fluorescence and X-ray fluorescence spectrometry (cf. molecular absorption spectrometry), whereas in flame photometry, arc/spark and plasma emission techniques, the sample is excited directly by thermal means. Diffraction gratings or prism monochromators are used for dispersion in all the techniques including X-ray fluorescence where a single crystal of appropriate lattice dimensions acts as a grating. Atomic fluorescence spectra are sufficiently simple to allow the use of an interference filter in many instances. Photomultiplier detectors are used in every technique except X-ray fluorescence where proportional counting or scintillation devices are employed. Photographic recording of a complete spectrum facilitates qualitative analysis by optical emission spectrometry, but is now rarely used. 

The design of a conventional atomic absorption spectrometer is relatively simple (Fig. 3.1), consisting of a lamp, a beam chopper, a burner, a grating monochromator, and a photomultiplier detector. The design of each of these is briefly considered. The figure shows both single and double beam operation, as explained below. 

FIGURE 9.10 An illustration of the single-beam flame atomic absorption optical path. The modulated light from the source is created either by a chopper or through electronic pulsing. 

FIGURE 9.14 An illustration of the light path for a single-beam atomic absorption spectrophotometer with a... 

How does a single-beam atomic absorption instrument differ from a double-beam instrument What advantages does one offer over the other ... 

Figure 26.1 Schematic Diagram of a Single-Beam Atomic Absorption Spectrophotometer.

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