Atomic absorption spectrophotometer principles

The early workers in this field built their own apparatus, often assembling these from suitable units available from instruments for other purposes. Emission flame photometers have been converted into atomic absorption spectrophotometers by appropriate attachments consisting of specific light sources, choppers, and lenses, a principle also employed by some manufacturers. 

We will not consider discussing the principles of ICP-MS here since not all environmental testing labs have such expensive instrumentation at present. You will most likely see atomic absorption spectrophotometers In labs that have older ICP-AES instruments, EPA contract lab requirements are that the elements As, Se, Tl, and Pb be determined by graphite furnace atomic absorption and the remaining priority pollutant metals be determined by ICP-AES. 

This technique is used mainly for determining elemental components in a sample. As the name implies, its principle of operation is based on absorption of photons emitted from a source and measurement of the extent of signal attenuation by the atomized sample that reaches a detector as shown schematically for a generic atomic absorption spectrophotometer (Figure 1.17a). Figure 1.17b shows a representation of an atomic absorption spectrometer (AAS) that was developed in the 1960s for measurement of uranium isotopes (Goleb 1966). 

In principle, any detection system which could be adapted for flow-through detection may be used as detectors for FIA. However, some detectors are inherently more suitable than others in the interfacing, and therefore are used more frequently in FI systems. These include the spectrophotometer (visible and UV), atomic absorption and ICP spectrometer, chemiluminescence and various electrochemical detectors, and will be discussed here in more detail. 

The principles of atomic absorption have been known since the early nineteenth century and the technique has been used by astronomers for the approximately quantitative analysis of stellar atmospheres for many years. However, apart from the determination of mercury vapour, it was not used by analytical chemists until 1955 when Walsh realised its potentialities and devised a simple apparatus which could be used for routine analysis. Since that time other equipment has been built and a commercial attachment is available for use with a spectrophotometer. 

The principle of the hollow cathode tube, production of a vapor of atoms by cathodic sputtering, has been employed by Gatehouse and Walsh (Gl) for sample vaporization. The sample is introduced into a vacuum chamber and is made the cathode which produces a cloud of activated atoms. The light of a separate hollow cathode tube is passed through this vapor and absorption is measured in a spectrophotometer. 

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