Atomic absorption spectroscopy

Key point: Almost every metallic element can be determined quantitatively by using the spectral absorption characteristics of atoms. The principles of atomic absorption spectroscopy are similar to those of UV–visible spectroscopy except that the absorbing species are free atoms or ions. Unlike molecules, atoms and ions do not have rotational or vibrational energy levels and the only transitions that occur are between electronic energy levels. Consequently, atomic absorption spectra con sist of sharply defined lines rather than the broad bands typical of molecular spectroscopy. Figure 8.39 shows the basic components of an atomic absorption spectrophotometer. The gaseous sample is exposed to radiation of a specific wavelength from a ‘hollow cathode’ lamp, which consists of a cathode constructed of a particular element and a tungsten anode in a sealed tube filled with neon. If a particular element is present in the sample, the radiation emitted by the lamp for that element is reduced in intensity because it stimulates absorption. By determining the level of absorption relative to standard materials a quantitative measurement can be made of the amount of the element. A different lamp is required for each element that is to be analysed.

Figure 8.39 The layout of a typical atomic absorption spectrophotometer.

The major differences in instrumentation arise from the different methods used to con vert the analyte (the substance being analysed) to free, unbound atoms or ions. In flame atomization the analyte solution is mixed with the fuel in a ‘nebulizer’, which creates an aerosol. The aerosol enters the burner where it passes into a fuel–oxidant flame. Typical fuel oxidant mixtures are acetylene–air, which produces flame temperatures of up to 2500 K, and acetylene–nitrous oxide, which generates temperatures of up to 3000 K. A common type of ‘electrothermal atomizer’ is the graphite furnace. The temperatures reached in the furnace are comparable to those attained in a flame atomizer but detection limits can be 1000 times better. The increased sensitivity is due to the ability to generate atoms quickly and keep them in the optical path for longer. Another advantage of the graphite furnace is that solid samples may be used. Because the ionization process may produce spectral lines from other components of the analyte, a monochromator is placed after the atomizer to isolate the desired wavelength for passage to the detector.

Almost every metallic element can be analysed using atomic absorption spectroscopy, although not all with high sensitivity or a usefully low detection limit. For example, the detection limit for Cd in a flame ionizer is 1 part per billion (1 ppb=1 in 109) whereas that for Hg is only 500 ppb. Limits of detection using a graphite furnace can be as low as 1 part in 1015. Direct determination is possible for any element for which hollow cathode lamp sources are available. Other species can be determined by indirect procedures. For example, PO₄³ reacts with MoO₄² in acid conditions to form H₃ PMo₁₂O₄₀, which can be extracted into an organic solvent and analysed for molybdenum. To analyse for a particular element, a set of calibration standards is prepared in a similar matrix to the sample, and the standards and the sample are analysed under the same conditions.

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مواضيع ذات صلة

Thermal analysis

X-ray fluorescence elemental analysis

CHN analysis

Ionization and detection methods

X-ray absorption spectroscopy

Photoelectron spectroscopy

Spectroscopic monitoring of titrations and kinetics

Absorption spectroscopy

Neutron diffractionNeutron diffraction

Single-crystal X-ray diffraction

Powder X-ray diffraction

X-ray diffraction