Page 6 - AAS & AES & FES 01082016_Neat

In the absence of any factor that could result in line broadening, the width of each

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spectral line for an atom or a mono-atomic ion is approximately 10 nm· Because it is nearly
impossible to avoid all of the factors that result in line broadening, atomic spectral lines are
usually about two orders of magnitude broader. Of the several reasons for line broadening, two

(Doppler and pressure) are nearly always encountered when atomic absorption
spectrophotometry is used. Broadening owing to the presence of a magnetic field is called the

Zeeman Effect.

I.2.1.1 Doppler Broadening:

The source of radiation that are used for most measurements of atomic absorption contain
energetically excited atoms that emit radiation upon returning to a lower energetic level. During

atomic emission and atomic fluorescence the radiation that is measured by the detector emanates
from excited atoms in the cell. In each case (atomic absorption , atomic emission , and atomic

fluorescence) the measured radiation is emitted from gaseous atoms that exhibit random motion

relative to the detector. The random rapid motion of the emitting species causes Doppler
broadening of the spectral line.

The velocity of the emitting atom toward the detector increases, the observed frequency
of the radiation also increases. Because the emitting atoms in the radiative source or in the cell

are in random motion, a series of overlapping spectral lines are observed by the detector. The
result is a broadened spectral line. Doppler broadening is the major cause of broadening when

atoms under low pressure emit radiation. The temperature of the source can affect Doppler

broadening.

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