Chemistry - Atomic absorption spectroscopy - AAS
Atomic absorption spectroscopy - AAS
Atomic absorption spectroscopy is a form of qualitative analysis which
makes use of the fact that specific quantities of energy are required to
promote electrons to higher energy levels. AAS is used throughout the world
to detect the presence of up to 70 metals.
The main components of an atomic absorption spectrometer are shown below:
In atomic absorption spectroscopy, the light source must contain
wavelengths that the element being tested for will definitely absorb. Hence
if testing the nickel content of an iron ore sample, a nickel cathode lamp
would be used. This is consistent with the fact that the wavelengths of
light emitted by excited Ni atoms will be exactly the same as those
absorbed by Ni atoms in their ground states.
The atomiser / flame combination is a key part of the instrument. A sample
of the substance being analysed, say iron ore for nickel content, is
dissolved in acid and sprayed into the flame where the atoms of nickel
absorb some of the light also passing into the flame.
When the light beam is shone through the flame the nickel atoms absorb
particular wavelengths and consequently the intensity of the light of those
wavelengths hitting the detector is less than the intensity with which they
left the lamp.
The monochromator acts as a wavelength (or colour) selector and controls
the wavelengths of light that reach the detector.
The amount of light absorbed is found by comparing the intensity of the
incident radiation, i.e. the light hitting the atomised sample, with the
intensity of the transmitted radiation, i.e. the light of particular
wavelength that passes through the atomised sample. The amount of light
absorbed is known as the absorbance. As in colorimetry, the absorbance is
directly related to the concentration of the absorbing atoms.
Knowing the absorbance for a particular sample of iron ore will not
however provide the nickel content unless the instrument has been
calibrated to determine the relationship between absorbance and amount of
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