Atomic absorption spectroscopy is an instrumental analytical technique for rapid trace metal analysis. It is based on element specific wavelength light absorption by state atoms within the flame or electro thermal graphite furnace.
This technique was introduced for analytical purpose in 1950s by Alan Walsh and Alkemade under the designation of atomic absorption spectroscopy. It is found to be superior to other technique because it often used to determine 50-60 elements from trace to large quantities. These may include metals and non-metals such as Al, Cd, Pb, Mo, Mg etc.
It is very sensitive technique as it can detect the elements up to parts per million (ppm) level and for some elements it can detect up to ppb or ppt level also.
PRINCIPLE OF AAS:
When a beam of electromagnetic radiations of a particular wavelength is passed through the vaporized atoms present in the flame, then atoms absorbs the radiations and decreases in the intensity of radiation will be directly proportional to the atoms present in the ground state. These very specific wavelength gives the technique excellent specificity and detection limits in the AAS analysis. Absorption for each element is specific; no other element absorbs this wavelength.
The principle of absorption of radiations is based on Beer-Lambert’s law. The absorption of radiations by the free atoms is directly proportional to their concentration.
ABSORBANCE= log10 IO/I = K.C.L
Log10 IO/I
C
Where,
IO= intensity of incident radiation
I= intensity of transmitted radiation
C= concentration of analyte
K= constant
L= path length
INSTRUMENTATION OF ATOMIC ABSORPTION SPECTROSCOPY
- RADIATION SOURCE: There’s radiation source which irradiates the atomized sample. The common source of radiation may be Hollow Cathode Lamp (HCL). This contains a tungsten anode and a cylindrical hollow cathode made from the element to be detected. These are sealed in a glass tube filled with an inert gas e.g., neon or argon at a pressure of between 1 N/m2 and 5 N/m2 . The ionisation of possible gas atoms occurs by applying a potential difference of about 300 – 400 V between the anode and therefore the cathode. These gaseous ions bombard the cathode and eject metal atoms from the cathode during a process called sputtering. Some sputtered atoms are in excited states and emit radiation characteristic of the metal as they fall back to the bottom state. These emitted radiations form incident radiations for the element under analysis.
The radiation source for atomic absorption spectroscopy should emit stable, intense radiation of the element to be determined, usually a resonance line of the element.
2. ATOMISER: Atomisation is an important step in Atomic absorption spectroscopy as helps in determining the sensitivity of the reading. As effective atomizer creates a large number of homogeneous free atoms, to reduce the sample in the atomic state. Atomization occurs and atomic vapor of elements to be analyzed is produced.
Nebulisation is that the mechanism by which the sample solution is introduced as fine spray into the flame. This process is instantly followed by atomization, wherein high energy like that of a flame converts molecules into atoms. The atomizers are used: flame atomizer, graphite furnace atomization, oxidants and fuels.
3. MONOCHROMATOR: It’s important that the instrument be capable of providing a narrow band width to separate the line chosen for determination from other undesirable lines. Usually used devices are gratings or prisms. Monochromator converts polychromatic light into monochromatic light. The essential elements of a monochromators are an entrance slit, a dispersing elements and an exit slit. The dispersing element could also be a prism or grating.
4. DETECTOR: the detector detects the signals / photons emitted from the slit.
Photomultipliers are commonly used as detectors in AAS. In some instruments additional filters and detectors are used to compensate for the fluctuations in the output of the source. The output of photomultiplier is amplified which helps in source modification.
METHOD OF ATOMIC ABSORPTION SPECTROSCOPY
The sample solution is aspirated into the flame or heated in a tube to convert them into atoms by the process known as atomization. The vaporization of the analyte ions or atoms is carried out in flame or graphite furnace as the samples are generally solids or liquids. The atoms then absorb radiations of characteristic wavelength, promoting the electrons from ground state energy level to excited state.
Each element absorbs radiations of a particular wavelength which forms the basis for qualitative analysis Depending on the number of atoms in the light path, the amount of light absorbed also changes. By detecting this amount of light, a quantitative determination of the analyte can be made. To excite the atom, one or more electrons often raised to the first or higher energy levels by the absorption of energy. This energy often supplied by photons or by collisions because of heat. Those electrons furthest from the nucleus require least energy to go from the bottom state E0 to the first energy level E1.
The energy E corresponds to the energy gap between the ground state and therefore the first energy level E = E1 – E0.The energy required for this transition often supplied by a photon with an energy given by:
E = h ν
Where, h is that the Planck’s constant and ν the frequency. This corresponds to a wavelength
(λ) of: λ = h c/ E
Where, c is that the speed of light.
APPLICATIONS OF AAS
- Analysis of lead in paint
- Determination of lead in petrol
- Determination of calcium, Magnesium, Sodium and potassium in blood
- Trace elements in raw material along ICP-MS
- Analysis of additives and purity in steels and other metal alloy
- Analysis of low-level contaminant
- In soil analysis
- Agriculture
- Oil and petroleum
- In case of paint chips, fibres and hair strands collected from the crime scene.
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