AAS, An analytical method based on the absorption of ultraviolet or visible radiation by free atoms in the gaseous state.

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M.Venkatasami
M.Tech (Processing and Food Engineering)
Department of food process engineering
AEC&RI, TNAU

2. Introduction
General principles
■ Energy transitions in atoms
■ Atomization
Instrumentation of AAS
Operation of a flame atomic absorption instrument
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AAS,
 An analytical method based on the absorption of ultraviolet or visible radiation by free atoms in the
gaseous state.
 Major role in the development of current database for mineral nutrients and toxicants
 Accurate measuring of mineral elements in trace amounts
 Advances : Food analysis, nutrition, biochemistry, and toxicology
 Replaced traditional wet chemistry methods for mineral analysis of foods
■ Exceptions: Calcium, chloride, iron, and phosphorus
 All of the elements in the periodic chart may be determined by AAS & AES (Mineral elements)

4.  AAS quantifies the absorption of electromagnetic radiation by well-separated neutral
atoms in the gaseous state
 AES measures emission of radiation from atoms excited by heat or other means
 AAS suitability is due to,
 Atomic spectra consist of discrete lines
 Every element has a unique spectrum
 Individual elements can be identified and quantified accurately and precisely
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 Production of atomic absorption spectra:
 When Ground state atoms (or ions) absorb energy from a radiation source.
 Absorption of a photon of radiation causes an outer shell electron to jump to a higher energy level,
an excited state
 The excited atom may fall back to a lower energy state, releasing a photon in the process.
 Atoms absorb or emit radiation of discrete wavelengths because the allowed energy levels of
electrons in atoms are fixed (not random).

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 The energy change associated with a transition between two energy levels is directly related to the frequency of the
absorbed radiation:
 Ee−Eg = hν
■ Ee = energy in excited state
■ Eg = energy in ground state
■ h = Planck’s constant
■ ν = frequency of the radiation
Rearranging, we have:
 ν = (Ee − Eg)/h
or, since ν = c/λ
 λ = hc/(Ee − Eg)
where:
■ c = speed of light
■ λ = wavelength of the absorbed or emitted light
 For absorption, transitions involve primarily the excitation of electrons in the ground state, so the number of
transitions is relatively small.

7.  AAS requires atoms of the element of interest to be in atomic state.
 In foods, all elements are compounds or complexes must be converted to neutral atoms
 Atomization: Separation of particles into individual molecules (vaporization) & breaking molecules
into atoms
 Exposing the analyte as fine mist to high temperatures , flame or plasma
 Solvent evaporates, leaving solid particles of the analyte that vaporize and decompose to atoms
 Atoms absorb radiation (atomic absorption) or become excited and subsequently emit radiation
(atomic emission).
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 Two types of atomization are commonly used in AAS: flame atomization and electrothermal
(graphite furnace) atomization.
Schematic representation of a double-beam atomic absorption spectrophotometer

9. 1. Radiation source, a hollow cathode lamp (HCL) or an electrode-less discharge lamp (EDL)
2. Atomizer, usually a nebulizer–burner system or an electrothermal furnace
3. Monochromator, usually an ultraviolet-visible (UV-Vis) grating monochromator
4. Detector, a photomultiplier tube (PMT) or a solid-state detector (SSD)
5. Computer
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10. HOLLOW CATHODE LAMPS
 A hollow tube filled with argon or neon, an anode ( tungsten), & a cathode (the metallic form of the
element being measured)
 Voltage applied across the electrodes, the lamp emits radiation characteristic of the metal in the
cathode
 If the cathode is made of iron, an iron spectrum is emitted.
 For a given electronic transition, either up or down in energy, the energy of an emitted photon is
exactly the same as the energy of an absorbed photon.
 HCLs available for about 60 metallic elements.
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11. CHOPPER
 A chopper is a disk with segments removed.
 The disk rotated at constant speed that the light beam reaching the flame is either on or off at
regular intervals.
 The flame also produces radiation but flame radiation is continuous
 The radiation reaching the detector consists of the sum of an alternating and a continuous
signal.
 Instrument electronics subtract the continuous signal & sends only the alternating signal to
the readout.
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ATOMIZERS
Types of atomizers are,
 Flame atomizers
 Electrothermal atomizers
 Cold vapor technique for mercury
 Hydride generation technique
FLAME ATOMIZER
Consist of a nebulizer and a burner.
Nebulizer converts sample solution into a fine mist or aerosol.
Sample mixed with fuel and an oxidant, and burned in a flame
Only 1% of the total sample is carried into the flame by the oxidant–fuel mixture
Flame atomizer

13.  Flame characteristics may be manipulated by, Adjusting oxidant/fuel ratios and by choice of oxidant
and fuel.
 Commonly used oxidant–fuel mixtures: Air-acetylene and nitrous oxide-acetylene
TYPES OF FLAMES
 Stoichiometric: the fuel and oxidant is completely consumed. yellow fringes.
 Oxidizing: Flame is produced from a fuel – lean mixture, hottest flame & clear blue
appearance.
 Reducing: Flame is produced from a fuel-rich mixture, relatively cool flame & yellow color
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Cont’d…

14.  Cylindrical graphite tubes connected to an electrical power supply, referred as graphite
furnaces.
 The sample is introduced through a microliter syringe (sample volumes normally range from
0.5 to 10 μl).
 The system is flushed with an inert gas to,
 Prevent the tube from burning
 Exclude air from the sample compartment.
 Tube is heated electrically.
 Stepwise increase in temperature, first the sample solvent is evaporated
 The sample is ashed & temperature is rapidly increased to 2,000–3,000⁰ C , quickly vaporize
and atomize the sample.
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This technique works only for mercury,
 Mercury, exist as free atoms in the gaseous state at room temperature.
 Reduced to elemental mercury by the action of stannous chloride (a strong reducing agent ).
 The elemental mercury is then carried in a stream of air or argon into an absorption cell
 Atomic absorption is measured the same way as it is in flame ionization and Electrothermal
instruments.
 This method has the advantage of very high sensitivity

16.  Volatile hydrides of elements are formed by reacting samples with sodium borohydride .
 Hydrides are carried into an absorption cell and heated to decompose into free atoms.
 Then atomic absorption measurements are carried out in the same manner as with other atomization
techniques
 Limited to a relatively few elements which are capable of forming volatile hydrides.
 E.g: As, Pb, Sn, Bi, Sb, Te, Ge, and Se.
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17.  Positioned in the optical path between the flame or furnace and the detector
 Its purpose is to isolate the resonance line of interest
 Only the desired wavelength reaches the detector.
 Monochromators of the grating type are used
 Two types of detectors
 Photomultiplier tubes
 solid-state detectors.
 Detectors convert the radiant energy into an electrical signal
 The signal is processed to produce either an analog or a digital readout
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Monochromator

18.  A generalized procedure that will be similar but not identical to procedures found in instrument
operating manuals:
1. Turn the lamp current control knob to the off position.
2. Install the required lamp in the lamp compartment.
3. Turn on main power and power to lamp. Set lamp current to the current shown on the lamp label.
4. Select required slit width and wavelength and align light beam with the optical system.
5. Ignite flame and adjust oxidant and fuel flow rates.
6. Aspirate distilled water. Aspirate blank and zero instrument.
7. Aspirate standards and sample.
8. Aspirate distilled water.
9. Shut down instrument.
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 Nielsen, SS. 2010. Food analysis: Springer.