Atomic absorption spectrometry (aas)
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Transcript of Atomic absorption spectrometry (aas)
Atomic Absorption Spectrometry (AAS)
ABU DARDAM.Sc.(polymer science and technology)
Contents
• What is AAS• Theory• Instrumentations• Principle of AAS• Applications
What is AAS ?Atomic absorption spectroscopy is a quantitative
method of analysis that is applicable to many metals and a few nonmetals.
The technique was introduced in 1955 by Walsh in Australia (A.Walsh, Spectrochim. Acta, 1955, 7, 108)
The application of atomic absorption spectra to chemical analysis
Concentrations range is in the low mg/L (ppm) range.
Theory
A much larger number of the gaseous metal atoms will normally remain in the ground state.These ground state atoms are capable of absorbing radiant energy of their own specific resonance wavelength.
If light of the resonance wavelength is passed through a flame containing the atoms in question, then part of the light will be absorbed.
The extend of absorption will be proportional to the number of ground state atoms present in the flame.
the gaseous metal atoms
specific resonance wavelength
the extend of absorption vs the number of ground state atoms present in the flame.
extend of absorption
AAS Instrument
The simple diagram for the AAS
1. We set the instrument at
certain wavelength
suitable for a certain element
2. The element in the sample
will be atomized by
heat
3. A beam of UV light will be focused
on the sample
5. The monochromator
isolates the line of interest
4. The element in the sample will absorb
some of the light, thus reducing its intensity
6. The detector measures the
change in intensity
7. A computer data system converts the change in intensity into an absorbance
Processes occurring during atomization
Flame atomization
Nebuliser - burner
To convert the test solution to gaseous atoms
Nebuliser --- to produce a mist or aerosol of the test solution
Burner head --- The flame path is about 10 –12 cm
Vaporising chamber --- Fine mist is mixed with the fuel gas and the carrier gas Larger droplets of liquid fall out from the gas stream and discharged to waste
1. Nebulizer:
1. mixes acetylene (the fuel) and oxidant
(air or nitrous oxide).
2. A negative pressure is formed at the end of the small diameter, plastic
nebulizer tube (aspiration).
3. The result is a heterogeneous mixture of gases (fuel + oxidant) and suspended aerosol (finely
dispersed sample).
4. The mixture flows immediately into the
burner head.
5. It burns as a smooth, laminar flame evenly distributed along a
narrow slot.
6. Liquid sample not flowing into the flame collects in the waste.
Note:When do we use NO2 ?
Elements that are highlighted in pink are detectable by AAS
Principle of Atomic Absorption Spectrophotometer
Atomized elements each absorb energy of a wavelength that is peculiar to that
element. The atomic absorption method uses as its light source a hollow cathode
lamp which emits light of a wavelength that is peculiar to each element. Elements
within a solution are heated in a flame or electrically (2000K to 3000K) and
subsequently determined using the fact that the degree of absorption will vary with
its concentration.
Light absorption process of atoms
Atomic Absorption Spectroscopy, AAS
Atomic Emission Spectroscopy, AES
Principle of Atomic Absorption Spectrophotometer
Ground state E0
Excited state E1
e
Absorption
Ground state E0
Excited state E1
e
Emission
ee
K0 - maximal absorption coefficientΔ - half width0 -central wavelength
Characters of the atomic absorption spectrum
Profile of the absorption line
It = I0νe -Kνl
The relationship between absorbance The relationship between absorbance and the concentration of atomsand the concentration of atoms
A = log ( II0ν0ν/ / It ) = 0.4343 K l
Beer’s law
It - intensity of the transmitted light
Io – intensity of the incident light signal
l – the path length through the flame (cm)
DilutionDilute the sample with purified water, dilute acid, or organic solvents.Examples: food products (e.g., dairy products), pharmaceuticals, and biological samples (e.g., blood, urine).
Types of Pretreatment
Dry DecompositionHeat the sample to a high temperature (400 to 500C), Decomposition is possible in a short time (a few hours) and operation is simple.Elements with low boiling points (e.g., Hg, As, Se, Te, and Sb) will vaporize
Wet DecompositionHeat the sample together with acid to a low temperature (approx. 300C). Suitable for volatile elements.A long time is required for the decomposition of organic substances.
Microwave DecompositionDecompose the sample at high pressure by heating it together with acid to a temperature in the range 100 to 200C in a sealed Teflon container.The decomposition process is sealed; there is little vaporization of elements with low boiling points; the decomposition time is short; there is little contamination from the operating environment and the reagent; and only a small amount of acid is required.Examples: Sediment, soil, dust, ceramics, living organisms, food products, etc.
Natural broadeningNatural broadening determined by the lifetime of the excited statedetermined by the lifetime of the excited stateand Heisenberg’s uncertainty principleand Heisenberg’s uncertainty principle ( 10-5 nm ) Doppler Broadening Doppler Broadening ( 10-3 nm )
results from the rapid motion of atoms as they emit or absorb radiation
Collisional BroadeningCollisional Broadening collisions between atoms and molecules in the gas phase lead to deactivation of the excited state and thus broadening the spectral lines
Characters of the atomic absorption spectrum
Doppler Broadening Doppler Broadening ( 10-3 nm )
results from the rapid motion of atoms as they emit or absorb radiation
Characters of the atomic absorption spectrum
Application of AAS
Pretreatment (dissolution) is required for solid samples.
AASAAS
Polished rice:0.118 ppm
Unpolished rice:0.070 ppm
0.1 ppm
Furnace method
Injected amount: 10 µL
Interference inhibitor: Pd 50ppm 5 µL
Ashing: 400C; Atomization: 1,800C
Results of Quantitative Analysis of Cd in Rice
Flame method
Air-C2H2
0.5 ppm
Polished rice : 0.118 ppm
Unpolished rice :0.073 ppm
The following 2 methods can be used to analyze unpolished and polished rice decomposed using acid:
AAS Interferences
AAS Advantages and Disadvantages'
Advantages1. High selectivity and sensitivity2. Fast and simple working3. Doesn’t need metals separation
Disadvantages1. Analysis doesn’t simultaneous2. Fragment have to form ready measure solution3. Limit types of cathode lamp (expensives)
THANK YOU
Questions
• How could atom’s collision• Sample preparation of AAS • The type of sample liquid or gas• Function of AAS of few nonmetals
examples• What happen to the sample in flame