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Atomic Absorption Spectroscopy
Presented byS.Saravanan M.Pharm (Pharmaceutics)Sri Ramachandra University
IntroductionAtomic absorption spectroscopy is a quantitative method of analysis that is applicable to many metals and elements. It is so sensitive that it can measure down parts per billion of a gram in a sample . Measurement of absorption of light passing through gaseous ground state atoms forms the basis of AAS.
Commonly encountered toxic heavy metals
Principle:When a solution containing metallic species is introduced into a flame, the gaseous metallic atom will be formed .some fraction of the metal atoms may get the thermal energy and goes to an excided state, and emit the characteristic wavelength of radiation of the corresponding metal atom. This phenomenon involved in the FES.
Whereas, a large percentage of the metal atom will be remain in the lower energy state(ground state). These ground state atom of a particular element are receptive of light radiation of their own specific resonance wavelength . Thus , when a light of this wavelength passed through a flame having atoms of metallic species, part of that light will be absorbed. This absorption intensity will be absorbed. This absorption intensity will be measured 4
This absorption intensity will be proportional to the concentration of the atom in the ground state. Mathematically the total amount of light absorbed may be given by the following equation.
At the lambda wavelength the total amount of light absorbed = (e2 / mc).Nf
Where, e= charge of electronM= mass of electronC= speed of lightN=total number of atom that can absorb at specific wavelength.5
Flow chart of principle of atomic absorption spectroscopy
Preparation of aqueous solution of sample
Spraying of sample solution(Nebulization)
Evaporation of solvent (Desolvation)
Formation of fine residues of metallic samples (Residue formation)
Formation of neutral atom (Atomization)
Neutral atom absorb specific wavelength of radiation from hallow cathode lamp( Absoption & Excitation)
Intensity of radiation absorbed is measured by photometric detector.Which is proportional to the concentration of sample6
the AAS instrument7
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 interest4. 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 absorbance9
10Difference between AAS & FES FESAASMeasurement of emitted radiation forms the basis of FES.Measurement of intensity of absorbed radiation is basis of AAS.Intensity of emitted radiation is directly proportional to the number of atoms in excited state.Intensity of absorbed radiation is directly proportional to the number of atoms in ground state.Here excitation process and signal response is influenced by flame temperature.Here absorption intensity and signal response is independent to temperature.Relationship between emission intensity Vs concentration in not that much linear.Absorption intensity Vs concentration is very much linear.
11Advantages of AASHighly specific in nature atom of a particular element can only absorb radiation of their own characteristic wavelength other elements cannot be interfered in the study. Ex: light of a particular wavelength can easily be absorbed by specific element to which it is characteristic.
Variation in flame temperature shows relatively less effect in AAS than FES.
12Disadvantages of AASNeed of separate lamp for each element to be determined is main limitation of AAS.This technique cannot be used very successfully for the elements which produce oxides in the flame. Ex: Al, Ti, W, Mo, Si.In aqueous solution, the predominant anion effect interfere the signal to a significant level.AAS is applicable to analysis of metals only.The one more major difficulty encountered with AAS is the presence of incompletely absorbed background emission from the source and scattered light from the optical system. As background becomes more intense relative to the absorption of the analyte, the precision of the measurement decreases dramatically..
Hollow cathode lamp (HCL)
Cathode -- in the form of a cylinder, made of the element being studied in the flameAnode -- tungsten13
A large voltage across the anode and cathode will cause the inert gas to ionize.
The inert gas ions will then be accelerated into the cathode, sputtering off atoms from the cathode.
Both the inert gas and the sputtered cathode atoms will in turn be excited by collisions with each other.
When these excited atoms decay to lower energy levels they emit a few spectral lines characteristic of the element of interest.
The light is emitted directionally through the lamp's window, a window made of a glass transparent in the UV and visible wavelengths.
The light can then be detected and a spectrum can be determined.
Burners:Mecker burner, Total consumption burner, Laminar flow burner.
Total consumption burner
Laminar flow burnerChopper:It is a rotating wheel lies between HCL and Atomizer. It is used to give Pulsating light. On rotation, it breaks the steady light from the lamp into an intermittent or pulsating light16
Common fuels and oxidants used in flame spectroscopy
The technique requires a liquid sample to be aspirated, aerosolized, and mixed with combustible gases, such as acetylene and air or acetylene and nitrous oxide.The mixture is ignited in a flame whose temperature ranges from 2100 to 2800 C.
Flame Atomic Absorption Spectroscopy:19
The process of lighting the AAS flame involves: turning on first the fuel then the oxidant and then lighting the flame with the instrument's auto ignition system. The flame breaks down the analyte's matrix create the elemental form of the analyte atom.
During combustion, atoms of the element of Interest in the sample are reduced to free, unexcited ground state atoms, which absorb light at characteristic wavelengths.
23Graphite furnace technique
24Graphite furnace technique
Small sample sizes ( as low as 0.5 uL)Very little or no sample preparation is neededSensitivity is enhanced ( 10 -10 10-13 g , 100- 1000 folds)
Direct analysis of solid samples
Graphite furnace technique
DisadvantagesBackground absorption effects Analyte may be lost at the ashing stageThe sample may not be completely atomizedThe precision was poor than the flame method(5%-10% vs 1%)
The analytical range is relatively narrow(less than two orders of magnitude)
The light passes from the HCL through the element in the sample to the monochromator. Its function is: It isolates the specific light of the element of interest from the other background lights and transfers it to the photomultiplier tube (detector).
Photomultiplier Tube (PMT)
Before an analyte is aspirated, a measured signal is generated by the PMT as light from the HCL passes through the flame. When analyte atoms are present in the flame--while the sample is aspirated--some of that light is absorbed by those atoms. This causes a decrease in PMT signal that is proportional to the amount of analyte.
The PMT detects the amount of reduction of the light intensity due to absorption by the analyte, and this can be directly related to the amount of the element in the sample.
The PMT converts the light signal into an electrical signal and a computer system translates it into absorbance.28
CurrentConvert light energy to electrical energy29
Estimation of trace elements in biological fluids (Eg: Urine, blood etc)
Estimation of elements like copper, Nickel and Zinc in food product.
Estimation of Magnesium, Zin etc in blood.
Estimation of Mercury in thiomersol solution.
Estimation of Lead in calcium carbonate, petrol etc.
Estimation of elements in soil samples, water supply, effluents, ceramics etc.
Applications:311. Qualitative Analysis:Different HCL lamp is been used for each element to be tested.As qualitative analysis involves the checking of one element at a time.It means that process is very laborious.2. Quantitative Analysis:This technique is based on the determination of the amount of radiation absorbed by the sample.If the value of radiation absorbed is substituted in equation,the number of absorbing atoms in light path is been determined.Total amount of light absorbed= e2/mc.NfCalibration Curve:The first job in quantitative analysis is preparation of calibration curve.In order to prepare this curve, the read out device should be adjusted to 100% transmission with blank and 0% transmittance when no radiation energy is entering