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Atomic Absorption spectroscopy
Principle
Atomic-absorption spectroscopy (AAS) uses the absorption of light to measure the concentration of gas-phase atoms. Since samples are usually liquids or solids, the analyte atoms or ions must be vaporized in a flame or graphite furnace. The atoms absorb ultraviolet or visible light and make transitions to higher electronic energy levels. The analyte concentration is determined from the amount of absorption. Applying the Beer-Lambert law directly in AAS is difficult due to variations in the atomization efficiency from the sample matrix, and non-uniformity of concentration and path length of analyte atoms (in graphite furnace AAS). Concentration measurements are usually determined from a working curve after calibrating the instrument with standards of known concentration. “Atomic Absorption Spectroscopy is an absorption method where radiation is absorbed by non- excited atoms in the vapour phase i.e., nebulized state.” The principle of AAS is based on the absorption of energy by ground state atoms in the gaseous state. When a solution having metal is introduced to flame the vapour of metal is obtained. Then some metal atoms may get raised to higher levels of energy to emit the radiations. But majority of atoms remain unexcited. When a light of specific resonance wavelength is passed through a flame having atoms of the metallic species, part of light will be absorbed & absorption will be proportional to the density of atoms in flame. Therefore, the total amount of light absorbed is
= ( e / mc) Nf
Whereas, e = charge on electrons m = mass of light N = total no. of atoms that can absorb at frequency ‘v’ in light path F = oscillar strength C = speed of light. From the above expression we can conclude that, absorption of light by an atom is independent of wavelength of absorption and the temperature of the atoms. But temperature effects the efficiency at which atoms are produced from a sample. The temperature of ignition is 2100° – 2800° C. The characteristic / specific resonance wavelength is obtained from a lamp whose cathode is made up of the same element that is to be determined. A Photomultiplier tube is used to detect the reduction in
23MPl intensity of light. The AAS is also called as Atomic Flame Photometry as sample solution is sprayed into the flame the whole technique is carried out in the UV-regions.
Interferences
AAS is less liable to be affected by interferences then compared to flame photometric techniques this technique is especially free from cationic interferences because the absorption of resonance lines from hollow cathode lamp. However, it has the following interferences which needs deep knowledge and practise. Since the concentration of the analyte element is considered to be proportional to the ground state atom population in the flame, any factor that affects the ground state population of the analyte element can be classified as interference. Factors that may affect the ability of the instrument to read this parameter can also be classified as interference. The following are the most common interferences: A) Spectral interferences are due to radiation overlapping that of the light source. The interference radiation may be an emission line of another element or compound, or general background radiation from the flame, solvent, or analytical sample. This usually occurs when using organic solvents, but can also happen when determining sodium with magnesium present, iron with copper or iron with nickel. B) Formation of compounds that do not dissociate in the flame. The most common example is the formation of calcium and strontium phosphates. C) Ionization of the analyte reduces the signal. This is commonly happens to barium, calcium, strontium, sodium and potassium. D) Matrix interferences due to differences between surface tension and viscosity of test solutions and standards. E) Broadening of a spectral line, which can occur due to a number of factors. The most common line width broadening effects are:
1. Doppler effect- This effect arises because atoms will have different components of velocity
along the line of observation.
2. Lorentz effect- This effect occurs as a result of the concentration of foreign atoms present in
the environment of the emitting or absorbing atoms. The magnitude of the broadening varies with the pressure of the foreign gases and their physical properties.
3. Quenching effect - In a low-pressure spectral source, quenching collision can occur in flames
as the result of the presence of foreign gas molecules with vibration levels very close to the excited state of the resonance line.
4. Self-absorption effect - The atoms of the same kind as that emitting radiation will absorb
maximum radiation at the center of the line than at the wings, resulting in the change of shape of the
23MPl It is widely used in Atomic Absorption Spectroscopy. The cathode consists of a hollow cup made up of the element which is to be determined. e.g., Na The Anode is a tungsten wire. The two electrodes are present in a tube containing inert gas. The lamp window is made up of quartz or silica or glass. When a current in the milliampere range arises, the inert gas is charged at the anode & charged gas is charged at anode & charged gas is attracted at high velocity to the cathode. The impact with cathode vaporizes some of the Sodium atoms. They get excited and when return to the ground state give rise to Na emission. The inert gas is used for – 1)It’s main source of current – carrying capacity in hollow cathode. 2)It dislodges atoms from the surface of cathode. 3)It’s primarily responsible for excitation of the ground state metal atoms. Removal of atoms by positive ions of inert gas that is bombarded on the cathode is called as Sputtering. The pressure is maintained at 1-5 torr, as higher pressure leads to unstable discharge and in lower Pressure vaporization of cathode metal increases & the operating temperature also increases.
1. IONIZATION 2. SPUTTERING 3. EXCITATION 4. EMISSION
In Atomic Absorption Spectrometry, the line spectrum of the element being analysed is emitted from a HCL and passes through the optics of the instrument. The atomized sample is introduced into the flame and absorbs resonance lines from the line spectra of the element. The decrease in the light intensity of the resonance lines is related to the concentration of the analyte by the Beer-Lambert Law in a similar fashion to other absorption methods, that is,
A = -log T = kc
As the name suggests the lamp consists of 2 parts (anode and cathode) and is filled with an inert gas (generally Ar or Ne). The HCL is connected to a regulated low voltage low current DC power supply. The flowing electrical current causes excitation of the element present on the cathode and generates the characteristic line spectra of that element. The shape and configuration of the HCL helps to focus the radiation into the optical path of the instrument. Generally, one uses the specific HCL for the element being analysed. Each specific HCL has specified current settings, so be sure to adjust the DC power supply to be within the operational parameters of that lamp. The reason for using the line source of the element for the radiation source is its very narrow and well-defined lines. The bandwidth of a selected line may approach 0.005 nm, something that most monochromators could not accomplish.
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B] Electrodeless Discharge Lamps:
Electrodeless discharge lamps (EDLs) are source of atomic line spectra and provides radiant intensities of order one to two orders of magnitude greater than hollow cathode lamps. The lamp is constructed from a sealed quartz tube containing little amount of inert gases like Argon & a small amount of metal / salt to be analysed. The lamp contains an intense field of microwave radiation instead of any electrodes. The inert gas Argon is ionized & the ions are accelerated by high- frequency component of the field until they have sufficient energy to excite the atoms of the metal. They are available for more than 15 elements.
Signal Modulation by Chopper:
In AAS to avoid interferences caused by emission of radiation by flame, the output of the source must be modulated to provide a fluctuating intensity at a constant rate. The detector receives two types of signals, an alternating one from the source and a continuous one from the flame. Later they are converted into the corresponding types of electrical responses. To achieve this kind of signal modification a circular metal disc or chopper is interposed in between the source and the flame. Alternate quadrants of the disk are removed to permit passage of light. Rotation of the disk at constant known rate produces a beam that is chopped to the desired frequency.
2) Flame Nebulizer (Atomizer):
In AAS to achieve absorption of atoms of the sample is reduced to its corresponding atomic state. This process of converting the sample into an atomic state is called Atomization or Neublization. It can be achieved by the following equipment’s –
a) Ovens: The sample is brought rapidly at high temperature.
b) Electric Arcs and Sparks: Here a high current or high voltage alternatively current spark is
applied to solid / liquid samples.
c)Sputtering Devices: In this, the sample held on a cathode is bombarded by positive ions of an
inert gas.
d)Flame Atomizers: Here the liquid sample is converted into gaseous state by use of a flame.
The fuel & oxidant gas are fed into a mixing chamber, where they proceed through series of baffles to the burner head. The flame orifice is in form of a long narrow slot, so that ribbon flame is produced. The sample in solution is aspired into the mixing chamber by a small air jet into a premixed gas air burner designed for a long path length. The radiation is passed into the monochromator and measured at the detector. The amount of radiation absorbed is passed into the concentration of the element in the sample. A calibration curve is prepared by measuring the absorbance of series of standard solutions. Flame in AAS: Some of the commonly used fuels and their relative data:
23MPl Ethyne/air (giving a flame with a temperature of 2200–2400°C) or ethyne/dinitrogen oxide (2600–2800°C) are often used. A flexible capillary tube connects the solution to the nebuliser. At the tip of the capillary, the solution is ‘nebulised’ – i.e., broken into small drops. The larger drops fall out and drain off while smaller ones vaporise in the flame.
Advantages of Flame Atomizers:
- They are convenient, reliable and relatively free of memory effects.
- Most of the flames used are noiseless and safe to operate.
- Burner systems are small, inexpensive and durable.
- Sample solutions are easily handled with simple nebulisers.
- A variety of flames can be used to select optimum working conditions for a wide variety of elements.
Disadvantages of Flame Atomizers:
- The sample volume available may be less than 3-10ml.Theanalyte concentration may be too low to allow dilution of the sample.
- Pneumatic nebulisers are only capable of introducing 10% of the sample into the flame.
- Flames are rarely able to atomise solid samples or viscous liquid samples directly.
- Flame background absorption and emission often requires some sort of correction procedure.
Electrothermal atomization :
It provides enhanced sensitivity as the entire sample is atomized in a short period and the average residence time of the atoms in the optical path is a second or more. A hollow graphite tube with a platform. 25 μl of sample is placed through the sample hole and onto the platform from an automated micropipette and sample changer. The tube is heated electrically by passing a current through it in a pre-programmed series of steps. The details will vary with the sample but typically they might be 30–40 seconds at 150°C to evaporate the solvent, 30 seconds at 600°C to drive off any volatile organic material and char the sample to ash, and with a very fast heating rate (1500 °C s) to 2000–2500°C for 5–10 seconds to vaporise and atomised elements (including the element being analysed). Finally heating the tube to a still higher temperature(2700°C) cleans it ready for the next sample. During this heating cycle the graphite tube is flushed with argon gas to prevent the tube burning away. In electrothermal atomisation almost 100% of the sample is atomised. This makes the technique much more sensitive than flame AAS.
Advantages of Electrothermal Atomisers:
- Unusually high sensitivity for small samples (0.5-10mg/ml)
- Decreased sample size.
- In situ sample treatment.
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- Direct analysis of solid samples.
- Unattended operation, safety of operation.
Disadvantages of Electrothermal Atomisers:
- Relatively poor precision compared to flame techniques (<1%).
- Furnace techniques are slow (5-10 mins) per sample.
- Small samples are not easily detected.
- More chances of interferences.
3) Monochromator & Filter:
The monochromator is used to select a given line in the emission spectrum of the light source and isolate it from all other lines. e.g.: - Prisms & Gratings. For AAS the instrument should produce a sufficiently narrow band width to separate the line chosen for determination from other undesirable lines that may either interface with the measurement or decrease the sensitivity of the analysis. For alkali metals having a few widely spaced resonance lines in visible region a glass filter is used.
4) Detectors:
For AAS; Photomultiplier Tube (PMT) is used. It has a good stability when used with a stable power supply. In PMT, there is an evacuated envelope which contains a photo cathode, a series of electrodes called dynodes and an anode. The photocathode is fixed to the terminal of power supply. When photon strikes the photocathode, an electron is dislodged and the photon is accelerated to dynode 1, which gives 2 or more electrons from dynode 1. Electron from dynode 1 are accelerated to dynode 2, which further gives more electrons. Thus, the current multiplied at each dynode and the resultant electron current is received by the anode to produce an EMF across the circuit which goes to the external amplifier & read out system.
5) Amplifier & Reader:
The PMT output is taken to an amplifier which itself operates at the source modification frequency. It includes digital displays & often graphical presentation of data is seen on video display unit or external computers. A hard copy can also be made on chart recorders. But care should be taken to prevent the use of highly luminous flames should always avoided; otherwise, the detector may be saturated due to continuous signal increase. Applications
23MPl Air Test Metal contamination or pollutants analysis Food Analysis AAS is also used to determine lead, mercury in soy sauce and cadmium in crab meat. Atomic emission spectroscopy (AES): Monitors food quality and determines elemental composition. ICP-OES: Determines toxic and nutritional elements in cereal and other plant seeds. It also analyses trace elements in whisky. Pharmaceutical (Drug Development & Quality Control) Pd: In carbenicillin sodium Cu, Pb, Zn: In activated charcoal Fe: In ascorbic acid Ag: In cisplatin Ph and Zn: In copper sulphate Petrochemical determine trace elements in petroleum products and feedstuffs. Forensics AAS analyse blood samples, brain and muscle tissue, and gunshot powder residue The following are the some of the important applications seen recent days in the field of Atomic Absorption Spectroscopy: -
- Qualitative Analysis: Due to requirement of separate hollow cathode lamp for each sample it is rarely used for Qualitative analysis.
- Quantitative Analysis: The AAS is very sensitive and is accurate than any other technique. The different factors effecting quantitative determination are – a) Flame System, b) Wavelength Selection, c) Solution.
- Simultaneous Multicomponent Analysis: If a multi-element emission source is available, then multi-component analysis can be carried out.
- Determination of metallic Elements in Biological Materials
- Determination of Metallic Elements in Food Industry: The toxic levels of Co, Ni & Zn levels in food items are detected by AAS. 4) Determination of Clacium, Magnesium, Sodium & Potassium in Blood Serum
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- Determination of Lead in Petrol: Tetra ethyl & Tetra methyl lead are present in petrol or the combination is present which is checked by AAS. 6) Determination of low levels of arsenic using Flame AAS & ultra-AA Lamps.
- **Extending analysis range of Gold using ultra-AA Lamps.
- Analysis of Iron ores.
- Analysis of Cement.
- Determination of Calcium in Saliva
- Application of atomic-absorption spectroscopy in physics research
- Determination of Cobalt in Steel, Alloy Steel and Nickel
- In Forensic Sciences** a. Determination of trace elements. b. Elemental profiles of biological samples. c. Trace elements in artificial fibres. d. Determination of the mode of poisoning. e. Hair analysis for heavy metal poisons. f. Determinations of ammunition manufactures. g. Discrimination of objects/Elements.
References
- Skoog, D.A., Holler, F.J. and Crouch, S.R. (2017), “Principal of Instrumental Analysis”, 7th Edition, Sunder College Publisher, New York. Page no.230-247.
- Chatwal, G.R. and Anand, S.K.J. (2003), “Instrumental Methods of Chemical Analysis”, Himalaya Publishing House, Mumbai, Page no.2.340-2.366.
- S M Khopkar, “Basic Concepts Of Analytical chemistry”, 5th^ Edition, New Age International (P)Limited, 2017, Page no. 344-
- G. Svehla, “Pearson Vogel’s Textbook of Quantitative Analysis”, Pearson Education; 6th edition, 2009, page no. 265-
