Atomic Absorption Spectrophotometer – Principle, Parts, Types

Atomic Absorption Spectrophotometer (AAS) is an analytical instrument used for finding the concentration of metals and metalloids in a sample. It is a sensitive method. It is mostly used when very small amount of element is present in the sample.

The instrument works on the absorption of light by free atoms. The atoms must be in ground state. Each element absorb light of its own fixed wavelength, so this wavelength is used for detecting that element.

In this method, the sample is first taken in liquid form. It is introduced into an atomizer. The atomizer may be flame or graphite furnace. Here the sample is converted into vapour and then into free atoms.

A hollow cathode lamp is used as the light source. It gives light of selected wavelength for the element. This light is passed through the free atom cloud. The atoms absorb some amount of light and the remaining light goes to the detector.

The detector measures the intensity of remaining light. If more atoms are present, more light is absorbed. According to Beer-Lambert law, the absorption of light is directly related with concentration of the element. So the concentration is calculated from the absorbed light.

Modern atomic absorption spectroscopy was developed in 1952 by Sir Alan Walsh and his team at Commonwealth Scientific and Industrial Research Organisation (CSIRO), Melbourne, Australia. Before this method, elemental analysis was mostly done by emission spectroscopy. But that method was much affected by flame temperature and solvent.

Walsh found that absorption measurement can give more stable and reliable result. In 1955, he published the paper “The application of atomic absorption spectra to chemical analysis”. After this, the method became important for chemical analysis.

The first commercial AAS instruments were prepared with parts made by Techtron Pty. Ltd. The instrument came in commercial use during the 1960s. Later, graphite tube atomizer and high resolution instruments were developed, which made this method more useful for trace metal analysis.

Principle of Atomic Absorption Spectrophotometer

Principle of Atomic Absorption Spectrophotometer is based on the absorption of radiation by neutral free atoms.

The sample is introduced into the flame or graphite furnace. Here the sample is evaporated. The salt or compound is dissociated and free atoms are produced.

Most of the free atoms remain in ground state. When radiation of proper wavelength is passed through these atoms, the atoms absorb the radiation. The electrons of the atoms are excited to higher energy level.

The radiation used in AAS is obtained from hollow cathode lamp. This lamp gives sharp line radiation of the element to be estimated. So the radiation absorbed is specific for that element.

The unabsorbed radiation is passed through the monochromator. It selects the required wavelength and removes unwanted radiation. The selected radiation then falls on the detector.

The detector measures the intensity of radiation. The reduction in intensity gives the absorbance of the sample. The absorbance increases with increase in number of atoms.

The concentration of the element is determined by Beer-Lambert law. The absorbance is proportional to the concentration of atoms present in the sample. Thus the metal concentration is measured from the absorbance value.

Instrumentation of Atomic Absorption Spectrophotometer (AAS)

1. Radiation source – It is used to give radiation of definite wavelength. The wavelength should be of the element which is to be estimated. Hollow cathode lamp (HCL) is commonly used. Electrodeless discharge lamp (EDL) and xenon lamp are also used in some instruments.
2. Hollow cathode lamp – It is the most important source in AAS. The cathode is made up of the same element which is to be analysed. When current is passed, the lamp emits sharp line radiation of that element.
3. Sample introduction system – It is used to introduce the sample into the instrument. The liquid sample is sucked into the system. It is then changed into fine mist or aerosol by nebulizer.
4. Nebulizer – It converts the liquid sample into very small droplets. These droplets are mixed with fuel and oxidant gas. The fine aerosol then enters into the burner or atomizer.
5. Atomizer – It is used to convert the sample into free neutral atoms. The atomizer removes solvent, vaporizes the sample and dissociates the compound. These free atoms then absorb the radiation.
6. Flame atomizer – In this type, flame is used for atomization of sample. The aerosol enters into flame and free atoms are produced. Air-acetylene or nitrous oxide-acetylene flame are commonly used.
7. Graphite furnace atomizer – It is also called electrothermal atomizer. In this method, the sample is placed inside a graphite tube. The tube is heated electrically and atomization takes place in steps.
8. Hydride generation system – It is used for some special elements which form volatile hydrides. Elements like arsenic, selenium and antimony can be analysed by this method. The hydride vapour is carried to the atomizer.
9. Optical system – It consists of lenses, mirrors and beam splitters. These parts direct the radiation from the lamp to the atom cloud. It also sends the transmitted radiation towards the monochromator.
10. Monochromator – It is used to select the required wavelength. It removes unwanted radiation and background light. A diffraction grating is generally used in the monochromator.
11. Detector – It receives the radiation coming from monochromator. It converts light energy into electrical signal. Photomultiplier tube (PMT), CCD and sCMOS detector may be used.
12. Background correction system – It is used to correct unwanted absorption or scattering by the sample matrix. Deuterium lamp, Zeeman correction or high current lamp pulsing may be used for this purpose.
13. Amplifier – It increases the weak electrical signal coming from the detector. The amplified signal becomes suitable for measurement. It is then sent to the readout system.
14. Computer and software – It processes the signal and gives final result. It prepares calibration curve and calculates the concentration of element. The result is shown on the screen and can be stored for record.

Types of Atomic Absorption Spectroscopy

1. Flame atomic absorption spectroscopy (FAAS) – It is the common type of atomic absorption spectroscopy. In this method, a continuous high temperature flame is used. The liquid sample is dried, vaporized and converted into free atoms in the flame. It is used for routine estimation of metals in ppm range.
2. Graphite furnace atomic absorption spectroscopy (GFAAS) – It uses an electrically heated graphite tube for atomization of sample. Only small amount of sample is needed in this method. It is more sensitive than flame method and used for trace metal analysis in ppb range.
3. Electrothermal atomic absorption spectroscopy (ET AAS) – It is also based on electrical heating of the sample. The sample is heated step by step inside the graphite tube. Drying, ashing and atomization takes place in the same tube. It is useful for very low concentration of metals.
4. Cold vapour atomic absorption spectroscopy (CVAAS) – It is a special non flame method used for mercury estimation. Mercury can remain as monoatomic vapour at room temperature. So mercury vapour is produced and its absorption is measured directly.
5. Hydride generation atomic absorption spectroscopy (HGAAS) – It is used for elements which form volatile hydrides. Arsenic, selenium, bismuth and antimony are commonly estimated by this method. The element is chemically converted into hydride gas and then carried into heated tube, where free atoms are formed.
6. Glow discharge atomization – In this method, low pressure argon gas is used. Argon ions are formed by electric field. These ions strike the conducting sample surface and neutral atoms are released from the surface. This process is called sputtering.
7. Line source AAS (LS AAS) – It is the traditional type of AAS. It needs a separate radiation source for each element. Hollow cathode lamp is mostly used in this method, which gives narrow line radiation of the selected element.
8. High-resolution continuum source AAS (HR-CS AAS) – It is an advanced type of AAS. It uses high intensity continuum source like xenon short-arc lamp. A high resolution double monochromator is used with it. It can measure different elements at different wavelengths.

Operating Procedure of Atomic Absorption Spectroscopy

1. The element to be estimated is selected first. The hollow cathode lamp (HCL) of that element is fitted in the instrument. This lamp gives the required wavelength of the selected element.
2. The sample is prepared in clear liquid form. Solid sample is first digested with acid or by heating. Then it is diluted properly and made free from solid particles.
3. The blank solution is prepared. It does not contain the element to be estimated. This blank is used for setting the base line reading of the instrument.
4. Standard solutions are prepared with known concentration of the element. Different concentration standards are made. These standards are used for preparing the calibration curve.
5. The instrument is switched on and allowed to become stable. The lamp current and wavelength are adjusted. The burner position, slit width and gas flow are set according to the selected element.
6. The blank solution is first aspirated into the atomizer. It may enter into the flame or graphite furnace. The blank reading is adjusted to zero.
7. The standard solutions are then aspirated one by one. In the atomizer, the liquid is dried, vaporized and converted into free ground state atoms. The absorbance of each standard is recorded.
8. The unknown sample solution is aspirated after the standards. The sample is atomized in the same way. The free atoms of the selected element absorb the radiation from the HCL.
9. The unabsorbed radiation passes through the monochromator. The monochromator removes unwanted radiation and allows only selected wavelength to reach the detector. The detector measures the decrease in light intensity.
10. The absorbance value of the unknown sample is recorded. The computer compares this value with the calibration curve of standards. From this, the concentration of the element is calculated.
11. After the reading, the system is washed with distilled water or suitable blank solution. This removes remaining sample from the nebulizer and burner system. It helps to prevent blockage and carry over.
12. The flame and gases are turned off carefully after completion of work. The lamp and instrument are then switched off. The burner head, sample tube and other parts are cleaned properly.

Applications of Atomic Absorption Spectrophotometer

• Atomic Absorption Spectrophotometer (AAS) is used in environmental monitoring. It is used for detection of heavy metals in water, soil and aquatic samples. Lead in drinking water and wastewater, mercury in soil and arsenic in groundwater are commonly estimated by this method.
• It is used for food safety testing. Toxic metals like lead, cadmium and mercury are detected in grains, fruits, vegetables and seafood. This is needed because these metals may be harmful even in small amount.
• It is used for nutritional analysis of food materials. Essential minerals like iron, calcium, magnesium and zinc are measured in food products. Dietary supplements are also analysed by this method.
• It is used in pharmaceutical industries for testing of metal impurities. Drug products are checked for toxic elemental impurities. Residual metal catalysts such as palladium and nickel are also measured in drug materials.
• It is used in clinical and biomedical studies. Blood serum, urine, hair and other body fluids are analysed for metal content. Magnesium in serum and zinc in hair can be measured by this method.
• It is used to study toxic metal exposure in human body. Heavy metals present in biological samples are estimated. It also helps in finding mineral deficiency.
• It is used in mining and metallurgy. Ores, alloys and smelting products are analysed by AAS. The amount of valuable metals and toxic metals present in these materials can be measured.
• It is used in agricultural analysis. Soil and crop samples are tested for toxic metals. Metals coming from pesticides, fertilizers and industrial runoff can be detected by this method.
• It is used in fertilizer formulation and testing. Nutrient metals present in fertilizers are estimated. This helps in maintaining proper composition of fertilizer.
• It is used in cannabis testing. Heavy metals present in cannabis sample are measured. This is done for safety checking of the product.
• It is used in forensic science. Graphite furnace AAS is useful for very small amount of sample. Barium and antimony in gunshot residue can be detected from cloth or other crime sample.
• It is used in petrochemical industries. Vanadium, nickel, sodium and iron are measured in fuel oils. Lead in gasoline is also estimated by this method.
• It is used in routine quality control laboratory. Raw materials and finished products are checked for metal concentration. This helps to control contamination and maintain product quality.

Advantages Atomic Absorption Spectrophotometer

• Atomic Absorption Spectrophotometer (AAS) gives accurate result for metal analysis. The result obtained is more reliable because each element is measured by its own specific wavelength.
• It is a sensitive method. Very small amount of metal can be detected by this method. By using graphite furnace atomizer, elements can be estimated even in ppb level.
• The error in this method is less. In most of the analysis, the error is about 0.5% to 5%. So it is useful for quantitative estimation of metals.
• It is highly selective method. The radiation absorbed by one element is different from other element. Therefore one particular metal can be estimated even when other metals are present in the sample.
• It can be used for complex sample also. Samples having high salt content can be analysed with less interference. This makes the method useful for biological, food and environmental samples.
• The running cost of AAS is low in comparison to advanced instruments like ICP-MS. The daily use chemicals and consumables are also less costly. So it is suitable for routine laboratory work.
• It can measure both trace amount and high concentration of elements. In many cases, repeated dilution of sample is not needed. This saves time during analysis.
• Different types of samples can be analysed by this method. Liquid samples, digested solid samples and some gaseous samples can be used. Thus it has wide application in many fields.
• It is useful in environmental monitoring, food testing, pharmaceutical analysis, clinical study and mining work. The same instrument can be used for many metal estimation work.
• Modern AAS instruments are compact and need less bench space. They also have software system for calculation and storage of result. Autosampler can also be used for large number of samples.
• It is a good method for quality control work. The instrument gives rapid and reproducible reading. Therefore it is commonly used for routine estimation of metals in laboratory.

Limitations Atomic Absorption Spectrophotometer

• Atomic Absorption Spectrophotometer (AAS) is used only for metals and some metalloids. It cannot be used for estimation of non-metals and organic compounds. So its use is limited to elemental analysis only.
• It generally measures one element at a time. For each element, separate hollow cathode lamp is required. Therefore analysis of many elements takes more time.
• The sample should be prepared properly before analysis. Most of the samples are converted into clear liquid form. Solid samples need acid digestion or heating, which takes more time.
• During sample preparation, contamination may occur. Glassware, acid, water or dust may add small amount of metal in the sample. This can give wrong result in trace metal analysis.
• It is affected by chemical interference. Some elements may form stable compounds in flame and they do not form free atoms easily. Due to this, less absorption is obtained.
• It is affected by ionization interference also. In very hot flame, some atoms may lose electrons and become ions. These ions do not absorb the radiation in same way as neutral atoms.
• Physical interference may occur due to difference in viscosity and surface tension of sample. Thick or salty sample may not enter properly into the flame. This changes the amount of sample reaching the atomizer.
• In Flame AAS, sample wastage is high. Only small part of the aspirated sample reaches the flame. About 90% to 95% of liquid sample may be wasted through drain.
• Flame method needs flammable gases. Acetylene and nitrous oxide are commonly used. These gases need careful handling, because they may cause fire hazard.
• Proper ventilation is required during flame operation. Fumes, heat and gas leakage may create safety problem. So the instrument should be used in well ventilated laboratory.
• It is not very suitable for fast multi-element analysis. Other techniques like ICP-MS can measure many elements together. But traditional AAS is slower for this type of work.
• Background absorption may affect the reading. Sample matrix may absorb or scatter light. Background correction is needed to reduce this error.

Precautions of Atomic Absorption Spectrophotometer

• Atomic Absorption Spectrophotometer (AAS) should be kept under proper exhaust system. The fumes and vapours formed during analysis should go outside the laboratory.
• The room should have good ventilation. The flame produces heat and combustion gases. These gases should not remain inside the room.
• Acetylene and nitrous oxide cylinders should be handled carefully. These gases are flammable. The regulator, pipe and pressure should be checked before use.
• The gas line should be checked for leakage. If gas smell is present, the instrument should not be started. Leakage may cause fire hazard.
• The flame should never be kept unattended. During flame operation, the operator should remain near the instrument. Open flame with gas supply is dangerous.
• The window of hollow cathode lamp should not be touched by hand. Finger mark decreases the light intensity. If touched, it should be cleaned with alcohol or acetone by lens tissue.
• The operator should not look directly into the lamp. The lamp and flame may give ultraviolet (UV) radiation. It may damage eyes and skin.
• The flame shield and chimney should be placed properly. These parts protect from flame, heat and radiation. They should not be removed during working.
• The lamp current should not be increased more than the given value. High current can damage the lamp. It also reduces the life of the lamp.
• The lamp compartment should be handled carefully. High voltage is present in this part. It should not be opened during operation.
• Before starting flame or furnace, the safety interlock should be checked. Cooling water, inert gas, burner fitting and drain trap should be proper.
• The drain trap should contain liquid. If it is empty or blocked, gas may pass back. This can create unsafe condition.
• The burner head and nebulizer should be clean. Salt deposit or blockage may disturb the flame. It also affects sample aspiration.
• The sample should be clear and free from solid particles. If particles are present, the sample should be filtered. Otherwise nebulizer may get blocked.
• Broken or expired hollow cathode lamps should not be thrown in normal waste. These lamps may contain toxic metals like lead, arsenic and cadmium. They should be disposed as hazardous waste.
• After completion of work, the flame should be closed first. Then gas supply should be turned off. The burner, sample tube and working area should be cleaned properly.

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