Atomic Absorption Spectrometry (AAS) is a powerful analytical technique used to determine the concentration of metal ions in a sample. It works by measuring the amount of light absorbed by atoms when they transition from the ground state to an excited state. Since each element absorbs light at specific wavelengths, AAS is highly selective, sensitive, and widely used for trace metal analysis in pharmaceuticals, biological fluids, environmental samples, and industrial materials.
Principle of Atomic Absorption Spectrometry
The principle of AAS is based on the absorption of electromagnetic radiation by free atoms. When ground-state atoms in the flame or furnace are exposed to radiation of a specific wavelength, they absorb light and become excited. The decrease in intensity of the incident radiation is measured and used to determine the concentration of the analyte.
Absorbance is directly proportional to the number of atoms in the ground state, making the technique ideal for quantitative analysis.
Steps Involved in Atomic Absorption Spectrometry
AAS involves several sequential processes:
- 1. Nebulization: Conversion of liquid sample into aerosol droplets.
- 2. Desolvation: Removal of solvent inside the flame or furnace.
- 3. Vaporization: Formation of gaseous molecules.
- 4. Atomization: Conversion of molecules into free atoms.
- 5. Radiation absorption: Atoms absorb specific light from the radiation source.
- 6. Measurement: The instrument measures loss of radiant energy.
Instrumentation of Atomic Absorption Spectrometry
AAS instruments are designed to produce atoms, illuminate them with element-specific light, and measure absorption accurately. The major components include:
1) Radiation Source
The light source provides the characteristic wavelength required for absorption. The common electrode-based sources are:
• Hollow Cathode Lamp (HCL)
- Cathode is made of the element being analyzed.
- Filled with an inert gas like argon or neon.
- Produces sharp, intense spectral lines with high specificity.
• Electrodeless Discharge Lamp (EDL)
- Operates using RF or microwave excitation.
- Provides higher intensity compared to HCLs.
- Useful for elements requiring stronger radiation.
• Solid-State Electrodes (rare)
Used in specialized instruments; less common in classical AAS.
• Reference Electrodes (not typically used)
While important in electrochemical techniques, they are not main components of AAS.
2) Chopper
A rotating mechanical chopper interrupts the light beam at regular intervals, converting continuous radiation into pulsed radiation. This helps distinguish between:
- Light absorbed by atoms
- Background flame emission
Chopping improves accuracy and minimizes signal noise.
3) Burners
Burners mix fuel (acetylene) and oxidant (air or nitrous oxide) to produce a stable flame for atomization. The flame type determines the temperature, which influences atom formation:
- Air–acetylene flame: Suitable for easily atomized elements.
- N2O–acetylene flame: Higher temperature for refractory metals.
4) Atomizer
The atomizer converts the sample into free atoms. Two types are used:
- Flame atomizer: Simple, rapid, ideal for routine analysis.
- Electrothermal atomizer (Graphite Furnace AAS): Provides higher sensitivity for ultra-trace metals.
5) Sample Cell
The sample cell or burner head is where aerosol droplets enter and atoms form. The flame shape affects path length and sensitivity.
6) Nebulizer System
The nebulizer draws the liquid sample into the burner and converts it to a fine mist. It controls:
- Droplet size
- Sample flow rate
- Aspiration efficiency
7) Monochromator
A monochromator isolates the narrow absorption line of interest by removing stray and scattered light. Grating monochromators offer excellent resolution for analytical wavelengths.
8) Detectors
Detectors convert light intensity into electrical signals. The most common type is the photomultiplier tube (PMT), which offers:
- High sensitivity
- Fast response
- Low noise
9) Readout Devices
The processed electrical signal is displayed digitally as absorbance or concentration. Modern instruments integrate microprocessor controls and automated calibration.
Instruments for Atomic Absorption Spectroscopy
A) Single-Beam AAS
- Light passes directly through the atomizer to the detector.
- Simpler and less expensive.
- More susceptible to fluctuations in lamp intensity.
B) Double-Beam AAS
- Beam is split into reference and sample paths.
- Compensates for lamp intensity variations.
- Provides greater stability and accuracy.
Interferences in AAS
AAS measurements may be affected by various interferences:
- Chemical interferences: Formation of refractory compounds reducing atom formation.
- Ionization interferences: Ground-state atoms ionize at high temperatures, reducing absorption.
- Spectral interferences: Overlapping absorption or emission lines.
- Background absorption: Caused by smoke, flame gases, or molecular absorption.
These can be minimized using:
- Releasing agents
- Protective agents
- Ionization suppressors
- Background correction techniques (e.g., deuterium lamp correction)
Applications of Atomic Absorption Spectrometry
- Determination of trace metals like copper, zinc, iron, lead, and cadmium
- Analysis of pharmaceutical raw materials and formulations
- Clinical analysis of serum calcium, magnesium, sodium, and potassium
- Environmental monitoring of soil and water contamination
- Food quality control for metal content
- Industrial alloy composition studies
Relationship Between Atomic Absorption and Flame Emission Spectroscopy
AAS and Flame Emission Spectroscopy (FES) are closely related techniques:
- Both involve atomization of sample in a flame.
- In FES, excited atoms emit radiation; measurement is based on emission intensity.
- In AAS, ground-state atoms absorb radiation; measurement is based on absorption intensity.
- AAS is generally more sensitive because most atoms remain in the ground state.
Experimental Procedure for Quantitative Analysis
A typical AAS analysis involves:
- Preparing standard metal solutions for calibration.
- Adjusting instrument parameters (lamp current, flame type, slit width).
- Aspiring blank solution to set the zero baseline.
- Aspiring standards to generate a calibration curve.
- Aspiring the sample and recording absorbance.
- Calculating concentration using the calibration graph.
Detailed Notes:
For PDF style full-color notes, open the complete study material below: