Atomic absorption spectroscopy is the most widely used tools in analytical chemistry.
It has high sensitivity for the determination of metals and metalloids.
By definition, atomic absorption spectroscopy is a type of quantitative analytical techniques where in the absorption of a specific wave length of light by the atoms in the neutral state is detected and recorded.
The more the number of the atoms in the given sample, the higher is the intensity of absorption and vice-verse.
This can also be called as metal analysis spectroscopy as it mainly used for metal analysis.
Atomic absorption Spectroscopy principle:
The method relies on the principle of absorption method of spectroscopy.
The liquid sample is allowed to convert into free atoms (desolvated and atomized). These free atoms absorb the light of a specific wavelength and remaining light is detected and recorded. The intensity of absorption is directly proportional to the concentration of the sample.
Atomic absorption Spectroscopy Instrumentation:
Atomic absorption spectroscopy, unlike other spectroscopy methods, has two additional requirements like a specially designed lamp to produce light of desired wave length and a burner to prepare the sample for absorption of light.
Additionally, the method also involves, spraying of the sample in the solution state over an atomizer (burner). This leads to evaporation of the solvent and leaves a fine dry residue behind which is has neutral atoms in the ground state.
The instrument is available as single and double beam instruments.
The instrumentation includes:
1. The atomizer (burner) to dry the sample and produce atoms.
2. Sample container.
3. Fuel and oxidant to burn the sample by heat.
4. Hallow cathode lamp to produce light of desired wave length.
5. Detector to detect the absorption intensity.
6. Amplifier and data recorder.
The burner (atomizer): Here the sample from the capillary rises to the tip of burner where it is burned with the flame produced by the fuel and oxidant combination. The sample after evaporation leaves a fine residue of neutral atoms.
Sample container: This is a beaker-like a container of sample which is placed below the burner preferably. A capillary tube drains the sample to the tip of burner.
Fuel and oxidant: This is a very important part of the entire process to be remembered. If the heat produced is not sufficient then the sample doesn’t form neutral atoms. If the heat of burner is more, the sample molecules may ionize instead of forming atoms. So both are undesirable for experimentation.Hence a proper combination of fuels and oxidant are to be used to produce recommended temperatures. Commonly used flues include propane, Hydrogen and acetylene and oxidants are mostly air or oxygen.
Fuel combinations chart:
|Fuel combinations||Flame temperature||Metals Analyzed|
|Acetylene + Air||2550 degrees||For most samples|
|Acetylene + Nitrous oxide||2900 degrees||Aluminum (Al), Molybdenum (Mo), Silicon (Si), Titanium (Ti)|
|Hydrogen + Air||2200 degrees||Lead (Pb), tin (Sn)|
Light source: The light source should produce a narrow spectrum with little back ground noise. Besides the light should be stable and have sufficient intensity.
Two types of light sources can be used based on the requirement.
1. Hallow cathode lamp: This is most widely used as light source. Inside the lamp, the cathode is coated with metal of analyte to be analyzed. For instance, if magnesium is to be analyzed in the sample, the cathode coated with magnesium is used.
Similarly, for all the other elements like Na, Ca, K, Zn etc. to be analyzed respective metal coated cathode is used in the lamp. The lamp is filled with inert gas like argon or neon which is ionized by electric arc. The ions get attracted toward cathodes and strike it leading to excitation of metal ions. This leads to emission of radiation with characteristic wave length of analyte metal.
Hollow cathode lamp advantages
2. Electrode-less discharge lamps: These lamps are less common in regular use but are essential of determination of Arsenic and selenium. A bulb containing element of interest (with argon gas) is present in the lamp. This element is excited using a microwave energy or radio frequency energy
Detector: The detector consists of photo multiplier tube or simple photo cell. The current or potential recorded for the sample absorption is recorded in a computer software and then analyzed.
Atomic Absorption Spectroscopy applications:
1. Atomic spectroscopy is used for quantitative analysis of metal elements in samples like soil, plant material.
2. It is especially useful to analyze ionic metal elements in blood, saliva, urine samples like sodium, potassium, magnesium, calcium and other body fluids.
3. To determine heavy metals like iron, manganese, copper, zinc, mercury, lead, nickel, tin etc in urine, blood etc. This analysis is essential in case of heavy metal poisoning as regular monitoring of poison levels in the blood are to be determined till patient recovery.
4. To determine metal elements in the food industry.
5. To estimate Lead in petroleum products.
6. To determine metal concentrations in ground water and bore well samplings before using for drinking and irrigation.