Atomic Absorption vs. Atomic Emission Spectroscopy

Spectroscopy is an advanced analytical technique. It is the study of the interaction of light with matter. Atomic absorption spectroscopy is used to evaluate the concentration of the analyte. In this technique the absorption of light by the ionized atoms of the analyte is studied whereas, atomic emission spectroscopy is the technique that quantifies the amount of sample present by the amount of light emitted by the group of the ionized atoms of a certain element.

Atomic absorption spectroscopy is a precise analytical technique. It detects at trace and ultra-trace levels. For example, up to ppm and ppb. are used for the quantification of metal atoms. On the other hand, atomic emission spectroscopy is a useful technique to find out inorganic constituents in various types of samples. Furthermore, it can also work on ppb levels.

Atomic Absorption vs Atomic Emission Spectroscopy

The basic principle is the same for both atomic absorption and atomic emission spectroscopy. Both of these follow Lambert Beers’ law.

“Greater the Concentration greater would be absorbance or greater would be emission”

But the main difference is absorbance and emission of radiation.

Principle of AAS

When monochromatic light irradiates the group of atoms, a specific wavelength is absorbed by the metal atoms under investigation. The light wavelength is absorbed by the electrons for excitation. Absorption is directly proportional to the concentration of that element.

By Beer’s Law

A ∝ c

Principle of AES/OES

Atomic emission spectroscopy is always coupled with the excitation source such as Inductively coupled plasma (ICP). The excited electrons when came back to the ground level emit discrete energy. This energy is detected with gamma camera detectors. The amount of emitted energy is directly proportional to the concentration of the analyte.

Working of AAS

• Solid and gaseous samples are converted into solution form.
• Nebulization of sample with the help of nebulizer.
• Vapourization of a sample.
• Atomization of a sample.
• Absorption of radiation leads to excitation of the sample.
• Emission of absorb wavelength and returns to stable ground state.

Working of AES

• The powered sample is placed in the cup-shaped graphite electrode. 
• High voltage is applied to vaporize the sample.
• This leads to the excitation of elements to higher energy levels.
• The excited state lasts for a very short period (10-8sec)
• Excited elements return to the ground state.
• Emission of absorbed radiation.

Instrumentation of AAS

1. Light source
2. Chopper
3. Atomizer
4. Monochromator
5. Detectors
6. Amplifier
7. Readout devices

Light source

A hollow cathode lamp is used as a radiation source in atomic absorption spectroscopy. It is enclosed in a glass envelope. It helps in the excitation of ground state electrons.

Chopper 

A chopper is a rotating wheel between a hollow cathode lamp and a flame.

This breaks steady light from the lamp into pulsating light.

Atomizer 

Atomizers consist of burners and flames. Two different types of burners are used, 

• Premix chamber burner
• Total consumption burner

Similarly, two different types of flames are used,

• Air acetylene flame.
• Nitrous oxide acetylene flame.

The purpose of the atomizer is to break of sample to free atoms by providing a high temperature.

Monochromators

Monochromators are used to select the specific wavelength which is absorbed by the sample and exclude all other wavelengths. The most common types of monochromators are,

• Prisms 
• Gratings 

Detectors

Detectors are used to convert electromagnetic waves into electronic currents. For example, films and photomultiplier tubes are most commonly used as detectors. 

Amplifiers

Electronic current from detectors is fed into an amplifier which amplifies the signal several times.

Readout devices

Chart recorders or digital readout devices are used in atomic absorption spectroscopy.

Instrumentation of AES

1. Sample cell
2. Atomizer
3. Monochromators
4. Detectors
5. Amplifiers
6. Readout devices

Sample Cell

Cup-shaped graphite electrode is acting as a sample cell.

Atomizer

Atomizers consist of flame and burners. This is used to vaporize the sample.

Different types of burners used are

• Total consumption burner
• Laminar flow burner 
• Mecker burner

Monochromators

Prism and diffraction grating acts as monochromators. These are used to chose the specific radiation emitted by the analyte.

Detectors

Photomultiplier tubes or photoemissive cells act as detectors in atomic emission spectroscopy.

Amplifiers

Amplifiers are used to amplify signals several times.

Readout devices

Computers are used as readout devices in atomic emission spectroscopy.

Related topics

• Mass spectrometry
• Chromatography

Quantification

Calibration of AAS

As “AAS” is not an absolute method for quantification. That’s why a calibration curve is used for the quantification of analytes. Calibration curve comparison of signals of the standard of known concentration and signals of unknown concentration. Commonly for standard and blank is used to construct a linear calibration curve of absorbance vs concentration of an analyte.

Calibration  of AES

In AES calibration curve is used for quantification. It is calibration curve is a graph of emission signals vs concentration. Atomic emission has longer linear regions of the calibration curve’s R² (103  to 105)  as compared to the calibration curve (102  to 103) of the atomic absorption spectrum.

Concepts berg

What is atomic absorption spectroscopy?

Atomic absorption spectroscopy is an analytical technique used for the determination of metal at trace and ultra-trace levels by measuring how much light is absorbed by metal atoms in the ground state.

What is atomic emission spectroscopy?

Atomic emission spectroscopy is an analytical technique used for the quantification of inorganic constituents in a given sample by measuring how much light is emitted by excited-state atoms.

What is the difference between atomic emission and atomic absorption?

Key Differences:

• Atomic absorption deals with the absorption of light whereas atomic emission spectroscopy deals with the emission of light 
• Atomic absorption spectroscopy depends upon a number of ground-state atoms whereas atomic emission spectroscopy depends upon a number of excited-state atoms.
• In atomic absorption, the spectroscopy electron moves from the ground state to the excited state, whereas, in atomic emission, the spectroscopy electron moves from excited to the ground state.

Why is atomic absorption spectroscopy preferable to atomic emission?

Atomic absorption spectroscopy is preferable to atomic emission spectroscopy because,

• It is a much more sensitive technique.
• It has fewer limitations.
• It is useful for the analysis of more than 70 different metals. It is a relative technique each metal is detected one by one with a reference. Atomic emission spectroscopy detects all the components together therefore, chances of uncertainty are much higher as compared with AAS.

What are the limitations of atomic absorption spectroscopy?

Limitations:

• It is a much more expensive technique
• It requires individual flame for every element
• It is used only for liquid samples.
• It can analyze only one atom at one time.
• It is a time-consuming technique.

What are the limitations of atomic emission spectroscopy?

Limitations:

• It is only used for alkali and alkaline earth metals because a very high temperature is required for the excitation of metals.
• It is a very expensive technique. 
• The operating procedure is more complicated than atomic absorption.

What is the advantage of direct solid analysis by atomic emission spectroscopy?

Direct solid analysis of a sample includes many advantages. A few of them are briefly explained below,

 Increased sensitivity 

 Because no dilution is required for the dissolution procedure.

Reduce the risk of contamination

Because no dissolution reagent is required.

Preparation time

It decreases the sample preparation time.

What are the advantages of atomic emission over atomic absorption spectroscopy?

Advantages:

1. Atomic emission is a cheap technique as compared to atomic absorption spectroscopy.
2. Atomic emission spectroscopy can be used for direct sample analysis.
3. More than one sample can be analyzed at one time by atomic emission spectroscopy whereas atomic absorption spectroscopy can only be used for one sample at one time.
4. Atomic emission is a quick technique of analysis as compared to atomic absorption.

References

• Atomic Absorption Spectrometry By Bernhard Welz and Michael Sperling
• Atomic Absorption Spectrometry edited by J.E. Cantle (Winsford, Cheshire, UK)