Difference Between AAS and AES


Difference between AAS and AES stems from their operating principles. AAS stands for ‘Atomic Absorption Spectroscopy‘ and AES stands ‘Atomic Emission Spectroscopy.’ Both of these are spectro-analytical methods used in Chemistry in order to quantify the amount of a chemical species; in other words, to measure the concentration of a specific chemical species. AAS and AES differ in their operating principle where AAS employs the method of absorption of light by the atoms and, in AES, the light emitted by the atoms is what is taken into consideration.

What is AAS (Atomic Absorption Spectroscopy)?

AAS or Atomic Absorption Spectroscopy is one of the most common spectral techniques used in analytical chemistry today to determine the concentration of a chemical species accurately. AAS employs the principle of absorption of light by the atoms. In this technique, the concentration is determined by a calibration method where the absorption measurement for known quantity of the same compound has been previously recorded. The calculations are made according to the Beer-Lambert Law and is used here to get the relationship between atomic absorption and the concentration of the species. Furthermore, according to the Beer-Lambert Law, it is a linear relationship that exists between the atomic absorption and the concentration of the species.

The chemical principle of absorption is as follows. The material under detection is first being atomized in the atomization chamber of the instrument. There are several ways of achieving atomization depending on the type of instrument used. These instruments are commonly known as ‘spectrophotometers‘. The atoms are then bombarded with monochromatic light matching its wavelength of absorption. Each type of element has a unique wavelength that it absorbs. And monochromatic light is a light which is especially adjusted to a particular wavelength. In other words, it is a single coloured light, in contrast to normal white light. The electrons in the atoms then absorb this energy and excites into a higher energy level. This is the phenomena of absorption, and the extent of absorption is directly proportional to the amount of atoms present, in other words, the concentration.

Difference Between AAS and AES

AAS Schematic Diagram Description – 1. Hollow Cathode Lamp  2. Atomizer  3. Species  4. Monochromator  5. Light Sensitive Detector  6. Amplifier 7. Signal Processor

What is AES (Atomic Emission Spectroscopy)?

This is also an analytical chemical method used to measure the quantity of a chemical substance. However, the underlying chemical principle, in this case, is slightly different to what is used in Atomic Absorption Spectroscopy. Here, the operating principle of the light emitted by the atoms is taken into consideration. A flame is generally used as the light source and, as mentioned above, the light emitted from the flame can be fine-tuned depending on the element under investigation.

The chemical substance has to be atomized first, and this process happens through the heat energy provided by the flame. The sample (substance under investigation) can be introduced to the flame in many different ways; some common ways are through a platinum wire, as a sprayed solution, or in gas form. The sample then absorbs heat energy from the flame and first splits into smaller components and get atomized upon further heating. Afterwards, the electrons within the atoms absorb a characteristic amount of energy and excite themselves to a higher energy level. It is this energy they release when they start to relax by coming down to a lower energy level. The energy released here is what is measured in Atomic Emission Spectroscopy.


ICP Atomic Emission Spectrometer

What is the difference between AAS and AES?

• Definition of AAS and AES:

• AAS is a spectro-analytical method used in Chemistry where the energy absorbed by atoms is measured.

• AES is a similar technique to AAS that measures the energy emitted by the atomic species under investigation.

• Light Source:

• In AAS, a monochromatic light source is used to provide energy for the excitation of electrons.

• In the case of AES, it is a flame that is often used.

• Atomization:

• In AAS, there is a separate chamber for atomization of the sample.

• However, in AES, atomization takes place step by step upon the introduction of the sample to the flame.

• Principle of operation:

• In AAS, when monochromatic light is bombarded through the sample the atoms absorb energy, and the extent of absorption is recorded.

• In AES, the sample which gets atomised in the flame then absorbs the energy through the electrons which get excited. Later this energy is released upon the relaxation of the atoms and is measured by the instrument as the emitted energy.


Images Courtesy:

  1. AAS spectrometer by Queyas (CC BY-SA 3.0)
  2. ICP atomic emission spectrometer via Wikicommons (Public Domain)
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    Difference between atomic spectroscopy and spectrometry