Emission spectroscopy

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Emission spectroscopy is a spectroscopic technique which examines the wavelengths of photons emitted by atoms or molecules during their transition from an excited state to a lower energy state. Each element emits a characteristic set of discrete wavelengths according to its electronic structure, by observing these wavelengths the elemental composition of the sample can be determined. Emission spectroscopy developed in the late 19th century and efforts in theoretical explanation of atomic emission spectra eventually lead to quantum mechanics.

There are many ways in which atoms can be brought to an excited state. Interaction with electromagnetic radiation is used in fluorescence spectroscopy, protons or other heavier particles in Particle-Induced X-ray Emission and electrons or X-ray photons in Energy-dispersive X-ray spectroscopy or X-ray fluorescence. The simplest method is to heat the sample to a high temperature, after which the excitations are produced by collisions between the sample atoms. This method is used in flame emission spectroscopy, and it was also the method used by Anders Jonas Ångström when he discovered the phenomenon of discrete emission lines in 1850s.^{[citation needed]}

Although the emission lines are caused by a transition between quantized energy states and may at first look very sharp, they do have a finite width, i.e. they are composed of more than one wavelength of light. This spectral line broadening has many different causes.

History

Emission lines from hot gases were first discovered^{[citation needed]} by Ångström, and the technique was further developed by David Alter, Gustav Kirchhoff and Robert Bunsen.

See spectrum analysis for details.

Experimental technique in flame emission spectroscopy

The solution containing the relevant substance to be analysed is drawn into the burner and dispersed into the flame as a fine spray. The solvent evaporates first, leaving finely divided solid particles which move to hottest region of the flame where gaseous atoms and ions are produced. Here electrons are excited as described above. It is common for a monochromator to be used to allow for easy detection.

On a simple level, flame emission spectroscopy can be observed using just a Bunsen burner and samples of metals. For example, sodium metal placed in the flame will glow yellow, whilst calcium metal particles will glow red, copper placed into the flame will create a green flame.

There are four main stages which occur during this flame emission spectroscopy. 1. Evaporation: The sample containing metal particles is dehydrated by the heat of the flame and the solvent is evaporated. 2. Atomisation: At this stage the metal ions that were in the solvent are reduced to metal atoms. For example, Mg2+(aq) + 2e → Mg(g). It is the electrons in metal atoms which absorb energy from the heat of the flame and move to higher energy levels. 3. Excitation: Electrons in these ground state metal atoms are now able to absorb energy from the heat of the flame. The quanta (amount) of energy absorbed depends on the electrostatic forces of attraction between the negatively charged electron and the positively charged nucleus. This in turn depends upon the number of protons in the nucleus. As electrons absorb energy they move to higher energy levels and are in the excited state. 4. Emission of radiation: Electrons in the excited state are very unstable and move back down to the ground state quite quickly. As they do so they emit the energy they absorbed. For some metals this radiation correspond to wavelengths of light in the visible region of the electromagnetic spectrum and are observed as a characteristic colour of the metal. As electrons from different energy levels are able to absorb light the flame color observed will be a mixture of all the different wavelengths emitted by the different electrons in the metal atom under investigation.

See also

• Atomic spectroscopy
• Electromagnetic spectroscopy
• Absorption spectroscopy
• Emission (electromagnetic radiation)
• Astronomical spectroscopy

Flame colorants

Dark red – Lithium chloride Red – Strontium chloride Orange – Calcium chloride (a type of bleaching powder) Yellow – Sodium Chloride (salt) Yellowish green – Borax (sodium Borate) Green – copper sulfate Blue – copper chloride Violet – 3:1 ratio, 3 parts potassium sulfate, and 1 part potassium nitrate Purple – Potassium chloride White – magnesium sulfate (epsom salts)