Atomic optical spectrometry

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Optical atomic spectrometry, a subset of atomic spectrometry (or atomic spectroscopy) includes technique analytical chemistry that involves the measurement of visible, infrared or ultraviolet light produced by the interaction of photons with the atoms of chemical elements. It is based on the properties of electrons at different energy levels within electrons, and the properties that cause light, of a charateristic wavelength, to be absorbed in moving an electron to a more energetic level, or to be emitted when electrons decay to a less energetic energy level.

There are three broad types of atomic optical spectrometry:[1]

  • Atomic absorption spectrometry: neasures the absorption, by the sample, of a given wavelength(s) of light
  • Atomic emission spectrometry: measures the emission, by the excited sample, of a given wavelength of light
  • Atomic fluorescence spectrometry: measures the emission of a second wavelength(s) of light, by an excited sample, when illuminated by a reference wavelength of light

Each have advantages and disadvantages, which must be judged in specific applications. That a particular method, for example, is good at the simultaneous analysis of multiple elements is a benefit for geological or forensic laboratories trying to identify specimens, but not especially important for clinical chemists that simply want to know the potassium or sodium level in a blood sample.

As with other techniques of analytical chemistry, one method or variant may displace another, but improvements in a displaced method may create a new and constructive competition. [2] For example, neutron activation analysis also does elemental analysis, but on nuclei rather than electrons. In its basic form, it has the restriction of needing access to a nuclear reactor. NAA, however, has especially good performance on elements with higher atomic numbers, as opposed to atomic emission spectrometry, which is strong for low-Z elements.

Common functions

All the methods require the sample to be broken into an aerosol, which is then both broken down to the atomic level and excited, usually by intense heat.

Aerosolization

Methods include: [3]

  • Pneumatic nebulizers
  • Ultrasonic nebulizers
  • Electrothermal vaporizers
  • Hydride generation
  • Cold vaporization

Atomization and excitation sources

"Analyte atoms in solution are aspirated into the excitation region where they are desolvated, vaporized, and atomized by a flame, discharge, or plasma. These high-temperature atomization sources provide sufficient energy to promote the atoms into high energy levels."[4] Excitation sources include:

Source Temperature[5]
Flame Natural gas/oxygen 2700-2900

Acetylene/air 2100-2400
Acetylene/oxygen 3050-3150
Hydrogen/air 2000-2100
Hydrogen/oxygen 2550-2700

Electrothermal vaporization 1200-3000
Direct-current plasma (DCP) with argon 4000-6000
Inductively coupled plasma (ICP) with argon 4000-6000
Microwave induced plasma (MIP) with argon 2000-3000
Glow discharge plasma 2000-3000
Microwave induced plasma (MIP) with argon nonthermal
Electric arc 4000-5000
Electric spark 40000
Laser induced breakdown (LIBS)
Laser induced plasma

Light source

AAS and AFS require a light source that emits sharply at the wavelength of interest. The two main types in use are hollow cathode lamps (HCL) and electrodeless discharge lamps (EDL).

Continuous spectrum light, however, may be used to cancel out background radiation

Polychromator

Detector

Analysis software

Atomic absorption spectrometry

A reference wavelength(s) of light is directed through vaporized atoms at ground state, and the attenuation of the intensity of that light measured.

Atomic emission spectrometry

The atoms decay back to lower levels by emitting light. Since the transitions are between distinct atomic energy levels, the emission lines in the spectra are narrow. The spectra of samples containing many elements can be very congested, and spectral separation of nearby atomic transitions requires a high-resolution spectrometer. Since all atoms in a sample are excited simultaneously, they can be detected simultaneously using a polychromator with multiple detectors. This ability to simultaneously measure multiple elements is a major advantage of AES compared to atomic absorption spectroscopy. [4]

Atomic fluorescence spectrometry

This technique is less popular than the other two, but has important niche applications, such as detection of trace pollutant minerals. It has both similarities and differences to emission and absorption:

  • Like absorption, the sample is illuminated by a specific wavelength
  • Like emission, the analyzer looks at light emitted from the sample, at a different wavelength than the illumination

While the intensity of the illumination, as long as it is adequate, is not too critical in absorption, in fluorescence, the more intense the illumination, the greater the sensitivity.[6]

References

  1. Atomic Spectroscopy: Atomic Absorption, Emission and Fluorescence Techniques, Andor Technology
  2. Report of an Advisory Group meeting held in Vienna (22–26 June 1998), Use of research reactors for neutron activation analysis, International Atomic Energy Agency, IAEA-TECDOC-1215
  3. David Ryan (2005), Lecture 10: Atomic Spectroscopy - AA & AF, Analytical Chemistry II, course UML 84.314, University of Massachusetts at Lowell, p. 3
  4. 4.0 4.1 Atomic Emission Spectroscopy (AES, OES), University of Vermont
  5. Bernhard Vogler, CH421/521 Atomic Spectroscopy, University of Alabama at Huntsville
  6. Ryan, p. 1