Chemical Sciences: A Manual for CSIR-UGC National Eligibility Test for Lectureship and JRF/Atomic spectroscopy

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Atomic spectroscopy is the determination of elemental composition by its electromagnetic or mass spectrum. Atomic spectroscopy is closely related to other forms of spectroscopy. It can be divided by atomization source or by the type of spectroscopy used. In the latter case, the main division is between optical and mass spectrometry. Mass spectrometry generally gives significantly better analytical performance, but is also significantly more complex. This complexity translates into higher purchase costs, higher operational costs, more operator training, and a greater number of components that can potentially fail. Because optical spectroscopy is generally less expensive and has performance adequate for many tasks, it is far more common. Atomic absorption spectrometers are one of the most commonly sold and used analytical devices.

Optical spectroscopy[edit]

Electrons exist in energy levels within an atom. These levels have well defined energies and electrons moving between them must absorb or emit an energy equal to the difference between them. In optical spectroscopy, the energy absorbed to move an electron to a more energetic level and/or the energy emitted as the electron moves to a lower energy level is in the form of a photon (a particle of light). Because this energy is well-defined, an atom's identity can be found by the energy of this transition. The wavelength of light can be related to its energy. It is usually easier to measure the wavelength of light than to directly measure its energy.

Optical spectroscopy can be further divided into absorption, emission, and fluorescence.

In atomic absorption spectroscopy, light is passed through a collection of atoms. If the wavelength of the light has energy corresponding to the energy difference between two energy levels in the atoms, a portion of the light will be absorbed. The relationship between the concentration of atoms, the distance the light travels through the collection of atoms, and the portion of the light absorbed is given by the Beer-Lambert law.

The energy stored in the atoms can be released in a variety of ways. When it is released as light, this is known as fluorescence. Atomic fluorescence spectroscopy measures this emitted light. Fluorescence is generally measured at a 90° angle from the excitation source to minimize collection of scattered light from the excitation source, often such a rotation is provided by a Pellin-Broca prism on a turntable which will also separate the light into its spectrum for closer analysis. The wavelength once again tells you the identity of the atoms. For low absorbances (and therefore low concentrations) the intensity of the fluoresced light is directly proportional to the concentration of atoms. Atomic fluorescence is generally more sensitive (i.e. it can detect lower concentrations) than atomic absorption.

Strictly speaking, any measurement of the emitted light is emission spectroscopy, but atomic emission spectroscopy usually does not include fluorescence and rather refers to emission after excitation by thermal means. The intensity of the emitted light is directly proportional to the concentration of atoms.

Mass spectrometry[edit]

Template:See details Atomic mass spectrometry is similar to other types of mass spectrometry in that it consists of an ion source, a mass analyzer, and a detector. Atoms' identities are determined by their mass-to-charge ratio (via the mass analyzer) and their concentrations are determined by the number of ions detected. Although considerable research has gone into customizing mass spectrometers for atomic ion sources, it is the ion source that differs most from other forms of mass spectrometry. These ion sources must also atomize samples, or an atomization step must take place before ionization. Atomic ion sources are generally modifications of atomic optical spectroscopy atom sources.

Ion and atom sources[edit]

Sources can be adapted in many ways, but the lists below give the general uses of a number of sources. Of these, flames are the most common due to their low cost and their simplicity. Although significantly less common, inductively-coupled plasmas, especially when used with mass spectrometers, are recognized for their outstanding analytical performance and their versatility.

For all atomic spectroscopy, a sample must be vaporized and atomized. For atomic mass spectrometry, a sample must also be ionized. Vaporization, atomization, and ionization are often, but not always, accomplished with a single source. Alternatively, one source may be used to vaporize a sample while another is used to atomize (and possibly ionize). An example of this would be laser ablation inductively-coupled plasma atomic emission spectrometry, where a laser is used to vaporize a solid sample and an inductively-coupled plasma is used to atomize the vapor.

With the exception of flames and graphite furnaces, which are most commonly used for atomic absorption spectroscopy, most sources are used primarily for atomic emission spectroscopy.

Liquid-sampling sources include:

  • Flames and sparks (atom source)
  • Inductively-coupled plasma (atom and ion source)
  • Graphite furnace (atom source)
  • Microwave plasma (atom and ion source)
  • Direct-current plasma (atom and ion source)

Solid-sampling sources include

  • Lasers (atom and vapor source)
  • Glow discharge (atom and ion source)
  • Arc (atom and ion source)
  • Spark (atom and ion source)
  • Graphite furnace (atom and vapor source)

Gas-sampling sources include

  • Flame (atom source)
  • Inductively-coupled plasma (atom and ion source)
  • Microwave plasma (atom and ion source)
  • Direct-current plasma (atom and ion source)
  • Glow discharge (atom and ion source)