Thermal ionization, also known as surface ionization or contact ionization, is a physical process whereby the atoms are

desorbed Desorption is the physical process where a previously adsorbed substance is released from a surface. This happens when a molecule gains enough energy to overcome the activation barrier of the bounding energy that keeps it in the surface. There ...

from a hot surface, and in the process are ionized. Thermal

ionization Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...

is used to make simple

ion source An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines. Electron ionization Electron ...

s, for

mass spectrometry Mass spectrometry (MS) is an analytical technique that is used to measure the mass-to-charge ratio of ions. The results are presented as a ''mass spectrum'', a plot of intensity as a function of the mass-to-charge ratio. Mass spectrometry is use ...

and for generating

ion beam An ion beam is a type of charged particle beam consisting of ions. Ion beams have many uses in electronics manufacturing (principally ion implantation) and other industries. A variety of ion beam sources exists, some derived from the mercury ...

s. Thermal ionization has seen extensive use in determining atomic weights, in addition to being used in many geological/nuclear applications.

Physics

The likelihood of ionization is a function of the filament temperature, the

work function In solid-state physics, the work function (sometimes spelt workfunction) is the minimum thermodynamic work (i.e., energy) needed to remove an electron from a solid to a point in the vacuum immediately outside the solid surface. Here "immediately" m ...

of the filament substrate and the

ionization energy Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged atom or molecule i ...

of the element. This is summarised in the Saha-Langmuir equation: :

\frac  =  \frac \exp \Bigg(\frac\Bigg)

where ::

\frac

= ratio of ion number density to neutral number density ::

\frac

= ratio of statistical weights (degeneracy) of ionic (g_+) and neutral (g_0) states ::

W

of surface ::

\Delta E_I

of desorbed element ::

k

Boltzmann's constant The Boltzmann constant ( or ) is the proportionality factor that relates the average relative kinetic energy of particles in a gas with the thermodynamic temperature of the gas. It occurs in the definitions of the kelvin and the gas constant, ...

T

= surface temperature Negative ionization can also occur for elements with a large

electron affinity The electron affinity (''E''ea) of an atom or molecule is defined as the amount of energy released when an electron attaches to a neutral atom or molecule in the gaseous state to form an anion. ::X(g) + e− → X−(g) + energy Note that this is ...

\Delta E_A

against a surface of low work function.

Thermal ionization mass spectrometry

One application of thermal ionization is

thermal ionization mass spectrometry Thermal ionization mass spectrometry (TIMS) is also known as surface ionization and is a highly sensitive isotope mass spectrometry characterization technique. The isotopic ratios of radionuclides are used to get an accurate measurement for the ele ...

(TIMS). In thermal ionization mass spectrometry, a chemically purified material is placed onto a filament which is then heated to high temperatures to cause some of the material to be

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

ized as it is thermally desorbed (boiled off) the hot filament. Filaments are generally flat pieces of metal around 1-2mm wide, 0.1mm thick, bent into an upside-down U shape and attached to two contacts that supply a current. This method is widely used in

radiometric dating Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon, in which trace radioactive impurities were selectively incorporated when they were formed. The method compares t ...

, where the sample is ionized under vacuum. The ions being produced at the filament are focused into an ion beam and then passed through a magnetic field to separate them by mass. The relative abundances of different isotopes can then be measured, yielding isotope ratios. When these isotope ratios are measured by TIMS, mass-dependent fractionation occurs as species are emitted by the hot filament. Fractionation occurs due to the excitation of the sample and therefore must be corrected for accurate measurement of the isotope ratio.Dickin, A.P., 2005. Radiogenic Isotope Geology 2nd ed. Cambridge: Cambridge University Press. pp. 21-22 There are several advantages of the TIMS method. It has a simple design, is less expensive than other mass spectrometers, and produces stable ion emissions. It requires a stable power supply, and is suitable for species with a low ionization energy, such as

strontium Strontium is the chemical element with the symbol Sr and atomic number 38. An alkaline earth metal, strontium is a soft silver-white yellowish metallic element that is highly chemically reactive. The metal forms a dark oxide layer when it is ex ...

and

lead Lead is a chemical element with the symbol Pb (from the Latin ) and atomic number 82. It is a heavy metal that is denser than most common materials. Lead is soft and malleable, and also has a relatively low melting point. When freshly cu ...

. The disadvantages of this method stem from the maximum temperature achieved in thermal ionization. The hot filament reaches a temperature of less than 2500 °C, leading to the inability to create atomic ions of species with a high ionization energy, such as

osmium Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mos ...

and

tungsten Tungsten, or wolfram, is a chemical element with the symbol W and atomic number 74. Tungsten is a rare metal found naturally on Earth almost exclusively as compounds with other elements. It was identified as a new element in 1781 and first isolat ...

. Although the TIMS method can create molecular ions instead in this case, species with high ionization energy can be analyzed more effectively with MC-ICP-MS.

References

{{DEFAULTSORT:Thermal Ionization Ion source Ionization

Physics

Thermal ionization mass spectrometry

See also

References