Template:ASMS ionization nomenclature

Transcluded in ASMS 1981 and ASMS 1984

Electron ionization

This is the term used to describe ionization of any species by electrons. The process may, for example, be written

M + e^- → M^+• + 2e^-

for atoms or molecules,

and

M^• + e^- → M⁺ + 2e^-

for radicals.

Photo-ionization

This is the term generally used to describe ionization of any species

by photons. The process may, for example, be written

[Note: Electrons and photons do not "impact" molecules or atoms. They interact with them in ways that result in various electronic excitations including ionization. For this reason it is recommended that the terms 'electron impact' and 'photon impact' be not used).

Field ionization

This term relates to the removal of electrons from any species by interaction with a high electrical field.

Field desorption

This term is used to describe the formation of ions in the gas phase from a material deposited ona solid surface (known as an 'emitter') in the presence of a high electrical field. 'Field desorption' is an ambiguous term because it implies that the electric field desorbs a material as an ion from some kind of emitter on which the material is deposited. There is growing evidence that some of the ions fanned are due to themal ionization, some to field ionization of vapor evaporated from material on the emitter. Because there is little or no ionization unless the emitter is heated by an electric current, 'field desorption' is a misnomer. The term is however firmly implanted in the literature and most users (by no means all) understand what is going on regardless of the implications of the term. In addition, no better simple term has been suggested to take its place and so; reluctantly, it is recommended that it be retained.

Chemi-ionization and chemical ionization are two terms which should not be used inter-changeably.

Chemi-ionization

refers to a process whereby gaseous molecules are ionized when they interact with other internally excited gaseous molecules or molecular moieties.

Chemical ionization

concerns the process whereby new ionized species are formed when gaseous molecules interact with ions. The process may involve transfer of an electron, proton or other charged species to or between the reactants. When a positive ion results from chemical ionization, the term may be used without qualification. When a negative ion results, the term negative ion chemical ionization should be used.

Surface ionization

takes place when an atom or molecule is ionized when it interacts with a solid surface. Ionization only.occurs when the work function of the surface, the temperature of the surface, and the ionization energy of the atom or molecule nave an appropriate relationship.

Thermal ionization

takes place when an atom or molecule interacts with a heated surface or is in a gaseous environment at high_ temperatures.: [Examples of the latter may be a capillary arc plasma, a microwave plasma, or an inductively coupled plasma].

Atmospheric pressure ionization

is an ambiguous term. In essence, it is used to describe chemical ionization at atmospheric pressure. It is recommended that use of the term should be discouraged.

Spark (source) ionization

occurs when a solid sample is vaporized and partially ionized by an intermittent electric discharge. Further ionization occurs in the discharge when gaseous atoms and small molecular moieties interact with energetic electrons in the intermittent discharge. It is recommended that the word 'source' be dropped from this term.

Auto-ionization

occurs when an internally supra excited atom or molecular moiety loses an electron spontaneously without further interaction with an energy source. (The state of the atom or molecular moiety is known as a pre-ionization state).

Associative ionization

occurs when two excited gaseous atoms or molecular moieties interact and the sum of their internal energies is sufficient to produce a single, additive ionic product.

Multi-photon ionization

occurs when an atom or molecule and their concomitant ions have energy states whereby the energy in two or more photons can be absorbed.

Penning ionization

occurs through the interaction of two or more neutral gaseous species at least one of which is internally excited.

Charge exchange (charge transfer ionization) ionization

occurs when an ion/atom or ion/molecule reaction takes place in which the charge on the ion is transferred to the neutral species without any dissociation of either.

Ion-pair formation

involves an ionization process in which a positive fragment ion and a negative fragment ion are the only products.

Ionization cross section

This is a measure of the probability that a given ionization process will occur when an atom or molecule interacts with an electron or a photon.

Electron attachment

A resonance process whereby an external electron is incorporated into an atomic or molecular orbital of an atom or molecule.

Ionization energy

This is the minimum energy of excitation of an atom, molecule or molecular moiety required to remove an electron in order to produce a positive ion.

Vertical ionization

This is a process whereby an electron is removed from a molecule in its ground or an excited state so rapidly that a positive ion is produced without change in the positions or momenta of the atoms. The resultant ion is often in an excited state.

Adiabatic ionization

A process whereby an electron is removed from the ground state of an atom or molecule producing an ion in its ground state.

Ionization

A process which produces an ion from a neutral atom or molecule.

Dissociative ionization

Im ionization process in which a gaseous molecule decomposes to form products, one of which is an ion.

Ionic dissociation

A decomposition of an ion into another ion of lower formula-weight and one or more neutral species.

Ionization efficiency

is the ratio of the number of ions formed to the number of electrons or photons used.

An ionization efficiency curve

shows the number of ions produced as a function of the energy of the electrons or photons used to_produce ionization.

Laser ionization

occurs when a sample is irradiated with a laser beam. In the irradiation of gaseous samples, ionization occurs via a single-or multi-photon process. In the case of solid samples, ionization occurs via_ a thermal process.

Positive ion

This is an atom, radical, molecule or molecular moiety which has lost one or more electrons thereby retaining an electrically positive charge. The use-of the term cation as an alternative is not recommended. The use of mass ion is not recommended.

Negative ion

An atom, radical, molecule or molecular moiety in the vapor phase which has gained one or more electrons thereby acquiring an electrically negative charge. The use of the term anion as an alternative is not recommended.

Singly-, Doubly-,

Triply-etc. Charged Ion These terms are used to describe an atom, molecule or molecular moiety which has gained or lost one, two, three or more electrons. The term multiply-charged ion is used to refer to ions that have gained or lost more than one electron where the number of electrons lost or gained is not designated.

Parent ion

An electrically charged molecular moiety which may dissociate to form fragment. one or more of which may be electrically charged, and one or more neutral species. A parent ion may be a molecular ion or an electrically charged fragment of a molecular ion.

Fragment ion

An electrically charged dissociation product of an ionic fragmentation. Such an ion may dissociate further to form other electrically charged molecular or atomic moieties of successively lower formula weight. (See also Daughter ion).

Daughter ion

An electrically charged product of reaction of a particular parent ion. In general such ions have a dfrect relationship to a particular precursor ion and indeed may relate to a unique state of the precursor ion. The reaction need not necessarily involve fragmentation. it could, for example involve a change in the number of charges carried. Thus, all fragment ions are daughter ions but not all daughter ions are necessarily fragment ions.

Rearrangement ion

An electrically charged dissociation product, involving a molecular or parent ion, in which atoms or groups of atoms have transferred from one portion of a molecule or molecular moiety to another during the fragmentation process.

Stable ion

An ion which is not sufficiently excited to dissociate into a daughter ion and associated neutral fragment(s) or to react further in any other way.

Unstable ion

An ion which is sufficiently excited to dissociate within the ion source.

Metastable ion

An ion which is sufficiently excited to dissociate into a particular daughter ion and neutral species during the flight from the ion source to the detector. The dissociation is most readily observed when it takes place in one of the field-free regions in a mass spectrometer.

Precursor ion

This term is synonymous with parent ion.

Product ion

This term is synonymous with daughter ion.

Molecular ion

An ion formed by the removal (positive ions) or addition (negative ions) of one or more electrons from a molecule without fragmentation of the molecular structure. The mass of this ion corresponds to the sum of the masses of the most abundant naturally occurring isotopes of the various atoms that make up the molecule (with a correction for the masses of the electrons lost or gained). For example, the mass of the molecular ion of ethyl bromide, C₂H₅Br will be 2 x 12 plus 5 x 1.0078246 plus 78.91839 minus the mass of the electron (m_e). This is equal to 107.95751 u - m_e, u being the unit of atomic mass based on the standard that the mass of the isotope ¹²C = 12u exactly.

Isotopic molecular ion

A molecular ion containing one or more of the less abundant naturally occurring isotopes of the atoms that make up the molecular structure. Thus, for ethyl bromide there exist molecular isotope ions such as ¹³CCH₅Br⁺ , C₂H₄D Br⁺ C₂H₅⁸¹Br⁺, ¹³C₂H₅⁸¹Br⁺, etc.

Isotopic ion

Any ion containing one or more of the less abundant naturally occurring isotopes of the elements that make up its structure.

Isotopically enriched ions

When the abundance of a particular isotope is increased above the level at which it occurs in nature and is incorporated in a molecule the term "isotopically enriched ion" is used to describe any ion containing the enriched isotope.

Dimeric ion

An ion formed either when a chemical species exists in the vapor phase as a dimer and can be detected as such, or when a molecular ion can attach to a neutral molecule within the ion source to form an ion such as [2M]ⁿ⁺. where M represents the molecule.

Protonated molecule

An ion formed by interaction of a molecule with a proton abstracted from an ion, as often occurs in Chemical Ionization according to the reaction : M + XH⁺ + MH⁺ + X. The symbolism [M+H]⁺ may also be used to represent the protonated molecule.

[Note : The widely-used term 'protonated molecular ion' to "describe the MH⁺ ion is not recommended. It suggests an association product of a proton with a molecular ion].

Adduct ion

An ion formed by interaction of two species, usually an ion and a molecule, and often within the ion source, to form an ion containing all the constituent atoms of one species as well as an additional atom or atoms.

Cluster ion

An ion formed by the combination of two or more molecules of a chemical species often in association with a second species. For example, [ (H₂O)_nH]⁺ is a cluster ion.

Radical ion

An ion containing an un-paired electron which is thus both an ion and a free radical. The presence of the odd electron is denoted by placing a dot alongside the symbol for the charge. Thus, C₆H₆^+. and SF₆^-. are radical ions.

Odd-electron ion

This term is synonymous with radical ion.

Even-electron ion

An ion containing no un-paired electrons, for example CH₃ in its ground state.