ASMS 1981

Report on the ASMS Nomenclature Committee Workshops held at the 29th Annual Conference on Mass Spectrometry and Allied Topics, Minneapolis, Minnesota, May 24-29, 1981, 978-815

ASMS Nomenclature Committee Workshops

Minneapolis, 1981 [1]

Two workshops were held on Monday, May 25, 1981 and Thursday, May 28, 1981. 84 members attended the first Workshop and 35 members the second. A copy of the list of terms and definitions for use in mass-spectrometry that had.been prepared following last year's Workshop was distributed to each attendee. The list had already been circulated in advance to 50 people who had attended last year's Workshop or who had written request.ing that the list-be sent to them and their suggested modifications had been incorporated.

Each term on the list was considered in detail. A copy of the terms finally approved at the workshops is attached.

Help was sought to prepare an additional list of terms for consideration at next year's Workshops. The following people volunteered to provide terms under the headings listed alongside their names

M.S. Story, Finnegan Instruments (Instrumentation detectors)

C.R. Lagergren, Battelle (Surface ionization, thermal ionization)

M. Elliott, V.G. Ltd. (Isotope mass spectrometry)

F.A. Elder, Xerox Corporation (Knudsen cell mass spectrometry)

C.M: Judson, University of Kansas (Sample introduction)

Several other people offered to send additional terms (with or without definitions) for inclusion in next year's list.

(The deadline for receipt of terms by me is August 31, 1981. This will allow time for circulation of a new list of terms to the attendees at this year's Workshops so that their committees can be included, and preparation of an agreed list for discussion at the 1982 Meeting in Hawaii).

The following decisions were taken unanimously:

l. That the.attached list of terms be submitted to the ASMS Board of Directors for their approval, with the following recommendations

(i) That if the list is approved, it be considered 'provisional' for one more year so as to give all members a final opportunity of commenting upon it.

(ii) That all comments received before December 31, 1981 be considered by the Chairman of the Nomenclature Committee who would then recommend to the Board, in the light of such comments, which terms should be issued as ASMS recommended terms.

{iii) That terms issued under the headings 'Vacuum' and 'Data System' should be separated into an Appendix to the list of recommended terms.

(iv) That the generosity of the American Vacuum Society in providing most of the terms under the heading 'Vacuum' should be acknowledged in the list.

2. That the same procedure as was used this year should be followed again so that a further list of terms can be submitted to the Board in a year's time.

J .H. Beynon

Chairman

Royal Society Research Unit

University College of Swansea

Singleton Park, SWANSEA SA2 SPP, UK

Ionization nomenclature

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.

Other Terms

Mass analysis

A process by which a mixture of ionic or neutral species is identified according to the mass-to-charge (m/z) ratios (ions) or their aggregate atomic masses (neutrals). The analysis may be qualitative and/or quantitative.

Detection of ions

In mass spectrometry this concerns the observation of the arrival of particular ionic species, at a detector under conditions that preclude or minimize ambiguities due to interferences. Ions may be detected by photographic or suitable electrical means.

Scanning method

This term refers to the sequence of control over operating parameters of a mass spectrometer which results in a spectrum of masses, velocities, momenta or energies.

Sample introduction

This refers to the manner in which a material which is to be subject to analysis is placed in the ion source of a mass spectrometer before and/or during such an analysis.

Vacuum system

Those components used to lower the pressure within a mass spectrometer are all parts of the vacuum system. This includes not only the various pumping componentsbut also valves, gauges and associated electronic or other control devices, the chamber in which ions are formed and detected and the vacuum envelope.

Data system

The components used to record and process information during the analysis of a sample are part of the data system. This includes electronic or other control devices, and recording storage and data manipulation devices.

Data processing

Once information is obtained with an appropriate data system, the information must be interpreted appropriate to the end use. Those steps which lead to this end use are of concern in data processing. Note that data processing does not necessarily include application of modern computer techniques.

Data reduction

The process of transforming the initial digital representation of a spectrometer output into a form which isn.amenable to interpretation; for example, a bar graph or a table of ion currents.

m/z

This abbreviation is used to denote the dimensionless quantity formed by dividing the mass number of an ion by the number of charges carried by the ion. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple of the electronic charge, e-. The abbreviation m/e is, therefore, not recommended. Thus, for example, for the ion C₇H₇²⁺, m/z = 45.5.

Ion/molecule reactions

Ion/neutral reaction

A process wherein a charged species interacts with a neutral reactant to produce either chemically different species or changes in the internal energy of one or both of the reactants. (NB. The term ion/neutral reaction is not ideal, simply because the word neutral is not a noun. However, any alternatives such as ion/neutral-species are so clumsy as to seem unlikely to be generally accepted).

Ion/molecule reaction

An ion/neutral reaction in which the neutral species is a molecule.

Charge inversion reaction

An ion/neutral reaction wherein the charge on the reactant ion is reversed in sign.

Charge transfer reaction

An ion/neutral reaction wherein the total charge on the reactant ion is transferred initially to the reactant neutral species so that the reactant ion becomes a neutral entity.

Partial charge transfer reaction

An ionn/neutral species reaction wherein the chargenon a multiply-charged reactant ion is reduced.

Charge stripping reaction

An ion/neutral reaction wherein the charge on the reactant ion is made more positive.

Charge permutation reaction

This is a general term to describe an ion/neutral reaction wherein there is a change in the magnitude and/or sign of the charges on the reactants.

(Note: Considering some of the possible reactions of ions M²⁺, M⁺ and M^- with a neutral species N these would be categorised on the basis of the above definitions as follows:

M²⁺ + N → M⁺ + N (Partial charge transfer)

M⁺ + N → M²⁺ + N + e^- (Charge stripping)

M^- + N → M⁺ + N + 2e^- (Charge stripping and charge inversion)

All are ion/neutral reactions and also charge permutation reactions].

Collision-induced dissociation

An ion/neutral process wherein the (fast) projectile ion is dissociated as a result of interaction with a target neutral species. This is brought about by conversion of part of the translational energy of the ion to internal energy in the ion during the collision.

Collisional activation

An ion/neutral process wherein excitation of a (fast) projectile ion is brought about by the same mechanism as in collision-induced dissociation. (The ion may decompose subsequently)n.

Collisional excitation

An ion/neutral process wherein there is an increase in the (slow) reactant ion's internal energy at the expense of the translational energy of either (or both) of the reacting species. The scattering angle may be large.

(Note: It is recommended that all three of the above terms should be retained).

Elastic scattering

An ion/neutral interaction wherein the direction of motion of the ion is changed, but there is no change in the total translational energy or internal energyof the collision partners.

Inelastic scattering

An ion/neutral interaction wherein the direction of motion of the ion is changed, and the total translational energy of the collision partners is reduced.

Elastic collision

A collision resulting in elastic scattering.

Inelastic collision

A collision resulting in inelastic scattering.

Superelastic collision

A collision in which the translational energy of the fast-moving collision partner is increased.

Ionizing collision

An ion/neutral reaction in which an electron or electrons are strippedfrom the ion and/or the neutral species in the collision. 'Generally, this term has come to be used to describe collisions cif fast moving ions with a neutral species in which the neutral species is ionized with no change in the number of charges carried by the ion. Care should be taken when this term is used to emphasize if charge stripping of the ion has taken place.

Association reaction (associative combination)

The reaction of a (slow moving) ion with a neutral species wherein the reactants combine to form a single ionized species.

Ion/neutral exchange reaction

In this reaction an association reaction is accompanied by the subsequent or simultaneous liberation of a different neutral species as product.

Translational spectroscopy

A technique to investigate the distribution of the velocity of product ions from ion/neutral reactions.

Ion energy loss spectra

Spectra that show the loss of translational energy of ions involved in ion/neutral reactions.

Impact parameter

The distance of closest approach of two particles if they had continued in their original direction of motion at their original speeds.

Interaction distance

The furthest distance of approach of two particles at which it is discernible that they will not pass at the impact parameter.

Charge exchange reaction

This term is synonymous with Charge Transfer Reaction.

Partial charge exchange reaction

This term is synonymous with partial charge transfer reaction.

Scanning of Spectra

Mass spectrum

A spectrum obtained when a beam of ions is separated according to the mass-to-charge (m/z) ratios of the ionic species contained within it.

[Note : A quadrupole mass spectrometer achieves separation of the various ionic species in this way].

Momentum spectrum

A spectrum obtained when a beam of ions is separated according to the momentum-to-charge ratios of the ionic species contained within it.

[Note: A sector magnetic field achieves separation of the various ionic species in this way. If the ion beam is homogeneous in translational energy, as is the case with sector instruments, separation according to the m/z ratios is also achieved].

Ion kinetic energy spectrum

A spectrum obtained when a beam of ions is separated according to the translational energy-to-charge ratios of the ionic species contained within it.

[Note: A radial electric field achieves separation of the various ionic species in this way].

Magnetic field scan

The usual method of producing a momentum (mass) spectrum in instruments.

Accelerating voltage (high voltage) scan

An alternative method of producing a momentum (mass) spectrum in magnetic deflection instruments. This scan can also be used, in conjunction with a fixed radial electric field to produce an ion kinetic energy spectrum.

Linked scan

A scan, l'.n an" instrument comprising two or more analysers, in which two or more of the analyser fields are scanned simultaneously so as to preserve a predetermined relationship between parameters characterising these fields. often, these parameters are the field strengths, but may also be the frequencies in the case of analysers in which alternating fields are employed.

Linked scan at constant B/E

A linked scan at constant B/E may be performed on a sector instrument incorporating at least one magnetic sector plus one electric sector. It involves scanning the magnetic sector field-strength B and the electric sector field strength E simultaneously, holding the accelerating voltage V constant, so as to maintain the ratio B/E at a constant value. This constant value is determined by the ratio of the two field strengths which transmit main-beam ions of predetermined mass:charge ratio; these preselected main-beam ions are the precursor ions whose fragment-ion spectrum is required. The fragmentation reactions so observed occur in a field-free region traversed before the two sectors scanned in this way.

[Notes: This terms should not be used without prior explanation of the meanings of B and E. The term "B/E linked scan" is not recommended. It may suggest that the ratio B/E varies during the scan].

Linked scan at constant E²/V

A linked scan at constant E²/V may be performed on a sector instrument incorporating at least one electric sector plus one magnetic sector. It involves scanning the electric sector field E and the accelerating voltage V simultaneously, so as to maintain the ratio E²/V at a constant value, equal to the value of this ratio which transmits the main beam of ions through the electric sector. The magnetic sector field is set at a fixed value such. that main-beam ions of a pre.determined mass:charge ratio are transmitted by the magnet; these preselected main.beam ions are the precursor ions whose fragment-ion spectrum is required. The fragmentation reactions so observed occur in a field-free region traversed before the two sectors scanned in this way.

[Notes : This term should not be used without prior explanation of the meanings of E and V.

The term "E2/V linked scan" is not recommended].

Linked scan at constant B²/V

A linked scan at constant B ²/E may be performed on a sector instrument incorporating at least one electric sector plus one magnetic sector. It involves holding the accelerating voltage fixed, and scanning the magnetic field Band the electric field E simultaneously so as to maintain the ratio B²/E at a constant value. This constant value corresponds to the ratio of the two fields which 'transmit main-beam ions of predetermined mass:charge ratio; these preselected main-beam ions are the fragment ions whose precursor-ion spectrum is required. The fragmentationreactions thus observed occur in a field-free region traversed before the two sectors scanned in this way.

[Notes : This term should not be used without prior explanation of the meanings of Band E. The term "B2 /Elinked scan" is not recommended] .

Linked Scan at Constant B l[ (E/E0)]l /E A 1 inked scan at constant B[l -(Ee/E0)]'/E may be performed on a sector instrument incorporating at least one electric sector plus one magnetic sector. It involves holding the accelerating voltage fixed, and scanning the magnetic field B and electric field E simultaneously, so as to maintain the quantity B[l -(E/Eo)]l/E at a constant value. This constant value is equal to B3/E0 , where E0 and B3 are respectively the electric sector field and magnetic sector field required to transmit m; ions in the main ion-beam; me3 represents the mass (m1 - m2) of the selected neutral fragment whose precursor ion spectrum is required. The fragmentation reactions so observed occur in a field-free region traversed before the two sectors scanned in this way.

[Note: This term should not be used without prior explanation of the meanings of B, E and E₀.

The term "B [1 -(E/E0)] '/E linked scan" is not recommended].

The above three definitions are merely examples of the types of linked scan that might be used. Any other linked scans can readily be defined in a similar manner.

Fixed precursor ion scans

1. Mass selection followed by ion kinetic energy analysis

If a precursor (parent) ion is selected, for example by a magnetic sector, all product ions formed from it in the field-free region between the magnetic sector and a following electric sector can be identified by scanning an ion kinetic energy spectrum.

2. Linked Scan at Constant B/E or at Constant E2/V

Both of these linked scans give a spectrum of all product (daughter) ions formed from a preselected precursor (parent) ion.

Fixed product-ion scans

1. High voltage scan.

2. Linked scan at constant B²/E.

Both 1. and 2. give. a spectrum of all precursor (parent) ions that fragment to yield a pre-selected product (daughter) ion.

Fixed neutral fragment scans

1. The linked scan at constant B[l -(E.'E0)]'/E gives a spectrum of all product (daughter)ions that have been formed by loss of a pre-selected neutralfcagment from any precursor(parent) ions.

[Note : The above definitions have all been given with reference to sector instruments; linked scans to give similar information have also been devised for instruments incorporating one or more quadrupoles].

2E Mass spectrum processes of the partial charge-transfer type

2+ + N+

m + N . m + occurring in a collision cell (containing a gas, N) located in a field-free region preceding a magnetic and electric sector combination placed in either order, may be detected as follows.

If the instrument slits are wide, and if the electric sector field Eis set to twice the value required to transmit the main ion-beam, the only ions to be trans.mitted will be those with a kinetic energy/charge ratio twice, or almost exactlytwice, that of the main ion-beam. The product ions of the process shown fulfill this condition. If the magnetic field Bis scanned, a mass spectrum of such singly.charged product ions, and thus of their doubly-charged precursors, is obtained. Such a spectrum is called a 2E mass spectrum.

E/2 mass spectrum processes of the charge-stripping type

m+ + N → m2+ + N + e- occurring in a collision cell (containing a gas N) located in a field-free regionpreceding a magnetic and electric sector combination placed in either order, may be detected as follows.

If the instrument slits are wide and if the electric sector field E is set to half the value required to transmit the main ion-beam, the only ions to be transmitted will be those with a kinetic energy/charge ratio half, or almost exactly half, that of the main ion-beam. The product ions of the charge-stripping process fulfill this condition. If the magnetic field B is scanned, a mass spectrum of such doubly-charged product ions, and thus of their singly-charged precursors, is obtained. Such a spectrum is called an E/2 mass spectrum.

. [Note Interference from product ions from processes of the type

+ m1 + -m2)

where m2 . 0.5 m1, can arise in E/2 mass spectra].

Charge inversion mass spectrum

Charge inversion processes of the types

or m-+ N → m+ + N + 2e.

respectively, occurring in a collision cell (containing a gas, N) located in a field free region preceding a magnetic and electric sector combination placed in either order, may be detected as follows

If the instrument slits are wide, and if the connections to the two sectors, appropriate to transmission of either positive or negative main-beam ions, are simply reversed, the negative or positive product ions of the two processes,respectively, will be transmitted. If the magnetic field is scanned, a spectrumof such product ions will be obtained, and this spectrum is called a charge-inversion mass spectrum. These spectra are sometimes referred to as -E and +Enspectra, respectively.

[Note : The terms "2E, E/2, -E or +E mass spectrum" should not be used without priorexplanation of the meaning of 2E, E, +E or -E]n.