ASMS 1997 Terms and Definitions Poster

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1997 Terms and Definitions Poster (Internet Archive)

ASMS Terms and Definitions

Poster presented 6/2/97, ASMS Conference in Palm Springs, CA by the Measurements and Standards Committee - Robert Voyksner, Philip Price, John Bartmess, James Little, and David Wang-Iverson

Why Terms?

Standard definitions of terms are important for clear communications. Mass spectrometry is a highly multi-disciplinary field. Originally, petroleum chemistry drove all commercial MS developments. Now, there are major influences from diverse areas such as biotechnology, geochemistry, forensics, catalyst development, and semiconductor studies. Automated document searches and archiving are common.

ASMS' goal is to provide an educational tool, an editorial guide, and a generally accepted set of definitions to foster clearer communications among mass spectrometrists (and with "outsiders"). Care must be taken to not arbitrarily add terminology to the main "Standard Definitions" document before adequate discussion and agreement among ASMS members. In many cases this process will prevent timely addition of new terms, but will avoid addition of terms only accepted by one faction of the community.

The "Standard Definitions" document must be kept up-to-date. This can only be done with your participation and discussion. The terms below are not meant as "provisionally acceptable", but are a diverse collection of input from ASMS members. An important part of the editorial process for the "ASMS Terms Document" is to have adequate discussion of preliminary suggestions. The Measurements and Standards Committee welcomes your comments.


1974 - first IUPAC MS nomenclature published
John Beynon - Chairman ASMS Nomenclature Committee
1978 - IUPAC additions to MS nomenclature
Chairmanship vacant 1975-1979 - was re-activated in 1979
John Beynon - Chairman ASMS Nomenclature Committee (there was no ASTM Nomenclature Committee)
Over 100 attendees at workshop.
About 10 pp. published in Proceedings.
Doug Cameron - Chairman of ASTM E-14.10 "Definitions and Terms"
"Definitions for Approval" (7 pp.) published in Proceedings
"The Board of Directors of ASMS approves the list of terms as presented in the Bound Volume of Abstracts for the 1981 meeting subject to periodic review and extension by the Nomenclature Committee and the Board, such that this shall be a living document. The Board recommends that authors’ usage conform to this nomenclature." -- Burnaby Munson, President, ASMS
Doug Cameron - Chairman of ASTM E-14.10 "Definitions and Terms"
Joint Workshop with ASMS
"Definitions for Approval" (brief) published in Proceedings
Frank Field - Chairman of ASMS’ "Nomenclature"
compiled list in proceedings, all SIMS terms to SIMS & AVS
1985, 86, 87
no activity
"Measurements and Standards Committee" - H. Fales, Chair.

1989 "Measurements and Standards Committee" - Rodger. Foltz, Chair.

circulated "Latest Discussions of MS Terms and Definitions" newsletter

"Measurements and Standards Committee" - Rodger. Foltz, Chair.

circulated "Latest Discussions of MS Terms and Definitions" newsletter
"Measurements and Standards Committee" - Mike Bowers, Chair.

"Measurements and Standards Committee" - Mike Bowers, Chair.

Workshop on "Nomenclature for Mass-to-Charge Ratio"
ASMS terms published in JASMS
1992 - 1996
"Measurements and Standards Committee" - Mike Bowers, Jack Henion,
Sharon Lias, Bob Voyksner, Chairs.

IUPAC MS nomenclature update published

"Measurements and Standards Committee" - Bob Voyksner, Chair
ASMS poster on terminology
Past ASMS article on terms and definitions: "Standard Definitions of Terms Relating to Mass Spectrometry, a Report from the Committee on Measurements and :Standards of ASMS", JASMS Vol. 2 #4, July/August 1991.[1]

Collected Terms Suggestions, grouped by category:


Double focusing - A combination of direction and velocity focusing in sector instruments used to achieve high resolution.

Tandem mass spectrometry (MS/MS) - Two-stage mass analysis experiment, used to study the chemistry of selected ions or individual components in mixtures.

Note from a reader: I despise "MS/MS spectra" -- Who has ever heard of an MS spectrum (a mass spectrometry spectrum). Prefer "tandem mass spectrum" or better "product-ion spectrum" or "precursor-ion spectrum". How do you handle a "metastable-ion spectrum"--please don't sanction a "metastable spectrum".

ion trap analyzer - type of mass analyzer whereby ions are confined in a region of space and analyzed, as opposed to a dispersive mass analyzer (See Paul Ion Trap and Penning Ion Trap) Note from a reader: The definition of "ion trap analyzer" is too limited. I realize that 99% of the mass spec community refers to a Paul, or Quadrupole Ion Trap simply as "an Ion Trap" thanks to Finnigan, but an FTICR/MS is also a type of ion trap based mass analyzer.

Paul ion trap - A type of mass analyzer in which ions are confined in space by means of a three dimensional, rotationally symmetric quadrupolar electric field. Sorting of ions is performed by changing the field conditions appropriately to destabilize an ion of a particular m/z. The destabilized ion is then detected when it exits the trap and strikes a collection device, e.g. an electron multiplier or conversion dynode. Penning ion trap - An ion trap that confines ions by placing them in a static magnetic field. Inside the field, the ions are subject to the Lorentz force which causes ions of a particular m/z to cyclotron at a specific frequency (cyclotron frequency).

Fourier transform ion cyclotron resonance (FTICR) analyzer - A type of mass spectrometer that uses a Penning ion trap to confine ions for mass analysis. Ions of all m/z values are excited by applying RF energy over a range of frequencies corresponding to the cyclotron frequencies of the ions to be detected. After cessation of the applied RF energy, all ions are detected simultaneously by measuring the current induced on the "detect" electrodes by the confined ions. The mass spectrum is obtained by application of the Fourier transform to the measured signal to extract the cyclotron frequencies of the ions. Once the cyclotron frequencies are known, the m/z values are calculated via the cyclotron equation.

Another reader’s definition: Fourier transform ICR - A method of obtaining data from an ion cyclotron resonance (Penning) trap, whereby all ions are translationally excited within a time much shorter than the ion/neutral collision time; the image current of the combined ions’ signal is detected; and the resulting time-domain signal is converted to a frequency (reciprocal mass) - domain signal by the Fourier Transform mathematical method. "FT Mass Spectrometry" is inappropriate. The FT method is not limited to ICR; FT time-of-flight has been demonstrated, and FT methods for other analyzers are possible.

Note from a reader: In the definition of "Ion cyclotron resonance analyzer," no mention is made of a magnet, a (in my humble opinion) grave omission. If you decide to incorporate our suggestions, you could have a note directing readers to "Fourier transform ion cyclotron resonance analyzer" and "Penning ion trap". Double resonance - In ICR, the irradiation of one ion at or near its cyclotron frequency, and observation of the effect on the intensities of other ions in the spectrum. Cyclotron motion - Cyclic rotation of an ion in a fixed magnetic field

Magnetron motion - Slow circular drift of an ion along a path of constant electrostatic potential; magnetron motion occurs in ICR as a result of the crossed radial electric field and axial magnetic field.

Time-of-flight mass spectrometer - An analytical instrument in which ions are formed and accelerated and their flight times measured to determine their mass. Time-of-flight mass spectrometers can be distinguished by those that accelerate ions to constant momentum (in which case flight times are linear with mass) and constant energy (in which case flight times are proportional to the square root of mass). Time-of-flight mass spectrometers were originally known as velocitrons.

Reflectron - A device used in a time-of-flight mass spectrometer that retards and then reverses ion velocities in order to correct for the flight times of ions having different kinetic energies. The reflectron is sometimes known as an ion mirror.

Single-stage reflectron - A reflectron in which a single retarding field is used to retard and then reverse ion velocities. Generally, a retarding field is used that is constant through the depth of the reflectron, and achieved by retarding voltages that increase linearly with reflectron depth. Such reflectrons provide first-order correction for differences in ion kinetic energy.

Dual-stage reflectron. - A reflectron in which two retarding fields are used to reverse ion velocities. Generally, both retarding regions are constant field, each achieved by retarding voltages that increase linearly with reflectron depth. In the most common embodiment, ions are retarded by two-thirds to three-quarters of their mean kinetic energies within the first retarding regions which comprises approximately 10% of the total reflectron depth.

Isotopic peak(ion) - Due to other isotopes of the same chemical but different isotopic composition.

Isobaric peak(ion) - of the same normal mass (integral).

Mass spectrum - Plot of ion abundance vs. mass-to-charge ratio normalized to most abundant ion.

Note from a reader: molecular weight - The text in the current document could be updated slightly to more clearly reflect some of the values relating to high mass (> 3,000) distributions, particularly when observed at less than unit resolution.

Note from a reader: mass unit - Should there be a comment on "µ " vs "amu" vs the 'biomedical term' "Dalton"? IUPAC currently defines "m = unified atomic mass (1/12 of the mass of an atom of nuclide 12C)".

Mass-to-charge ratio (m/z) - Daltons/electronic charge.

Note from a reader: on Thomson - the fluid dynamics people have already used that one; it is listed in the CRC Handbook and IUPAC documents. ASMS should be doing things in addition to or clarifying points mentioned (or not) in IUPAC. However, we should be cautious about doing anything that actually opposes or conflicts with IUPAC documents.

Mass spectrometry - This is the study of mass spectra obtained by using a mass spectrometer. The term "mass spectroscopy" should be avoided, because this implies optical dispersion. Possible exceptions might be in the case of photoplate or various optical methods of detection.

Mass Selective Detector - a detector that only monitors ion currents at certain m/z values.

Mathieu stability diagram - Diagram showing the solutions to the Mathieu equation which correspond to stable ion trajectories and displayed as a function of parameters related to operating voltages, mass and charge of the trapped ions.

Secondary ion mass spectrometry - Mass spectrometry based on analysis of particles that are emitted when a surface, usually a solid, although sometimes a liquid, is bombarded by energetic (~ keV) primary particles (e.g. Ar+ and Cs+).

Flowing Afterglow - a reactor for observing ion-molecule reactions, in which ions are introduced to a bath gas containing a neutral reactant, and flowing rapidly down a vacuum system, where neutral pressure and distance become the reaction variables. Detection of the ions is by mass spectrometry through a leak at the product end of the system.

High Pressure Pulsed MS - a combined reactor and detector for observing ion-molecule reactions. A chemical ionization (CI) source is operated in a time-resolved fashion, where the ions are generated in one point in time, allowed to react in the CI source, and extracted and analyzed by MS at some later time.

Sample Introduction

Ion source - Device used to generate sample ions by electron impact, chemical ionization, etc.

Gas chromatography-mass spectrometry (GC-MS) - combined technique for mixture analysis in which the separated GC components are passed continuously into the MS.

Desorption ionization - Method used to ionize non-volatile solid samples by impact of energetic particles or photon beams.

Spray ionization - Method used to ionize liquid samples directly by electrical, thermal, or pneumatic energy through formation of a spray of fine droplets.

Liquid chromatography-mass spectrometry (LC-MS) - combined technique for mixture analysis.

CE/MS - The combining of capillary electrophoresis with mass spectrometry.

CEC/MS - The combining of capillary electrokinetic chromatography (or capillary "electrochromatography"?) with mass spectrometry.

CE/MS Interface - The interface used between the capillary electrophoresis (CE) and the mass spectrometer to provide a continuous introduction of effluent from the CE into the MS. Common interfaces include: (1) layer flow interface, (2) liquid junction interface, and (3) direct interface. Another opinion: Instead of "layer flow interface" it should be "sheath-flow" interface.

Layer Flow Interface - An approach using coaxial concentric tubes to add solvent and/or solvent modifiers post-column (outer tube) to the column effluent (inner tube) for improved CE/MS or LC/MS operation.

Liquid Junction Interface - An interface used to combine CE with MS in which a reservoir supplies the additional solvent flow to the CE effluent in order to achieve stable electrospray operation.

Ion Molecule reactions

Collision-induced dissociation - Process whereby a mass-selected ion is excited and caused to fragment by collision with a target gas, especially in MS/MS.

Inelastic collision - Collision in which internal energy is not conserved.

Ionization Nomenclature

Radical ion (odd-electron ion) - Charged open-shell molecule with at least one unpaired electron.

Odd-electron ion - See radical ion.

Parent ion (m1+) - Any ion (including negatively and doubly charged ions) that gives fragments.

Product ion (m2+) - Ion generated by fragmentation of any parent ion.

Metastable ion - Ion that fragments slowly after emergence from the ion source but before it reaches the detector; in sector instruments, metastable ions give rise to signals which appear at unique m/z values related to the parent and product ion masses.

Surface-induced dissociation - Process whereby a mass-selected ion is excited and caused to fragment by collision with a target surface.

Ionization energy(IE) - Minimum energy required to remove an electron from a molecule. Endothermicity of the process M ® M.+ .

Glow discharge - Method used to ionize solid samples for elemental analysis by applying an electric field to create an energetic plasma.

Inductively coupled plasma - Method used to ionize solution samples for elemental analysis by a plasma.

Electron affinity - Enthalpy change for the process M- ® M + e-.

Proton affinity - Enthalpy change for the process MH+ ® M + H+.

Electron capture - Ionization process in which a molecule or atom captures a thermal energy electron typically in a CI source, and generates the molecular radical anion.

Electron impact - Ionization method in which molecules are ionized directly by energetic electrons (usually 70 eV) at low pressure (<10-5 torr).

Even-electron ion - Ion with even number of electrons commonly with a closed shell electronic configuration.

Distribution of internal energy - Analogue of Boltzmann distribution for molecules not in thermal equilibrium.

Resonance electron capture of thermal electrons - Should be referred to as "electron capture ionization" or "ECI". The processes which ECI describes (Chemical Ionization Mass Spectrometry, Harrison, 2nd ed., 1992, p. 24-25):

                    1)           MX + e-   ®   MX-                electron capture
                    2)           MX + e-   ®   M. + X-           dissociative electron capture
       Case (1) leads to formation and detection of a molecular anion, while case (2) leads to detection of a fragment with loss of a radical, for example, pentafluorobenzyl ester fragmentation.

{ Note from submittor:  Whether or not ECI/MS is acceptable to the committee, I hope that some consensus can be reached to describe the two processes I have described above.  I have seen them described as NCI, NICI, ECCI, ECNI and ECNICI.  I think its time to pick one.  By the way, my second choice would be ECNI (electron capture negative ion).}

The term ECI should be separate from the "chemical ionization" section of nomenclature and the term "electron attachment" should be dropped or changed to "electron capture" to avoid confusion. This description should be limited to only those processes which involve thermal electron capture as the principle mode of ionization. Other negative ionization procedures which are the result of ion/molecule interactions, such as chloride attachment, should still be referred to as negative ion chemical ionization. Under electron capture conditions, the description of the "reagent" gas should be changed to "moderating gas" or "buffer gas". As the gas is not acting as a reagent, these terms should be more description of the process involved. Appearance energy - Endothermicity of process AB+ ® A+ + B Atmospheric pressure chemical ionization - A variant of chemical ionization performed at atmospheric pressure.

A reader’s comment: Atmospheric pressure ionization (API) should remain a general term, for any form of ionization at atmosphere. The definition given is specific for chemical ionization at atmosphere. The definition given should be for a separate term, atmospheric pressure chemical ionization (APCI). This was merely the first atmospheric pressure ionization means that was commercialized. API would then properly include, electrospray (ESI), APCI, Ionspray (a coined term, and more generally this is pneumatically-assisted electrospray), and flame ionization (in some of the early papers on API this was the mode of ionization). Lots of others have been tried including microwave, etc.

Atmospheric Pressure Chemical Ionization (APCI) - The formation of ionized species when gaseous molecules interact with ions (reagent ions) at atmospheric pressure. The reagent ions are formed by a corona discharge of the vaporized solvent introduced into the system.

Atmospheric Pressure Ionization (API) - Ionization technique(s) that occur at atmospheric pressure. Specific API ionization techniques include electrospray, pneumatically assisted electrospray and atmospheric pressure chemical ionization and is often used to couple LC to MS.

Chemical ionization - Method in which neutral molecules are ionized by ion-molecule reactions to generate a parent ion at a pressure of about 1 torr.

Breakdown curve - Plot of ion abundance vs. ion internal energy normalized at each energy; shows mass spectrum as a function of internal energy.

Charge exchange - Process whereby one particle transfers an electron to another,

e.g.  M + A+.   ®   M+.  + A, used in chemical ionization.

Spray ionization - Methods used to ionize liquid samples directly by electrical, thermal, or pneumatic energy through formation of a spray of fine droplets.

Electrospray Ionization (ESI) - A technique used to produce gas phase ions from molecules in solution. The process makes use of strong electric fields for both nebulization and charging of a liquid, drying to reduce the size of the charged droplets to increase the field strength, and ion evaporation. Ionization occurs when the field strength in the droplets exceeds the solvation energy of the molecule in solution.

Another opinion: I object to the use of the term "electrospray ionization". However, I think everyone will use it anyway. Technically, electrospray is a spray process which eventually produces gas phase ions via ion evaporation. The latter is the ionization process. However, I think the former term will continue to be used.

Another opinion: Electrospray - Generation of a fine mist of droplets by spraying solutions through an electrically biased capillary. In mass spectrometric applications ("electrospray MS"), provision is made for solvent evaporation from the sprayed droplets, and resulting ions are sampled for mass analysis via a differential pumping system. In some instances, the ions are formed as a result of electrochemistry driven by the capillary bias ("electrospray ionization" or ESI), but in most cases the ions are present in the solution a priori (ES).

Ion Evaporation - A desorption ionization process which brings ions from a liquid to the gas phase when the electric field strength exceeds the solvation energy of an ion in solution. Ion evaporation processes are found in the techniques of electrospray, pneumatically assisted electrospray and thermospray.

Pneumatically Assisted Electrospray - An electrospray process where pneumatic nebulization (e.g., N2) assists in the initial nebulization of the liquid introduced into the electrospray system (vs. nebulization through charging of the liquid). The process is often used to obtain electrospray spectra at higher flow rates (e.g., >0.2 mL/min) and with solvents with high surface tensions (e.g., water).

Ion Spray - Manufacturer’s trade name for pneumatically assisted electrospray.

Ion Spray - Also known as "pneumatically assisted electrospray," a variation on ES in which the droplet formation process is facilitated by incorporation of a sheath nebulizing gas passing through a second capillary concentric with and larger than the ES capillary. The nebulizing gas helps accommodate larger flow rates and/or use of liquids of relatively high surface tension (e.g., aqueous solutions without co-solvents, which would otherwise require spray potentials above the threshold for electric discharge). In common usage, the distinction between ES and ion spray has blurred. Most commercial "ES" sources use nebulizing gas, but do not use the "ion spray" terminology [which may therefore be superfluous].

Thermospray - A process where liquid is thermally vaporized in a capillary and the ions in the resulting aerosol are transferred from the liquid to the gas phase. The ionization process can involve ion evaporation or chemical ionization (the reagent ions are formed by a filament or discharge) ionization of the solvent. The technique is often used in LC/MS.

Particle Beam - An interface often used for LC/MS where the sample is separated from the solvent and then introduced into the MS for ionization. The process involves: (1) solution nebulization, (2) vaporization of the solvent to obtain unsolvated sample molecules (particles), (3) momentum separation of the particles of sample from solvent gas, and (4) introduction of the particles into the MS for ionization (EI, CI, or desorption technique).

Thermobeam (or Thermabeam) - A trade name for particle beam which uses thermal nebulization vs. pneumatic nebulization to form the initial aerosol. Particle beam should be used in place of this term.

MAGIC (Mono disperse aerosol generation interface for combined LC/MS) - A trade name for a particle beam approach in which the initial droplet formation is carefully controlled to generate a uniform aerosol. Particle beam should be used in place of this term.

LC/MS Interfaces - It has been suggested by several ASMS members that there are three ways to make a spray or aerosol for LC/MS: with a gas, heat, or electricity. Hence the terms: (1) aerospray; (2) thermospray; and (3) electrospray. Under this model:

MAGIC ® aerospray Particle Beam ® aerospray Thermobeam ® aero-thermospray ion spray ® aero-electrospray

       All this may be a bit "extreme", since it does not closely follow what is in the literature, and  excludes APCI.  However, this whole issue should be addressed, and a few terms settled on.

EH - A methodology by which ions in solution can be desorbed from a liquid meniscus directly into an evacuated chamber by application of a suitable electrical bias to a metal capillary in which the solution is contained. In contrast to ES, droplet spraying is avoided in EH MS applications (by using very low flow rates), since there is no mechanism (other than metastable or collision-induced dissociation) for solvent removal subsequent to ion desorption. This latter feature lends to EH the possibility for studying condensed-phase ion solvation less invasively than other MS methods. Flow rates are restricted by using small capillaries and/or viscous solvents. In applications as a source of heavy primary ions for SIMS, flow rates are increased and/or solution compositions are adjusted to promote emission of massive clusters.

Thermal ionization - Method used to ionize solid samples on a hot surface of a metal filament.

Unimolecular rate constant k - in s-1, dependent upon internal energy (epsilon) , which is shown explicitly.

Scanning of Spectra

Reconstructed ion Chromatogram (RIC) - Sometimes also referred to as "extracted ion current profiles" (EICP) - This is a chromatographic plot of the intensity of a single m/z (or range, or selected values) versus scan number, or time. This plot is produced by re-processing scanned data.

Another reader’s opinion: Please make it clear in the definition of terms that "MRM" is not the correct term for SRM. I am frustrated by manufacturers that invent their own terms.

Selected ion monitoring (SIM) - experiment in which mass analyzer is used to detect one or a few ions as a function of time.

Base peak - The most intense peak in the mass spectrum (only in the mass range plotted?), hence 100% relative abundance.

Relative abundance(RA) - Normalization relative to the base peak.

Neutral loss scan - An MS/MS experiment which records all parent ions which lose a particular neutral fragment.

Parent ion - Any ion that fragments to a product ion.

Parent scan - An MS/MS experiment which records all parent ions which produce a particular product ion.

Product ion scan - An MS/MS experiment which records all product ions derived from a single parent ion.

Types of Ions, Ion Structure

Distonic ion - Radical ion in which the charge and radical sites are formally located on different atoms in the molecule.

Fragmentation pattern - Set of reactions leading from the molecular ion to fragment ions.

Fragment ion - ion not generated by direct ionization of a neutral molecule.

Ion internal energy - Total electronic, vibrational, and rotational energy referenced to ground state of the ion.

Molecular ion - Ion derived from the neutral molecule by loss or gain of an electron or other simple unit e.g., (M+H)+, (M+Cl)-, (M-H)-.

Multiple-charged ions - Ion bearing more than a single charge and having correspondingly reduced mass/charge ratios.

Photodissociation - Process in which an ion fragments by absorption of one or more photons.

Charge-remote fragmentation (remote site fragmentation) - Decompositions that occur without any obvious involvement of the charge site. These reactions may be of closed-shell species and have thermal analogies or be of radical ions and be radical-site induced. One requirement is a stable charge site, which is usually closed shell, stable, and localized (e.g., -COO-, -COOLi2+, -OHNa+, -SO3-, etc.). The reactions are particularly useful in locating functional groups in aliphatic chains such as in fatty acids, surfactants, lipids, steroids but also occur for peptides and other biomolecules. Many charge-remote fragmentations require high-energy collisional activation but others have low-energy requirements and are seen at metastable-ion decompostions or under low-energy collisional conditions.

Resolution of the Confusion on Peak Separation

Mass resolving power and mass resolution have been used interchangeably throughout the literature, so the confusion surrounding their exact meaning is understandable. In his forthcoming book, "Guide to Mass Spectrometry," Ken Busch advocates definitions that are consistent these proposed terminologies for mass resolution and mass resolving power. In most disciplines, resolution is understood to be the smallest observable change in a quantity, whereas resolving power, i.e. the ability to distinguish two closely spaced quantities, is inversely proportional to resolution.

Proposed definitions:

mass resolution - the mass (actually, m/z) difference, Dmx that exists between two adjacent peaks in a mass spectrum that are of equal size and shape (Gaussian, Lorentzian, triangular) with a specified amount of overlap, where the subscript "x" denotes the overlap criterion (10% valley, Full Width at Half Height [FWHH], etc.) See Usage Note for mass resolving power and theoretical mass resolving power

mass resolving power - m/Dmx, where Dmx is the mass resolution See Usage Note for theoretical mass resolving power

Usage note: Although the definition of mass resolution is contingent upon two adjacent, mass spectral peaks of equal size and shape, which is almost never the case experimentally, it is acceptable to calculate the mass resolving power or mass resolution from a single peak. An assumption is made about the peak shape, whereby the peak width at 5% height for a single peak would be approximately equivalent to the distance between the apexes of two peaks with a 10% valley between them. This assumption is not unreasonable for most common peak shapes encountered in mass spectrometry. Therefore, the mass resolving power that is obtained by dividing the mass (m/z) value at the apex of a peak by the peak width at 5% of the peak height could be indicated as m/Dm10%V theoretical mass resolving power -

Usage note: Theoretical mass resolving power is useful for determining the relative difficulty in separating two peaks in a mass spectrum. The "masses" are actually m/z values, and the subscript "d" indicates that the criterion used to determine Dm is simply the difference in mass between the two peaks. One should be careful to notice the subtle distinction between Dmd, a quantity that is independent of instrumental performance, and Dmx, a quantity that is determined by instrumental performance. It is important to realize that the theoretical mass resolving power makes no peak shape assumptions. Therefore, the choice of overlap criterion, i.e., 10% valley, full width half height, etc. is the link between the theoretical mass resolving power and the experimentally measured "mass resolving power." For an instrument to be capable of separating two particular ions, the instrument must possess a mass resolving power (over the range m + Dm) that is greater than the theoretical mass resolving power calculated for the ions in question. For example, if it is desired to determine whether or not a particular mass spectrometer is capable of resolving 41K+ from 40Ar1H+, determine the theoretical mass resolving power:

Next, the instrumental mass resolving power of the instrument at m/z = 41 is compared with the theoretical mass resolving power. For a quadrupole based instrument, a 10% valley overlap would correspond to a Dm of approximately 1 Da, assuming typical scan rates are used. For a peak at m/z = 41, this corresponds to a "mass resolving power" = 41. Therefore, this particular instrument does not possess mass resolving power capable of separating these two species. >From the preceding discussion, it is apparent that even greater mass resolving power would be required for a separation if two adjacent peaks if the peaks are not of equal size and shape. The lesser peak could be lost in the "wings" of the larger peak.

Another comment: Note that resolving power is dimensionless, but when defined as peakwidth, it usually has units of "parts-per-million" (of mass). Thus, a resolution of 10,000 corresponds to 100 ppm.

The Committee is indebted to Kenneth E. Milgram and John R. Eyler for extensive work on the Resolution section.

The following section contains a variety of terms and subjects that need clear definitions:

Initial time distribution

Initial spatial distribution

Space focus plane

Dual-stage extraction

Initial kinetic energy distribution

Time-lag focusing

Gridless reflectron

Quadratic reflectron

Post-source decay

Delayed extraction

In-source decay

Note from a reader: MS/MS scans - The MS/MS scanning definitions in the ASMS document are stated in terms of sector instruments. This should be updated to include hybrids, quadrupoles, traps, TOF, and FT/MS. Both Richard Martinez (Rapid Communications in MS, Vol. 3, #12, pp427-31 (1989)) and Graham Cooks have had specific suggestions.

abbreviations - Several people have commented about a need for an "accepted & official list" of MS abbreviations, acronyms, and syntax (GC-MS vs. GC/MS, GC/MS/MS, etc. Some of these are covered in the ASMS document, more work could be done. IUPAC Guidelines clearly (and correctly) state that "An acronym, abbreviation, or invented jargon should only be used after a full explanation of its meaning has been given in the text".

The issue of syntax/-punctuation was one that was avoided by the committee during the initial editing process, but something that should be addressed. Aside from esthetics, it has the potential to affect computer searches of abbreviations. The collected thoughts at the time were:

Slash indicates the connection of two techniques or instruments. It connotes some sort of physical or temporal connection. Thus GC/MS is a GC connected to an MS. MS/MS is more subjective, but represents one mass spectrometer (or separation process) connected in tandem to another. "FT-MS" should probably used, since "FT" modifies how the "MS" is done; an "FT" isn't connected or attached to an "MS". But, try to convince someone that uses an FTMS or FT/MS. A dash is the conventional English Grammar punctuation used for "compound words" and "unit modifiers" (see pp.31-34 of the ACS Style Guide, 1986). Thus "frit-FAB MS" would be similar the ACS example of "water-soluble polymer". However, what do you do about LSIMS, SIMS, FABMS, and all the others that have been "out there" for so long? What do you do with something complicated like: "high-resolution capillary GC/MS"? How does one best clarify whether the technique being described is high GC resolution or high MS resolution? The questions:

       - when does one put a slash, as in MS/MS?
       - when does one put a dash, as in frit-FAB?
       - when does one run it all together, as in ITMS?

ICR & FTMS - There are very few "ICR related" terms.

TOF - There are few terms relating to time-of-flight MS.

inorganic MS - Should ASMS deal with microwave induced plasma MS, glow discharge, etc.?

ion abundances - One reviewer commented on the awkward terms often used to describe ion abundances in a spectrum. "Strong" and "weak" should probably refer to GC peaks. "Abundant" is OK, but "scarce" is strange.

Ionization nomenclature - FAB, LSIMS, dynamic SIMS, static SIMS - all need attention.

laser ionization - A suggestion was made that there is insufficient treatment of this area, particularly addressing multi-photon phenomena. Related terms to define might include RIMS and SNMS. Are there any recommendations? There are no entries relating to MALDI.

Other chromatography/MS - CZE/MS, CE/MS, GPC/MS, SFC/MS (or SCF/MS), etc.

survivor ion MS - See article by Frantisek Turecek (OMS, V. 27, 1335-6 (1992)).

isotopomers & isotopologs - See letter to C. & En. News, p. 2, Dec. 7, 1992.

Biochemistry & biotechnology - Given the ever-increasing influence of these disciplines on MS, to what extent should we get into these issues? One possibility would be to cite some specific reference (as was done on "vacuum terminology" by citing a document by the American Vacuum Society).

A reader’s comment: Data System - This section of the document is terribly out-of-date. Maybe some terms should be dropped, but many new terms could be added.

sensitivity - Should this definition be modified to include image current detectors, array detectors, etc.? Should the definitions relating to GC/MS - sensitivity, S/N, internal standards etc. be considered? (see paper by Bob Boyd - Rapid Communications in MS V. 7, 257-71 (1993)).

"EPA-related" terms - Things like MDL, PQL, etc. These are "in common use", yet are often only poorly understood.

"FDA-related" terms - Things relating to GLP and GALP.

Criteria for detection, confirmation and quantification - "We feel that ASMS should adopt a leadership posture in this area since NCCLS, SOFT, AOAC, etc. are actively engaged in establishing standards." from an ASMS workshop on Nov. 2, 1996.


ASMS should assume a leadership role in these matters. Such discussions could be undertaken by the ASMS Committee on Measurements and Standards, or by an ad hoc group of specialists appointed under the aegis of this Committee. The ad hoc committee, preferably sponsored by ASMS as described in Recommendation 1, should be constituted to include both professional statisticians concerned with definitions of limits of detection and related concepts as well as experienced trace analytical mass spectrometrists. This committee would be charged with examining the possibility of defining appropriate levels and limits which are both theoretically sound and are practical in an operational sense for busy analytical laboratories.

It is important for ASMS to resolve these issues. ASMS is very interested in your comments on the "collected terms" gathered from society members above. Please send all comments and recommendations to:

Finally, the Committee on Measurements and Standards wishes to thank all the many members of the society who have donated time and thought to this project.