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Resolution and resolving power terminology in mass spectrometry

ASMS 2022
Poster MP 113
Kermit K Murray

Premise

Nomenclature inconsistencies and conflicts can best be resolved through a detailed understanding of the origin and development of terms. The goal of this project is to investigate the origins and use as well as prior and current definitions of resolution and resolving power in order to make informed recommendations on the controversial and in some cases conflicting terminology.

Current definitions

In mass spectrometry, two peaks in a mass spectrum are resolved if they are distinguishable as separate. The degree to which the peaks are resolved can be quantified using the peak width or the separation between two peaks and is represented by Δ(m/z) where m/z is the mass-to-charge ratio. For singly charged ions, this can be expressed as Δm or, in older publications, as ΔM. The smallest value of Δm for which peaks are resolved is the limit of resolution. There are two general methods to determine Δm: peak width and valley:

Peak width: Δm is the peak width at a specified fraction of the peak height, for example at 50% Δm is the full width at half maximum

Valley: Δm is the separation between two equal height peaks that produces a valley a specified fraction of the height, for example 10%.

The 10% valley Δm is comparable to the 5% peak height Δm and approximately half that obtained from the FWHM. There are three general interpretations of the definitions of resolution and resolving power:

  • the terms are equivalent and represented by m/Δm (Meyerson 1975, Murray 2013)
  • resolution is m/Δm and resolving power is Δm (Price 1991, Todd 1991)

Historical use

Prior to the Second World War, the term resolving power, defined as M/ΔM, was used almost exclusively. Resolution was used as a binary variable or as the limit of resolution. In the second half of the 20th century, the two terms were increasingly used interchangeably.

F.W. Aston used resolution as a binary variable and resolving power as a quantitative measure, for example, “the instrument will resolve beams of different masses if the change in ϕ for change of mass is greater than the geometrical spread, and the greater ϕ for a given mass and given spread the greater the resolving power” (Aston 1922). In his book Mass Spectra and Isotopes, Aston defines resolving power as M/ΔM (Aston 1933).

J. Dempster defined limit of resolution as Δm/m (Dempster 1918) and, like Aston, often used the construct “one in [mass]” for resolving power, as in “resolving power with this comparatively wide slit is 1 in 1000” (Dempster 1935).

K. T. Bainbridge stated that “resolving power is defined as the ratio M/ΔM for complete separation of two lines and so is more stringent than the optical definition” (Bainbridge 1936).

J. Mattauch defined resolving power as M/ΔM and resolution as ΔM/M (Mattauch 1936)

W. Bleakney used the term resolving power in a 1929 publication (Bleakney 1929) but defined resolution as m/Δm in a 1949 publication (Mariner 1949).

A. O. Nier used both resolving power (Nier 1936) as well as resolution (Nier 1960).

J. H. Beynon in his textbook Mass Spectrometry and its Applications to Organic Chemistry writes “’resolution’ and ‘resolving power’ have been used a great deal in the above discussion. It has been assumed that the doublet is ‘resolved’ when its constituent ion species are ‘separated’ and that the difficult of separation or ‘resolving power’ necessary to separate the adjacent mass peaks is given by M/ΔM” (Beynon 1960)

K. Biemann in his textbook Mass Spectrometry: Organic Chemical Applications, states that “the term resolution is used in different ways – Throughout this book resolution will be considered as M/ΔM” (Biemann 1962).

ASMS Definitions

Subcommittee 10 on Definitions and Terms of ASTM Committee E-14 on Mass Spectrometry was established in 1970 and presented a compendia of terms at the 1974 ASMS meeting (Meyerson 1975). The ASMS Nomenclature Committee presented a list of terms at the 1982 ASMS meeting in Honolulu (Cameron 1982) and terms assembled by the ASMS Measurements and Standards Committee were published in 1991 (Price 1991) which closely paralleled the contemporary IUPAC recommendations (Todd 1991).

IUPAC Definitions

There have been four IUPAC recommendations for mass spectrometry terminology in the past five decades produced by the IUPAC Analytical Chemistry Division Commission on Analytical Nomenclature (Robertson 1974), the IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy (Beynon 1978), the IUPAC Physical Chemistry Division Commission on Molecular Structure and Spectroscopy Subcommittee on Mass Spectroscopy (Todd 1991), and the IUPAC Physical and Biophysical Chemistry Division (Murray 2013). The IUPAC Compendium of Chemical Terminology “Gold Book” gives definitions of resolution (valley and width) from Todd 1991 and gives two conflicting definitions for resolving power, one from Todd 1991 (also Robertson 1974) that defines resolving power as Δm and one from Beynon 1978 that defines resolving power as m/Δm.

Recommendations

Terminology recommendations for resolution and resolving power must take into account the current interchangeable use of the terms as well as the longstanding use of resolving power as m/Δm. It is the opinion of the author that resolution should be used as a binary variable, resolving power defined as m/Δm be encouraged, and limit of resolution defined as Δm/m be used where necessary.

Resolution: The use of resolution as a quantitative measure is discouraged: use resolving power or limit of resolution as appropriate.

Resolving power: The observed m/z value divided by the smallest difference Δ(m/z) for two peaks that can be separated: (m/z)/Δ(m/z).

Limit of resolution: The smallest difference Δ(m/z) for two peaks that can be separated divided by m/z: Δ(m/z)/(m/z).

The recommendations above are those of the author who hopes that these concepts will be considered when developing the next list of terminology.

References

Aston, F.W.: Some problems of the mass-spectrograph. Philos. Mag. 43, 514 (1922)

Aston, F.W.: Mass Spectra and Isotopes, Arnold, London, (1933).

Bainbridge, K.T., Jordan, E.B.: Mass Spectrum Analysis. Phys. Rev. 50, 282 (1936)

Biemann, K: Mass Spectrometry: Organic Chemical Applications, McGraw-Hill, New York (1962).

Bleakney, W.: A New Method of Positive Ray Analysis and Its Application to the Measurement of Ionization Potentials in Mercury Vapor. Phys. Rev. 34, 157 (1929)

Beynon, J.H.: Recommendations for Symbolism and Nomenclature for Mass Spectroscopy. Pure Appl. Chem. 50, 65 (1978)

Beynon, J.H. Mass Spectrometry and its Applications to Organic Chemistry, Elsevier, (1960)

Cameron, D.: ASMS Nomenclature Committee Workshop. Annual Conference on Mass Spectrometry and Allied Topics Abstracts. 30, 901 (1982).

Dempster, A.J.: A new method of positive ray analysis. Phys. Rev. 11, 316 (1918)

Dempster, A.J.: New Methods in Mass Spectroscopy. Proc, Am. Phil. Soc. 75, 755 (1935)

Mariner, T., Bleakney, W.: A large mass spectrometer employing crossed electric and magnetic fields. Rev. Sci. Instrum. 20, 297 (1949)

Meyerson, S.: Definitions and terms in mass spectrometry. Biomed. Mass Spectrom. 2, 59 (1975)

Mattauch, J.: A Double-Focusing Mass Spectrograph and the Masses of N15 and 018. Phys. Rev. 50, 617 (1936)

Murray, K.K., Boyd, R.K., Eberlin, M.N., Langley, G.J., Li, L., Naito, Y.: Definitions of terms relating to mass spectrometry, Pure. Appl. Chem. 85, 1515-1609 (2013)

Nier, A.O.: A Mass-Spectrographic Study of the Isotopes of Argon, Potassium, Rubidium, Zinc and Cadmium. Phys. Rev. 50, 1041 (1936)

Nier, A.O.: Small General Purpose Double Focusing Mass Spectrometer. Rev. Sci. Instrum. 31, 1127 (1960)

Price, P.: Standard definitions of terms relating to mass spectrometry. J. Am. Soc. Mass Spectrom. 2, 336 (1991)

Robertson, A.J.B.: Recommendations for Nomenclature of Mass Spectrometry. Pure Appl. Chem. 37, 469 (1974)

Todd, J.F.J.: Recommendations for Nomenclature and Symbolism for Mass-Spectroscopy. Pure. Appl. Chem. 63, 1541 (1991)