User talk:Vera Kehrli/m over z
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m/z is a term used in mass spectrometry. It used to be a symbol for the mass-to-charge ratio exclusively used by mass spectrometrists (all other fields of science used the ISO 31 symbols m/q or the outdated symbol m/e). In 1998 the m/z has been redefined and its use for mass-to-charge ratio is deprecated. This is confirmed by the latest IUPAC document[1].
Definitions of m/z
Recent IUPAC Definition in Final Draft
In 2006 a work group of the IUPAC Analytical Chemsitry Division [2] came up with a new definition of m/z [3]:
- m/z
- Three-character symbol m/z is used to denote the dimensionless quantity formed by dividing the mass of an ion in unified atomic mass units by its charge number (regardless of sign). The symbol is written in italicized lower case letters with no spaces.
- Note 1: The term mass-to-charge ratio is deprecated. Mass-to-charge-ratio has been used for the abscissa of a mass spectrum, although the quantity measured is not the quotient of the ion's mass to its electric charge. The three-character symbol m/z is recommended for the dimensionless quantity that is the independent variable in a mass spectrum
- Note 2: The proposed unit thomson (Th) is deprecated.
Remarks:
- mass spectrometers measure mass-to-charge ratio and not mass per charge number
- according to the rules in the IUPAC green book the m/z as defined above has dimension mass and therefore is not dimensionless
- m/z would therefore require a unit of mass, in this case e.g. dalton, Da
- according to the IUPAC green book Ch 1.1: Neither the name of the physical quantity, nor the symbol used to denote it, should imply a particular choice of unit. Which means this definition of m/z is illeagal.
- the three charater symbol m/z is also illegal according to the IUPAC green book Ch 1.3. which states:
- The symbol for a physical quantity should generally be a single letter of the Latin or Greek alphabet (see p.143)'. Capital and lower case letters may both be used. The letter should be printed in italic (sloping) type. When no italic font is available the distinction may be made by underlining symbols for physical quantities in accord with standard printers' practice. When necessary the symbol may be modified by subscripts and/or superscripts of specified meaning. Subscripts and superscripts that are themselves symbols for physical quantities or numbers should be printed in italic type; other subscripts and superscripts should be printed in roman (upright) type.
- It is now possible to send comments on this draft definition to the IUPAC: details on this page
Definition in IUPAC gold book (1998)
In the Compendium of Chemical Terminology, also called The Gold Book, Second Edition, the current entry is [4]:
- mass-to-charge ratio (in mass spectrometry), m/z
- The abbreviation m/z is used to denote the dimensionless quantity formed by dividing the mass number of an ion by its charge number. It has long been called the mass-to-charge ratio although m is not the ionic mass nor is z a multiple or the elementary (electronic) charge, e. The abbreviation m/e is, therefore, not recommended. Thus, for example, for the ion C7H72+, m/z equals 45.5.
Remarks:
- mass spectrometers measure mass-to-charge ratio and not mass number per charge number
- according to the rules in the IUPAC green book the symbol for mass number is A, not m, which is reserved for mass.
Definition in IUPAC orange book (1997)
In the Compendium of Analytical Nomenclature, also called Orange Book, 3rd edition, the entry is [5] :
- Mass/charge ratio
- (m/z) ratio.
Remarks:
- this definition, though brief, widely complies to the rules in the IUPAC green book
- according to the IUPAC green book Ch 2.3: the symbol Q is recommended as a symbol for charge and therefore m/Q are the recomended symbols for indicating a mass-to-charge ratio.
- m/z is not a single symbol, but rather a division of two symbols, as described in Ch 1.5. of the IUPAC green book
- there is no mentioning of charge number nor mass number
- according to this definition, the dimension of m/z is mass/charge and possible units are kg/C, u/e, Da/e, Th (the unit thomson).
Definition in literature
Usage of m/z
The big picture
The wider scientific community uses m/Q or m/q to denote a mass-to-charge ratio (see some examples below) which is also in line with the international conventions summarized in the IUPAC green book. It is hard to understand why mass spectrometrists need to use the non-standard m/z notation which does not facilitate the communication with other fields of science.
Books on ion optics and symbols used
- Electron and Ion Optics by Miklos Szilagyi, Plenum Press, ISBN 0-306-42717-6.
- This book about ion optics and electron optics uses m for mass, Q for charge, and m/Q for the mass-to-charge ratio.
- Introduction into optics of charged particles by J. Grosser, Teubner, ISBN 3-519-03050-0.
- This book about particle optics uses m for mass, q for charge, and m/q for the mass-to-charge ratio.
- Applied charged particle optics, edited by A. Septier, Academic Press, ISBN 0-12-014574-X.
- This book uses m for mass, Q for charge, and m/Q for the mass-to-charge ratio.
Usage in mass spectrometry
Even within the mass spectrometry community m/z is not used coherrently. Some use it as a dimensionless quantity mass number per charge number, some use it as a quantity of dimension mass, and some use it as a mass-to-charge ratio with dimension mass/charge. Not to mention that many mass spectrometrists use m/q, m/Q, and even still m/e.
m/z used as a dimensionless quantity
- [6] ASMS poster
m/z used as a quantity of dimension mass
m/z used as a quantity of dimension mass/charge
Websites:
- [8] "For a given amplitude of the dc and rf voltages, only the ions of a given m/z (mass to charge) ratio will resonate, have a stable trajectory to pass the quadrupole and be detected. Other ions will be de-stabilized and hit the rods.
- [9]"Matrix-Assisted Laser Desorption/Ionisation Time Of Flight (MALDI-TOF), Surface-Enhanced Laser Desorption/Ionization Time Of Flight (SELDI-TOF), and Liquid Chromatography (LC-MS/MS) are the main techniques. They separate gas phase ions according to their m/Z (mass to charge ratio) values producing huge volumes of data. MS output is represented, as a (large) sequence of value pairs, containing a measured intensity, which depends on the quantity of the detected biomolecules and a mass to charge ratio m/Z, which depends on the molecular mass of detected biomolecules. A MS experiment usually generates one or more datasets, said spectra, that contains a huge quantity of measurements (m/Z, intensity) said peaks."
- [10]"m/z: Mass to charge ratio, synonymous with "m/e"."
- [11]"The mass spectrometers operate at an optimal m/z (mass to charge ratio) range of 50-2000. Because electrospray ionization typically produces multiply charged ions, it is possible to analyze compounds of up to MW 150,000 Daltons"
- [12]"M = m/z (mass-to-charge ratio)"
Journals
- Nature!"This shows the total ion chromatogram with recovery standard (SS) and internal standard (IS) peaks, and partial ion chromatograms for the metabolites 2-naphthoic acid (mass to charge ratio, m/z = 155); 5,6,7,8-tetrahydro-2-naphthoic acid (m/z = 131) and two isomers of decahydro-2-naphthoic acid (m/z = 164)."
- BMC Bioinformatics"Mass spectrometry (MS) can then be used to separate the co-elutants according to mass-to-charge ratio (m/z). "
- Protein Science"Abbreviations: CID, low-energy collision-induced fragmentation • Da/kDa, Dalton/(kilo)Dalton = atomic mass units • ESI, electrospray ionization • IMAC, immobilized metal affinity chromatography • 5-IU, 5-iodouracil • LRF, laboratory reference frame • MALDI, matrix-assisted laser desorption/ionization • m/z, mass-to-charge ratio • MS, mass spectrometry • MS/MS, tandem mass spectrometry • NTA, nitrilotriacetic acid • PAGE, polyacrylamide gel electrophoresis • SDS, sodium dodecyl sulfate • SSB, single-stranded DNA-binding protein • TOF, time-of-flight"
with unit thomson