Editing Mass spectrum (section)

==X-axis: ''m/z'' (mass-to-charge ratio)==
The [[x-axis]] of a mass spectrum represents a relationship between the mass of a given ion and the number of elementary charges that it carries. This is written as the [[IUPAC]] standard ''m/z'' to denote the quantity formed by dividing the mass of an ion (in daltons) by the [[dalton (unit)|dalton unit]] and by its charge number (positive absolute value).<ref name="GoldBook-mz">{{GoldBookRef|title=mass-to-charge ratio|file= M03752}}</ref><ref>{{cite journal|title=Recommendations for nomenclature and symbolism for mass spectroscopy | doi=10.1016/0168-1176(95)93811-F | volume=142 | journal=International Journal of Mass Spectrometry and Ion Processes | year=1995 | pages=209–240| bibcode=1995IJMSI.142..209T | last1=Todd | first1=John F.J. | issue=3 }}</ref><ref>{{cite web|url=http://www.iupac.org/publications/analytical_compendium/TOC_cha12.html|title=TOC_cha12.html|work=iupac.org}}</ref> Thus, ''m/z'' is a dimensionless quantity with no associated units.<ref name="GoldBook-mz"/> Despite carrying neither units of mass nor charge, the ''m/z'' is referred to as the [[mass-to-charge ratio]] of an ion. However, this is distinct from the mass-to-charge ratio, m/Q (SI standard units kg/C), which is commonly used in physics. The ''m/z'' is used in applied mass spectrometry because convenient and intuitive numerical relationships naturally arise when interpreting spectra. A single ''m/z'' value alone does not contain sufficient information to determine the mass or charge of an ion. However, mass information may be extracted when considering the whole spectrum, such as the spacing of [[isotope]]s or the observation of multiple charge states of the same molecule. These relationships and the relationship to the mass of the ion in daltons tend toward approximately [[rational number]] values in ''m/z'' space. For example, ions with one charge exhibit spacing between isotopes of 1 and the mass of the ion in daltons is numerically equal to the ''m/z''. The IUPAC Gold Book gives an example of appropriate use:<ref name="GoldBook-mz"/> "''for the ion C<sub>7</sub>H<sub>7</sub><sup>2+</sup>, m/z equals 45.5''".

===Alternative x-axis notations===
There are several alternatives to the standard ''m/z'' notation that appear in the literature; however, these are not currently accepted by standards organizations and most journals.  ''m/e'' appears in older historical literature. A label more consistent with the [[IUPAC green book]] and [[ISO 31]] conventions is ''m/Q'' or ''m/q'' where ''m'' is the symbol for mass and ''Q'' or ''q'' the symbol for charge with the units u/e or Da/e. This notation is not uncommon in the physics of mass spectrometry but is rarely used as the abscissa of a mass spectrum. It was also suggested to introduce a new unit [[Thomson (unit)|thomson]] (Th) as a unit of ''m/z'', where 1 Th = 1 u/e.<ref>Cooks, R. G. and A. L. Rockwood (1991). "The 'Thomson'. A suggested unit for mass spectroscopists." Rapid Communications in Mass Spectrometry 5(2): 93.</ref> According to this convention, mass spectra x axis could be labeled ''m/z'' (Th) and negative ions would have negative values. This notation is rare and not accepted by [[IUPAC]] or any other standards organisation.

===History of x-axis notation===
[[Image:Dempster mass spectrum.gif|right|thumb|300 px|Mass spectrum of sodium and potassium positive ions from [[Arthur Jeffrey Dempster|Arthur Dempster]]'s 1918 publication "A new Method of Positive Ray Analysis " ''Phys. Rev.'' '''11''', 316 (1918)]]
In 1897 the mass-to-charge ratio <math>m/e</math> of the [[electron]] was first measured by [[J. J. Thomson]].<ref>{{cite web|url=http://web.lemoyne.edu/~giunta/thomson1897.html|title=J. J. Thomson 1897|work=lemoyne.edu}}</ref> By doing this he showed that the electron, which was postulated before in order to explain electricity, was in fact a particle with a mass and a charge and that its mass-to-charge ratio was much smaller than the one for the hydrogen ion H<sup>+</sup>. In 1913 he measured the mass-to-charge ratio of [[ion]]s with an instrument he called a parabola spectrograph.<ref>{{cite web|url=http://web.lemoyne.edu/~giunta/canal.html|title=Joseph John Thomson|work=lemoyne.edu}}</ref> Although this data was not represented as a modern mass spectrum, it was similar in meaning. Eventually there was a change to the notation as ''m/e'' giving way to the current standard of ''m/z''.{{Citation needed|date=July 2011}}

Early in mass spectrometry research the [[Resolution (mass spectrometry)|resolution]] of mass spectrometers did not allow for accurate mass determination. [[Francis William Aston]] won the Nobel prize in Chemistry in 1922.<ref>{{Cite web |url=http://nobelprize.org/chemistry/laureates/1922/aston-lecture.pdf |title=Archived copy |access-date=18 April 2006 |archive-date=13 May 2006 |archive-url=https://web.archive.org/web/20060513215609/http://nobelprize.org/chemistry/laureates/1922/aston-lecture.pdf |url-status=dead }}</ref> "For his discovery, by means of his mass spectrograph, of isotopes, in a large number of non-radioactive elements, and for his enunciation of the [[Whole Number Rule]]." In which he stated that all atoms (including isotopes) follow a whole-number rule<ref>{{cite web|url=http://web.lemoyne.edu/~giunta/aston.html|title=F. W. Aston|work=lemoyne.edu}}</ref> This implied that the masses of atoms were not on a scale but could be expressed as integers (in fact multiple charged ions were rare, so for the most part the ratio was whole as well). There have been several suggestions (e.g. the unit thomson) to change the official mass spectrometry nomenclature <math>m/z</math> to be more internally consistent.