Difference between revisions of "Mass/Charge Ratio Comments"

From MS Terms Wiki
Jump to navigation Jump to search
(→‎Background: clean up using AWB)
(Redirected page to M/z)
 
Line 1: Line 1:
= Enter Your Discussion Items Below =
+
#REDIRECT [[m/z]]
 
 
Please sign your comments.
 
 
 
 
 
= Th, never use Da, no units?. An ion of m/z or an ion at m/z? =
 
 
 
{{Sugdef|Mass/charge Ratio|The physical quantity mass/charge has the symbol ''m/q''. The SI units would be '''kg'''/'''C''', however, in mass spectrometry the [[Atomic Mass Unit]] '''u''' is used instead of the kilogram, and the [http://www.example.com atomic unit] '''e''' is used instead of the Coulomb. '''e''' is the elementary charge. In short this is indicated by:
 
: [''m/q''] = '''u'''/'''e'''
 
 
 
'''u''' is sometimes called Dalton ('''Da'''). '''u'''/'''e''' is sometimes called Thomson ('''Th'''). Hence, the mass spectra x-axis should be labeled
 
: ''m/q''  ('''u'''/'''e''') or ''m/q''  ('''Da'''/'''e''') or ''m/q''  ('''Th''')}}
 
 
 
== Background ==
 
 
 
Contrary to what the name implies, mass spectrometers do not measure the mass of an ion, they measure another [http://en.wikipedia.org/wiki/Physical_quantity physical quantity], the mass/charge ratio. This simple fact is the source of much confusion.
 
 
 
Using the correct [http://en.wikipedia.org/wiki/International_system_of_units SI units], the mass/charge ratio sould be indicated in kg/C (kilograms/Coulombs). In mass spectrometry, however, this [http://en.wikipedia.org/wiki/System_of_units system of units] is hardly ever used. Instead the mass is indicated in the [[Atomic Mass Unit]] '''u''' and the charge is indicated in multiples of the [http://en.wikipedia.org/wiki/Elementary_charge elementary charge] '''e'''.
 
 
 
Hence the correct labling of "mass spectra" would be ''m/q'' '''(u/e)''' where ''m'' is the symbol for mass, ''q'' the symbol for charge, '''u''' the unit of mass in atomic mass units (also called Dalton, '''Da'''), and '''e''' the unit of charge in elementary charge units. This notation, however, is hardly ever used. The reason is that one could argue that '''e''' is not an elementary charge unit but an elementary charge constant. The difference seems artificial, however, whereas the atomic mass unit '''u''' is accepted by the [[CPGM]], the elementary charge seems to lack such an acceptance. This is rather surprising considering that the elementary charge is a natural fundamental unit, whereas the atomic mass unit is defined somewhat arbitrarily. Also, the elementary charge unit is often used not only by mass spectrometrists but by many (other) physicist.
 
 
 
As a way out of this confusion, many mass spectrometrists do not use ''m/q''. Instead they use a somewhat artificial unit ''m/z'' where ''z'' is considered a unitless property indicating the number of elemental charges ''e'' missing in a molecule. This questionable approach, however, leads to the problem that the unit of ''m/z'' becomes the same as the unit for a mass ''m'', thereby creating even more confusion.
 
 
 
To avoid this, some people go on and declare also ''m'' as a unitless property, whereby the ''m/z'' also becomes unitless. Following this approach, however, one has to wonder why not all measurments (length, time, speed, ...) are declared unitless.
 
 
 
In order to find a way out of the mess, it was proposed to introduce a new unit by 1 '''u'''/'''e''' = 1 '''Th''' ([[Thomson]] - see Cooks & Rockwood, Rapid Commun. Mass Spectrom. ''5'', 93, '''1991''' and Sparkman, ''Mass Spec Desk Reference'' p. 27).
 
 
 
According to this convention, mass spectra x axis should be labled ''m/q'' '''(Th)'''. However, it will take years to change the commonly used ''m/z'' into ''m/q'' and to introduce the still not well known unit '''Th'''. 
 
 
 
Since Th is derived from the elemental charge unit '''e''', this unit should get official status. Therefore I propose installing a new unit for charge that is equal to the elementary charge. A good name would be Millikan ('''Mi''').
 
: [''q''] = '''Mi'''
 
 
 
 
 
 
 
* [http://www.sizes.com/units/atomic_mass_unit.htm history of amu]
 
* [http://www.iupac.org/goldbook/U06554.pdf amu]
 
* [http://www.iupac.org/goldbook/D01514.pdf Dalton]
 
* The [http://en.wikipedia.org/wiki/Elementary_charge elementary charge] is a constant, unfortunately not (yet) a unit.
 
* [http://en.wikipedia.org/wiki/Robert_Millikan Robert Millikan] first measured the elementary charge '''e'''.
 
* [http://en.wikipedia.org/wiki/Natural_units natural units]
 
 
 
[[Category:Comment]]
 

Latest revision as of 19:57, 8 June 2013

Redirect to: