Re: MS - m or Mr or m/z or what?
Ioannis Papayannopoulos (iap@iname.com)
Fri, 19 Jun 1998 07:58:25 -0400
The "ASMS [American Society for Mass Spectrometry] Terms and Definitions"
poster, presented ath the 6/2/97 ASMS conference (check the ASMS home pag=
e,
"www.ASMS.org") defines "mass-to-charge ratio" as "Daltons/electronic
charge", which obviously has units, although when one labels the x-axis o=
f
a mass spectrum or when one refers to a particular measured m/z value the
units are invariably omitted (e.g. "...the fragment ion at m/z 222.2...").
There has been discussion among mass spectrometrists (and, therefore,
probably of no interest to anyone else) whether one should call "m/z" a
"Thomson", except that, as someone noticed, the name has already been
appropriated by the fluid dynamics people, which is probably just as well.
One (1) m/z is usually refered to as "u" or "=B5" (this is the greek lett=
er
"mu"); it's not clear to me whether this is a result of the confusion
generated by using "u" to designate "mu" since older word processing
software (or typewritesrs, remember those?) did not support non-latin
alphabet characters. Hence, the difference in mass between two species i=
n
a mass spectrum would be given as "u", e.g., "...the ion at 18 u below th=
e
protonated molecule m/z corresponds to loss of water...".=20
My preference, is to use u when I discuss mass spetral data, but use Da
when I discuss calculated masses for biomolecules (e.g., "...a 25 kDa
protein...). I believe that if one refers to molecular weight (MW) no
units are used (e.g. "The moleculer weight of this peptide is 444.4").
It's not clear to me whether Mr is the same as MW (hence unitless) or it =
is
expressed in Da. The issue of monoisotopic vs. average mass calculations
needs to be considered, especially when one compares mass spectral data
(which could be either, depending on the resolution of the instrument) to
calculated values (which could also be either, depending on what software
one uses). I am not sure any of this makes the issue any clearer, but
perhaps others will contribute their thoughts as well.
Ioannis Papayannopoulos
CytoMed, Inc.
Cambridge, MA
At 11:48 AM 6/19/98 +1000, Ken Mitchelhill wrote:
>Colleagues,
>
>I am trying to put a "style page" together for the Journal of Biomolecul=
ar
>Techniques and am scratching my head a bit about how to represent mass.
>
>The Journal of Biological Chemistry allows the use of two mass terms, "m=
"
>and "Mr" where:
>
>"Molecular mass (symbol m) is expressed in daltons (Da); one dalton is 1=
/12
>of the mass of carbon 12. Molecular weight (Mr, relative molecular mass)=
is
>the ratio of the mass of a molecule to 1/12 of the mass of carbon 12 and=
is
>dimensionless. Hence, it is not correct to express Mr in daltons."
>
>There is no mention of the term "m/z" although I am used to seeing this =
in
>a whole host of mass spectrometric output.
>
>I was curious as to what the feeling out there was with regard to the
>"correct" way to express mass in the context of our experiments?
>
>Ken
>
>********************************
>
>Ken I. Mitchelhill
>The John Holt Protein Structure Laboratory
>St. Vincent's Institute of Medical Research
>41 Victoria Parade
>Fitzroy 3065 Victoria
>AUSTRALIA
>
>Telephone: 61-3-9288 2480
>Facsimile: 61-3-9416 2676
>
>Email: k.mitchelhill@medicine.unimelb.edu.au
>
>Laboratory: http://www.medstv.unimelb.edu.au/WWWDOCS/SVIMRdocs/JHPSL.htm=
l
>ABRF: http://www.abrf.org
>
>***********************************
>
>
>