This is easy. "Mr" is a molecular weight (i.e. neutral molecule), while
"m/z" refers to the mass-to-charge ratio of a peak observed in a mass
spectrum. Consider the following sentence:
"The mass spectrum of this unknown insulin variant revealed a peak at m/z
5736, suggesting that this corresponds to the MH+ ion of a variant with Mr
of 5735 Daltons."
Note that one does not use "daltons" when using "m/z", since this is a
ratio and not a mass. "Mr" is a mass and therefore has units of daltons.
Alternatively, one can use "MW" in place of "Mr", but probably not in
Biological Chemistry.
Please relay this.
Bob Cotter
----------
From: Amina[SMTP:amina@welchlink.welch.jhu.edu]
Sent: Friday, June 19, 1998 9:31 AM
To: 'Robert J. Cotter'
Subject: FW: MS - m or Mr or m/z or what?
Bob:
Could you please answer this question. I think you could do it better than
I could.
Amina
----------
From: Ken Mitchelhill [SMTP:k.mitchelhill@medicine.unimelb.edu.au]
Sent: Thursday, June 18, 1998 9:49 PM
To: Recipients of ABRF List
Subject: MS - m or Mr or m/z or what?
Colleagues,
I am trying to put a "style page" together for the Journal of Biomolecular
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.html
ABRF: http://www.abrf.org
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