Re: MS - m or Mr or m/z or what?
Jeffrey A Kowalak (jkowalak@codon.nih.gov)
Fri, 19 Jun 1998 11:45:17 -0400
Hi Everyone,
Here's my two cents. The JBC nomenclature is indeed correct, the
problem seems to be that people use the terms "molecular mass" and
"molecular weight" interchangably. "Molecular mass" is a physical property
and is expressed in SI units of mass. In freshman chemistry we learned
that a Dalton (Da) was defined as the mass of a hydrogen atom (1.007825),
but has since been redifined to be 1/12 the mass of C12 (or 1.000000).
This is an important distinction because under the old definition 10 kDa =
10,078.25 vs 10,000.00 for the new def. Modern mass spectrometers can
easily differentiate 10,078 from 10,000. The term "molecular weight" seems
to be a contraction of "relative molecular weight" or Mr, which is a ratio
and hence demensionless. Further, the distance between m/z 10 and m/z 11
is one atomic mass unit (formerly amu, now u). "m/z" is obviously
mass/charge, most commonly the charge is gain or loss of a proton.
For the obsessive/compulsives, Mr is italic "M"; "r" is subscript,
but not italicized. "m/z" is italicized. Finally, the ordinate of a mass
spectrum should be labeled "Relative Abundance". In addition to the ASMS
ref that Ioannis mentioned (a very good source of MS nomenclature), I
believe the ACS Style Guide covers this topic a well.
Regards,
Jeff Kowalak
Jeffrey A Kowalak, Ph.D.
jkowalak@codon.nih.gov
Section on Metabolic Analysis and Mass Spectrometry
National Institute of Child Health and Human Development
Building 10 Room 9D52
Bethesda MD 20892-1580
telephone: 301-594-3678
facsimile: 301-480-5460