Here is the message:
"Dear Colleagues,
The attached [several messages posted on the ABRF board] was forwarded to
me by Ken Standing.
The biological community appears to have embraced the use of the Dalton as the
unit of atomic mass, although this has (to the best of my knowledge) never
been adopted by IUPAC, and not by IUPAP. The statement regarding usage that is
given by the Jopurnal of Biological Chemistry reflects this state of affairs.
The symbol "u" was agreed on by IUPAC and IUPAP in 1959-60, when they took a
coordinated action to move from the old "physical" (16O = 16 mass units or
amu) and "chemical" (O = 16 mass units) scales to the present one based on 12C
= 12 u. The symbol "u" was chosen to indicate that it was the "unified" mass
scale. The IUPAP commission has further refined the definition to include the
fact that the 12C is considered to be an isolated atom in the nuclear and
atomic ground states. (This is reflected in the notes on the definition of the
mole in SI as adopted by the General Conference on Weights and Measures, 1971
-- see B. Petley, Metrologia 33 (1996) 261).
The use of other abbreviations for the atomic mass unit (e.g. "mu" or "amu")
are unfortunate and not recognized.
The IUPAC CAWIA has always carefully distinguished between the dimensionless
ratios and the quanitites expressed in u (e.g. the matter of "molecular
weight").
The use of "m/z" by mass spectroscopists is a little different. The "m" part
of it is in "u". The "z" is the exact integer multiple of elementary charges
on the ion under study and is a dimensionless number. The actual net charge on
the ion is "ze", where e = 1.602 exp(-19) Coulombs. Hence "m/z" is still
properly expressed in "u".
With best regards to all,
Bob Barber
Assoc. Secretary-General, IUPAP"