# Why atomic masses are quoted with reference to an isotope?

Is there any fundamental reason why atomic masses are quoted with respect to a certain element such as an arbitrary C-12 isotope whose mass has been set to 12 unit exactly? Previously, chemists have no other way to measure absolute masses hence they have been using a reference such as H or O which were set to 1 and 16 exactly for more than a century. I am a chemist and asked several people (chemists) as to why mass spectrometric measurements cannot lead to "absolute atomic masses" and they were of the opinion that it is difficult to calibrate a mass spectrometer and determine exact electric fields and magnetic fields of the setup. I don't feel that the calibration issue is the sole reason for using relative atomic masses still today. My point is that at least in principle, one could measure the m/e ratio of an isotopic and quote its absolute mass.

I was thinking to this crude analogy: If we had a hypothetical atomic balance, which could weigh atoms, currently we have calibrated that balance by asking it to read the weight of a C-12 atom as 12 units (as many digits as the balance can read to infinity). All atoms are then weighed with reference to this balance. Although we know that the weight of C-12 atom is not exactly 12.0000000000000000000000000 units because humans set it arbitrarily. This approach looks very artificial.

Opinions and perspectives of physicists would be helpful in understanding the logic behind relative atomic masses vs absolute atomic masses?

EDIT.

I found one reference from Precision Measurements and Fundamental Constants, here W. H. Johnson makes the makes the following point shown in a rectangle that experience has shown that measuring relative masses are more accurate and useful. So it seems like an experimental issue to use a reference isotope.

• Wikipedia discusses the topic, and references a paper proposing the same, i.e. that we get rid of the reference isotope ("Why the dalton should be redefined exactly in terms of the kilogram" (2012)). I suppose that the concern is that this would seem to break tradition, while anyone who does want to refer to mass in an absolute relationship with grams is freely able to do so without using atomic mass units. Kind of a neat quasi-political topic, though. – Nat Feb 9 at 23:16
• Particle physicists don't use AMU, and quote isotope masses in $\mathrm{eV}$ instead. Not because electron-volts are morally superior, but because it means we are working in a consistent set of units. – dmckee Feb 9 at 23:22
• IUPAC has gotten rid of mass based on a particular isotope. On 20 May 2019 Avogadro's Constant will have a value of exactly $6.02214076 \times10^{23}$. This will cause the mass of C-12 to be a bit different. – MaxW Feb 9 at 23:39
• @Alchimista - No, both Avogadro's Constant and the kilogram will be redefines on 20 May 2019 to particular absolute values. $^{12}C$ will be 12 g/mol, within experimental error of current measurements, but as measurements improve the mass of $^{12}C$ will vary. – MaxW Feb 10 at 15:15
• @Alchimista - (cont) The overall idea is to break cascading variations where, for example, an improvement in the measurement of the speed of light meant that multiple "fundamental" constants changed. So the SI's set of fundamental constants are now independent. (Unless someone comes up with a grand theory of everything, then it will be back to the drawing board... ) – MaxW Feb 10 at 15:16