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Atomic mass cannot be measured directly; for that matter, NO mass can be
measured directly. In the case of macroscopic objects, masses are determined in
relation to the Standard
Kilogram, a cylinder of platinum-iridium alloy in
Paris.
In the case of atomic masses, measurements must be made in relation to the mass of some particular atom. In this case, it is the mass of the isotope 12C, carbon-12, which is defined to be exactly 12. 12 what, you ask? In one sense, it doesn't matter; any mass is a ratio--including the masses of macroscopic objects, which as I said are compared to the Standard Kilogram--but we often use the term "atomic mass unit" (amu). The term "Dalton" is often used in biochemistry as a synonym for amu, but it has no recognized standing.
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