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I understand that true (non-relative) masses of atoms are calculated with a mass spectrometer. using this relationship: Centripetal Force = Force due to magnetic field(B)

But I was wondering how masses of atoms were calculated before the mass spectrometers were invented.

(Please don't get this mixed up with relative atomic mass. That's not what I'm talking about.)

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Absolute mass can be determined electrochemically, once the coulomb is defined in terms of numbers of electrons, as through the Millikan oil-drop experiment, to be $6.24×10^{18}$ electrons per coulomb.

See Determination of Avogadro's Number and Absolute Determination of the Electrochemical Equivalent and the Atomic Weight of Zinc.

One thing that caused confusion in the determination of atomic mass is natural isotopic separation, e.g. sulfur derived from condensation from vulcanism vs. from mineral compounds, so that there were slightly different experimental results depending on source.

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