By measuring the cyclotron frequency ratio of ${}^4{\mathrm{He}}^{+}$ to ${{}^{12}\mathrm{C}}^{3+}$ in a Penning trap the mass of ${}^4\mathrm{He}$ has been determined to be 4.002 603 254 665(36) u. [The corresponding mass of the alpha particle is 4.001 506 179 662 (36) u]. This opens the possibility of deriving the atomic mass of the electron at a relative precision of $1\times {10}^{-11}$ from proposed measurements of the Landé $g$-factor of ${}^4{\mathrm{He}}^{+}$.

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• Relativistic & quantum electrodynamic effects in atoms, molecules,& ions
• Trapped ions
• Penning traps