A Double-Focusing Mass Spectrograph and the Masses ofN15andO18

After a short summary of the theory of double focusing a mass spectrograph which works as an "achromatic lens" for all masses is described in some detail. It is found to work according to the theoretical expectations. The differences of "packing fraction" (i.e., $\frac{\Delta M}{M}\times {10}^4$) of doublets containing the rare isotopes ${\mathrm{N}}^{15}$ and ${\mathrm{O}}^{18}$ have been determined. It is found that for the doublet at mass number 15 (${\mathrm{C}}^{12}$ $\mathrm{H}_3^{}{}_{}{}^1$—${\mathrm{N}}^{15}$): $\frac{\Delta M}{M}\times {10}^4={15.8}_6\pm 0.05$, $\Delta M=0.02382\mathrm{}$ and for the 18-doublet (${\mathrm{O}}^{16}$ $\mathrm{H}_2^{}{}_{}{}^1$—${\mathrm{O}}^{18}$): $\frac{\Delta M}{M}\times {10}^4={6.9}_8\pm 0.10$, $\Delta M=0.01257\mathrm{}$. With Aston's masses of ${\mathrm{C}}^{12}$ and ${\mathrm{H}}^1$ the isotopic weights referred to ${\mathrm{O}}^{16}$ = 16 are: ${\mathrm{N}}^{15}$ = 15.0040±0.0008 and ${\mathrm{O}}^{18}$ = 18.0037±0.0007. The measurement of ${\mathrm{N}}^{15}$ gives the rare chance to compare the energy release of a nuclear reaction viz.: ${\mathrm{N}}^{14}$+${\mathrm{H}}^2$=${\mathrm{N}}^{15}$+${\mathrm{H}}^1$ with its value derived entirely by massspectrographic determinations.