# Determining stoichiometry in ICP-OES data analysis

I have questions regarding data analysis for an inductively coupled plasma (ICP) experiment from a paper by Wang et al. [1].

The generic formula for the chemical compound is $$\ce{Na_{2−x}Fe[Fe(CN)6]}$$.

In the paper, it was mentioned that chemical formula and ICP data are

$$\begin{array}{lrr} \text{Formula} & \ce{Na} / \mathrm{ppm} & \ce{Fe} / \mathrm{ppm} \\\hline \ce{Na_{1.53}Fe[Fe(CN)6].{4.2}H2O} & 112300 & 357200 \\ \ce{Na_{1.67}Fe[Fe(CN)6].{3.9}H2O} & 116100 & 339400 \\ \ce{Na_{1.73}Fe[Fe(CN)6].{3.8}H2O} & 118400 & 332400 \\ \ce{Na_{1.68}Fe[Fe(CN)6].{3.9}H2O} & 112000 & 332700 \\ \end{array}$$

My question is how they determined the $$\ce{Na}$$ number ($$1.53$$, $$1.67$$, and so on) on each cases based from the $$\mathrm{ppm}$$? I tried to make a $$\mathrm{ppm}$$ ratio of $$1:2$$ for $$\ce{Na}$$ and $$\ce{Fe}$$ however the number does not match with the paper.

### Reference

1. Wang, W.; Gang, Y.; Hu, Z.; Yan, Z.; Li, W.; Li, Y.; Gu, Q.-F.; Wang, Z.; Chou, S.-L.; Liu, H.-K.; Dou, S.-X. Reversible Structural Evolution of Sodium-Rich Rhombohedral Prussian Blue for Sodium-Ion Batteries. Nature Communications 2020, 11 (1), 980. DOI: 10.1038/s41467-020-14444-4

$$\pu{R = }\frac{\pu{112300 ppm } /\ \pu{22.98977 g mol^{-1} } }{\pu{357200 ppm } / \ \pu{55.845 g mol^{-1} } } = \pu{0.7637 }$$