什么是 Isotope reference standard？

An isotope reference standard is a material whose isotope ratio defines the zero of a $\delta$ scale. Two are used in the Earth sciences. VSMOW (Vienna Standard Mean Ocean Water) defines $\delta\mathrm{D}$ and $\delta^{18}\mathrm{O}$ of waters, silicates and most other materials; its absolute ratios are $\mathrm{D}/\mathrm{H} = 1.5575 \times 10^{-4}$ and $^{18}\mathrm{O}/^{16}\mathrm{O} = 2.0052 \times 10^{-3}$. VPDB (Vienna Pee Dee Belemnite) defines $\delta^{13}\mathrm{C}$ and the $\delta^{18}\mathrm{O}$ of low-temperature carbonates; its absolute ratios are $^{13}\mathrm{C}/^{12}\mathrm{C} = 1.1237 \times 10^{-2}$ and $^{18}\mathrm{O}/^{16}\mathrm{O} = 2.0672 \times 10^{-3}$.