IB Chemistry SL 1.4.2 Relative Atomic Mass Ar and Relative Formula Mass Mr Topic Practice

The element boron has two naturally occurring isotopes, ${ }^{10} \mathrm{~B}$ and ${ }^{11} \mathrm{~B}$.

The percentage abundance of the isotopes of boron can be determined with a mass spectrometer. The diagram shows the operation of a mass spectrometer.

Identify the species that is used as the scale for the mass of the isotopes.