  Measurements of the isotopic composition of single-charged cosmic rays
provide important insights in the propagation processes. However, the isotopic
identification is challenging due to the one hundred times greater abundance of
protons when compared to deuterons, the only stable isotope of hydrogen. Taking
advantage of the precise measurements of the velocity and momentum in the Alpha
Magnetic Spectrometer (AMS-02), a particle physics detector operating aboard
the International Space Station since May 2011, we describe a parametric
template fit method, which takes into account systematic uncertainties such as
the fragmentation of particles inside AMS-02 and eventual differences between
data and simulation through the use of nuisance parameters. With this method we
are also able to assess the AMS-02 performance in terms of mass resolution,
showing that it is able to separate the isotopes of hydrogen up to 10 GeV/n.
