  Atom Probe Tomography (APT) analysis is being actively used to provide
near-atomic-scale information on the composition of complex materials in
three-dimensions. In recent years, there has been a surge of interest in the
technique to investigate the distribution of hydrogen in metals. However, the
presence of hydrogen in the analysis of almost all specimens from nearly all
material systems has caused numerous debates as to its origins and impact on
the quantitativeness of the measurement. It is often perceived that most H
arises from residual gas ionization, therefore affecting primarily materials
with a relatively low evaporation field. In this work, we perform systematic
investigations to identify the origin of H residuals in APT experiments by
combining density-functional theory (DFT) calculations and APT measurements on
an alkali and a noble metal, namely Na and Pt, respectively. We report that no
H residual is found in Na metal samples, but in Pt, which has a higher
evaporation field, a relatively high signal of H is detected. These results
contradict the hypothesis of the H signal being due to direct ionization of
residual H$_2$ without much interaction with the specimen's surface. Based on
DFT, we demonstrate that alkali metals are thermodynamically less likely to be
subject to H contamination under APT-operating conditions compared to
transition or noble metals. These insights indicate that the detected H-signal
is not only from ionization of residual gaseous H$_2$ alone, but is strongly
influenced by material-specific physical properties. The origin of H residuals
is elucidated by considering different conditions encountered during APT
experiments, specifically, specimen-preparation, transportation, and
APT-operating conditions by taking thermodynamic and kinetic aspects into
account.
