  So far no designated mission to either of the two ice giants, Uranus and
Neptune, exists. Almost all of our gathered information on these planets comes
from remote sensing. In recent years, NASA and ESA have started planning for
future mission to Uranus and Neptune, with both agencies focusing their
attention on orbiters and atmospheric probes. Whereas information provided by
remote sensing is undoubtedly highly valuable, remote sensing of planetary
atmospheres also presents some shortcomings, most of which can be overcome by
mass spectrometers. In most studies presented to date a mass spectrometer
experiment is thus a favored science instrument for in situ composition
measurements on an atmospheric probe. Mass spectrometric measurements can
provide unique scientific data, i.e., sensitive and quantitative measurements
of the chemical composition of the atmosphere, including isotopic, elemental,
and molecular abundances. In this review paper we present the technical aspects
of mass spectrometry relevant to atmospheric probes. This includes the
individual components that make up mass spectrometers and possible
implementation choices for each of these components. We then give an overview
of mass spectrometers that were sent to space with the intent of probing
planetary atmospheres, and discuss three instruments, the heritage of which is
especially relevant to Uranus and Neptune probes, in detail. The main part of
this paper presents the current state-of-art in mass spectrometry intended for
atmospheric probe. Finally, we present a possible descent probe implementation
in detail, including measurement phases and associated expected accuracies for
selected species.
