  Imaging of liquids and cryogenic biological materials by electron microscopy
has been recently enabled by innovative approaches for specimen preparation and
the fast development of optimised instruments for cryo-enabled electron
microscopy (cryo-EM). Yet, Cryo-EM typically lacks advanced analytical
capabilities, in particular for light elements. With the development of
protocols for frozen wet specimen preparation, atom probe tomography (APT)
could advantageously complement insights gained by cryo-EM. Here, we report on
different approaches that have been recently proposed to enable the analysis of
relatively large volumes of frozen liquids from either a flat substrate or the
fractured surface of a wire. Both allowed for analysing water ice layers which
are several microns thick consisting of pure water, pure heavy-water and
aqueous solutions. We discuss the merits of both approaches, and prospects for
further developments in this area. Preliminary results raise numerous
questions, in part concerning the physics underpinning field evaporation. We
discuss these aspects and lay out some of the challenges regarding the APT
analysis of frozen liquids.
