  We demonstrate a novel method to ionize molecules or molecular clusters by
proton transfer at temperatures below 1 K. The method yields nascent ions and
largely eliminates secondary reactions, even for notoriously "delicate"
molecules. Protonation is achieved inside liquid helium nanodroplets (HNDs) and
begins with the formation of (H$_{2}$)$_{m}$H$^{+}$ ions as the proton donors.
In a separate and subsequent step the HNDs are doped with a proton acceptor
molecule, X. Proton transfer occurs between X and the cold proton donor ions
inside a helium droplet, an approach that avoids the large excess energy that
is released if HNDs are first doped and then ionized. Mass spectra, recorded
after stripping excess helium and hydrogen in a collision cell, show that this
method offers a new way to determine proton affinities of molecules and
clusters by proton-transfer bracketing, to investigate astrochemically relevant
ion-molecule reactions at sub-kelvin temperatures, and to prepare XH$^{+}$ ions
that are suitable for messenger-tagging action spectroscopy.
