In practice, the internal energy of the ion packet is influenced by the ion source and the ion transfer insights into water-containing new particles but do not provide quantitative thermodynamic parameters of water binding, which is vital to comprehend water's role in particle growth. Our group recently probed the thermochemistry of hydration of lanthanide complexes by VTMS, where the lanthanide complexes were hydrated step-by-step in a variable temperature ion trap in the presence of a water-seeded buffer gas.( ) By monitoring the partial pressure of water and ion intensities of both dry and any hydrated complexes, we were able to carry out van't Hoff analyses to derive the free energies required to bind water to these complexes. Ours is a modified, low temperature additive version of the VT method employed by Froyd and Lovejoy. Adapting this approach to determine the thermodynamics of vapor uptake of atmospherically-relevant molecular clusters can provide a quick way to validate computational predictions as well as complement structural information obtained from IR-MS.