The nucleation pathway for calcium phosphates is an important topic that many scientists are interested in, but also grappling with. As a result, there is no agreed upon method to control the nucleation pathway to date. We have experienced ourselves that a multitude of factors influence this pathway in unexpected ways, such as the order of mixing components and the angle at which one pipettes. We decided to reproduce the calcium phosphate clusters studied by Epasto, et al 7 , however due to the inherent delicate nature of these processes, a few small modifications of the Kurzbach sample preparation method were necessary to observe PNCs under our laboratory conditions (Supporting Information Methods). Our replicate mSBF 31 P solution NMR experimental conditions (Figure ) exhibit a faster transition timescale, where the splitting of the 31 P solution NMR lineshape offers evidence for the formation of approximately 2 nm size PNCs exchanging with free dissolved phosphate according to Kurzbach and coworkers 7 . We could not perform DLS on our mSBF recipe due to the presence of HEPES agent, which obscures other similar or smaller sized particles around 1 nm. It is well known among groups that study these processes that PNC formation is a kinetic process that is highly sensitive to any experimental variation, including the choice of buffer, temperature, certainly the exact composition of the mSBF and even the order and method of salt mixing. Further complicating interpretation, the solution 31 P NMR chemical shift of monophosphate salt is dependent on the total ionic strength of the solution in a nonlinear relation (likely due to effects such as water hydration structure and extent of hydration), while conversely related to the pH and temperature of the solution in a linear relation (Figure ). Given the complexity of these processes that require more scrutiny, the elucidation of PNC formation pathways is beyond the scope of our study. Instead, we present a proof of principle study of MQ-SC under DNP conditions of PNCs to showcase the feasibility of such an approach, especially of solution state clusters in vitrified solution.