HDX has been utilized as a technique to investigate the structure of proteins and solvent accessibility. This is achieved by examining whether a protein retains its native folded conformation in a particular solution formulation. If the components of the formulation fail to stabilize the protein, its conformation may be perturbed, leading to additional aggregation or degradation. By employing HDX, it becomes possible to identify conformational changes, thereby providing insights into the physical stability of the molecule. Understanding protein-excipient interactions is important to optimize protein formulations and identify specific excipients that effectively enable their solid-state stability. ssHDX can be applied as a solid characterization method for protein conformation in the dried formulation by exposing protein solids to the D2O vapor and followed by mass spectrometry (MS) analysis. In a study of myoglobin, sucrose formulation demonstrated less level of deuterium uptake and correlated well with better storage stability. The correlation between protein stability and the H/D exchange rates has been observed in various proteins, including BSA, lysozyme, and β-lactoglobulin. Later on, the extent of hydrogen/deuterium H/D exchange (X∞) is proposed as a good short-term measurement predictive of long-term stability, indicating the rigidity of structure and dynamics of the protein formulations. Rui et al. found that the level of H/D exchange (X∞) was reduced in the protein that was freeze-dried using a sucrose matrix compared to a trehalose matrix under conditions of 40ºC and 11% relative humidity. This finding indicates that the protein retained greater stability in the sucrose formulation, at least under these specific conditions. 209 However, ssHDX might not be capable of detecting differences among various excipients when the presence of excipients does not affect the protein backbone or if excipients interact with the protein through certain mechanisms other than hydrogen bonding. ssNMR has been widely used to study the structure of insoluble membrane proteins and amyloid fibrils. 211 However, it faces challenges in acquiring high-resolution structural information for amorphous protein solids as they are highly randomly oriented. Nonetheless, ssNMR is still a sensitive spectroscopic tool to provide atomic-resolution structural insights and several studies have evaluated the structure of biological samples including peptides, proteins, and antibodies in the solid-state form. Zhou et al. utilized two-dimensional spectra ( 15 N-1 H and C-1 H) and characterized the structure of lyophilized aprotinin and insulin in different physical forms including lyophilized solid, microcrystalline suspension, and fibril forms. Compared to the relatively simple structure of peptides, there is also growing interest in understanding the higher-order structure of monoclonal antibodies in solid-state form after lyophilization or spray drying. Jacqueline et al. demonstrated the feasibility of ssNMR 1 H-13 C cross-polarization buildup structural fingerprinting on the lyophilized mAbs at natural isotopic abundance using the NIST mAb as a model system.