Table 2.

Summary of solid-state characterization methods.

Characterization Method	What does it measure?	Information Provided	Advantage	Disadvantage
DSC	Change in enthalpy and heat capacity when heating the solid sample	Tg; matrix dynamics	Routine measurement; useful for drying cycle development	Global information on matrix; little information on local protein structure
Neutron Scattering	Energy and intensity of scattered neutrons	Matrix dynamics; molecule distribution in solid state	In-situ detection without extra sample preparation	Time-consuming data acquisition and analysis; limited accessibility of the technique
ssFTIR	Vibrational frequency of chemical bonds shown in fingerprint regions	Protein secondary structure	Fast and easy to access information on protein secondary structure	Global structure information; difficult to quantitatively describe structural disruption
Solid-state Fluorescence Spectrometry	Protein intrinsic fluorescence from aromatic residues	Protein tertiary structure	Detect tertiary structure disruption in-situ	Not proved to be correlated with physical stability in protein formulations
ssNMR	Chemical shifts of isotopes	Protein conformation; matrix and protein dynamics; protein-excipient interactions	Provide comprehensive information for both protein and matrix in solid state	Data acquisition and interpretation can be time-consuming
ssHDX	Change in protein backbone based on the disruption of hydrogen bonding	Disruption of protein conformation; heterogeneity of protein in solid state	Characterize hydrogen bonding patterns between protein and sugar-based excipients; correlated with long-term stability of protein formulations	Poor correlation in formulations with ionic excipients; does not work well to distinguish different drying process
ssPL	Distribution of the molecules in the solid matrix near protein	Protein interactions with excipient, salt or residual water	Labeling available for both protein and excipient; does not require specific type of interactions between protein and excipient	Extra sample preparation; complicated quantitation process