Reference value assignment was performed using a nominal 0.5 mm path length cuvette from Starna Cells, Inc. (Atascadero, CA) and constructed from Spectrosil fused silica. Primary decadic attenuance measurements, which is analogous to absorbance except without light scatter correction, of the mAb were measured using the NIST Material Measurement Laboratory (MML) Transfer Spectrophotometer (Cary 6000i spectrometer (Agilent)). Traceability is to the decadic logarithm of the derived International System of Units (SI unit) of regular spectral transmittance (expressed as decadic attenuance) via the national reference instrument for optical absorbance (HAS-2) [14, 15]. Traceability is achieved using transfer standards SRMs 2034 Holmium Oxide Solution Wavelength Standard (240 nm to 650 nm); 930, 1930 and 2930 Glass Filters for Spectrophotometry; and 2031 Metal-on Fused--Silica Neutral Density Filters (250 nm to 635 nm). Calibration of the wavelength scale was done using SRM 2034. The Cary instrument was allowed to warm up for 30 min before measurements were taken to stabilize the UV lamp. The instrument was operated in double beam mode with a decreased slit height, temperature of 22 °C, and a spectral bandwidth of 0.8 nm, which are the typical operation conditions used to match the HAS II reference spectrophotometer. The decadic attenuance spectra were measured from 240 nm to 340 nm, at a scan rate of 20 nm/min and wavelength interval of 0.5 nm.