Reference decadic attenuance values determined for each lot of NISTmAb are given in Table S2 in the ESM. The combined standard uncertainty associated with the Dcorr measurement includes multiple contributions as described in the ESM [14, 15]. Metrological traceability is to the decadic logarithm of the derived unit of regular spectral transmittance through the NIST Transfer Spectrophotometer (TS), which is qualified against the HAS II National Spectrophotometer via control standard SRM 2031 at 280 nm. The Dcorr reference values reflect measurements conducted using the specific quartz cuvette (b = 0.05092 cm). In an effort to report a more universal reference value amenable to direct comparison with other spectrophotometric determinations (e.g. alternative path length and/or dilutions), Dcorr was further used along with Eq. 1 to calculate reference mass concentration values reported in Table S3 (see ESM).