NISTmAb charge heterogeneity was evaluated by the qualified CZE assay , wherein mAb charge variants are separated according to differential electrophoretic mobility in free solution within a uniform electric field applied across a buffer-filled fused silica capillary. The results of the CZE analysis of RM 8671 were consistent in all salient features to those observed previously with PS 8670, having three charge groups: the main group, which comprises the majority of the sample; the basic variants, which migrate toward the cathode more rapidly than the main group; and the acidic variants, which migrate toward the cathode less rapidly than the main charge group . The charge purity of the NISTmAb is given as the relative abundance of the main charge group with respect to all detected charge species. The results for RM 8671 compared to PS 8670 method qualification results are presented in Table S6 (see ESM) and Fig. 3.Fig. 3Homogeneity of RM 8671 lots by charge purity measured by CZE. Error bars represent ±3uc based on ANOVA analysis for each lot (n = 3 for RM 8671 lots) or from intermediate precision data for PS 8670 Qualification 