Table 2. Comparison of RP-UPLC quantification of purified protein with elemental analysis for cobalt.

Sample	Reported color	Conc. (g/L)	ICP-MS B12 (mg/kg)1	RP-UPLC B12 (mg/kg)	ng B12/mg protein2
Fc-Fusion: Protein A Pool	Very pink	12.8	N/T3	44.53	3478.77
mAb1: Lot 1 DS	Pink	70	4.4	3.40	48.54
mAb1: Lot 2 DS	Non-pink	70	0.2	< LOQ4	< LOQ4
mAb2: Protein A Pool	Pink	12.6	0.7	0.70	55.82
mAb3: Protein A Pool	Pink	16.5	0.7	0.67	40.42
mAb3: Column 2 Pool	Non-pink	25	0.3	0.29	11.47
mAb3: DS	Peach	72.9	0.8	0.68	9.35

¹ For ease of comparison, elemental ICP-MS results were converted to B₁₂ by ratio of Co:CN-Cbl molar mass.² Assuming a molecular weight of 150 kD for mAbs, a conversion factor of 11 070 may be used to obtain B₁₂-Protein molar ratio.³ N/t, Not tested.⁴< LOQ, Peak below limit of quantification.