Iodixanol impurity C – 5-[acetyl[3-[[3,5-bis[(2,3-dihydroxypropyl)carbamoyl]-2,4,6-triiodophenyl]amino]-2-hydroxypropyl]amino]-N,N’-bis(2,3-dihydroxypropyl)-2,4,6-triiodobenzene-1,3-dicarboxamide