A systematic evaluation of Flow Field Flow Fractionation and single-particle ICP-MS to obtain the size distribution of organo-mineral iron oxyhydroxide colloids.

•FlFFF distinguishes between Fe-OM complexes and organo-mineral FeOx colloids.•Size detection limit of sp-ICP-MS/MS compromises sizing of FeOx colloids.•Fe(II) oxidation with OM results in decreased colloid size with increasing OM/Fe ratio.•Small Fe-OM colloids prevail in peat soil and larger Fe-rich colloids in mineral soil.