Study of the size-based environmental availability of metals associated to natural organic matter by stable isotope exchange and quadrupole inductively coupled plasma mass spectrometry coupled to asymmetrical flow field flow fractionation

The determination of the isotopically exchangeable fraction of metals in environmental solid samples (soils, composts, sediments, sludges, etc.) is used to know the amount of metal potentially available (E-value). Stable isotopes can be used for determination of E-values through the analysis of the aqueous phases from spiked suspensions. However, the presence of isotopically non-exchangeable metal forms in the aqueous phase led to overestimation of the E-values. In this paper, a method for monitoring the degree of isotopic exchange in function of the molecular mass and/or size of the metal form has been developed based on the direct coupling of asymmetrical flow field flow fractionation (AsFlFFF) with inductively coupled plasma mass spectrometry (ICP-MS) for on-line isotope ratio measurements. ICP-MS data acquisition parameters were stressed to avoid degradation of isotope ratio precision. Two sets of fractionation conditions were selected: a colloids separation, which allowed the separation of substances up to 1 μm, and a macromolecules separation, designed to resolve small size substances up to 50 kDa. The methodology was applied to study the environmental availability of copper and lead in compost samples, where metals are mainly associated to different forms of organic matter. No significant differences on isotopic exchange were observed over the size range studied, validating the E-values determined by direct analysis of the aqueous phases.