Development of sedimentation field-flow fractionation-inductively coupled plasma mass-spectrometry for the characterization of environmental colloids

Abstract

A relatively new hyphenated technique for the simultaneous size separation and elemental analysis of colloids has been further developed and applied to the characterization of soil colloids. Sedimentation field-flow fractionation (SdFFF) was directly interfaced to an inductively coupled plasma-mass spectrometer (ICP-MS) to provide high-resolution sizing and elemental analysis of colloids in the range 0.05–1.0 μm. For this work our existing SdFFF instrument was modified by addition of an upgraded motor and software for centrifuge speed control and data collection. Analytical techniques were developed for the calibration and drift correction of the ICP-MS data collected during on-line SdFFF-ICP-MS analyses. Software was developed to allow off-line computation of drift-corrected, elemental concentrations across the colloid size range. SdFFF-ICP-MS examination of two colloid samples isolated from surface soil horizons showed significant enrichment in iron-containing phases in both the smaller and larger colloids relative to intermediate particle sizes (≈0.3 μm). Element ratios obtained by SdFFF-ICP-MS indicated that the relative amounts of kaolinite, vermiculite, and illite vary across the colloid size range, with kaolinite less concentrated in the larger sizes (>0.3 μm). These results demonstrate the utility of SdFFF-ICP-MS for examination of soil chemistry and mineralogy and suggests the technique will have application to other environmental and geochemical studies.