Selective determination of Cr(Ⅵ) and non-chromatographic speciation analysis of inorganic chromium by chemical vapor generation-inductively coupled plasma mass spectrometry

A novel and sensitive method for the selective determination of Cr(VI) and non-chromatographic speciation of Cr(VI) and Cr(III) was developed based on chemical vapor generation (CVG) in KBH₄-acid system for sample introduction into an inductively coupled plasma mass spectrometer (ICP-MS) for detection. The CVG of Cr(VI), rather than Cr(III), was found to be remarkably enhanced in the presence of sodium diethylaminodithioformate (DDTC). After the oxidation of Cr(III) to Cr(VI) by KMnO₄, the quantitation of Cr(III) could be obtained based on the difference between the concentration of total chromium and that of Cr(VI). Parameters affecting the CVG reaction and determination of Cr(VI) were evaluated in detail, including the concentrations of DDTC, hydrochloric acid and KBH₄, the sample flow rate, as well as the length of reaction and transferring tubing. Under optimal conditions, the CVG efficiency and the limit of detection (LOD) of Cr(VI) were found to be 28% and 0.2 ng mL^-1, respectively. The relative standard deviations for seven replicate measurements of 20 ng mL^-1 of Cr(Ⅵ) was 1.8%. Furthermore, with excess DDTC (100 μg mL^-1) added to the test solutions, possible interferences from Cu²⁺ (up to 400 ng mL^-1) could be eliminated. The proposed method was thus successfully applied to the determination of Cr(VI) in three real water samples and one certified reference water sample, as well as two simulated water samples of Cr(VI) and Cr(III), all with satisfactory results. The possible reasons were discussed for the varied degrees of enhancement between Cr(III) and Cr(VI).

Colloidal mobilization from soil and transport of uranium in (sub)-surface waters

An analytical methodology was developed to characterize the colloidal distribution of trace elements of interest in environmental waters sampled in a same site and enables the different colloidal distributions from waters to be compared. The purpose was to provide consistent information related to the origin and nature of colloids responsible for the transport of trace element(s). The work was motivated by the observed enhanced mobility of uranium in soil. The colloidal size continuum was investigated by a multi-technique approach involving asymmetric flow field-flow fractionation (AF4) coupled with ultraviolet spectroscopy (UV), multi angle light scattering (MALS), and atomic mass spectrometry (ICPMS). To take into consideration the size and shape variability specific to each sample, the size distributions were established from the gyration radii measured from MALS, also considering the size information from standard nanospheres fractionated by AF4. A new parameter called "shape index" was proposed. It expresses the difference in hydrodynamic behavior between analytes and spherical particles taken as reference. Under AF4 diffusion conditions, it can be considered as an evaluator of the deviation from the sphericity of the fractionated analytes. AF4-UV-MALS-ICPMS enabled the dimensional and chemical characteristics of the colloidal size continuum to be obtained. As a "proof of concept", the developed methodology was applied at a field scale, in a reference study site. In order to have a "dynamic understanding", the investigation was based on the joint characterization of colloids from surface waters and soil leachates from static and dynamic processes. In the water samples of the study site, the continuum of gyration radius ranged from a few nanometers up to 200 nm. Colloids containing iron, aluminum, and organic carbon were involved in the uranium transport in the soil column and surface waters. The colloidal uranium concentration in the surface water increased from the upstream location (approximately 13 ng (U) L^-1) to the downstream location (approximately 60 ng (U) L^-1).