Multielement characterization of metal-humic substances complexation by size exclusion chromatography, asymmetrical flow field-flow fractionation, ultrafiltration and inductively coupled plasma-mass spectrometry detection: a comparative approach.

The use of three different separation techniques, ultrafiltration (UF), high performance size exclusion chromatography (HPSEC) and asymmetrical flow field-flow fractionation (AsFlFFF), for the characterization of a compost leachate is described. The possible interaction of about 30 elements with different size fractions of humic substances (HS) has been investigated coupling these separation techniques with UV-vis absorption spectrophotometry and inductively coupled plasma-mass spectrometry (ICP-MS) as detection techniques. The organic matter is constituted by a polydisperse mixture of humic substances ranging from low molecular weights (around 1kDa) to significantly larger entities. Elements can be classified into three main groups with regard to their interaction with HS. The first group is constituted by primarily the monovalent alkaline metal ions and anionic species like B, W, Mo, As existing as oxyanions all being not significantly associated to HS. The second group consists of elements that are at least partly associated to a smaller HS size fraction (such as Ni, Cu, Cr and Co). A third group of mainly tri- and tetravalent metal ions like Al, Fe, the lanthanides, Sn and Th are rather associated to larger-sized HS fractions. The three separation techniques provide a fairly consistent size classification for most of the metal ions, even though slight disagreements were observed. The number-average molecular weight (Mn), the weight-average molecular weight (Mw) and the polydispersity (rho) parameters have been calculated both from AsFlFFF and HPSEC experiments and compared for HS and some metal-HS species. Differences in values can be partly explained by an overloading effect observed in the AsFlFFF experiments induced by electrostatic repulsion effects in the low ionic strength, high pH carrier solution. Size information obtained from ultrafiltration is not as resolved as for the other methods. Molecular weight cut-offs (MWCO) of the individual filter membranes refer to globular proteins and molecular weight information may therefore, deviate from that given by the other methods after calibration with polystyrene sulfonate (PSS) standards.

Slurry sampling electrothermal atomic absorption spectrometry as screening method for chromium in compost.

Ultrasonic slurry sample introduction was applied to the determination of total chromium in composted materials by electrothermal atomic absorption spectrometry (ETAAS). The effect of grinding on the heterogeneity of the test samples and on the attainable precision was studied. The repeatability was influenced by the heterogeneity of the test samples at the mug-level, the R.S.D. of the measurements being 15%. The reproducibility depended on the heterogeneity of the test sample at the mg level, and it could be improved from 11 to 7% by increasing the grinding time. The characteristic mass was 2.6pg and the detection limit for the optimised procedure at the 0.04% (w/v) slurry concentration, 370ngg(-1). Good agreement with a certified reference material and with the conventional microwave assisted digestion method was found by using external calibration with aqueous standards. The performance of the method for screening purposes was evaluated.