Characterization of suspended particulate matter from freshwater : validation of a field filtration technique

Sammanfattning: A field filtration method for the concentration and separation of suspended particulate matter (SPM) from freshwater systems and the subsequent determination of major, minor, trace and ultra trace elements (Al, As, Ba, Be, Ca, Cd, Co, Cr, Cs, Cu, Fe, Ga, Hf, K, Mg, Mn, Mo, Na, Nb, Ni, P, Pb, Rb, S, Sb, Sc, Si, Sn, Sr, Ta, Th, Ti, Tl, U, V, W, Zn and Zr) is validated with respect to detection limits, precision, and bias. The validation comprises the whole procedure including filtration, sample digestion and instrumental analysis. The method includes two digestion procedures (microwave acid digestion and alkali fusion) in combination with inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma quadrupole mass spectrometry (ICP-QMS). Total concentrations of these 38 trace and major elements have been determined in suspended particulate matter (SPM) from lake and river water with low levels of suspended solids (<2 mg l-1 DW), and a wide range of element concentrations. The precision of the method including filtration, digestion and instrumental determination ranges between 8 and 18% RSD for most elements on a dry weight basis. Higher recovery after acid digestion is found for some elements, probably because of volatilization or retention losses in the fusion procedure. Other elements show higher recovery after fusion, which is explained by more efficient decomposition of mineral phases relative to the non-total acid digestion. Non-detectable concentrations of some elements are reported due to small differences between blank filter levels and the amounts of elements present on the filters after sampling. The calculated sums of main inorganic components, expressed as oxides, ranges between 94.0 and 98.0% ash weight. The method limits of detection range between 0.7 ng and 100 µg, as estimated from the blank filter samples. These detection limits are 3-1000 times higher compared to the corresponding instrumental limits of detection. The accuracy and bias of the overall analytical procedure was assessed from replicate analysis of certified reference materials. A better knowledge of the mechanisms of filter clogging in sampling of suspended matter is important in order to extend the applicability of the method. For the sample types investigated in this study, the amount of inorganic material in the suspended particulate matter (SPM) seems to be the most important factor controlling the maximum volume of filterable water, and Fe is presumable the most important clogging regulating parameter in the group of elements included in the inorganic matter. A critical evaluation of the instrumental capabilities of the ICP- QMS instrumentation in comparison with the sensitive double focusing sector field plasma mass spectrometry technique (ICP-SFMS) is also included. It was found that a modified microwave acid digestion procedure in combination with ICP-SFMS could replace ICP-AES determinations and fusion digestions for most of the investigated elements. Guidelines and limitations for this powerful and relatively simple and less time consuming procedure, covering most elements in one single determination, are discussed.