Validation of a field filtration technique for characterization of suspended particulate matter from freshwater. Part I. Major elements

Abstract

A field filtration method for the concentration and separation of suspended particulate matter (SPM) from freshwater systems for subsequent determination of major elements (Si, Al, Ca, Fe, Mn, Mg, Na, K, P, Ti and S) is validated with respect to 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). Total concentrations of the elements have been determined in suspended particulate matter 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 10 and 14% RSD for most elements on a dry weight basis. 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 3 and 100 μg, as estimated from the blank filter samples. These detection limits are 3–1000 times higher compared to the corresponding instrumental (ICP–AES) limits of detection. 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 presumably the most important clogging regulating parameter in the group of elements included in the inorganic matter. A critical evaluation of the indirect method of calculating concentrations in SPM as the difference between unfiltered-digested and filtered subsamples is also included.