The magnetic stir bar as a source of impurities in laboratory blank solutions of the environmental samples subjected to microwave-assisted acid digestion

Teflon-coated magnetic stir bars are suspected to be a source of impurities for the laboratory blank solutions. To evaluate the ability of magnetic bars to absorb and release different elements at sub-ppb and above-ppb (µg L-1) level the experiment with new and used bars was conducted. The bars were subjected to microwave-assisted acid digestion with the soil sample and a series of successive cleaning procedures. The ICP-OES/MS analysis of the obtained extracts revealed that most of the elements may be released during subsequent extractions in the ppb-level concentrations, whereas the following elements: Cr, Cu, Sb, Sn, and Pb may be extracted even in elevated concentrations. SEM/EDS inspection of Teflon surfaces demonstrated the multiple defects that probably increase the absorption of the elements. We concluded that the concentration of mineral impurities in the laboratory blank solution prepared in the randomly selected one vessel per sample preparation batch is hardly propagated to the other used vessels. The alternative concept "blank per vessel" was proposed especially for ultra-trace analysis. Also, an additional step in the bar washing procedure was proposed.