Direct determination of ultra-trace levels of metals in fresh water using desolvating micronebulization and HR-ICP-MS: application to Lake Superior waters

Abstract

We report a method for the rapid ultra-trace (<1 ppb to < 1 ppt) quantification of P, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Mo, Cd, Re, Tl and Pb in fresh water. High sensitivity (>2 × 10⁶ cps ppb⁻¹ In) combined with a careful blank control provide unprecedented detection limits (e.g., Fe = 0.81 ppt and Pb = 0.014 ppt) that are approximately 10 fold better than previously published methods. Such low detection limits are required for the investigation of biogeochemical cycling of elements in some natural systems. One example is Lake Superior, where dissolved trace element and phosphorus concentrations in offshore waters are all less than 1 ppb, with some metals (such as Co, Re, Tl and Pb) reaching levels lower than 1 ppt. Depth profiles from Lake Superior exhibit the lowest concentrations of certain metals (Fe, Mn, Co, Zn, Re, Tl and Pb) ever measured in natural fresh waters. The precision of the method is better than ±20% (2σ) for all analytes of interest except Co (30%; 2σ) and accuracy determined from the analysis of certified reference material SLRS-4 indicates excellent (within certified 2σ) agreement for all elements.