Characterization and Implementation of a Continuous Flow Trace Metal Pre-Concentration System for Inductively Coupled Plasma Atomic Emission Spectrometry

Robert N. Sibley, Colby College


A continuous flow pre-concentration system for analysis of trace metals in natural waters was characterized and used to determine the copper(II) and lead(II) concentrations in seawater from Penobscot Bay, Maine. Pre-concentrated samples were analyzed by inductively coupled plasma atomic emission spectrometry (ICP-AES). System characterization involved elucidation of the thermodynamic complexation constants, KCu(II) and Kpb(II), for Cu(II) and Pb(II) with the pre-concentration column chromatographic material. This was done in aqueous media of known pH and ionic strength. The column contained 8-hydroxyquinoline immobilized on Toyopearl HW-75 fractogel chromatographic resin. The buffer system used for metal binding experiments was an equimolar mixture of acetic acid (HOAc) and tris(hydroxymethyl)aminomethane (TRIS). This combination ensured maximum buffer capacity in the pH region in which the column was being characterized. Scatchard plots were generated to determine KCu(lI) and Kpb(II). These were 9.17 and 9.68, respectively, at 25.0° C and at ionic strengths between 0.01 and 0.001 M. A computer based interactive model for metal speciation in aqueous electrolyte solutions was developed and employed for these determinations. Incorporating the new thermodynamic complexation constants into the speciation model allowed prediction of the optimal pH for preconcentration of Cu(II) and PbCII) in seawater. Two pK values for aqueous PbC03 in seawater are available in the literature. If carbonate (6e-4 M) is treated as the predominant ligand of Cu(II) and Pb(II) in seawater, then predicted metal recoveries for the system at pH 7.553 (about 71 % for Cu and 47% for Pb) are consistent with the observed values (70.5% for Cu and 48.5% for Pb). If Cl-(0.54 M) is considered to be the most important ligand of Pb(m in seawater, then the predicted recovery of Pb from the system is 100%. In Penobscot Bay, Cu(Il) concentrations were found to be between 30 and 60 nM near the surface and sediment. and about 10 nM at mid depth. Pb(II) concentrations were found to decrease from about 32 nM at the surface to less than 5 nM near the sediment. The detection limit for Cu and Pb by this method is in the low nanomolar to high picomolar range.