Environmental calibration standard

Metal-ion concentrations were quantified by using an Agilent 7900 inductively coupled plasma–mass spectrometer housed in the Center for Environmental Health Sciences Bioanalytical Core Facility at MIT. The instrument was operated in the helium mode. The instrument was calibrated before each analysis session using a series of five serially diluted (1:10) samples of the Environmental Calibration Standard (Agilent, part # 5183-4688) in 5% nitric acid and a 5% nitric acid–only standard. The concentrations of Mg, Ca, Mn, Fe, Co, Ni, Cu, and Zn were quantified, and terbium (1 ppb Tb; Agilent, part # 5190-8590) was used as an internal standard. Samples were prepared in 15-mL Falcon tubes, and 2-mL samples were transferred to ICP–MS polypropylene vials (Perkin Elmer, B3001566) and analyzed.

Acid HNO₃ and hydrogen peroxide H₂O₂ were purchased from Merck (Darmstadt, Germany) and Sigma-Aldrich (Buchs, Switzerland). Diluted HNO₃ (5%, v/v) was used to clean all glassware and plastic. Prior to use, glassware and plastic were rinsed with ultrapure water. For all dilutions, ultrapure water (18.2 MΩ/cm resistivity) was produced using the Direct-Q^® 5 UV System (Millipore Corporation Merck, Darmstadt, Germany).
Certified multielement standards composed of Al, As, Cd, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Pb, Se, and Zn with 99.99% purity for all elements (10 mg/L, Environmental Calibration Standard, Agilent Technologies, Santa Clara, CA, USA) were used for ICP-MS instrument calibration. A standard stock solution for ICP-MS analysis was prepared by dissolving the multielement standard mixture solution with ultrapure water. Further, working solutions were prepared by serial dilution of the stock solution with 5% (v/v) HNO₃ and kept at room temperature until further use.
The certified standard consisting of Bi, In, Sc, Y, and Tb (20 mg/L; Inorganic Ventures, Blacksburg, VA, USA) was used as the internal standard for ICP-MS correction of sensitivity drift and the matrix effect. Certified reference material DORM-4 fish protein (National Research Council Canada, Ottawa, Ontario, Canada) was used for the quality control of results.