Isotope-ratio measurements of lead in NIST standard reference materials bymultiple-collector inductively coupled plasma mass spectrometry

The capability of a second-generation Nu Instruments multiple collector ind uctively coupled plasma mass spectrometer (MC-ICP-MS) has been evaluated fo r precise and accurate isotope-ratio determinations of lead. Essentially th e mass spectrometer is a double-focusing instrument of Nier-Johnson analyze r geometry equipped with a newly designed variable-dispersion ion optical d evice, enabling the measured ion beams to be focused into a fixed array of Faraday collectors and an ion-counting assembly. NIST SRM Pb 981, 982, and 983 isotopic standards were used. Addition of thallium to the lead standard s and subsequent simultaneous measurement of the thallium and lead isotopes enabled correction for mass discrimination, by use of the exponential corr ection law and Tl-205/Tl-203 = 2.3875. Six measurements of SRM Pb-982 furni shed the results Pb-206/Pb-204 = 36.7326(68), Pb-207/Pb-204 17.1543(30), Pb -208/Pb-204 36.7249(69), Pb-207/Pb-206 = 0.46700(1), and Pb-208/Pb-206 0.99 979(2); the NIST-certified values were 36.738(37), 17.159(25), 36.744(50), 0.46707(20), and 1.00016(36), respectively. Direct isotope lead analysis in silicates can be performed without any chemical separation. NIST SRM 610 g lass was dissolved and introduced into the MC-ICP-MS by means of a micro co ncentric nebulizer. The ratios observed were in excellent agreement with pr eviously reported data obtained by TIMS and laser ablation MC-ICP-MS, despi te the high Ca/Pb concentration ratio (200/1) and the presence of many othe r elements at levels comparable with that of lead. Approximately 0.2 mug le ad are sufficient for isotope analysis with ratio uncertainties between 240 and 530 ppm.