LEAD ISOTOPE RATIO MEASUREMENT OF NIST 610 GLASS BY LASER ABLATION INDUCTIVELY COUPLED PLASMA MASS-SPECTROMETRY

The isotopic composition of lead within National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 610 glass has been measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A Nd:YAG laser operated in the Q-switched mode with an energy output of approximately 2 mJ per shot was used to generate craters less than 40 microns in diameter and 60-80 microns deep. The total amount of ablated lead per crater was approximatety 9 x 10(-11) g. An ICP source coupled to a double focusing magnetic sector mass analyser equipped with seven Faraday detectors was used to measure the isotope ratios. Simultaneous measurement of Hg-202, Tl-203, Hg-204 + Pb-204, Tl-205, Pb-206, Pb-207 and Pb-208 isotopes was made. The ratio of lead atoms ablated to lead ions detected was approximately 450:1. Measurement of the Tl-205:Tl-203 ratio was used to determine the value of mass bias, and this value was then used to correct the measured lead isotope ratios. The Hg-204 interference with Pb-204 was corrected by measurement of the Hg-202 isotope. Each analytical period included the measurement of approximately 12 craters in the SRM 610 glass, and this was repeated a further five times. Each crater was ablated and measured for a 5 s period. Measured values were Pb-208:Pb-204 = 36.948 +/- 0.038, Pb-207:Pb-204 = 15.509 +/- 0.018, Pb-206:Pb-204 = 17.051 +/- 0.016, Pb-208:Pb-206 = 2.1670 +/- 0.0018 and Pb-207 : Pb-206 = 0.9096 +/- 0.0008. All errors are given as two standard deviations. The measured lead isotope ratios agree with those determined by thermal ionization mass spectrometry (TIMS). The high sensitivity of our measurement system and the potential of our methods for solid isotope ratio analysis of natural materials by laser ablation has been shown.