Characterisation and Correction of Instrumental Bias in Inductively Coupled Plasma Quadrupole Mass Spectrometry for Accurate Measurement of Lead Isotope Ratios

The accuracy and external precision of isotope ratio measurements can be limited by any of a number of instrumental bias factors which include mass bias, pulse pile-up, background effects and mass scale shift. Measured isotope ratios are generally biased from their true value by the interaction of several of these instrumental bias factors. Attainment of accurate isotope ratio measurement requires careful consideration of all possible causes of instrumental bias and adoption of methods for their elimination or correction. The various forms of instrumental bias observed in measurement of the Pb isotope ratios for NIST SRM 981 Natural Lead (Isotopic) were minimised to reveal 204 Pb: 206 Pb=0.0592±0.0002, 207 Pb: 206 Pb=0.9148±0.0007, 208 Pb: 206 Pb=2.1709±0.0015 ( n =5) compared with certified values of 0.059042±0.000037, 0.91464±0.00033 and 2.1681±0.0008, respectively. The thallium mass bias concentration procedure was employed in obtaining this data. All uncertainties given as 2 σ .