An isotopic perspective on mass bias and matrix effects in multi-collector inductively-coupled-plasma mass spectrometry

Abstract

Lead and Tl isotope intensities and ratios were measured in radial and axial plasma profiles using multi-collector inductively-coupled-plasma mass spectrometry (MC-ICP-MS). Signal profiles for ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb display mass dependent radial and axial distributions in the plasma. In radial profiles lighter isotopes exhibit greater dispersal from the plasma axis than heavier isotopes, in agreement with ICP-MS elemental observations. However, signal maxima (I_max) for heavier Pb isotopes occur closer to the load coil than I_max for lighter Pb isotopes, the reverse of ICP-MS elemental observations. These mass dependent distributions predict that the radial dispersion and axial I_max locations for ²⁰³Tl and ²⁰⁵Tl isotopes should occur interspersed with the values for simultaneously measured Pb isotopes. Instead however, their values are appropriate for masses greater that ²⁰⁸Pb, indicating that the observed profiles are not the product of a single stage mass dependent process and that other element properties also effect the distributions. Isotope ratio (and thus mass bias) variation with sampling depth reflects both the mass dependent separation of the numerator and denominator isotope I_max along the axis of the plasma and the distance along the axis of the plasma over which the isotope signals are distributed. Addition of matrix to the plasma significantly reduces the variability of these two characteristics suggesting that more accurate correction of instrumental mass bias and more accurate normalization of sample ratios to standard values could be achieved by addition of a common matrix to samples and standards.