Determination of ultra-low 236U/238U isotope ratios by tandem quadrupole ICP-MS/MS

Abstract

Isotope ratios of ²³⁶U/²³⁸U were measured at levels below 10⁻⁷ by single collector ICPMS with a tandem quadrupole mass separation mechanism. Peak tailing of the prominent ²³⁸U⁺ ion beam on the ²³⁶U⁺ peak was reduced to the level of ∼10⁻¹⁰ by use of two quadrupole mass filters. The ²³⁵UH⁺ interference on ²³⁶U⁺ was efficiently reduced to a UH⁺/U⁺ ratio of 1 × 10⁻⁸ by an ion–molecule reaction between UH⁺ and O₂ in a collision/reaction cell placed between the two quadrupoles. The resultant detection limit for ²³⁶U/²³⁸U measurement was better than those reported by any other ICPMS study. The ²³⁶U/²³⁸U ratios, measured as ²³⁶U¹⁶O⁺/²³⁸U¹⁶O⁺, were determined in the range 10⁻⁹ to 10⁻⁷ without correction for spectral interference. Accurate measurements of ²³⁶U/²³⁸U to as low as 1 × 10⁻¹⁰ are projected.