EVALUATING THE IMPACT OF TRIPLE QUADRUPOLE LA-ICP-MS ACQUISITION METHODS ON ZIRCON U/PB ANALYSIS PRECISION

Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a valuable zircon uranium-lead (U/Pb) dating method due to rapid data collection and the small amount of sample that is destroyed. The recently developed triple quadrupole mass spectrometer design uses two quadrupoles to isolate specified isotopes, and between them is a reaction/collision cell which can remove interferences using reaction chemistry or ion collisions, at the cost of lower signal intensity. We investigated the effect of different instrument operating conditions on zircon U/Pb age precision. The precision values reported here are percent uncertainties for a weighted mean age using 40 spot analyses on zircon reference material 91500.

Higher ablation rates increase signal intensity, but require shorter ablation durations (thus, fewer measurement cycles) to maintain the same pit volume. To evaluate the precision tradeoff of these factors, we tested different combinations of ablation rate and duration, while keeping pit volume, beam energy densities, laser spot size and mass counting times constant. Low repetition rates (5 Hz) yield lower-precision U/Pb dates (± 0.4 %) compared to medium and high repetition rates (10 Hz, ± 0.3 % and 15 Hz, ± 0.3 %). Increasing repetition rates from 5 Hz to 15 Hz increases sensitivity by 59% (from ~38000 CPS to ~93000 CPS on ²³⁸U).

We also investigated the impacts of different reaction cell modes on signal intensity and measurement precision: single quadrupole mode (using one quadrupole as a mass filter), MS/MS mode (using both quadrupoles as mass filters), and MS/MS mode with helium (He) in the reaction cell). Changing from single quadrupole mode to MS/MS mode decreased signal intensity by 53 to 71 % depending on the isotope (42000 CPS to 16500 CPS for ²³⁸U, 390 CPS to 90 CPS on ²⁰⁷Pb, and 6000 CPS to 2800 on ²⁰⁶Pb). Adding helium to the reaction cell (1 ml/min He) does not significantly impact signal intensity (16500 CPS to 16700 CPS on ²³⁸U, 90 CPS to 140 CPS on ²⁰⁷Pb, and 2800 CPS to 2500 CPS on ²⁰⁶Pb). Uranium-lead dates collected in single quadrupole mode had lower uncertainties (± 0.4 %) compared to ratios collected in MS/MS mode (± 0.5 %) and MS/MS mode with He (± 0.5 %). Evaluation of the accuracy benefits of MS/MS interference removal is currently in progress.