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Oxide interference can be problematic for trace element and isotopic analyses using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Yet, the oxide production mechanism remains poorly understood. Here, we determined the oxide production rates (MO+/M+ ratios) of 15 rare earth elements (REEs), Th, and U by LA-ICP-MS analyses of synthetic and natural phosphates as well as NIST SRM 610 glass and natural zircon. We found two contrasting oxide production behaviors against the Ar sample gas flow rate, depending on the MO+ dissociation energy (D0(MO+)): the MO+/M+ ratios of elements with D0(MO+) ≥7 eV increased with the gas flow rate from 0.85 to 1.00 L min−1, whereas those with D0(MO+)

We present an improved U–Pb age dating method for zircon and monazite crystals using 193 nm excimer laser ablation and 200/266 nm femtosecond laser ablation (200/266FsLA) multiple-Faraday collector inductively coupled plasma-mass spectrometry (MFC-ICP-MS). Optimization of a 266 fs laser beam enabled an analysis of 207Pb/206Pb and 206Pb/238U ratios with an in-run precision of 1–2% from a crater of dimensions 50 μm × 10 μm (diameter × depth) at a repetition rate of 2 Hz for 30 s. The same in-run precision was obtained from a 30 μm × 20 μm crater using a 200 fs laser beam of 20 μm in diameter rastered along the circumference of a circle with a 7 μm radius at 25 Hz for 15 s. With an enhanced sensitivity ion interface, the sensitivity for the total amount of Pb was ∼2 mV ppm−1 or ∼125 000 cps ppm−1 using the above crater setup. The use of high gain amplifiers equipped with a 1012 Ω register enabled the determination of the U–Pb age of zircon and monazite crystals with an internal and intermediate precision comparable to that obtained from sensitive high resolution ion microprobe (SHRMP) techniques. We analysed standard zircon crystals using a 91500 zircon crystal (1065.4 ± 0.6 Ma determined by isotope dilution thermal ionization mass spectrometry (ID-TIMS)) as a bracketing standard. Ages determined from TEMORA2, Presovice, and OD-3 zircons compared very well with their reference ages determined by ID-TIMS and/or SHRIMP. Thompson Mine and Monangotory standard monazites, dated using a 44069 monazite crystal (424.9 ± 0.4 by ID-TIMS) as a standard, also reproduced the U–Pb ages determined by ID-TIMS/LA-MFC-ICP-MS, but at a sample volume four times smaller than that required for zircons. Zircon and monazite ages are accurate given the small offsets from ID-TIMS ages, 0.15–0.7% for zircons and 0.2–0.7% for monazite well within internal precision from the primary standard in the analytical session and competitive with an internal precision of 0.43–0.6% for zircon and 0.2–0.8% for monazite. More easily obtaining high resolution age data is useful for the precise determination of the U–Pb age.

In this study we have investigated the suitability of silicagels from four companies (Merck, Sigma-Aldrich, Nissan Chemical and Alfa Aesar) as emission activators for ionisation of lead in thermal ionisation mass spectrometry (TIMS). We have tested U and Pb blank levels, ionisation efficiency, signal stability, degree of fractionation, and reproducibility and accuracy of the Pb isotopic ratios measured with these emitters. Further tests were undertaken to evaluate the dependency of ionisation efficiency on the particle size and concentration of silicagels and the mixing ratio of silicagel and phosphoric acid. Mass fractionation, reproducibility and stability of the ion beam were monitored using 300 pg of the lead isotopic standard SRM-981. The Merck and Sigma-Aldrich silicagels have the best performance as Pb ion emitters. They yielded accurate Pb isotopic ratios with precisions of ∼0.05% (2SD) for 207Pb/206Pb. We find that the ionisation efficiency does not depend on the particle size, but on silicagel concentration and proportion of silicagel to phosphoric acid. The highest ionisation efficiency (9.2 ± 2.2%, 2SE) was obtained using 0.004 ml of a silicagel from Sigma-Aldrich at a concentration of 0.4 wt% of SiO2 loaded with 0.05 ml of 0.02 N phosphoric acid. These results clearly indicate that the Sigma-Aldrich emitter is an excellent alternative for Pb isotopic measurements to the widely used, but no longer produced Merck silicagel. The silicagels from Alfa Aesar and Nissan Chemical have a high U background concentration compared to the Merck and Sigma-Aldrich gels, and produce less accurate Pb isotopic ratios.