Development of laser ablation multi-collector inductively coupled plasma mass spectrometry for boron isotopic measurement in marine biocarbonates: new improvements and application to a modern <i>Porites</i> coral

RATIONALE: Laser Ablation coupled to Multi-Collector Inductively Coupled Plasma Mass Spectrometry (LA-MCICPMS)
is a powerful tool for the high-precision measurement of the isotopic ratios of many elements in geological
samples, with the isotope ratio (11B/10B) of boron being used as an indicator of the pH of oceanic waters. Most geological
samples or standards are polished and ablation occurs on flat surfaces. However, the shape and the irregularities of
marine biocarbonates (e.g., corals, foraminifera) can make precise isotopic measurements of boron difficult. Even after
polishing, the porosity properties and the presence of holes or micro-fractures affect the signal and the isotopic ratio
when ablating the material, especially in raster mode.
METHODS: The effect of porosity and of the crater itself on the 11B signal and the isotopic ratio acquired by LA-MCICPMS
in both raster and spot mode was studied. Characterization of the craters was then performed with an optical
profilometer to determine their shapes and depths. Surface state effects were examined by analyzing the isotopic
fractionation of boron in silicate (NIST-SRM 612 and 610 standards) and in carbonate (corals).
RESULTS: Surface irregularities led to a considerable loss of signal when the crater depth exceeded 20 μm. The stability
and precision were degraded when ablation occurred in a deep cavity. The effect of laser focusing and of blank correction
was also highlighted and our observations indicate that the accuracy of the boron isotopic ratio does not depend on the
shape of the surface. After validation of the analytical protocol for boron isotopes, a raster application on a Porites coral,
which grew for 18 months in an aquarium after field sampling, was carried out.
CONCLUSIONS: This original LA-MC-ICPMS study revealed a well-marked boron isotope ratio temporal variability,
probably related to growth rate and density changes, irrespective of the pH of the surrounding seawater.