Methods
Boron isotope methodology
Sample preparation:
Aqueous samples are loaded directly or preconcentrated via low-temperature, partial evaporation in oven in vertical laminar flow clean hood equipped with boron-free PTFE HEPA filtration.
Mass spectrometry:
1 to 4 µL samples (containing a total of ~1 to 5ng of boron) are directly loaded onto outgassed single rhenium filaments along with 2 µL of activator solution containing Na, Mg, Ca, and K (roughly in proportions of seawater), mixed from high-purity single-element standard solutions in 5% HCl matrix. Loads are evaporated to dryness at low current (~0.4A), typically taking 8 to 15 minutes depending sample volume. After drying, current is gradually raised to ~0.7A and gradually decreased to 0.0A over ~25 sec. All sample loading is carried out in a vertical laminar flow clean hood equipped with boron-free PTFE HEPA filtration. Data on standards (NIST951, OISL Atlantic seawater, and IAEA Groundwater B-3) loaded using this method are presented in (Figure 1) and yield external precision of approximately 0.5‰ δ11B. Total loading blank is <15pg B as determined by isotope dilution (NIST952). The load solution delivers ionization efficiency similar to seawater and has negligible CNO- (mass 42) interference, based on negligible signal at proxy mass 26 (CN-). Beam intensities (11BO2-) range from 0.3V to 3V.
Strontium isotope methodology
Sample preparation:
Liquid samples (waters or digested solids) are evaporatively preconcentrated in HEPA filtered clean hood and re-digested in 0.6mL of 3.5N HNO3 from which strontium is separated using Eichrom Sr-specific ion exchange resin.
Mass spectrometry:
Approximately 1 to 10µg Sr is loaded onto out-gassed single rhemium filaments along with TaO activator solution. Samples and standards are gradually heated to obtain a 88Sr beam intensity of ~3V, after which 300 cycles of data are collected, yielding a typical internal precision of ~ 0.000004 for 87Sr/86Sr ratios (1 sd). External reproducibility on standard NIST987 yields a mean 87Sr/86Sr ratio of 0.710233 ± 0.000009 (1 sd) since installation (Figure 2).