Accurate and precise determination of the boron isotope ratio by QQQ-ICP-MS: application to natural waters and carbonates

• Published in: Journal of analytical atomic spectrometry 2022-06, Vol.37 (6), p.1327-1339
• Main Authors: Chanakya, I. V. Satya, Misra, Sambuddha
• Format: Article
• Language: English
• Subjects: Blanks; Boron; Boron isotopes; Carbonates; Distillation; Isotope ratios; Isotopes; Seawater
• ISSN: 0267-9477; 1364-5544
• DOI: 10.1039/d2ja00051b

We present a new method for the accurate and precise (±0.4‰) determination of the boron isotope ratio by single collector QQQ-ICP-MS (Agilent™-8900). The key advantages of our δ 11 B determination method are pico-gram levels of boron blanks, rapidity of sample preparation and analyses, a low mass requirement of 1.25 ng per analysis, and a relatively high tolerance for ICP-MS matrix mismatch. We utilized a mixture of HF and HNO 3 as the ICP-MS matrix for rapid washout of boron and high sensitivity. The long-term instrumental accuracy and precision of δ 11 B determination are identical to the published results: AE-121 = 19.69 ± 0.26‰ (2 σ , n = 40); AE-120 = 20.18 0.23 (2 , n = 16); and AE-122 = 39.60 ± 0.36 (2 σ , n = 8). This is the first reported boron isotope determination technique based on QQQ-ICP-MS and our accuracy and precision are on par/better than published single collector methods. We also report an improved micro-distillation method, characterized by low procedural blanks (4 3 pg, n = 9) and quantitative boron recovery (98.7 5.5%), for boron purification from carbonate and seawater matrices. The average seawater boron isotopic composition ( 11 B SW ) of 39.63 ± 0.40‰ (2 σ , n = 51) determined on micro-distilled samples is analytically indistinguishable from published values. Additionally, we report identical δ 11 B SW values of 39.68 ± 0.40‰ (2 σ , n = 11) and 39.67 ± 0.42‰ (2 σ , n = 18) for the smallest (0.5 μl/2 ng-B) and the largest aliquots (30 μl/120 ng-B) of seawater samples analyzed. However, we report a systematic offset in δ 11 B SW between aliquots micro-distilled in the HCl matrix (35.43 ± 2.34‰, 2 σ , n = 17) and HNO 3 matrix (39.63 ± 0.40‰, 2 σ , n = 51). The long-term precision of δ 11 B determination of carbonate samples (δ 11 B Coral ), determined through repeat analyses of our in-house coral standard, is 24.44 ± 0.44‰ (2 σ , n = 83). The δ 11 B Coral of the smallest sample (∼2 mg coral/15-40 ng-B) analyzed (24.36 ± 0.55‰, 2 σ , n = 7) is identical to that of the largest mass (∼20 mg coral/100-150 ng-B) sample analyzed (24.49 ± 0.34‰, 2 σ , n = 19). Our external precision of ± 0.38‰ (2 σ ) for foraminiferal δ 11 B determination, based on seven replicates of Orbulina Universa samples (18.59 ± 0.38‰, 2 σ ) from ODP Site 664 in the Atlantic Ocean, would enable pH reconstruction at a resolution (ΔpH) of 0.035 units. In summary, this method can be utilized for applications requiring δ 11 B precision of ≥ 0.4‰ irrespective of boron m