Application of 10¹³ ohm Faraday cup current amplifiers for boron isotopic analyses by solution mode and laser ablation multicollector inductively coupled plasma mass spectrometry

Rationale: Boron isotope ratios (δ¹¹B values) are used as a proxy for seawater paleo-pH, amongst several other applications. The analytical precision can be limited by the detection of low intensity ion beams from limited sample amounts. High-gain amplifiers offer improvements in signal/noise ratio and can be used to increase measurement precision and reduce sample amounts. Methods: 10¹³ ohm amplifier technology has previously been applied to several radiogenic systems, but has thus far not been applied to non-traditional stable isotopes. Here we apply 10¹³ ohm amplifier technology for the measurement of boron isotope ratios using solution mode MC-ICP-MS and laser ablation mode (LA-)MC-ICP-MS techniques. Precision is shown for reference materials as well as for low-volume foraminifera samples. Results: The baseline uncertainty for a 0.1 pA ¹⁰B⁺ ion beam is reduced to <0.1 ‰ for a typical measurement period. The external precision is better than 0.2 ‰ (2SD) for δ¹¹B measurements for solution samples containing as little as 0.8 ng total boron. For in situ microanalyses with LA-MC-ICP-MS, the external precision of ¹¹B/¹⁰B from an in-house calcite standard was 1 ‰ (2SD) for individual spot analyses, and 0.3 ‰ for the mean of ≥10 replicate spot analyses. Conclusions: 10¹³ ohm amplifier technology is demonstrated to offer advantages for the determination of δ¹¹B values by both MC-ICP-MS and LA-MC-ICP-MS for small samples of biogenic carbonates, such as foraminifera shells. 10¹³ ohm amplifier technology will also be of benefit to other non-traditional stable isotope measurements.