In-Depth Method Investigation for Determination of Boron in Silicate Samples Using an Improved Boron–Mannitol Complex Digestion Method by Inductively Coupled Plasma Mass Spectrometry

This study was carried out using an Agilent 7900 ICP-MS (Agilent, Santa Clara, CA, USA), which consisted of a concentric nebulizer, a Scott chamber with a temperature-controlled system, a standard quartz torch, an assemblage of Ni sample/skimmer cones (1.0/0.45 mm), a quadrupole mass analyzer, and a peristaltic pump. To enhance signal sensitivity, this ICP-MS equipped with a Pt shielding plate and a silicon shielding cap.

The ICP-MS instrument worked under standard instrumental settings (1550 W of forward power, 15.0 L/min of plasma gas, 1.0 L/min of auxiliary gas, 1.05 L/min of nebulizer gas, and 10 mm of sampling depth) and was optimized daily to obtain the highest possible sensitivities for low to high mass isotopes. After the system had been subjected to warm-up for at least one hour, tunings for torch axis, EM, lens, and mass resolution/axis were accomplished using a tuning solution that contained 1.0 ng/mL of Li, Y, Ce, and U. Here, oxide formation (CeO⁺/Ce⁺) and doubly charged species (Ce²⁺/Ce⁺) were well controlled lower than 2.0%. Thereafter, the P/A factor for pulse and analog modes of the detector was calibrated using a 50 ng/mL of multi-element solution. Before quantification, a solution of digested silicate sample was introduced to flush the whole system for at least 30 min. During the measurement, a standard solution as the drift monitor was repeatedly quantified every five unknown samples. To minimize the memory effect on quantification accuracy, the system had been continuously washed in 2% HNO₃ (v/v) solution, with a signal recovery of 25 ng/mL of the internal standard Rh checked. Boron determination was done using no gas mode of this ICP-MS. The data were read under peak jumping mode with the detector set as the dual mode and isotope dwell time fixed at 0.3 s.

In this work, ultrapure acids were used to reduce the procedure blanks. The commercially available acids including HNO₃ (68% v/v, AR grade), HF (40% v/v, AR grade), and HCl (36% v/v, AR grade) were purified twice using sub-boiling distillation in Teflon stills (Savillex DST-1000-PFA, Eden Prairie, MN, USA). All solutions were prepared using ultrapure water with a resistivity of 18.2 MΩ·cm, which was produced by passing deionized water through a Milli-Q water purification system (Millipore, Bedford, MA, USA).

The solutions used in this work were prepared by the gravimetric method. The 2% mannitol (wt.) solution was prepared by dissolving 1.0 g of mannitol (AR grade) in ultrapure water with a final solution weight of 50 g. The 8% HNO₃ (wt.) solution and 6% HCl (wt.) solution were obtained by diluting 0.4 mL of concentrated HNO₃ and 0.3 mL of concentrated HCl with ultrapure water to 50 g, respectively. Boron solutions with concentrations of 5, 15, 30, 50, and 100 ng/mL of boron) in 2% HNO₃ (v/v), which were used as the external calibrators, were prepared progressively from a boron single-element standard solution of 1.0 mg/mL purchased from the National Institute of Standards and Technology, China. A solution of 2% HNO₃ (v/v) without adding boron standard solution was utilized as the blank external calibrator. Here, to exclude any possible assay bias from long-term storage, all standard solutions were prepared freshly.

Three silicate reference materials from basic to acid compositions, including diabase W-2, basalt JB-2a, and rhyolite JR-2, were selected in digestion method and long-term stability study of boron quantification in this work. The W-2 is a mafic geochemical reference material from the Geological Survey of U.S. (Reston, VA, USA), JB-2a and JR-2 are silicate standard materials from the Geological Survey of Japan (Tsukuba, Ibaraki Prefecture, Japan).

In this work, the evaporation and heating processes were finished in a boron-free ULPA filtration hood in a hundred clean room. The utilized PFA beakers were immersed in aqua regia (HNO₃-HCl, 3:1, v/v) and ultrapure water sequentially, with each step performed at 120 °C for 24 h. Prior to usage, the labware was carefully rinsed three times with ultrapure water. The silicate samples were digested using a boron–mannitol complex strategy. Samples with a quantity of 25–50 mg (±0.5 mg) were weighed in 15 mL of PFA beakers, then 0.6 mL of concentrated HF, 30 μL of concentrated HNO₃ and 0.5 mL of 2% mannitol were added into the samples gently. After the ternary reagents had mixed with samples, the beakers were capped tightly, and the samples were digested according to the following procedures (see Table 1). (1) The beakers were placed in an ultrasonic bath for pretreatment 4 h (M-method 1 to M-method 3). (2) The beakers were then transferred to the hotplate and heated overnight at 65 (M-method 1 and M-method 4), 100 (M-method 2), or 140 °C (M-method 3). (3) With samples evaporated to incipient dryness, 0.6 mL of 8% HNO₃ was added, and samples were continuedly fluxed overnight. (4) Thereafter, the samples were evaporated to incipient dryness again, and 0.6 mL of 6% HCl was added with samples fluxed overnight. (5) When becoming incipiently dry, the samples were fortified with 2.0 mL of 40% HNO₃ (v/v) and fluxed 4 h. (6) After aging overnight, the solutions were transferred to PET bottles and then gravimetrically diluted to 50 ± 0.5 g using 2% HNO₃ (v/v) solution. Finally, the sample solutions were taken for boron quantification by ICP-MS directly.

The samples were also digested using a laboratory high-pressure closed acid method developed by Tan and Wang [25] with small modifications. In brief, 1.0 mL of HF and 0.5 mL of HNO₃ were added in Teflon bombs with 50 ± 0.5 mg of samples. The samples were evaporated to incipient dryness, which was called hotplate pressure relief, at 140 (H-method 1), 100 (H-method 2), or 60 °C (H-method 3). Thereafter, 0.5 mL of HF and 1.0 mL of HNO₃ were added into the samples, and the bombs were sealed and transferred into an oven at 185 °C for 12 h. After cooling, the samples were evaporated to incipient dryness at 140 (H-method 1), 100 (H-method 2), or 60 °C (H-method 3). Then, the samples were fortified with 1.0 mL of HNO₃ and again evaporated to incipient dryness. With 2.0 mL of 40% HNO₃ (v/v) added, the residues were re-dissolved at 135 °C for 6 h with bombs in metal jackets and then aged overnight. The final solutions were transferred to PET bottles, and then gravimetrically diluted to 50 ± 0.5 g using 2% HNO₃ (v/v) solution. Samples were digested using H-method 4, which was similar to H-method 1 but skipped the step of hotplate pressure relief.