A Review on the Development of Boron Isotope Analytical Techniques

Boron (B) is a light and fluid-mobile element. It has two stable isotopes: ¹⁰B and ¹¹B. The two isotopes fractionate significantly in nature due their relatively large mass difference. Therefore, B isotopes are one of the non-traditional stable isotope tracers, which have been used in the research areas of chemistry, environmental, bioscience, earth and planetary sciences. In the last twenty years, the analytical methods of B isotopes have been continuously improved and many important advances have been made. However, there are still some challenges to obtaining high-quality B isotope data. The techniques of B isotope analysis are quite different among laboratories, which arise principally from three stages: sample digestion, purification, mass spectrometry.

Because B is volatile and isotopic fractionation may be induced by different coordination in different pH environments, sample digestion, and purification have a great impact on the high-precision measurement of B isotopes. Four digestion methods have been applied for extracting B from samples, including pyro-hydrolysis, acid dissolution, alkali fusion, and ashing. Pyro-hydrolysis requiring large volumes of water is time-consuming. Acid dissolution is one of the most popular techniques due to the small volumes of reagents needed and hence lower levels of contamination. Samples are dissolved with different acids such as hydrochloric, nitric, hydrofluoric, and perchloric. Painstaking attention is required with hydrofluoric acid since BF₃ is highly volatile and easily lost in nature. Suitable amounts of mannitol are added during acid dissolution to form a stable boron-mannitol complex to prevent the loss of B and avoid B isotope fractionation.

Alternatively, alkali fusion is a dissolution method for solid rock samples. High purity fluxing agent is needed, such as K₂CO₃, Na₂CO₃, NaOH, NaOH, and Na₂O₂. As all the B would be present as borate in the resulting alkaline solution, alkali fusion eliminates the risk of B isotope fractionation due to evaporation. The advantage of this method is that it is rapid and relatively large numbers of samples can be processed. The ashing is mainly used to digest plant samples. Ashing was chosen for plant sample decomposition because ashing removes the organics and avoids the use of reagents carrying a B blank or generating isobaric interferences.

Once a sample is dissolved, it is necessary to purify B before analysis. There are two principal methods currently in use, which are ion exchange and microsublimation. The ion exchange techniques can be divided into those involved in using B-specific resin Amberlite IRA 743 and those using cation (AG50W-X8/AG50W-X12) or anion (Bio-Rad AG MP-1) cation exchange resins. Microsublimation is an effective and simple method to purify B. It is used to purify B from organic-enriched solutions. Microsublimation appears advantageous in terms of matrix removal efficiency and low procedural blank, however the technical challenges involved are also great.

There are two main types of B isotope analytical methods: in-situ and solution methods. Solution methods analyse B ratios using thermal ionization mass spectrometry (TIMS) method or multiple collector inductively coupled plasma-mass spectrometry (MC-ICP-MS). The in-situ method, uses secondary ion mass spectrometry (SIMS) method or laser ablation multiple collector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS) to measure samples with high B concentration. The accurate and precise determination of the B isotope composition is still a difficult task. For solution methods, the difficulty arises principally from the near ubiquitous level of B contamination in most standard clean laboratories, the light mass of the element, the occurrence of only two stable isotopes, and the large mass difference between them. For in-situ approaches, the difficulty arises principally from a lack of reference materials, surface contamination, limited precision in low-concentration samples, and limitations in reproducibility in high-concentration samples. On the whole, MC-ICP-MS is the dominant method for B isotopic analysis, which is still has the challenges of matrix effect, memory effect, and mass bias.

The relevant techniques inherent to the three stages of B isotope analysis are summarized and the advantages and disadvantages of the different techniques are discussed. The aim of the work contained in this paper is to further promote the progress and development of domestic and foreign scholars in the research of B isotope geochemistry.