Development of boron isotopic composition analysis method using SIMS and its application to geochemical cycle studies

我们尝试建立了一种利用二次离子质谱仪（SIMS）测定溶液样品硼同位素组成的方法，并将其应用于标准样品NIST 951和自然样品如海水、温泉水和大气样品的冷凝。将溶液样品在硅片上用红外灯烘干，进行纳米SIMS （NanoSIMS）分析。在样品干燥过程中，以前的研究建议添加无硼海水来抑制样品中硼的损失。然而，我们的研究表明，在NIST 951标准和温泉样品中添加无硼海水与未添加无硼海水相比，明显降低了硼同位素的光束强度。我们认为无硼海水加入法对NanoSIMS分析效果不佳。对于海水样品，我们尝试了带/不带Au（金）涂层的样品制备。未镀金样品的δ^<11> B值为+32 ~ +73，与报道的海水样品δ^<11> B值（+39.5）有较大差异。另一方面，用同样方法处理的NIST 951数据计算的Au涂层样品的δ^<11 b> B值为+44 ~ +46，与海水的报告值基本一致。虽然还需要进一步的研究，但金包覆法可能是利用NanoSIMS精确分析硼同位素组成的有效方法。温泉样品δ^<11> B值为+2.6和+5.6，在日本火山气体δ^<11> B值范围内。东京大学海洋研究所大气样品的凝结δ^<11> B值为-5.1、-1.8和+5.0，与同一地点样品的δ^<11> B值（+1.7）基本一致。这些事实表明，NanoSIMS可以用于自然溶液样品硼同位素的精确测定。

We tried to develop a method for determining boron isotopic composition of solution sample using secondary ion mass spectrometer (SIMS) and applied it to a standard sample, i. e., NIST 951 and such natural samples as seawater, hot spring waters and condensation of atmospheric sample.The solution samples were dried on silicon wafer by an infrared lamp for SIMS (NanoSIMS) analysis. During drying of the samples, previous studies have recommended an addition of boron-free seawater for suppressing loss of boron in the samples. However, our study indicates that the addition of boron-free seawater to NIST 951 standard and hot spring samples apparently decreases the beam intensity of boron isotopes compared with those without the addition of boron-free seawater. We think that the boron-free seawater addition method is not effective for NanoSIMS analysis.As for seawater samples, we tried with/without Au (gold) coating sample preparations. The samples without Au coating showed δ^<11> B values ranging +32〜+73, which were quite different form the reported value for seawater (+39.5). On the other hand, δ^<11> B values of sample with Au coating calculated from data of NIST 951 treated by same manner showed +44〜+46, which were nearly identical with the reported value of seawater. Although further studies are required, the Au coating method may be effective for accurate boron isotopic composition analysis using NanoSIMS.δ^<11> B values of the hot spring sample were +2.6 and +5.6, which were within the range of those of volcanic gases of Japan. The condensation of atmospheric sample collected at Ocean Research Institute, University of Tokyo showed δ^<11> B values of -5.1, -1.8 and +5.0, which is almost same as δ^<11> B value (+1.7) of sample collected at the same location.These facts suggest that NanoSIMS could be used for accurate boron isotopic determination of natural solution samples.