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

建立了二次离子质谱仪（西姆斯）测定溶液样品硼同位素组成的方法。例如，NIST 951标准以及海水、温泉沃茨、大气冷凝水等天然样品，在硅片上用红外灯干燥后进行二次离子质谱（NanoSIMS）分析。在样品干燥过程中，先前的研究建议添加无硼海水以抑制样品中硼的损失。然而，我们的研究表明，添加到NIST 951标准和温泉样品的无硼海水明显降低了硼同位素的束流强度相比，没有添加无硼海水。我们认为，无硼海水添加方法对NanoSIMS分析无效。至于海水样品，我们尝试了有/无Au（金）涂层样品制备。没有Au涂层的样品显示δ^<11>B值为+32 +73，这与海水的报道值（+39.5）有很大差异。用<11>同样方法处理NIST 951数据，计算出Au涂层样品的δ^ B值为+44 ~+46，与海水的报道值基本一致。虽然需要进一步的研究，Au涂层的方法可能是有效的，准确的硼同位素组成分析使用NanoSIMS。<11>温泉样品的δ^ B值为+2.6和+5.6，这是在日本的火山气体的范围内。在东京大学海洋研究所采集的大气样品中，冷凝态的δ^<11>B值分别为-5.1、-1.8和+5.0，与<11>同一地点采集的样品中的δ^ B值（+1.7）基本一致，表明NanoSIMS可用于天然溶液样品中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.