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

建立了用二次离子质谱仪(SIMS)测定溶液样品中硼同位素组成的方法，并将其应用于标准样品NIST 951以及海水、温泉水和大气冷凝样品等天然样品。溶液样品在硅片上用红外灯干燥后进行SIMS分析。在样品干燥过程中，以前的研究建议添加无硼海水，以抑制样品中的硼损失。然而，我们的研究表明，在NIST 951标准样品和温泉样品中添加无硼海水后，与不添加无硼海水相比，硼同位素的束流强度明显降低。我们认为无硼海水添加方法不适用于纳米SIMS分析。对于海水样品，我们尝试了有/没有金(金)涂层的样品制备。没有镀金的样品的δ^&lt；11&gt；B值在32~73之间，与海水的报道值(39.5%)相差很大。而用同样方法处理的NIST951数据计算的镀金样品的δ^&lt；11&gt；B值为44~46，与海水的报道值基本一致。虽然还需要进一步的研究，但镀金方法可能是利用纳米SIMS准确分析硼同位素组成的有效方法，温泉样品的δ^&lt；11&gt；B值分别为2.6和5.6，处于日本火山气体的范围内。在东京大学海洋研究所采集的大气样品的冷凝结果显示，δ^&lt；11&gt；B值分别为-5.1、-1.8和5.0，与同一地点采集的样品的δ^&lt；11&gt；B值(1.7)基本一致。这些事实表明，纳米SIMS可以用于天然溶液样品的准确硼同位素测定。

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.