Vol.9 No.2 2016
Research paper : Development and utilization of geochemical reference materials (T. OKAI)−65−Synthesiology - English edition Vol.9 No.2 (2016) stage and reached its mature stage leading to changes in the sample selection, standard value determination methods, and distribution methods.3.3.2 Progress in analytical methodsFigure 4 shows the changes in the analytical methods that were used in the chemical analysis of geological materials. The initial instrumental analyses required large instruments, had high cost, and the analytical accuracy was insufficient. Therefore, except for trace elements with which quantitative analysis with the wet method is difficult, the most common method employed was the wet method. However, analytical equipment evolved rapidly with development in X-ray fluorescence analysis and atomic absorption spectrometry, and in the 1970s, instrumental analysis quickly became widespread. With the development in the 1980s of atomic emission spectrometry and mass spectrometry using inductively coupled plasma (ICP), the majority of chemical analyses were conducted through instrumental analysis. The wet method, which achieved high accuracy, remained as the official method of JIS and other organizations, but the main analysis methods had shifted toward instrumental analysis. The development and distribution of reference materials are strongly associated with the distribution of such general-purpose methods of instrumental analysis. For example, X-ray fluorescence analysis combines several geochemical reference materials to prepare a calibration curve for quantitative analysis, thus accurate quantitative analysis is impossible without geochemical reference materials. In the atomic absorption spectrometry and the ICP method, the use of the geochemical reference materials is essential for evaluating the effects from matrix and coexisting components and managing analytical accuracy. The development and progress of analytical instruments and the development and distribution of reference materials have evolved together.4 Underlying technology for reference material development4.1 Sample selection4.1.1 Initial sample selection (rocks that represent Japan)Based on the basic concept of rocks that represent Japan, volcanic rocks (silicate rocks) that constitute the Japanese Islands were prepared first. As the first two types of reference materials, JG-1(J for Japan and G for Granite), which is a granitic rock with high silicon dioxide content, and JB-1 (B for Basalt), which is a mafic basalt (high iron and magnesium) with relatively low silicon dioxide content, were selected. The next question was from which area the samples should be collected, and based on ample geological research materials, granodiorite from Sori, Gunma Prefecture, was chosen for JG-1, and alkaline basalt from Sasebo, Nagasaki Prefecture, was chosen for JB-1. For the actual sample collection, because rocks exposed at the surface are affected by weathering and pollution, fresh samples that had not been exposed to the atmosphere were sampled from quarries and rock pits (Fig. 5). For the preparation of reference materials, about 200 kg of a sample is needed. Specifically, for the first two types, about 400 kg of the source rock was sampled, Trace componentsMain targetsMain analytical methodsGeneral-purpose instrumental analysisEarly stage instrumental analysisWet methodAnalytical methodCharacteristicsCurrent situationThis method produces accurate values, but it takes a long time and requires expertise.Gravimetry,Volumetry, ColorimetryMajor components and trace components at high contentNot used other than for some elements(SiO2, FeO)Trace components that could not be analyzed with the wet method can be analyzed and multiple elements can be analyzed simultaneously. However, during development, reference materials for comparison were rare and it took much expertise to acquire accurate values. Equipment had a high cost. Emission spectrometry,Spark method,Arc methodIt was abandoned for a while, but with improvement in equipment, some methods were revived.The techniques are highly versatile, and the number of reference materials had increased by the time of development. With a certain amount of experience, accurate values can be obtained.X-ray fluorescence spectrometry,Atomic absorption spectrometry,ICP emission spectrometry,ICP mass spectrometryMajor components, Trace componentsThese are the mainstream methods for most components.Fig. 4 Main chemical analysis techniques for geological materials and changes through time Temporal changes in analytical methods are shown from left to right.