Principal Research Results

R & D on Analysis of Trace Metal Elements and Chemical Forms
by Synchrotron Radiation
Background
XAFS (X-ray absorption fine structure) analysis ＊ 1 using the powerful synchrotron X-ray emitted by the SPring-8 (SuperPhoton ring-8GeV) is advanced analysis of a local crystal structure around an atom using the specific XAFS spectrum of a material
and is expected to prove to be a useful technique to analyse trace metal elements and their chemical forms in wide-ranging research
fields (Fig. 1). However, there is hardly any precedence of identifying chemical forms with XAFS analysis regarding such complicated material systems involving multiple compounds as coal and fly ash and the standard understanding is that the application of XAFS
analysis to such complicated systems is extremely difficult.

Objectives
We apply XAFS analysis to such complicated material systems involving multiple compounds as coal and fly ash using theSpring-8 industrial consortium beamline, and clarify the potential of this analysis for trace metal elements and their chemical forms.

Principal Results
Application of XAFS analysis to a standard bituminous coal sample (NBS1632c) and standard fly ash sample (NBS1633b)of the US National Institute of Standards and Technology (NIST) produced the following results.
1. Development of Technique to Identify Chemical Forms with Transmission Mode XAFS Measurement
With transmission mode XAFS measurement ＊ 2, a lower limit analytical concentration of 103 - 104 mg/kg was achieved bystabilising the X-ray beam (Fig. 2-A)＊ 3. Identification of the chemical form and quantification of the mixing ratio of iron, etc. at thisconcentration level were proven to be feasible by comparing the XAFS spectrum of the sample with that of the standard material (Fig. 3).
2. Development of Technique to Identify Chemical Forms with X-Ray Fluorescence Mode XAFS
Measurement(1) Improvement of lower limit analytical concentration through optimisation of intake angle
With X-ray fluorescence mode XAFS measurement, improvement of the S/N ratio by more than one figure was achieved
through optimisation of the intake angle of the XAFS spectrum to a solid state detector (SSD) which reduced the scattered X-ray
noise. In this way, the feasibility of this mode of XAFS measurement for analysis of a concentration of some 10 mg/kg, a level difficult to achieve by the transmission mode, was established (Fig. 2-B).
(2) X-ray fluorescence mode XAFS measuring using high resolution detector
Using the fluorescence X-ray detector ＊ 4 jointly developed by the CRIEPI and companies participating in the industrial consortium beamline to the highest level in the world (Fig. 4), a lower limit analytical concentration of less than 1 mg/kg was achieved forelements ranging from titanium to celium for which the excitation of the inner shell electrons is possible by X-ray fluorescence mode
XAFS measurement using the industrial consortium beamline (Fig. 2-C). Moreover, the feasibility of identifying chemical forms at
this concentration level through comparison with the standard material was established (Fig. 5).
Based on the above results, the ranges of the measurable concentration for complicated material systems involving multiple
compounds by the above-mentioned two modes of XAFS measurement were established, making it possible to analyse the chemicalforms of trace metal elements in compounds.

Future Developments
The next steps are verification of the applicability of XAFS analysis using a high energy X-ray to heavy elements and thedetection limit of low concentration elements using the newly introduced high sensitivity X-ray detector and to expand the range of XAFS analysis by creating a database which includes data for standard materials.
Main Researcher: Tohru Yamamoto, Ph.D.,
Research Scientist, Materials Functions and Mechanism Creation Sector, Materials Science Research Laboratory
Reference“Application of Synchrotron Radiation to X-Ray Analysis of Industrial Materials - Study on X-Ray Analysis and Technology Using
Industrial Consortium Beamline at SPring-8”, CRIEPI Report (W01027 in Japanese)“Analysis of Trace Metal Elements and Chemical Forms by Synchrotron Radiation Light - Application of X-Ray Absorption Fine
Structure Analysis”, CRIEPI Report (W03034 in Japanese)＊1：XAFS analysis is an analytical technique made possible by the use of the SPring-8.
＊2：Yamamoto, et. al, CRIEPI Report W01027. X-ray transmission mode XAFS measurement measures the strength of an X-ray transmitting
through a sample. Meanwhile, X-ray fluorescence mode XAFS measurement measures a fluorescent X-ray generated on the sample surface
and, therefore, can be applied to non-transmissive samples or extremely thin samples of which measurement by the transmission mode is difficult.
＊3：The lower limit analytical concentration at each mode is set as equivalent to 10 - 30 times the actually measured S/N ratio.
＊4：This fluorescent X-ray detector equipped with a high resolution X-ray spectral crystal offers the world’s highest detection sensitivity (108 1010 atoms/cm2) (Awaji and Yamamoto, etc. al., Jpn. J. Appl. Phys., 39, pp. L1252 - 1255 (2000)).

118

10. Advanced Basic Technologies - Nano-technology

X-Ray Absorption and Energy (KeV)

Adjacent
AtomTarget Atom
X-RayCombustion (mg/kg)

Measurable range
by fluorescencemode XAFS
measurement using
the newly developed
fluorescent X-raydetector

Fig.1 Principle of XAFS Analysis

X-Ray Detector (SC)
Slit
Fluorescent X-Ray
X-RaySample

Newly Developed High
Resolution Detector

Atomic Number N

Fig.2 Identifiable Range of Chemical Forms by XAFS
Analysis Achieved by the Present Research
The measurable concentration ranges of transmission mode XAFS
measurement (A), fluorescence mode XAFS measurement (B) andfluorescence mode XAFS measurement using the newly developed highresolution detector (C) are shown. Also shown are the quantitative
analysis results of NBS1632c (○) and NBS1633b (△) by the NIST andthe elements with which XAFS analysis was conducted in the present
research (●, ▲).
Strength of Fluorescent X-Ray

Crystal SpectographMeasurable range byfluorescence mode
XAFS measurementn/a

The interference between the wave released
from the atom which has absorbed the X-ray
and the reflected wave from the adjacentatom makes it possible to observe theoscillation structure in the XAFS spectrum.
Analysis of the XAFS spectrum produces
information on the state of the electrons of
the target atom and the type, distance andcoordination number of the adjacent atom.

Slit

Measurable range bytransmission mode
XAFS measurement

Solid
StateDetector(SSD)

Measurement Results ofSolid State Detector

Fluorescent
X-Ray
X-Ray
SampleMeasurement Results ofHigh Resolution Detector

Conventional
Solid StateDetector

Energy (KeV)

Fig.4 Comparison between Newly Developed High Resolution Detector and Conventional
Solid State Detector

Standardised X-Ray Absorption CoefficientStandardised X-Ray Absorption CoefficientThe development of a high resolution detector equipped with a crystal spectrograph and slits has significantly improved the energy
resolution and S/N ratio, both of which are essential for the XAFS analysis of the trace metal elements of a compound.

Measurement Value
Fitting ResultError(Iexp-Ifit)

Standard Sample: SeVI
Standard Sample: SeIV
Standard Sample: Se0

Photon Energy (KeV)

Fig.5 XAFS Measurement Results for Selenium(Se) Using High Resolution Detector

Photon Energy (KeV)

Fig.3 XAFS Measurement Results for Fe Tn
NBS1633b
The iron contained in NBS1633b is identified as a compoundof which the main constituents are FeO (OH) and Fe(OH)3 as
shown in the inserted figure.

Using the XAFS spectrum, it is established that the state ofselenium oxidisation is low oxidised compounds, such as
IVFeSe2, in the case of NBS1632c or a mixed compound of Se
VIand Se in the case of NBS1633b.

119