TECHNICAL NOTE
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X-ray Fluorescence Analysis of Hexavalent Chromium Using KβSatellite Peak Observed as Counterpart of X-ray Absorption
Near-Edge Structure Pre-Edge Peak
Isao Tsuyumoto* and Yoshihiro Maruyama

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Department of Applied Chemistry, College of Bioscience and Chemistry, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi,Ishikawa 921-8501, Japan
ABSTRACT: A Kβ satellite peak in X-ray ﬂuorescence (XRF)
spectra is observed as a counterpart of the pre-edge peak of theX-ray absorption near edge structure (XANES). In Kβ emission of chromium, a small satellite peak is observed at 5.983
5.988 keV only for hexavalent chromium compounds such as
CrO3, Na2CrO7 3 2H2O, Na2CrO4 3 4H2O, K2Cr2O7, K2CrO4,Zn2CrO4(OH)2 3 2H2O, PbCrO4, and BaCrO4, together withthe main peak at 5.947 keV, while trivalent chromium compounds such as Cr2O3 and Cr(OH)3 3 nH2O show only themain peak at 5.947 keV. This corresponds to the fact that the K
pre-edge peak in XANES is observed only for the hexavalentchromium compounds. The electronic level causing the satellite
peak is almost at the same energy level as that causing the preedge peak. Our ﬁnding not only aﬀects the interpretation of the origin of the pre-edge peaks but also leads to the simple speciation
method of chromium compounds using XRF.

An understanding of valence states of heavy metals in theenvironment is essential to assess potential hazards because
the toxicity of the heavy metals varies greatly and is dependent on
the chemical states.1 It is well-known that there is a great
diﬀerence in toxicity between hexavalent and trivalent chromium
compounds, and in many countries only the hexavalent chromium is strictly regulated. Since the hexavalent chromium
compounds have higher solubilities in water than the trivalentchromium compounds, discrimination between them have been
generally made by combination of elution tests and element
analyses in the area of environmental analysis. Analysis of trace
amounts of hexavalent chromium (∼6.4 mass ppm of Cr(VI) in
58.4 mass ppm of total Cr) has been also achieved by using X-ray
absorption near-edge structure (XANES) spectroscopy.2,3 Because hexavalant chromium exhibits a distinct Cr K pre-edge
peak about 12 eV prior to the main Cr K absorption edge asshown in Figure 1, the intensity of the Cr K pre-edge peak has
been used for quantitative analysis of the hexavalent chromium.
This pre-edge absorption resonance has been considered to bedue to electronic transitions from 1s to 3d and sometimes
characteristic to transition metals with empty d orbitals. These
pre-edge features are also observed for Ti(IV), V(V), Mn(VII),and etc. and have been extensively investigated by many
researchers.413 Although the transition from 1s to 3d is a dipole
forbidden transition, it can gain intensity by a quadrupoletransition from 1s to 3d or a dipole transition from 1s to adp hybrid orbital. Numerous precise measurements and theoretical calculations have been performed to estimate ther 2011 American Chemical Society

Figure 1. XANES K-edge spectra for Cr2O3, Cr(OH)3 3 nH2O, CrO3,Na2Cr2O7 3 2H2O, Na2CrO4 3 4H2O, and PbCrO4. The spectra arevertically deviated for clarity.

contributions of such mechanisms to the pre-edge peaks, and it
has been deduced that the electric quadrupole transition from 1s
to 3d is an erroneous interpretation.14 The pre-edge is related tothe coordination structure of the central transition metal ion, andReceived: June 23, 2011
Accepted: August 26, 2011
Published: August 26, 2011
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Analytical Chemistry
it can be interpreted through the Oh and Td character tables ofgroup theory. The same discussion is also applicable to Kβ X-rayﬂuorescence spectra.15,16
Many researchers have intensively investigated the ﬁne structures of X-ray ﬂuorescence (XRF) spectra.1720 It has been reported that the oscillation similar to the extended X-ray absorption ﬁne structure (EXAFS) was observed in lower energy thanthe main Kα peak of XRF and that the similar information as
EXAFS spectra was obtained without using a synchrotronradiation facility.2125 Fluorescence spectra generally showcounterparts of absorption spectra because both involve transitions among the same electronic levels. There have, however,been no reports on the ﬁne structures of the XRF spectra
showing the counterpart of the XANES pre-edge peak. In thisstudy, we present the ﬁrst observation of the counterpart of thepre-edge peak of Cr K-edge absorption spectra in Cr Kβ emission
spectra.

’ EXPERIMENTAL SECTION
Two types of trivalent chromium compounds and eight types ofhexavalent chromium compounds were measured: Cr2O3, Cr(OH)3 3 nH2O, CrO3, K2Cr2O7, K2CrO4, Na2Cr2O7 3 2H2O, Na2CrO4 3 4H2O, Zn2CrO4(OH)2 3 2H2O, PbCrO4, and BaCrO4(Kanto Chemical Co., Inc., Tokyo, Japan; reagent grade). Eachsample was pressed into a pellet of 13 mm o.d. In order to examine
the relationship between the Cr(VI)/total Cr ratio and the satellite
peak intensity, ﬁve types of a mixed pellet of Cr2O3 and Na2CrO4 34H2O were also measured. As for Cr(OH)3 3 nH2O, BN wasmixed with Cr(OH)3 3 nH2O to improve the stiﬀness of the pellet
at the ratio of 1 part BN to 4 parts Cr(OH)3 3 nH2O by weight.
XRF spectra were taken with a wavelength dispersive X-ray
ﬂuorescence spectrometer (XRF-1700; Shimadzu Co., Ltd.,
Kyoto, Japan). A rhodium anode X-ray tube was used for excitation with 40 kV and 95 mA. The dispersive crystal was LiF (200)(2d = 0.40267 nm), and the X-ray detector was a scintillation
counter with NaI (Tl). The 2θ scan range was from 57° to 65° by
a 0.01°step, and the counts were accumulated for 50 s for eachstep. The XRF spectra were ﬁtted with a pseudo-Voigt function
and a Lorentz function using a nonlinear least-squares method
based on the generalized reduced gradient algorithm. XANES
spectra, for comparison, were also taken for Cr2O3, Cr(OH)3 3
nH2O, CrO3, Na2Cr2O7 3 2H2O, Na2CrO4 3 4H2O, and PbCrO4 atbeamline BL38B01 of Spring-8 (Japan Synchrotron Radiation
Research Institute, JASRI; Hyogo, Japan).3 The spectra werecollected in transmission mode using two ion chambers for I0 and I.
’ RESULTS AND DISCUSSION
In the XANES spectra, a single peak was observed in the preedge region of the spectra of the hexavalent chromium compounds, at about 12 eV smaller than the Cr K absorption edge,
and the pre-edge peak was not observed for the trivalent chromium compounds, as shown in Figure 1. Figure 2 shows Cr Kβ
XRF spectra after background subtraction and normalization. Allthe spectra were almost the same around the main peak Kβ1,3(2P3/2, 2P1/2 f 2S1/2) at 5.947 keV. However, a small shoulder
was observed only for hexavalent chromium compounds in thetail region of 5.976.00 keV. The spectral curves for CrO3,K2Cr2O7, K2CrO4, Na2Cr2O7 3 2H2O, Na2CrO4 3 4H2O, Zn2CrO4(OH)2 3 2H2O, and PbCrO4 were almost the same, whereas thecurve for BaCrO4 showed a larger shoulder than the otherhexavalent compounds. Similarly the spectral curves for Cr2O3TECHNICAL NOTE

Figure 2. Kβ emission spectra for Cr2O3, Na2CrO4, and BaCrO4. The
inset is a magniﬁed image of satellite regions.

Table 1. Peak Parameters of Kβ Emissions for Chromium
Compoundsa
main peaksatellite peaku (eV)σ (eV)hηu (eV)σ (eV)h

CrO35947

13

1.026

0.355

5986

90.036

Na2CrO45947

13

1.078

0.459

5986

100.043

Na2Cr2O75947

13

1.044

0.355

5986

100.040

K2CrO45948

13

1.053

0.354

5987

110.045

K2Cr2O75947

13

1.036

0.342

5986

100.043

ZnCrO4
PbCrO45948
5947

13
13

1.040
1.060

0.382
0.383

5987
5988

10120.041
0.042

BaCrO45947

131.030

0.4545983

110.069

Cr2O35947

13

1.057

0.327

Cr(OH)35947

13

1.056

0.366

u, peak position; σ, FWHM; h, peak height; and η, ratio of Lorentz
function in pseudo-Voigt function.

aand Cr(OH)3 3 nH2O were almost the same. The main peak waswell ﬁtted with the pseudo-Voigt function, except for the small
shoulder region of hexavalent chromium compounds. The residual error after subtracting the ﬁtted pseudo-Voigt function fromthe experimental data indicated that a small satellite peak (we callthis here Kβ00 ) existed around 5.99 keV in the spectra of thehexavalent chromium compounds. Thus, the double-peak ﬁtting,the pseudo-Voigt function for the main peak and the Lorentzianfunction for the satellite peak, was performed. The residual sumof squares for the hexavalent chromium compounds were substantially reduced by using the double-peak ﬁtting, whereas thosefor the trivalent chromium compounds remained almost thesame as the case of the single peak ﬁtting. The ﬁtting parameters
are summarized in Table 1. The satellite peak of hexavalentchromium compounds except for BaCrO4 is centered at 5.986
5.988 keV, about 40 eV higher than the main peak at 5.947 eV,
and the intensity is about 4% of the main peak. As for BaCrO4,
the satellite peak is centered at 5.983 keV, and the intensity is
about 7% of the main peak. Figure 3 shows the relationshipbetween the Cr(VI)/total Cr ratio and the satellite peak area.
The satellite peak was estimated to show small intensity also forthe sample with no hexavalent chromium because of the smalldeviation between the experimental data of the main peak andthe ﬁtted pseudo-Voigt function. Such phenomena are usually
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TECHNICAL NOTE

Figure 3. The satellite peak area vs hexavalent chromium content forﬁve mixtures of Cr2O3 and Na2CrO4. The satellite peak area is normalized to the main peak area. The open square, 0, shows the observed peakarea (Iobs) containing the contribution from the trivalent chromium,
which is derived from the deviation between the main peak and the ﬁtted
function. The closed square, 9, shows the net peak area (Inet) calculated
by canceling the apparent contribution of the trivalent chromium.

Figure 4. Schematic illustration of the origin of the Kβ00 satellite
emission and the K-pre-edge absorption along with the ordinary Kβ1,3emission and K-edge absorption.

observed in the multiparameter ﬁtting process. In order to cancel
the apparent contribution of the trivalent chromium derived
from the deviation between the theoretical and experimental
curves, the net satellite peak area (Inet) was estimated bysubtracting the satellite peak area at the fraction of Cr(VI) = 0
from the observed peak area (Iobs),
Inet ¼ Iobs  ð1  fhex ÞI0ð1Þ

where fhex is the fraction of Cr(VI) and I0 is the observed satellite
peak area at fhex = 0. The calibration curve for the fraction ofCr(VI) vs the net satellite peak area showed a linear relationship.
This indicates that the quantitative analysis of valence states will
be possible by the analysis of the satellite peak in the XRF spectra.
Figure 4 shows the comparison between the energy levels of Kabsorption and Kβ emission. The energy level to which an
electron transits in the K pre-edge absorption (5.993 keV) isvery close to the energy level from which an electron transits inthe present Kβ00 satellite emission (5.9835.988 keV). Therefore, the satellite peak observed only for the hexavalent chromium compounds is a counterpart of the pre-edge peak ofXANES observed only for the same hexavalent chromium
compounds as well. This is the ﬁrst observation of the counterpart of the pre-edge peak of XANES in the XRF spectra. The
origin of the pre-edge peak in XANES has been considered to bedue to the electric quadrupole transition from 1s to 3d or theelectric dipole transition from 1s to a dp hybrid orbital. Recent
studies based on polarized spectra, group theory, and theoretical

calculations indicated that an intense pre-edge peak is attributable to the electric dipole transition to p-character in the dphybrid orbital and that the electric quadrupole transition from 1sto 3d is the erroneous interpretation.14The pre-edge features are characteristic to the transitionmetals with empty 3d orbital such as Cr(VI), Ti(IV), V(V),and Mn(VII). It is interesting to note that not only the transition
to the electronic level corresponding to the pre-edge feature in
XANES but also the transition from the same electronic level wasobserved in XRF. Because the 3d orbital is empty, the quadrupoletransition from 3d to 1s in XRF is considered to be improbable.
Our results also contradict the quadrupole transition and support
the electric dipole transition from the dp hybrid orbital relating
to the symmetry of the central transition metal ion. We expectthe analysis of the satellite peaks in XRF spectra would be very
informative to clarify the origin of the pre-edge features, because
they contain useful information on the electronic levels relating
to the XANES.
The main peaks observed in the present study were rather
broad (fwhm, 0.33°, 0.013 keV), and the satellite peaks weresuperimposed on the tail of the main peaks. The energy resolution would be greatly improved by employing a double crystal
X-ray monochromator or the Bragg-Brentano geometry,26 andsimple and easy analysis without using the synchrotron radiation
facility would be possible. The pre-edge peaks in XANES reﬂect
various information on the chemical states of metal compounds
such as symmetry, valence, and coordination number.11 The
analysis of the ﬁne structure of XRF spectra would be an eﬀective
tool alternative to the conventional XANES analysis to obtainsuch information. We believe that our present ﬁndings in the ﬁne
structure of XRF spectra would promote the ﬁeld of chemical
state analysis.

’ CONCLUSIONS
We ﬁrst found that the ﬁne structures of Kβ emission in XRF
spectra provide the same information as the K pre-edge feature ofXANES spectra. In Kβ emission, a small satellite peak at 5.983
5.988 keV was observed only for hexavalent chromium compounds, about 40 eV higher than the main peak at 5.947 keV,corresponding to the fact that K pre-edge peaks are observed at
5.993 keV only for hexavalent chromium compounds in XANESspectra. As for the origin of the pre-edge peaks and thecorresponding satellite peaks, our results contradict the quadrupole transition and support the electric dipole transition from thedp hybrid orbital. Since the pre-edge peaks in XANES reﬂect
the chemical states of metal compounds such as symmetry,valence, and coordination number, it is expected that the analysisof the ﬁne structures of XRF would be a new useful tool forchemical state analysis.
’ AUTHOR INFORMATION
Corresponding Author*E-mail: tsuyu@neptune.kanazawa-it.ac.jp.

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