usability of portable x-ray spectrometer for discrimination of

USABILITY OF PORTABLE X-RAY SPECTROMETER FORDISCRIMINATION OF VALENCE STATES

I.A.Brytov, R.I.Plotnikov ,B.D.Kalinin,NPP Bourevestnik, Malookthinsky Pr., 68, St. Petersburg, Russia

ABSTRACTSpectral resolution and line positioning reproducibility of the SPARK-1M portable X-rayspectrometer have been determined and the usability of the instrument for the study ofchemical shifts of K-lines in the emission X-ray spectra of transition metals has beenevaluated.The spectrometer has been shown to reproduce goniometer positioning within afraction of an angular second, which corresponds to the line shift on the order of 0.01 eV. Inthe case of iron, chromium and uranium compounds with different oxidation states, theusability of the spectrometer for measuring chemical shifts and for the assay of transitionmetal oxidation states has been demonstrated.

INTRODUCTIONIn certain analytical problems, a ratio of different oxidation states of a given chemical elementhas to be determined, such as the ratio of sulfate and sulfide in coal, Fe+2 и Fe+3 inkimberlites, mica and other minerals, UO2

+2 and U+4 in nuclear materials, etc. Precision X-rayspectrometers allow to solve these problems by studying the fine structure of X-ray emissionspectra and measuring X-ray line shifts (chemical shift) [1,2]. Usually, chemical shifts do notexceed 0.5-1 eV and are difficult to measure. The use of conventional Soller-type scanningspectrometers (mass-produced for elemental analysis) to study chemical shifts has beenreported [3,4]. This present paper reports on the use of a portable X-ray spectrometer tomeasure chemical shifts of Kβ-lines of transition metals and U Lα line. Measurement errorsare calculated.

SPECTROMETER CHARACTERISTICSSPARK-1M is an automatic short-wave Johansson-type X-ray spectrometer with a150 mm focal radius. A BKh-7 X-ray tube capable operating with an anode voltage up to 45kV and output power up to 10 W was used as an excitation source. The anode of BKh-7 tubeis a thin silver coating (about 5 µm thick), located directly on the tube beryllium window.This design allows one to bring the sample within 3-5 mm from the focus and to obtain highcount rates, which is only ten times lower than those on spectrometers with 3 kW reflective-anode X-ray tubes.

The scanning mechanism simultaneously rotates the crystal and the detector (a xenon-filledproportional counter) at the correct ratio within the range 2θ = 24…88о. With the mostcommon crystal, LiF(200), this range corresponds to the wavelengths from 0.83 to 2.8 Å,covering the K-series of elements in the range from Z=21 (Sc) to Z=51 (Sb) (using first andsecond orders of reflection) and the L-series of elements with Z≥56 (Ва). A stepper motorscans the spectrum in 0.00125Å steps throughout the entire wavelength range.

Line half-width was measured and calculated as a geometric sum of four components:intrinsic line width, entrance slit width (τ=0.2 mm), mosaic non-uniformity of the crystal (δ=3’) and vertical divergence of the beam (crystal width of 20 mm).

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For wavelengths over 1.3 Å, full agreement is observed between calculated and experimentaldata (from 8 eV for TiKα to 67 eV for Zn Kβ). For shorter wave lengths (U Lα line in thesecond order), measured values significantly exceed the calculated ones (98 and 56 eV),which may be explained by the passage of radiation through the slit blades and penetration ofthe radiation into the crystal.

Reproducibility error of the goniometr was evaluated by repeated measurements of the countrates on the line slope with repositioning of the goniometer after each measurement. Theaverage result of 10 goniometer repositioning cycles r.m.s. reproducibility was 0.012 eV or0.2 angular seconds, which can compete with the reproducibility of precision spectrometers.Similar results were obtained for several individual SPARK-1M spectrometers. With thereproducibility and resolution demonstrated, line shifts due to change in chemical state of anelement may be measured.

X-RAY SPECTRA OF Cr Kβ, Fe Kβ, AND U Lα LINESChemical shifts was evaluated by measuring parameters of a differential spectrum (differencebetween normalized profiles of the lines in the compounds with different oxidation states ofthe given element), or by measuring the ratio of intensities on the left and right slopes of theline. The latter procedure substantially reduces measurement time and is especiallyconvenient when the sample contains the given element in both oxidation states. In this case,the emission line is a superposition of two lines with different shift, and the shift of themeasured line gives direct quantitative information on the ratio between the oxidation states.

Figs. 1 and 2 present the comparison of spectra of iron (III) oxide Fе2O3 vs. iron (II) oxalateFeC2O4 (Fig.1) and chromium (III) oxide Сr2O3 vs. potassium chromate K2CrO4 (Fig.2) in theKβ spectral region for iron and chromium, respectively.


Fig. 3 presents the Lα1 line of uranium oxides UO2 and UO3 in the second order ofdiffraction.

These were measured the powder probes of all mentioned compounds. A voltage of 35 kVand current of 0.2mA were used. Peak count rates were ~40,000 s-1 for iron, ~12,000 s-1 and~4,000 s-1 for chromium in oxide and chromate, respectively, and ~4,000 s-1 for uranium. Thespectra were scanned in 0.0005 Å steps with 10 second exposure at each point. For everysample, an average result of 5 measurement cycles was used.Upon normalization of thespectra (the value at every point was divided by the sum of all points), differential count ratesJ1–J2 were determined and plotted vs. wavelength together with the line profiles (differentialcurves for chromium and iron are magnified for a better view).


As Fig. 1 shows, differential count rate JFe+3 – JFe+2 (dotted line) has a fairly complex trend; itsS-shaped profile testifies to the shift of the main Fe Kβ1,3 peak.A peak around 1.753 Å andadditional peaks superimposed onto the main curve around 1.755 – 1.76 Å are due to the Fe Kβ2,5 line whose intensity and position depend on the oxidation state of iron. A similar structurearound longer wavelengths (1.76 – 1.77 Å) is due to the non-diagram line Fe Kβ′ with theposition and intensity also substantially affected by the oxidation state of the metal.

For chromium, the differential curve JCr+3 – JCr+6 (Fig. 2) has a simpler structure. As thespectrum of chromate lacks the Cr Kβ′ line, the long-wave end of the curve reveals only onepeak corresponding to that line of chromium oxide. Approximating the line shape with aGaussian curve and assuming the shift ∆ to be small compared to the line width H, the shiftmay be approximately calculated as ∆= 0.35H(Amax–Аmin)/A) , where (Amax–Аmin)/A is theratio of the difference between the maximum and the minimum of the S-shaped differentialcurve to the magnitude of the main peak. The values E, H and σ in this and followingequations can be expressed in units of wave-length (Å) or energy units (eV), what is moreconvenient for estimation of a chemical shift.

The use of this formula for the examples shown in Figs. 1 and 2 yields the shift of 0.17 eVbetween Fe2O3 and FeC2O4, and 0.32 eV between K2CrO4 and Cr2O3. The largest shift wasobserved for uranium, about 1.3 eV.

Similar results were obtained by evaluating the shift from the count rate ratio on the lineslopes.Using the Gaussian approximation of the line shape, we can write

]E

21[EXP

N

Nk

2

0

σ

−== , or kln2E −σ= (1)

where N is the count on a line slope at the distance Е (in energy units) from the peak and N0 isthe maximum peak count, σ is the dispersion which can be expressed as

355.2H

2ln22H 2/12/1 ==σ (2)

Consider the ratio of pulse counts NR and NL on the right and left slopes of the reference lineat ЕR and EL from the peak, respectively.

]]EE[21[EXP

NN

RL

2

R

2

L

R0

σ

σ

−−== (3)

If the line is shifted by ∆ (in the ∆<< Н limit), the new ratio may be expressed as

]]EE[21[EXPR

L

2

R

2

©

L

©

R1 N

N

σ

σ

∆−−

∆+−== (4)

Then the logarithm of the ratio R1/R0 is

)EE()E2E2(2R

RlnRln LR2LR20

1 +∆

−=+∆

−==σσ

(5)

Using (1), we can then write


)ln2ln2(2ln22RlnH

kk LR

2/1

−+−−=∆ (6)

or, assuming kR and kL to be approximately equal,

kln*2ln8RlnH 2/1

−=∆ (7)

A shift measurement error ∆E is caused by the error of R, which accordingly depends on themeasurement errors of count rates on the slopes of the line. The latter errors consist of severalcomponents: statistical variation of pulse count, goniometer position reproducibility, samplereproducibility, and instrumental error of measurement. The contribution of each factordepends on the spectrometer characteristics and experimental strategy.

In the present study of line shift using SPARK-1M spectrometer for multiple alternatemeasurements of each sample with repeated repositioning of the goniometer to the lineslopes, the main factors of error were statistical variation of the count and goniometer positionerror.Taking a derivative of (7) over R and assuming the statistical error of R to be

Nk2stat}

R(

0

R =σ (8)

(as the four values of N are very close),and the goniometer positioning error to be

E2/10

R

KHkln*2ln4E*

dEdN*

Nk1pos)

R( σ=σ=σ (9)

where K is the number of repositions, we can derive the respective error components for theshift:

NkRlnE2)stat(

0

E∆

=σ∆ (10)

and

EKH

kln*2lnE8)pos(2/1

E σ−∆

=σ∆ (11)

Substituting (7) into (10) and (11), one can see that the statistical component is proportionalto the line width, and the goniometer positioning component does not depend on instrumentresolution.

Table 1 shows the line shifts measured for the same sets of samples and the estimates for thecomponent content assay error, σС, calculated as

%100

01

Rc RR

×−σ=σ


Table 1Estimated errors of the oxidation state assay from the count rate ratio on two wavelengths.

Compounds Wavelength, A

λL λR

Ratio

R=R1/R0

∆, eV σR, eV σС, ,%

Fe2O3 FeC2O4 1.758 1.762 0.971 -0.15 0.0043 2.8

K2CrO4 Cr2O3 2.088 2.092 0.911 -0.30 0.013 4.3

UO3 UO2 0.907 0.912 0.9358 -1.18 0.042 3.5

The table shows the error of oxidation state analysis using the SPARK-1M spectrometer to liewithin 3–5%, which corresponds to the data from the analysis of the oxidation states of ironby measuring the shift of L-series lines using the CAMEBAX micro-analyzer [5].

CONCLUSIONHigh goniometer positioning reproducibility of the SPARK-1M spectrometer allows usingthis instrument to study chemical shifts of X-ray lines. A possibility of oxidation state studyby comparison of the K-spectra of transition metals and uranium L-spectrum has beendemonstrated. When different oxidation states of a transition metal are present in a sample,their content can be estimated within 3–5% accuracy from the ratio of count rates on theslopes of the Kβ line.

REFERENCES1. Brytov I.A., Obolensky E.A., Goldenberg M.S., Rabinovich L.G., Antoyeva T.M., MagdinYu.A. Apparatura i metody rentgenovskogo analiza (Equipment and methods of X-rayanalysis), Leningrad, 1983, No. 29,p. 14 – 18.2. Blokhin M.A., Nikiforov I.Ya. Apparatura i metody rentgenovskogo analiza (Equipmentand methods of X-ray analysis),. Leningrad, 1972, No 10,p. 89 – 94.3. Li Z.; Ruqin Y.; Shi L.; Wang Q. Anal. Chim. Acta, 15 Jul 1991, 248 (1), 257-2614.Yoichi Tamaki X-ray Spectrom., ,1995,24,235-2405. Hoffer H.E., Brey G.P., Schulz-Dobrick B., Oberhansli R. Eur J. Mineral., 1994, 6, 407-418


usability of portable x-ray spectrometer for discrimination of

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