ed(p)xrf: screening analysis and quantitative analysis with polarized … · 384 ed(p)xrf:...

384

ED(P)XRF: SCREENING ANALYSIS AND QUANTITATIVE

ANALYSIS with POLARIZED X-RAYS

R. Schramm’, J. Heckel’, K. Molt 2

‘Spectra Analytical Instruments, Kleve, Germany.

2Gerhard-Mercator- University Duisburg, Germany.

Screening is a fast method for qualitative or semi-quantitative analysis, in contrast to high

precision quantitative analysis. This paper presents an instrumental design to handle both:

FAST SCREENING ANALYSIS AND HIGH PRECISION ANALYSIS. The analyses of

solids and liquids with real times between 150 and 600 s and a relative error < 10 % for the

elements Na up to U will prove the screening method. The analysis of main components in

Mn-concentrates within 600 s with a precision in the range of 10.06% will prove the high

precision quantitative analysis.

INTRODUCTION

Screening analysis is a traditional application of energy dispersive x-ray fluorescence [l-3]. In

the past, screening meant qualitative analysis or,’ at best, semi-quantitative analysis. In recent

times, the aim of further development of EDXRF spectrometers was improvement of detection

limits and sensitivities of the analyzing elements. This resulted in quantitative analysis

comparable with atomic emission or absorption methods for several applications [4]. One of the

great disadvantages of common EDXRF systems is long measurement times to get low detection

limits and high precision. Also the use of one x-ray tube in a specific excitation arrangement

cannot give optimal excitation conditions for light anJ heavy elements. The use of primary beam

filters combined with varying tube voltages can improve the situation.

The main goal would be to combine the quantitative analysis with short measurement times to

get a quantitative screening analysis. One way this result can be reached is using polarized x-rays

Copyright (C) JCPDS-International Centre for Diffraction Data 1999ISSN 1097-0002, Advances in X-ray Analysis, Volume 41

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This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website – www.dxcicdd.com

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ISSN 1097-0002, Advances in X-ray Analysis, Volume 41

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385

for excitation in an EDXRF system [5-lo]. The background reduction by using polarized x-rays

for excitation results in very low detection limits and high sensitivities. This allows for

conducting a quantitative screening analysis within a real time between 150 and 600s with a

relative error below 10 % for all elements between Na and U.

To get a flexible screening method valid for a large number of different matrices the fundamental

parameter algorithm was combined with an empiric matrix calibration. This allows the

determination of components in different matrices (eg oil and water) using the same calibration.

Thus the quantitative screening method achieves results with a relative error < 10% although the

matrix of the sample is unknown.

The high stability of an XRF system generally allows analysis with a precision better than 0.15%

for main component analysis. In contrast to WDXRF systems, the lower sensitivity of common

EDXRF systems leads to extremely longer counting times. By using a BRAGG polarizer in the

ED(P)XRF system, the total measurement time (real time) for main components like Mg - Fe in a

light matrix (eg glass bead) could be decreased to less than 600s [ 111.

EXPERIMENTAL SET-UP

The instrumentation used was a SPECTRO XLAB 2000 equipped with a 400 W Rh end window

tube and a Si(Li) detector with a resolution of 148 eV (1000 cps Mn Ka). The available targets

were A1203 and B& used as BARKLA polarizer, an HOPG (High Oriented Pyrolitic Graphite) -

crystal used as BRAGG polarizer and Al, MO and Co used as secondary targets. The irradiation

chamber could be operated under vacuum or by using a gas purge either with nitrogen or helium.

The automatic sample changer could be equipped with 20 samples maximum.

SAMPLE PREPARATION The analytical target of analysis defines the necessary sample preparation technique. For

quantitative screening analysis with a relative error < 10% the direct measurement of powders

should be sufficient. However, if there is an interest in elements like Na, Mg and Al, the

preparation as a pellet would be necessary. For liquids, simple pouring into a cuvette can be used.

For high precision analysis the samples were prepared as glass beads.


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RESULTS AND DISCUSSION

QUANTITATIVE SCREENING ANALYSIS

For screening analysis, a main target is saving time and receiving a fast overview of the sample

contents. Nevertheless the high performance of the polarized excitation will achieve

concentrations down to the low ppm level. Table 1 shows the excitation conditions used for

screening analysis of liquids, powders and pellets.

Table 1: Excitation conditions for screening analysis.

target type of target excited elements voltage curren? I

MO secondary &03 Barkla

HOPG Bragg

Cr - Zr (K), Pr - Bi (L) Y-Ce Na-V

35 kV 4mA 49.5 kV 6mA

10’ / 20L kV 2mA

‘solids, *liquids, 3current is optimized for each measurement to maximum countrate.

The evaluation of the resulting spectra is done by a method based on fundamental parameters.

The non-detectable elements with Z < 11 can be taken into account by using an empiric

calibration of the mass absorption coefficient versus the Compton intensity. This is analogous to

the well-known Compton-Method [ 12- 141.

The standard samples shown in Table 2 and 3 are used to calibrate the mass absorption

coefficient of the sample versus the Compton - scattered MO Ka radiation of the MO target for

powders and liquids.

Table 2: Standards used for calibration of powders.

No Standard Term

with the following standards Si, P, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Y, Zr, calibrated elements Nb, MO, Ag, Cd, In, Sn, Sb, Te, I, Cs, Ba, Ce, Pr, Nd, Sm, La, Ta, W, TI, Pb, Bi, Th,

U 1 876-i Ash 2 BCS372/1 Portland Cement 3 BCS382ll Basic Slag 4 GSDll Sediment 5 GSR07-09 Rock 6 GSS6 Soil 7 NBS120c Phosphate Rock 8 NIST696 Bauxite 9 NaCI, NaBr

10 SARM05 Pyroxenite


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Table 3: Standards for calibration of liquids.

No Standard Cone, range. Elements

1 Conostan S-21 1 - 900 l.lg/g Ag, Al, Ba, Ca, Cd, Cr, Cu. Fe, Mg, Mn, MO, Na, Ni, P, Pb, Si, Sn, Ti, V, Zn

2 MBH SMIN-5%-4 0,001 - 5,0% s

3 Laboratorystd. 0,001 - lO,O% Cl, Br, I

4 ICP Multi Element Standard IV 1 - 1000 pg/g Ag, Al, Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Ga, In, K, Mn, Ni, Pb, Sr, TI, Zn (Merck)

5 Multi Element 1 - 500 pg/g As, Hg, P, S, Sn, Sb, Te, TI Standardsolution (Bernd Kraft)

Figure 1 and 2 show the calibration curves for Zn in powders and Cu in liquids as examples.

Figure 1: Calibration of zinc in powder (real time: 150s).

10.0 100.0 1000.0

Cu [pg/g] certified

Figure 2: Calibration of copper in water- and oil based liquids (real time: 200s).

Tables 4 - 6 show the results of different samples : powder, monophase liquid and polyphase

liquid measured as ‘unknown’ and evaluated with the calibrations mentioned.


388

Table 4: Results BCR-176 (ash) prepared as powder (real time: 450s).

.

Table 5: Results of ICP multi element standard solution I (Merck) (real time: 600s).

Element XLA B 2000 certified concentrations cont. ’ &g/g] tw4l1

Cr 25 + 3 25 r2.5 Mn 7.4 + 3 5 + 0.5 Fe 16k3 15 *1.5 co 22 f 2 20 *2 Ni 60*2 50*5

Zn Ga Ag Cd Pb

’ counting error (20).

22.2 f 1 2oi2 161 i2 15oi15 47.2 + I 5oi5

21.1 io.5 2oi2 221*2 200 f 20

Table 6: Results of a two liquid phased sample, real time 6OOs, 3g sample + lg charcoal as adsorbing material.

Element XLA B 2000 known concentrations cont. tls/sJ f/J&71

*impurity of charcoal.

If there is interest in light elements like Na, Mg, Al and Si, the samples must be prepared as

pellets. This avoids surface effects, which have a high influence on the determination of light

elements. The calibration used is similar to the powder calibration, apart from preparing the


389

standards as pellets and measuring in a vacuum. Table 7 shows the results of the standard GSR-

02 (andesite) prepared as pellet in comparison to the certified values.

Table 7: Results of GSR-02 prepared as pellet (real time 150s).

’ counting error (20).


390

HIGH PRECISION ANALYSIS

The development of ED(P)XRF also makes it possible to achieve high precision analysis [ 151.

The use of a HOPG - crystal as a BRAGG-polarizer resulted in a decrease of measurement time

(real time) below 10 minutes. One example is the analysis of the main components in Mn-

concentrates, a common raw material for production of Fe-alloys. Table 8 shows the excitation

conditions used for the analysis. The samples were prepared as glass beads. The calibration used

was based on the alpha normalization method due to Keith Norrish [16]. Table 9 shows the

calibration parameter for the elements of interest. The calibration of the main component Mn is

presented in detail in figure 3. For a mean value of 26.9% Mn, the RMS of 0.19% corresponds to

a relative error of 0.7%. The main uncertainty is the error in the chemical analysis performed to

achieve the reference values. To get the precision of the XRF method, one standard was

measured 10 times (table 10). This gives a RSD for Mn of 0.13 % which points out the

performance and stability of the system. An overview of the achievable precision as a function of

concentration is shown in table 11.

Table 8: Measurement conditions for analysis of Mn-concentrates.

target type of target

Al Secondary HOPG Bragg

excited elements

Na, Mg Al - Fe

vo/tage

14.5 kV 14.5 kV

current

12.5 mA 12.5 mA

real time

250s 290s

Table 9: Calibration data for Mn concentrates prepared as glass beads.


391

20.00 30.00

certified values for Mn rh]

Figure 3: Calibration curve for Mn in Mn-concentrates. RMS = 0.19%

Table 10: precision determined for a standard sample of raw material, prepared as glass bead, measured 10 times (given error corresponds to 95% confidence limit).

component calculated value certified value I%1 tw

MgO 8.628 k 0.06 8.530 A1203 3.830 ?r 0.04 3.800 SiO2 15.97 + 0.05 16.10

S 0.310 k.002 0.319 CaO 10.24 f 0.03 10.14 Mn 36.90 F 0.05 36.90 Fe 8.28 f 0.05 8.50

Table 11: Precision concerning different concentration ranges.

Concentration range Standard deviation 20 1x1 [%]

70-99 < 0.06 40-70 s 0.06 3-40 I 0.06 0.5 - 3 10.013 < 0.5 2 0.01


392

CONCLUSION

Screening analysis and high precision analysis with the same instrumental design is no longer

only a concept, but a reality. The applications presented serve as examples of the high flexibility

concerning different analytical problems and the high performance of the technology.

Nevertheless, the applications can be improved by expanding the number of standards containing

additional elements. Sample preparation greatly influences the accuracy of the results achieved.

The choice between pellets and fusions has to be done carefully, especially for high precision

analysis.

REFERENCES

(1) S. Piorek, XRF Technique as a Method of Choice For On-Site Analysis of Soil Contaminants and Waste Material, Adv. X-Ray Anal., Vol. 33,639 (1990).

(2) A.R. Harding, J.P. Walsh, Application of Field Mobile EDXRFAnalysis to Contaminated Soil Characterization, Adv. X-Ray Anal., Vol. 33,647 (1990).

(3) N.C. Jacobus, Screening of Hazardous Waste with an Energy Dispersive X-Ray Fluorescence Spectrometer, Adv. X-Ray Anal., Vol. 33,655 (1990).

(4) P.A. Russell, R. James, Journal of Analytical Atomic Spectrometry, 12,25 (1997). (5) T.G. Dzubay, B.V. Jarrett, J.M. Jaklevic, Nucl. Jnstrum. Methods 115,297 (1974). (6) E. J. Taggart, Adv. X-Ray Anal. 28, 17 (1985). (7) J. Heckel, M. Haschke, M. Brumme, R. Schindler, Principles and Applications of Energy-dispersive X-

ray Fluorescence Analysis With Polarized Radiation, J. Anal. Atom. Spectrom. 7,281 (1992). (8) J. Heckel; M. Brumme; A. Weinert and K. Jrrner, Multi-Element Trace Analysis of Rocks and Soils by

EDXRF Using Polarized Radiation, X-ray Spectrom., 20 (1991) 287. (9) J. Heckel, Using Barkla polarized X-ray radiation in energy dispersive X-RayjZuorescence analysis

(EDXRF), J. Trace Microprobe Tech., 13(2) (1995) 97. (10) R. Schramm, J. Heckel, Controle d’entre’e de rejets organiques et d’hydrocarbures halogen& par

EDXRF, Spectra Analyse 196, May - June (1997). (11) R. Schramm, Diploma thesis, Gerhard-Mercator-University Duisburg, Germany (1995). (12) G. Andermann and J.W. Kemp, Scattered X-Rays as Internal Standard in X-Ray Emission

Spectroscopy, Anal. Chem. 30, 1306 (1958). (13) R.C. Reynolds, Matrix Corrections In Trace Element Analysis by X-Ray Fluorescence: Estimation of

the Mass Absorption Coefficient by Compton Scattering, Jr. Am. Mineral. 48, 1133 (1963). (14) C.E. Feather and J.P. Willis, A Simple Method for Background and Matrix Correction of Spectral

Peaks in Trace Element Determination by X-Ray Fluorescence Spectrometry, X-Ray Spectrom. 5,41 (1976).

(15) J. Heckel, R. Schramm, Bragg and Barkla Polarization in EDXRF, Adv. X-ray Anal. Vol. 40, accepted for publication (1996).

(16) K. Norrish and J.T. Hutton, An accurate X-ray spectrographic method for the analysis of a wide range of geological samples, Geochim. Cosmochim Acta 33,43 1 (1969).


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