ED(P)XRF: SCREENING ANALYSIS AND QUANTITATIVE ANALYSIS with POLARIZED

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1 384 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

2 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 ICDD Website -

3 385 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.

4 386 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? MO secondary Cr - Zr (K), Pr - Bi (L) 35 kv 4mA &03 Barkla Y-Ce 49.5 kv 6mA HOPG Bragg Na-V 10 / 20L kv 2mA solids, *liquids, 3current is optimized for each measurement to maximum countrate. I 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 [ 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 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

5 387 Table 3: Standards for calibration of liquids. No Standard Cone, range. Elements 1 Conostan S 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 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 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) 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.

6 388 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 r2.5 Mn Fe 16k3 15 *1.5 co 22 f 2 20 *2 Ni 60*2 50*5 Zn Ga Ag Cd Pb counting error (20) f 1 2oi2 161 i2 15oi I 5oi 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

7 389 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).

8 390 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 Mnconcentrates, 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 excited elements vo/tage current real time Al HOPG Secondary Bragg Na, Mg Al - Fe 14.5 kv 14.5 kv 12.5 ma 12.5 ma 250s 290s Table 9: Calibration data for Mn concentrates prepared as glass beads.

9 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 k A ?r SiO S k CaO f Mn F Fe 8.28 f Table 11: Precision concerning different concentration ranges. Concentration range Standard deviation 20 1x1 [%] < s I <

10 392 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).