Reduction of the Sample Size in the Analysis of Rock by EDXRF

Transcription

1 Copyright (C) JCPDS International Centre for Diffraction Data Reduction of the Sample Size in the Analysis of Rock by EDXRF Riidiger Harmel, Ulrike Otto, Olaf Haupt, Clemens Sch$er and Walter Dannecker, University of Hamburg, Institute of Inorganic and Applied Chemistry, Martin-Luther-King-Platz 6, Hamburg, Germany Introduction The advantages of the XRF-technology, the easy preparation, the non-destructive measurement, the fast multi-element analysis, lead to a growing interest in this technology even in fields otherwise covered by ICP-MS or AAS because after the XRF-analysis it is possible to do further investigations with the same specimen. In the analysis of samples from ancient buildings and historic monuments, like churches, chapels and castles, the composition of stones from damaged and yet undamaged parts is of interest to find out if the weathered stones have lost salts by elution or get new ones by capillary effects, if the initial composition of the stone is still present or has changed. During the restoration of these buildings there is often only a small amount of material to analyze due to the protection of the monuments. Therefore it was necessary to make efforts to reduce the usual sample amount of 3-4 g which is needed for a pellet. Instrumentation In conventional operation, specimens with a diameter of 32 mm and an area of 8.06 cm2 were analyzed using an energy-dispersive XRF spectrometer (X-lab, Spectra A.I., Germany) containing secondary targets. The specimen was fixed in a rotating cup positioned on a plate for 18 cups. The primary source was a rhodium side-window tube, and the IS0 Debyeflex 300 x-ray generator (Seifert, Germany) produced high-voltage potentials between 8 and 60 kv. The emitted fluorescence radiation produced current pulses in a Si(Li)-semiconductor with an active area of 10 mm2 and an effective thickness of 3 mm. Reduction of the Sample Size Small Pellets In the instrument the radiation beam comes from underneath of the specimen with the detector at right angles to the incident radiation (figure 1): sample holder /,/I- -\,, / / x-ray \, / / I radiation \, x-ray tube / d&or secondary targets fig. 1: small pellet holder (side view) The radiation excites only a part of the usual 32 mm-pellet. Therefore a specimen rotation is used to cover the whole area of the pellet. But with this rotation turned off a smaller pellet can

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3 874 be placed directly in the excited area (13 mm diameter). A special Teflon ring was constructed to minimize backscattering and the small pellet was arranged asymmetrically in the ring due to the geometry of the device (figure 2). teflon ring pelle fig. 2: small pellet holder (top view) The results in mg/kg obtained by this method with the usually used pellet calibration show approximately 60% of the concentration of a normal size pellet but vary dependent on the matrix. Further investigations on these effects didn t take place because the reduction from 4 g to 660 mg is not very remarkable. Dry Filter Preparation In our institute a calibration for quartz fiber [1,2] and membrane filters [3] was developed usually for use with aerosol sampling. However, due to the grain size difference of an aerosol and a finely ground stone is not very remarkable, the evaluation of a filter coated with powdered stone with this calibration is possible. To avoid in-deptheffects membrane filters were used (Sartorius, Germany, cellulose nitrate filter, 0.45 pm pore size, 50 mm diameter). The experimental set-up shows figure 3. The sample was placed at the bottom of the device, swirled up with purified compressed air and sucked through the filter. The big advantage of this method is the sample amount of only 4 mg. That is only the thousandth part of the original pellet. The disadvantage is the loose filter coating. It is necessary to handle the filters very carefully in order not to lose parts of the coating. Since the radiation in the spectrometer comes from beneath (figure 1) the specimen has to be placed upside down into it. Therefore a fixation of the coating is necessary. Several attempts with different methods have been tested. Up to now hair spray seems to deliver the best results but puyp / i., filter +.,..-..~_-- air admittance ---J fig. 3: dry filter preparation unit (25 I/min) -,: f is not an optimum. To avoid the loose coating another idea has been tried.

4 875 Wet Filter Preparation A suspension of the sample was prepared in a flask, stirred very well and a part of it was given into the instrumental set-up shown in figure 4. To obtain a homogeneous distribution over the whole filter area it is necessary to mix the sample suspension with the suspension liquid. The liquid was sucked through the filter and a stable coating of only 4 mg has been obtained. As liquid both, water and toluene have been used. With toluene the elution effects of the sample are reduced, but the handling is more difficult due to the smaller surface tension. In order to avoid in-depth-effects, to get a coating on top of the filter, a membrane filter with a pore size of 0.1 pm was used (Sartorius, Germany, cellulose nitrate filter, 50 mm diameter). The largest possible disadvantage of the wet filter preparation method, the elution of the sample by the liquid, is found to be below 3% with the samples tested yet. This error is lower than the complete mistake of the method. suspension liquid \ filter-2 ~ Pump waste I &%z?z?& Ii!I I fig. 4: wet filter preparation unit Weighing With an aerosol filter calibration the result is given in ng/cm2. The XRF-result of a stone is expected to be in mg/kg. Therefore the area weight of the filter has to be determined (equation 1). % (1) ;; = zg = ;; = resu~;af;;j;tion 1 _[ cm2 The area weight is calculated with the weight of the coating and the area of the coating. By weighing the empty filter and the coated filter five times each the coatings weight is obtained. The area is 32 mm in diameter (8.06 cm2). It is cut out from the whole 50 mm filter. This cut results in a mathematical correction which is described later. The membrane filters used have a weight of about 100 mg, but the coating is only 4 mg. For example a precision of 1% results in a repeatability of 40 pg. With a special micro-balance (Sartorius, Germany, MSP-OOOVOOl) including a faraday-cage it is possible to reach a precision of 2% for at least 5 measurements.

5 876 Results Dry preparation Figure 5 shows the measured concentration which is obtained by the filter calibration in ng/cm2 versus a reference concentration calculated by equation 2: [ 1 rock content z * coating s weight [ mg] [ ng 1 (2) reference concentration - = cm2 filter area [ cm* 1 The rock content was obtained by ICP-OES and ICP-MS. The correlation coefficient shows a relevant linear link between the measured concentration and the reference as expressed exemplary for Aluminum and Potassium. Other elements like Mg, Ca, Ti, Mn, Fe, Zn, Sr and Pb show a comparable behavior reference[ng/cm2] fig. 5: correlation curve - dry preparation Wet Preparation Nevertheless, figure 6 shows a similar correlation comparable to the dry preparation. Again, the reference concentration is calculated by equation 2.

6 Copyright (C) JCPDS International Centre for Diffraction Data reference [ng/cm*] Fig. 6: correlation curve - wet preparation There s a linear correlation between the measured and the calculated reference concentration. Hence it is possible to calculate a correction equation for each element. With these equations it is possible to calculate the concentration of other specimen prepared with the same method. The coated area of the wet preparation method is larger than the area of the dry preparation. This is due to the instrumental set-up. Therefore the sensitivity of the wet preparation method seems to be smaller than the sensitivity of the dry preparation method because the area which is cut out is the same (32 mm diameter),. Comparison with ICP-OES For the ICP-OES values shown in figure 7 the specimen was prepared by wet digestion. The XRF results were calculated by the concentration obtained from the former calibration curve (equation 3) and the weight of the coating as shown in equation 4. (3) cont. (cal curve) = cont. (measured) *a, + b, (x = element) (4) concentration (Cal. curve) Figure 7 presents only four elements with similar concentration ranges. But for other elements like Sr and Ti the results are in their range comparable.

7 x s g z$ T Mg Al K Fe ICP-OES digestion q XRF wet filter / Fig. 7: comparison ICP-OES / XRF The difference of the values ranges between 1 O-20% which is sufficient for a rough estimation of the main rock components. Conclusion The decrease from 4 g for the pellet method down to 4 mg with the filter method means a reduction by 99,9% in sample size. Nevertheless, due to the small sample size, trace element concentrations might be lower than the detection limit. Hence, for applications where a fingerprint of the analyzed stone from the main components is of interest the method is pretty good. And the differences of the ICP-OES and the XRF results shown in figure 7 might get lower if the calibration curve is enlarged with more samples. Further investigations have to take particle size distribution and different matrices into account. References 1 0. Haupt, B. Klaue, C. Schafer and W. Dannecker, Preparation of Quartz Fibre Filter Standards for X-Ray Fluorescence Analysis of Aerosol Samples, X-Ray Spectrometry 24, (1995) Haupt, C. Schafer, S. Strauss and W. Dannecker, Production of calibration standards for x-ray fluorescence analysis of aerosol particles precipitated on different filter materials, Fresenius J. Anal. Chem. 355,375 (1996) 3. C. Schafer, 0. Haupt and W. Dannecker, Comparison of two different approaches to calibrate an energy-dispersive x-ray fluorescence spectrometer for the analysis of aerosolloaded filters, Fresenius J. Anal. Chem. 355,379 (1996)