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USEFULNESS OF A DUAL MACRO AND MICRO ENERGY DISPERSIVE X-RAY FLUORESCENCE SPECTROMETER TO DEVELOP QUANTITATIVE METHODOLOGIES FOR HISTORIC MORTAR AND RELATED MATERIALS CHARACTERIZATION Cristina García-Florentino, Maite Maguregui, Miriam RomeraFernández, Ignasi Queralt, Eva Margui, and Juan Manuel Madariaga Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b00216 • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018
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Analytical Chemistry
USEFULNESS OF A DUAL MACRO AND MICRO ENERGY DISPERSIVE X-RAY FLUORESCENCE SPECTROMETER TO DEVELOP QUANTITATIVE METHODOLOGIES FOR HISTORIC MORTAR AND RELATED MATERIALS CHARACTERIZATION Cristina García-Florentino†*, Maite Maguregui‡, Miriam Romera-Fernándezδ, Ignasi Queraltγ, Eva Marguiλ, Juan Manuel Madariaga† †
Department of Analytical Chemistry, Faculty of Science and Technology, University of Basque Country UPV/EHU, P.O. Box 644, 48080 Bilbao, Basque Country, Spain, *email:
[email protected] ‡ Department of Analytical Chemistry, Faculty of Pharmacy, University of the Basque Country UPV/EHU, P.O. Box 450, 01080, Vitoria-Gasteiz, Basque Country, Spain δ MINERSA Group, C/Jaén nº1, 3º, 48012 Bilbao, Basque Country, Spain γ Institute of Environmental Assessment and Water Research. IDAEA-CSIC. Jordi Girona 18-26, 08034 Barcelona, Spain. λ Department of Chemistry, University of Girona, Faculty of Sciences, C/ M.Aurèlia Campmany, 69, Girona, Spain. ABSTRACT: Wavelength Dispersive X-ray fluorescence (WD-XRF) spectrometry has been widely used for elemental quantification of mortars and cements. In this kind of instruments, samples are usually prepared as pellets or fused beads and the whole volume of sample is measured at once. In this work, the usefulness of a dual Energy Dispersive X-ray fluorescence spectrometer (EDXRF), working at two lateral resolutions (1 mm and 25 µm) for macro and micro-analysis respectively, to develop quantitative methods for the elemental characterization of mortars and concretes is demonstrated. A crucial step before developing any quantitative method with this kind of spectrometers is to verify the homogeneity of the standards at these two lateral resolutions. This new ED-XRF quantitative method also demonstrated the importance of matrix effects in the accuracy of the results being necessary to use Certified Reference Materials as standards. The results obtained with the ED-XRF quantitative method were compared with the ones obtained with two WD-XRF quantitative methods employing two different sample preparation strategies (pellets and fused beads). The selected ED-XRF and both WD-XRF quantitative methods were applied to the analysis of real mortars. The accuracy of the ED-XRF results turn out to be similar to the one achieved by WD-XRF, except for the lightest elements (Na and Mg). The results described in this work proved that µ-ED-XRF spectrometers can be used not only for acquiring high resolution elemental map distributions, but also to perform accurate quantitative studies avoiding the use of more sophisticated WD-XRF systems or the acid extraction/alkaline fusion required as destructive pretreatment in Inductively Coupled Plasma Mass Spectrometry based procedures.
1. INTRODUCTION A realistic quantification of mortars by means of X-ray fluorescence (XRF) is not an easy task due to their high inhomogeneity and thus, a previous homogenization of this kind of samples must be carried out.1 In order to ensure the accuracy and the reproducibility of the results, materials are usually crushed or grinded into fine powder and then pellets at high pressure are prepared. Usually together with the mortar, a binding agent is added before the pelletization with the aim to obtain more resistant pellets. An adequate binding agent must be free from significant contaminant elements, it must present low X-ray absorption, it must be stable under vacuum and irradiation conditions and it should not introduce significant inter-element interferences. 1, 2 Some commonly used binders, which meet with all these characteristics are boric acid, cellulose and waxes.1, 3 Generally, XRF quantitative analysis is
carried out by external calibration-curve method. This kind of calibrations curves, comparing directly intensities with concentrations can only be used for a limited concentration range of an element and when the standards and sample matrix compositions are nearly similar. 4 In this way, net intensity (y-axis) is assumed to be linearly related to concentration (x-axis) through the general linear regression equation. However, for some applications it is difficult to find sufficient certified standards with similar matrices to those of the samples. In those cases, the use of synthetic standards prepared in the laboratory with commercially available pure elements or compounds, trying to reconstruct somehow synthetically the matrix of the samples, has proved to be in some cases an efficient calibration alternative, which is cheaper and can be easily prepared.
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Elemental quantification of building materials such as cements or concrete is mainly performed using Wavelength Dispersive X-ray fluorescence (WD-XRF) spectrometers in industrial laboratories; in fact special equipment and in-house methods are specifically designed for their characterization.5 In the research field, WD-XRF has been used for the characterization of mortars. 6,7 However, due to recent developments of benchtop Energy Dispersive X-ray fluorescence (ED-XRF) spectrometers, which offer easier operation, lower cost and more compact design in comparison to WD-XRF have promoted their use for characterization of mortars. 8 In most of the cases, the ED-XRF quantification is only performed by semiquantitative methods based on Fundamental Parameters equations. 8 There are other works in which different empirical calibration methodologies have been developed for elemental quantification in different matrixes such as vegetables 9 and rice 10, or in totally different samples such as nuclear fuels to quantify U and P. 11 In the field of Cultural Heritage, the characterization of building materials is commonly carried out by means of ED-XRF following an in situ qualitative strategy.12-16 There is an study where an empirical calibration based on standards was developed for the characterization of calcareous building materials with high Cultural Heritage value. 17 Anyway, all these calibration methodologies were in all the cases developed for different ED-XRF systems specifically designed for quantification. To the authors’ knowledge, there are no works where empirical calibration methodologies are successfully designed in a µ-ED-XRF spectrometer with the X-ray beam collimated at few microns or at millimeter. Nowadays is still a challenge to perform quantifications by means of µ-ED-XRF.18, 19 This kind of spectrometers are more frequently used to obtain the distribution maps of the elements in the sample (imaging study).20 If accurate quantification methods could be developed using this kind of ED-XRF spectrometers, the quantitative information of the elements present in the sample and their distribution maps could be obtained using the same device in contrast to other works in which two different spectrometers were necessary.9 In this work, the usefulness of a dual ED-XRF spectrometer working at two different lateral resolutions (1 mm and down to 25 µm) for macro and micro imaging studies was tested to develop quantitative methods for the elemental characterization of mortars and related materials (e.g. concrete) used in buildings. Two calibration strategies were selected. The first calibration method was based on the use of synthetic standards prepared as pellets using a mixture of different oxides, carbonates and sulfates, trying to synthetically reproduce these kind of matrices; additionally, in these mixtures elements at trace levels (mg/kg) were added as liquid solutions. The second one was based on the preparation of pellets of a set of different Portland Cements and rocks Certified Reference Materials (CRMs). The accuracy of the quantitative results achieved with both methods will be compared with the ones obtained with two WD-XRF quantitative methods employing two sample preparation strategies (pellets and fused beads); the accuracy of these last methods will be also checked using the same CRMs. The developed quantitative ED-XRF and both WD-XRF methods will be applied to the quantification of real mortars and concrete samples extracted from a historical
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building of the early 20th century, Punta Begoña Galleries (Getxo, Basque Country, Spain).
2. EXPERIMENTAL PROCEDURE 2.1. Calibration standards description and preparation 2.1.1. Synthetic standards For the empirical calibration based on the use of synthetic standards different oxides such as MgO >99% pure (Panreac, Barcelona, Spain), SiO2 99% pure (Carlo Erba, Sabadell, Spain), Fe2O3 99% pure (Sigma-Aldrich, St. Louis, MO), Al2O3 99% pure (Fluka Analytical, Sigma-Aldrich, St. Louis, MO) and TiO2 (anatase), 99% pure (Merck, Darmstadt, Germany) were employed. Trying to obtain calibration standards with similar matrixes to the calcite based mortar samples, calcium was added as CaCO3 (calcite) 99% pure (SigmaAldrich, St. Louis, MO) and as pure CaSO4·2H2O (gypsum) (kindly suministrated by a private enterprise). This gypsum was also taken as the sulfur source for calibration of this element together with BaSO4 99% pure (Fluka Analytical, Sigma-Aldrich, St. Louis, MO). Trying to avoid the instability of some oxides, Na was added as NaCl 99% pure (J.T Baker, Thermo Fisher Scientific, Madrid, Spain), being this also the source for Cl quantification. K was added as KNO3 99% pure (Merck, Darmstadt, Germany). Stock solutions of Mn and Rb of 1000 mg/L in HNO3 0.5 M (Romil Pure Chemistry, Cambridge, UK) and Zn of 1000 mg/L in HCl 1% w/w (Fluka Analytical, Sigma-Aldrich, St. Louis, MO) were used to add different small volumes of the elements expected at trace concentrations (
