Comparison of a portable micro-X-ray fluorescence spectrometry with inductively coupled plasma atomic emission spectrometry for the ancient ceramics analysis

Spectrochimica Acta Part B 59 (2004) 1877 – 1884 www.elsevier.com/locate/sab

Comparison of a portable micro-X-ray fluorescence spectrometry with inductively coupled plasma atomic emission spectrometry for the ancient ceramics analysis D.N. Papadopouloua, G.A. Zachariadisa, A.N. Anthemidisa, N.C. Tsirliganisb, J.A. Stratisa,* b

a Laboratory of Analytical Chemistry, Faculty of Chemistry, Aristotle University, GR-54124, Thessaloniki, Greece Archaeometry Laboratory, Cultural and Educational Technology Institute, Tsimiski 58, GR-67100, Xanthi, Greece

Received 10 February 2004; accepted 24 August 2004 Available online 12 October 2004

Abstract Two multielement instrumental methods of analysis, micro X-ray fluorescence spectrometry (micro-XRF) and inductively coupled plasma atomic emission spectrometry (ICP-AES) were applied for the analysis of 7th and 5th century B.C. ancient ceramic sherds in order to evaluate the above two methods and to assess the potential to use the current compact and portable micro-XRF instrument for the in situ analysis of ancient ceramics. The distinguishing factor of interest is that micro-XRF spectrometry offers the possibility of a nondestructive analysis, an aspect of primary importance in the compositional analysis of cultural objects. Micro-XRF measurements were performed firstly directly on the ceramic sherds with no special pretreatment apart from surface cleaning (micro-XRF on sherds) and secondly on pressed pellet disks which were prepared for each ceramic sherd (micro-XRF on pellet). For the ICP-AES determination of elements, test solutions were prepared by the application of a microwave-assisted decomposition procedure in closed high-pressure PFA vessels. Also, the standard reference material SARM 69 was used for the efficiency calibration of the micro-XRF instrument and was analysed by both methods. In order to verify the calibration, the standard reference materials NCS DC 73332 and SRM620 as well as the reference materials AWI-1 and PRI-1 were analysed by micro-XRF. Elemental concentrations determined by the three analytical procedures (ICP-AES, micro-XRF on sherds and micro-XRF on pellets) were statistically treated by correlation analysis and Student’s t-test (at the 95% confidence level). D 2004 Elsevier B.V. All rights reserved. Keywords: Ancient ceramics; Micro-XRF; Inductively coupled plasma; Multielement methods

1. Introduction The study of cultural objects, such as ancient ceramics, involves the application of a wide range of instrumental methods of analysis which enable the extraction of objective information. Such scientific information, in conjunction with the archaeological information, offers an integrated scientific methodology that may be effectively used in provenance or characterization studies of archaeological materials [1,2]. Inductively coupled plasma atomic emission spectrometry (ICP-AES) is often used for the multielement analysis of ancient ceramics because it presents some excellent ana* Corresponding author. Tel.: +30 2310997843; fax: +30 2310997719. E-mail address: [email protected] (J.A. Stratis). 0584-8547/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sab.2004.09.001

lytical characteristics such as high precision, sensitivity and selectivity [3–5]. However, in the ICP-AES analysis, the samples should be in an aliquot form thus a part of the original ceramic sample is permanently destroyed by the application of a digestion method. X-ray fluorescence (XRF), on the other hand, is a multielement, sensitive and nondestructive method that is commonly used for the analysis of archaeological materials [6–8]. In addition, the development of portable energy-dispersive X-ray spectrometers has received considerable scientific attention. Several spectrometers have been constructed and numerous applications have been reported many among them concerning archaeological materials [9]. The emphasis that was given to the development of these systems was based on the expectations that portable X-ray spectrometers would create new

1878

D.N. Papadopoulou et al. / Spectrochimica Acta Part B 59 (2004) 1877–1884

fields of application and contribute significantly to the progress of provenance studies [9]. More recently, microscopic analytical methods (microX-ray fluorescence spectrometry, electron probe microanalysis and secondary ion spectroscopy) have rapidly been developed. These methods offer new abilities in the material analysis and are more frequently included in archaeometric studies [10,11]. Micro-XRF is the microscopic equivalent of bulk XRF and it is a suitable analytical method for the analysis of cultural objects because, apart from its nondestructive character, micro-XRF is a fast, accurate, sensitive, universal, versatile and multielement analytical method [1,10–13]. Its principal advantages are the minimization of sample preparation, the ability to operate in air and to perform a localised nondestructive analysis of microscopic areas over the surface of large objects [11,14]. Various forms of micro-XRF may be used depending on the scientific questions of each investigation. Synchrotron micro-XRF has the advantage of offering intense radiation; it is often used in XRF microprobe analysis of ancient ceramics, but it requires big and expensive facilities [15,16]. On the other hand, laboratory micro-XRF instruments can operate with either X-ray tubes or radioisotopic sources. The most recent development in the micro-XRF technique is the construction of small, compact and portable micro-XRF instruments in contrast with the bench-top instruments [17]. These portable instruments are expected to contribute, to a large extent, to the analysis of archaeological findings because they offer the advantage of their in situ analysis in places where the artefacts are normally located (museums, galleries and archaeological sites). The construction of small and portable micro-XRF instruments involves a critical selection of the appropriate components such as X-ray sources, focusing optics and detectors [18]. In their primary form, microbeams were produced by a collimator system. However, after the most recent developments in the field of capillary optics, polycapillary and monocapillary lenses have become an essential part of the micro-XRF instrumentation [19–21]. Ancient ceramics are the most common archaeological findings and they carry a significant historical content. A large variety of analytical methods can be applied for their analysis and comparative results between different analytical methods are often reported in the scientific literature [22,23]. Micro-XRF instruments with a compact and portable construction have been used for the analysis of historical metallic objects, multicolored items, industrial materials, forensic glasses, gold artifacts, bronze statuettes and paint layers [17,19,24,25]. However, no applications have still been reported on ancient ceramics by a similar instrument with monocapillary optics. In addition, micro-XRF has not been fully characterized in comparison with more traditional instrumental methods, such as ICP-AES, for the analysis of ancient ceramics.

The aim of the present work was to investigate and evaluate the ability of the employed portable micro-XRF instrument for the in situ multielement and quantitative (Si, Fe, Ca, K, Mn, Ti) analysis of ancient ceramic samples. The obtained results were compared with those from ICP-AES analysis after microwave-assisted digestions. The mean elemental concentrations were compared by correlation analysis and Student’s t-test. In addition, the standard reference material (SARM 69) was used for the efficiency calibration of the micro-XRF instrument and was analysed by both methods. In order to verify the calibration of the micro-XRF, the reference materials NCS DC 73332, SRM620, AWI-1 and PRI-1 were analysed.

2. Experimental 2.1. Instrumentation and operational conditions The XRF spectrometer used in this work is a portable and compact micro-XRF unit based on the prototype model that was developed under the EU project bCOPRAQ (Compact Portable Roentgen Analyser Project), with the difference that the X-ray optics include a straight monocapillary lens instead of a polycapillary one. Straight monocapillaries act as wave-guides and X-rays are transported without the 1/r 2 losses that normally occurs in a collimator. The instrument consists of a side-window X-ray tube with Mo anode (Series 5011 XTF, Oxford Instruments) and maximum voltage/ current of 50 kV/1 mA. The nominal beam diameter is b150 Am at the position of the sample. The micro-XRF spectrometer and the experimental setup are shown in Figs. 1 and 2, respectively. The samples (either in sherd or pellet

Fig. 1. Micro-XRF spectrometry system.

D.N. Papadopoulou et al. / Spectrochimica Acta Part B 59 (2004) 1877–1884

1879

Table 1 Micro-XRF spectrometer operating conditions Tube conditions Tube Real time acquisition Beam diameter (at sample position) X-ray optics Detector

Fig. 2. Schematic diagram of the experimental micro-XRF geometry.

form) are placed on a rotating holder mounted on a computer-controlled XYZ-motorized stage which can move in steps of 0.1 mm and have a maximum travel distance of 5 cm in the X, Y direction and 2.5 cm in the Z direction. The angle of incidence of the primary X-ray beam on the sample surface is 488 (relative to the surface). Thus, the geometry can be accurately reproduced in each measurement. A solid state Si(Li) Peltier-cooled detector (8 Am Be window, 3.5 mm2 active area, 300 Am nominal thickness and resolution of 149–166 eV at the Mn K a energy) is used for the detection of the secondary fluorescence X-rays positioned at 908 relative to the primary beam. The relative angle between the X-ray tube and the detector is fixed by the manufacturer and cannot be altered (except by using specific setup procedures). A long-distance optical microscope is placed on the detector and X-ray tube plane and is used in order to locate the focal spot on the sample surface. All micro-XRF measurements were performed in a point scan mode on several points which were selected so as to cover the whole surface of the sample (in either sherd or pellet form). In particular, 10 measurements were performed per each sample (average dimension of sherds was 22 cm2 and the diameter of pellets was 30 mm). Reported concentrations are mean values of the 10 measurements per sample. The elements of interest were identified by their characteristic X-ray lines in the energy range of 0–25 keV. The operating conditions during the micro-XRF measurements are summarised in Table 1. The current and voltage values were chosen based on the results of a preliminary investigation concerning the effect of voltage value on the intensity of the characteristic peak areas. For the six elements that were determined in the present study, we found that the characteristic peak area values approach an upper limit value (cps) around a voltage of 30 kV and any further increase in voltage does not influence the intensity of the characteristic X-ray lines significantly. Although for elements such as Mn and Fe, we noticed a further increase in the characteristic peak areas after a respective increase in the voltage value, the recorded

35 kV, 0.9 mA Mo mini-focus 300 s b150 Am Monocapillary Si(Li), Peltier cooled

count rate is sufficiently high, at around 30–35 kV, so as any further increase in the voltage was not regarded as essential. Finally, the required increase in the acquisition time, in order to improve the low-intensity characteristic X-ray lines of trace elements (such as Ni, Zn, Cu, etc.) is not reasonable. Thus, the acquisition time was kept in the present value and their quantification was omitted. A Perkin-Elmer Optima 3100XL Series spectrometer was used for the ICP-AES analysis. The elements determined by ICP-AES were detected in two different emission lines. Based on the precision and sensitivity observed in each emission line, the following were chosen: Si at 251.611 nm, Fe at 238.204 nm, Ca at 317.933 nm, Ti at 336.121 nm and Mn at 259.372 nm. Potassium was determined by FAES because, in the available ICP instrument, the polychromator range was 165–403 nm. The operating conditions of the ICP-AES spectrometer are described in Table 2. The microwave-assisted digestions were performed in a MARS 5 microwave oven (CEM, USA, 1200 W). The device is equipped with a Teflon-coated cavity and a removable 12-position sample carousel. The samples were digested in high-pressure closed Teflon PFA vessels (HP500 Plus type, maximum temperature=483 K, maximum pressure=2.4 MPa, volume=100 ml) which are placed in high-strength outer sleeve assemblies. The microwave power output is managed through direct feedback from temperature (EST-300 Plus probe) and pressure (ESP-1500 Plus) probes, providing precise control of the chemical reactions. The vessels are equipped with special caps (Autovent Covers) that are used for the secure release of the excess pressure. Table 2 Operating conditions of Perkin Elmer Optima 3100 XL ICP-AES instrument RF generator RF incident power Sample uptake rate Argon flow rates Air flow rate (shear gas) Viewing mode Torch, injector, id Nebulizer Sample propulsion Sample flow rate Spray chamber Polychromator/resolution Detector

40 MHz, free-running Optimized 1 ml min 1 Auxiliary 0.5 l min 1; nebulizer 0.8 l min 1; plasma 15 l min 18 l min 1 Axial Fassel type, Alumina, 2.0 mm Gem tip cross flow Peristaltic pump, three channel Optimised Scott double pass Echelle/0.006 nm at 200 nm Segmented-array charge-coupled (SCD)

1

1880

D.N. Papadopoulou et al. / Spectrochimica Acta Part B 59 (2004) 1877–1884

2.2. Samples and reference materials Six ceramic sherds (A2, A4, A14, A17, A25 and A35) from the excavation in the ancient cemetery of Abdera (North Greece) were used. Samples A2, A4, A25 and A35 date back to 7th century B.C. and samples A14 and A17 date back to the 5th century B.C. None of the sherds presented decoration over its surface. Samples were dried overnight at 105 8C prior to the analysis. The standard reference material SARM 69 (prepared and distributed by MINTEK South Africa and the Department of Geology, University of Free State) was used as a calibration standard in the micro-XRF and was analysed by both micro-XRF and ICP-AES. Moreover, the synthetic silicate standard reference material, NCS DC 73332 (China National Analysis Center for Iron and Steel), the SRM620 (NIST, Soda Lime Flat Glass) and the reference materials AWI-1 and PRI-1 (both from Group of Instrumental Geochemistry, Fonds National de la Recherche Scientifique/Nationaal Fonds voor Wetenschappelijk, FNRSNFWO) were analysed by micro-XRF in order to verify the calibration. 2.3. Sample preparation The micro-XRF measurements were performed directly on the ancient ceramic sherds and on pressed pellets. In the first case, the only preparation step followed was the removal of the external layer with a drill and a tungsten carbide cutter in order to eliminate the possible surface contamination effects. The sample surface was not polished. This was not considered necessary since in microXRF the beam size is in the order of microns (Am). Moreover, although this treatment partially revokes the nondestructive character of the micro-XRF analysis, it was followed at the present work for comparison reasons. In the second case, subsamples were cut-off from the ceramic samples and they were finely powdered in an agate mortar. The powdered ceramic samples were then sieved and the fraction with an average grain size b93 Am was isolated. Pressed pellets were finally prepared by thoroughly mixing this fraction of the powdered ceramic samples with a cellulose binder in a 4:1 (sample/binder) ratio and pressing was performed using a 11-ton hydraulic press. The grain size fraction of b93 Am was selected to match the grain size of the calibration standard. In the calibration and quantification procedure of a micro-XRF spectrometer, it is important that the analyzed standards and samples have similar grain size distributions. All standard reference materials were prepared in pressed pellets by a similar procedure as the ceramic samples. For the ICP-AES analysis, subsamples were cut off from the initial sherds and were then finely powdered in an agate mortar. Decomposition of the silicate matrix was performed in a microwave oven in closed pressurized vessels. In particular, 0.1 g of each powdered sample was placed in a

Teflon PFA digestion vessel and a mixture of 5 ml HF 40% m/m and 5 ml HNO3 69.5% m/m was added. The exact decomposition procedure followed has been described previously [26]. The standard reference material SARM69 was analysed in a similar way. Multielement, matrixmatched standards were used for the quantitative determinations (in 0.5 mol l 1 in HNO3).

3. Results and discussion 3.1. Micro-XRF calibration and spectra evaluation The evaluation of micro-XRF spectra in order to extract quantitative results involves taking into account a number of factors each possessing an impact on the final result [1,27,28]. Heterogeneity of the different particle sizes and elemental distributions of the samples can influence significantly the final results in the micro-XRF analysis. In practice, micro-X-ray spectra that are acquired from several points on the surface of a ceramic sherd can present qualitative (different peaks) and quantitative (different intensity) differences. On the contrary, when Xray spectra are acquired from several points on the surface of a pellet, the spectra are always qualitatively and quantitatively similar (Figs. 3 and 4, respectively). The acquired spectra in the current measurements include also the Compton and Rayleigh scattering lines of Mo as well as the K a line of Zr, which is a component element of the detector collimator. The calibration procedure in the micro-XRF spectrometry, for the quantitative analysis of ceramics is a complicated procedure since a number of parameters such as surface irregularities, crystal structure, grain size, geometry and absorption of the X-rays, influence the final result [1,20,29]. Moreover, there is a lack of ceramic SRMs specifically for micro-XRF analysis. After each measurement, the spectra were deconvoluted by a dedicated software (WinAxil software package v 4.0.1.) in order to determine the fluorescence intensities of chemical elements [30,31]. These intensity data were then quantified automatically by a fundamental parameter correction procedure. In particular, spectral data are processed by the method of least squares fitting, where the optimum values of x 2 (the weighted sum of squares of the differences between a chosen model and the measured spectrum) are found iteratively (Marquardt algorithm). A suitable background estimation model was also chosen (smooth filter). This is an important step in the quantitative evaluation of samples since several fitting procedures may be applied [32–34]. Smooth filter model compares the content y i of a channel i with the mean m i of the two neighboring channels (channels i 1 and i+1, respectively). If the value m i is smaller than the channel content y i , the content of channel i is replaced by the mean m i . Repetition of this process for the whole spectrum gradually causes the peak to be stripped away. The used

D.N. Papadopoulou et al. / Spectrochimica Acta Part B 59 (2004) 1877–1884

1881

Fig. 3. Micro-XRF spectrum acquired from two different points (a and b) on the surface of sample A2 in the form of sherd.

software allows the calibration of the system by the fundamental parameters method with a single standard and the quantitative analysis of samples in a compare mode where it is assumed that the calibration standard and samples have similar matrix compositions. The type of the X-ray tube excitation is calculated and stored by the software, after relevant data (type of tube window, measurement conditions) are entered. The obtained precision of micro-XRF analysis was high for all six elements (in terms of relative standard deviation, RSD%) and it ranged between 0.15% and 1.04%. The precision was determined by taking several measurements on different points (12 points) over the calibration standard and by repeating the measurements several times in each point (10 repetitions in each point). All spectra were then fitted and precision was determined by calculating weighted average peak areas and the respective weighted standard deviation for each element. In order to verify the overall quality of the calibration and to compare micro-XRF with an already accepted method, the calibration standard was reevaluated as a sample using the obtained calibration parameters. The reevaluated concentration values together with the certified values and

obtained values by ICP-AES analysis are given in Table 3. Mean elemental concentrations determined by micro-XRF present a slight positive deviation. Light elements like Na and Al cannot be analysed by the available micro-XRF system. It is important to notice, that laboratory built A-XRF instruments are most often used for the analysis of low to medium atomic number elements (Si–Cu), while heavier elements that are present in low concentrations are not often investigated [1,29]. Although a higher number of elements were determined for the SARM69 by the ICP-AES, we report only the concentrations of six analytes in Table 3 for comparison purposes. For the verification of the micro-XRF calibration, a synthetic silicate (NCS DC 73332) and two sedimentary rock reference materials (AWI-1 and PRI-1) were analysed. In addition, micro-XRF was applied for the analysis of SRM620. Although this is a glass standard reference material, it is important to observe the performance of the present micro-XRF spectrometer for the analysis of another silicate matrix. The results of the above analyses are given in Table 4. The relative errors of the micro-XRF mean values range between 0% and 10% for the majority of cases.

Fig. 4. Micro-XRF spectrum acquired from two different points (a and b) on the surface of sample A2 in the form of pellet.

1882

D.N. Papadopoulou et al. / Spectrochimica Acta Part B 59 (2004) 1877–1884

Table 3 Comparison among micro-XRF, ICP-AES and certified values for the standard reference material SARM 69 (concentrations in %m/mFS.D.) micro-XRF ICP-AES Certified

SiO2

K2 O

CaO

TiO2

MnO

Fe2O3

69.1F0.8 67.3F3.6 66.6F0.5

2.20F0.01 2.00F0.03 1.96F0.04

2.33F0.03 2.38F0.02 2.37F0.04

0.889F0.039 0.699F0.011 0.777F0.008

0.149F0.001 0.119F0.002 0.129F0.002

8.36F0.06 6.35F0.21 7.18F0.075

3.2. Analysis of ancient ceramic sherds Results from the quantitative analysis (in %m/m oxide concentrationFS.D.) of the six ancient ceramic sherds determined by the three analytical procedures, that is, ICP-AES after microwave-assisted digestion, micro-XRF applied directly on the ceramic sherds (micro-XRF-s) and micro-XRF applied on pressed pellets (micro-XRF-p), are presented on Table 5. Precision of micro-XRF analysis for the six different samples (in terms of relative standard deviation, RSD%), ranged between 1.1% and 5.6% for Si, between 2.9% and 4.7% for Fe, between 1.1% and 5.8% for Ca, between 0.8% and 3.9% for Ti, between 0.1% and 4.2% for K and between 0.2% and 5.8% for Mn. For the ICP-AES analysis, precision ranged between 1.2% and 5.6% for Si, between 2.9% and 4.7% for Fe, between 1% and 5.8% for Ca, between 0.8% and 3.9% for Ti, between 0.2% and 4.2% for K and between 0.2% and 5.8% for Mn. The mean oxide concentrations were subjected to linear correlation analysis, and linear correlation coefficients for each element oxide were calculated in each one of the three comparison pairs: ICP-AES vs. micro-XRF-s, ICP-AES vs. micro-XRF-p and micro-XRF-s vs. micro-XRF-p. For the first two comparison pairs, it was found that correlation coefficients are low for the majority of elements. Notable exceptions were noticed for SiO2 and K2O (correlation coefficients 0.76 and 0.84 respectively). In addition, for CaO, TiO2 and K2O in the pair micro-XRF-s vs micro-XRF-

p, strong correlation was noticed (correlation coefficients 0.97, 0.99 and 0.81 respectively), while for Fe2O3 in the same pair, a moderate correlation was noticed (correlation coefficient 0.71). In overall, elemental concentrations that were determined by micro-XRF both on ceramic sherds and on pressed pellets correlate strongly for three out of the six elements (CaO, TiO2 and K2O) and moderately for one element (Fe2O3). Only for Mn, the correlation coefficients were very low for all comparison pairs. This differentiation can be attributed to spectral interferences that exist in the microXRF analysis. In particular, the K b1 X-ray line of Mn at 6.490 keV may interfere with the K a1 and K a2 lines of Fe at 6.403 and 6.390 keV, respectively [32]. The interference is affecting primarily Mn while for Fe the effects are limited due to its significantly higher concentration (1:100 ratio in the particular samples). For the same comparison pairs, mean oxide concentrations for each ceramic sample, were treated with a Student’s t-test, at the 95% confidence level. In the majority of cases, the mean oxide concentrations from the three different analytical procedures differed significantly for all comparison pairs (that is ICP-AES vs. micro-XRF-s, ICP-AES vs. micro-XRF-p and micro-XRF-s vs microXRF-p). Only for samples A2, A4 and A17, the concentration of TiO2 as it was determined by micro-XRF-s and micro-XRF-p presented statistically not significant differences. Moreover, for the same comparison pair and for

Table 4 Comparison between micro-XRF, certified and recommended values for the reference materials NCS DC 73332, SRM620, AW-I and PR-I (concentrations in %m/mFS.D.) SiO2

K2O

CaO

SRM620 Micro-XRF Certifieda

78.30F0.36 72.08F0.08

0.35F0.00 0.41F0.03

5.96F0.09 7.11F0.05

NCS DC 73332 Micro-XRF Certifiedb

67.20F0.90 (72)

1.05F0.01 (1.35)

AW-I Micro-XRF Recommended

59.45F0.90 60.46F0.55

PR-I Micro-XRF Recommended

64.50F0.98 68.60F0.35

TiO2

MnO

Fe2O3

1.11F0.04 (1.80)

0.120F0.005 0.100F0.003

0.088F0.002 0.100F0.002

3.89F0.02 (4.0)

3.06F0.04 3.06F0.09

0.73F0.07 0.69F0.07

0.95F0.03 0.92F0.05

0.16F0.00 0.14F0.01

7.93F0.06 7.21F0.17

3.40F0.04 3.79F0.21

2.70F0.07 2.49F0.16

0.67F0.02 0.71F0.03

0.06F0.00 0.04F0.01

3.32F0.02 3.32F0.13

a SRM620 is not certified for MnO oxide concentration. The certified concentrations for TiO2 and Fe2O3 oxide are low (0.018% and 0.043%, respectively) and were not quantified by the current micro-XRF measurement conditions. b Values in parenthesis are recommended values.

D.N. Papadopoulou et al. / Spectrochimica Acta Part B 59 (2004) 1877–1884

1883

Table 5 Comparative results of the chemical composition of ancient ceramic sherds from ancient Abdera (%m/mFS.D.) Sample

Method

SiO2

Fe2O3

CaO

MnO

TiO2

K2Oa

A4

ICP-AES micro-XRF-s micro-XRF-p ICP-AES micro-XRF-s micro-XRF-p ICP-AES micro-XRF-s micro-XRF-p ICP-AES micro-XRF-s micro-XRF-p ICP-AES micro-XRF-s micro-XRF-p ICP-AES micro-XRF-s micro-XRF-p

51.35F2.89 39.91F0.45 41.11F0.45 54.01F0.62 60.46F0.61 41.81F0.47 56.74F0.81 54.40F0.75 43.34F0.54 52.00F1.37 48.56F0.79 39.40F0.49 49.91F2.50 38.25F0.53 40.01F0.52 53.96F0.66 46.85F0.52 41.06F0.48

3.49F0.11 6.04F0.03 6.66F0.03 3.69F0.13 5.25F0.03 5.65F0.03 4.52F0.15 5.77F0.04 6.27F0.03 5.26F0.25 6.80F0.05 7.05F0.04 4.80F0.17 4.70F0.03 6.26F0.04 4.97F0.14 5.74F0.03 5.96F0.03

2.11F0.12 3.84F0.10 3.39F0.07 1.04F0.01 1.42F0.04 1.61F0.04 0.37F0.06 2.35F0.08 1.99F0.04 1.68F0.08 1.25F0.05 1.48F0.03 3.63F0.11 3.04F0.09 3.13F0.08 2.05F0.06 2.70F0.12 2.60F0.06

0.031F0.002 0.070F0.002 0.095F0.002 0.025F0.001 0.068F0.002 0.085F0.002 0.026F0.000 0.089F0.003 0.094F0.002 0.047F0.001 0.080F0.003 0.098F0.002 0.036F0.001 0.049F0.001 0.186F0.004 0.032F0.001 0.076F0.002 0.080F0.001

0.646F0.005 0.609F0.013 0.604F0.013 0.490F0.007 0.573F0.015 0.598F0.014 0.594F0.008 0.553F0.016 0.565F0.010 0.720F0.023 1.000F0.030 0.824F0.020 0.598F0.006 0.532F0.014 0.535F0.013 0.636F0.025 0.525F0.012 0.535F0.009

0.27F0.00 0.83F0.01 1.04F0.01 1.62F0.07 2.03F0.03 1.73F0.01 1.17F0.02 1.68F0.02 1.51F0.01 1.56F0.01 2.05F0.02 1.80F0.01 1.06F0.00 0.81F0.01 1.03F0.01 1.80F0.04 3.13F0.02 1.64F0.01

A25

A17

A35

A2

A14

a

Determined by FAES.

sample A14 mean concentrations of CaO presented statistically not significant differences. Finally, Fe2O3 mean concentration did not differ significantly between ICPAES and micro-XRF on pellets for sample A2. These differences are justified by the fact that in ICP-AES the bulk volume of the sample is used in the sample preparation procedure, while, in micro-XRF applied on pellets, the grain size was restricted to a certain fraction, and, in micro-XRF applied directly on sherds, the external layer is analyzed and heterogeneity greatly affects the final analytical result.

4. Conclusions A good agreement was observed between elemental mean values that were determined by the current micro-XRF instrument and certified or recommended values, for five reference materials (SARM69, NCS DC 73332, SRM620, AW-I and PR-I). Multielement and nondestructive analysis constitutes an objective of increasing interest in the archaeometry research field. Six different ceramic samples from ancient Abdera, Northern Greece were analysed by micro-XRF spectrometry (both as sherds and as pressed pellets) and also by ICP-AES. Elemental mean concentrations show statistically significant differences (95% CL) when comparisons are performed among the different analytical procedures used. These differences could be attributed to the different parameters that affect the analytical results in each procedure. The choice between a destructive (ICP-AES) or a non-destructive (micro-XRF) method of analysis depends on the specific demands of each investigation. Most importantly, sample preparation plays a crucial role not only between different analytical methods, (ICP-AES or micro-XRF), where the sample may be

analysed in the form of aliquot or solid but also in the same analytical method (micro-XRF) where the sample may be analysed in pellet form or as an untreated sherd. Nevertheless, a strong correlation was noticed between the mean elemental concentration obtained by micro-XRF analysis applied on sherds and by micro-XRF applied on pellets for three out of six elements and a moderate correlation for one element. Micro-XRF applied directly on ancient ceramics is beneficial with respect to time of analysis and is nondestructive. However, heterogeneity problems that affect the quantification procedure are more prominent in this case. Consequently, the portable microXRF spectrometer employed is a promising tool that may be used for a quick screening analysis of ancient sherds, without the need for time-consuming sample preparation procedures.

Acknowledgments The current project is part of Mrs D.N. Papadopoulou PhD thesis. It was funded by the Greek General Secretariat of Research and Technology and the EU, in terms of the programme PENED 2001 (project 01ED240). The authors wish to express their gratitude to Mrs K. Kallintzi, director of the excavation in the cemetery of ancient Abdera, and Mrs M. Chrisafi, archaeologist, for their assistance in the sampling of the ancient ceramic sherds.

References [1] K. Janssens, F. Adams, A. Rindby, Microscopic X-ray Fluorescence Analysis, Wiley, Chichester, 2000. [2] A.M. Pollard, C. Heron, Archaeological Chemistry, RSC, Cambridge, 1996.

1884

D.N. Papadopoulou et al. / Spectrochimica Acta Part B 59 (2004) 1877–1884

[3] B. Bruno, M. Caselli, M.K. Curri, A. Genga, R. Striccoli, A. Traini, Chemical characterization of ancient pottery from south of Italy by inductively coupled plasma atomic emission spectrometry (ICP-AES), Anal. Chim. Acta 410 (2000) 193 – 202. [4] J. Perez-Arantequi, M.I. Urunuela, J.R. Castillo, Roman glazed ceramics in the Western Mediterranean: chemical characterization by inductively coupled plasma atomic emission spectrometry of ceramic bodies, J. Archaeol. Sci. 23 (1996) 903 – 914. [5] L. Paama, I. Pitkanen, P. Peramaki, Analysis of archaeological samples and local clays using ICP-AES, TG-DTG and FTIR techniques, Talanta 51 (2000) 349 – 357. [6] R.E. Van Grieken, A.A. Markowicz, Handbook of X-ray Spectrometry, Marcel Dekker, New York, 1993. [7] A.E. Pillay, C.K. Punyadeera, L. Jacobson, J. Eriksen, Analysis of ancient pottery and ceramic objects using X-ray fluorescence spectrometry, X-ray Spectrom. 29 (2000) 55 – 62. [8] M. Mantler, M. Schreiner, X-ray fluorescence spectrometry in art and archaeology, X-ray Spectrom. 29 (2000) 3 – 17. [9] O. Williams-Thorpe, P.J. Potts, P.C. Webb, Field-portable nondestructive analysis of lithic archaeological samples by X-ray fluorescence instrumentation using a mercury iodide detector: comparison with wavelength-dispersive XRF and a case study in British stone axe provenancing, J. Archaeol. Sci. 26 (1999) 215 – 237. [10] F. Adams, A. Adriaens, A. Aerts, I. De Raedt, K. Janssens, O. Schaim, Micro and surface analysis in art and archaeology, J. Anal. At. Spectrom. 12 (1997) 257 – 265. [11] P.A. Pella, M. Lankosz, Highlights of X-ray spectrometry for microanalysis, X-ray Spectrom. 26 (1997) 327 – 332. [12] G.J. Havrilla, Applications of X-ray microfluorescence to materials analysis, X-ray Spectrom. 26 (1997) 364 – 373. [13] K. Janssens, L. Vincze, J. Rubio, G. Bernasconi, F. Adams, Microscopic X-ray fluorescence analysis, J. Anal. At. Spectrom. 9 (1994) 151 – 157. [14] K. Janssens, G. Vittiglio, I. Deraedt, A. Aerts, B. Vekemans, L. Vincze, F. Wie, I. Deryck, O. Schalm, F. Adams, A. Rindby, A. Knochel, A. Simionovici, A. Snigirev, Use of microscopic XRF for non-destructive analysis in art and archaeology, X-ray Spectrom. 29 (2000) 73 – 91. [15] F. van Langevelde, R.D. Vis, Trace element determination using a 15-keV synchrotron X-ray microprobe, Anal. Chem. 63 (1991) 2253 – 2259. [16] F. Adams, K. Janssens, A. Snigirev, Microscopic X-ray fluorescence and related methods with laboratory and synchrotron radiation sources, J. Anal. At. Spectrom. 13 (1998) 319 – 331. [17] G. Vittiglio, K. Janssens, B. Vekemans, F. Adams, A. Oost, A compact small-beam XRF instrument for in-situ analysis of objects of historical and/or artistic value, Spectrochim. Acta, Part B 54 (1999) 1697 – 1710. [18] S. Bichlmeier, K. Janssens, J. Heckel, D. Gibson, P. Hoffmann, H.M. Ortner, Component selection for a compact micro-XRF spectrometer, X-ray Spectrom. 30 (2001) 8 – 14. [19] R. Lu, A. Le, Y. Gu, G. Wu, J. Zhu, Precious metals assay by means of microbeam XRF technology, Nucl. Instrum. Methods, B 104 (1995) 595 – 601.

[20] S. Bichlmeier, K. Janssens, J. Heckel, P. Hoffmann, H.M. Ortner, Comparative material characterization of historical and industrial samples by using a compact micro-XRF spectrometer, X-ray Spectrom. 31 (2002) 87 – 91. [21] K. Janssens, B. Vekemans, L. Vincze, F. Adams, A. Rindby, A microXRF spectrometer based on a rotating anode generator and capillary optics, Spectrochim. Acta Part B 51 (1996) 1661 – 1678. [22] F. Cariati, P. Fermo, S. Gilardoni, A. Galli, M. Milazzo, A new approach for archaeological ceramics analysis using total reflection Xray fluorescence spectrometry, Spectrochim. Acta Part B 58 (2003) 177 – 184. [23] A. Tsolakidou, V. Kilikoglou, Comparative analysis of ancient ceramics by neutron activation analysis, inductively coupled plasma–optical-emission spectrometry, inductively coupled plasmamass spectrometry and X-ray fluorescence, Anal. Bioanal. Chem. 374 (2002) 566 – 572. [24] G. Vittiglio, S. Bichklmeier, P. Klinger, J. Heckel, W. Fuzhong, L. Vincze, K. Janssens, P. Engstrom, A. Rindby, K. Dietrich, D. Jembrih-Simburger, M. Schreiner, D. Denis, A. Lakdar, A. Lamote, A compact A-XRF spectrometer for (in situ) analyses of cultural heritage and forensic materials, Nucl. Instrum. Methods, B 213 (2004) 693 – 698. [25] B. Kanngieger, W. Malzer, I. Reiche, A new 3D micro X-ray fluorescence analysis set-up–first archaeometric applications, Nucl. Instrum. Methods, B 211 (2003) 259 – 264. [26] D.N. Papadopoulou, G.A. Zachariadis, N.A. Anthemidis, N.C. Tsirliganis, J.A. Stratis, Microwave-assisted versus conventional decomposition procedures applied to a ceramic potsherd standard reference material by inductively coupled plasma atomic emission spectrometry, Anal. Chim. Acta 505 (2004) 173 – 181. [27] R. Jenkins, R.W. Gould, D. Gedcke, Quantitative X-ray Spectrometry, Marcel Dekker, Inc., New York, 1995. [28] M. Milazzo, C. Cicardi, Simple methods for quantitative X-ray fluorescence analysis of ancient metal objects of archaeological interest, X-ray Spectrom. 26 (1997) 211 – 216. [29] M. Milazzo, Radiation applications in art and archaeometry X-ray fluorescence applications to archaeometry. Possibility of obtaining non-destructive quantitative analyses, Nucl. Instrum. Methods, B 213 (2004) 683 – 692. [30] B. Vekemans, K. Janssens, L. Vincze, F. Adams, P. van Espen, Analysis of X-ray spectra by iterative least squares (AXIL): new developments, X-ray Spectrom. 23 (1994) 278 – 285. [31] P.H. Abbott, F. Adams, AXIS: automated XRF interpretation of spectra, X-ray Spectrom. 26 (1997) 125 – 131. [32] B. Vekemans, K. Janssens, L. Vincze, F. Adams, P. van Espen, Comparison of several background compensation methods useful for evaluation of energy-dispersive X-ray fluorescence spectra, Spectrochim. Acta Part B 50 (1995) 149 – 169. [33] B. Kanngieger, Quantification procedures in micro X-ray fluorescence analysis, Spectrochim. Acta Part B 58 (2003) 609 – 614. [34] D. Wegrzynek, A. Markowicz, E. Chinea Cano, Application of the backscatter fundamental parameter method for in situ element determination using a portable energy-dispersive X-ray fluorescence spectrometer, X-ray Spectrom. 32 (2003) 119 – 128.