Nuclear Instruments and Methods in Physics Research B 540 (2023) 122–128 Contents lists available at ScienceDirect Nuclear Inst. and Methods in Physics Research, B journal homepage: www.elsevier.com/locate/nimb Rare-earth element comparative analysis in chosen geological samples using nuclear-related analytical techniques P.J. Adeti a,*, G. Amoako a, J.B. Tandoh d, O. Gyampo b, H. Ahiamadjie b, A.S.K. Amable e, C. Kansaana c, Ruth A.T. Annan b, A. Bamford d a Physics Department, University of Cape Coast, P.M.B Post Office Cape Coast, Ghana b Ghana Atomic Energy Commission, P.O. Box LG 80, Legon, Accra, Ghana c Nuclear Regulatory Authority, Box AE 50, Atomic-Kwabenya, Ghana d Graduate School of Nuclear and Allied Sciences, University of Ghana, P.O. Box AE1, Atomic-Kwabenya, Ghana e University of Health and Allied Sciences, School of Basic and Biomedical Sciences, Department of Basic Sciences, PMB31 Ho, Ghana A R T I C L E I N F O A B S T R A C T Keywords: In this study, four analytical techniques for the detection of Sc, La, Ce, Nd, Sm, Eu, Tb and Lu in volcanic rock X-ray fluorescence spectrometry specimens from Ghana have been compared. These rock samples were examined using inductively coupled Am-241 excited-based system plasma mass spectrometry, X-ray fluorescence spectrometry, and instrumental neutron activation analysis. The Ag-anode X-ray Tube Am-241 excitation-based X-ray fluorescence accuracies were good for Y, La, Ce, Nd, Eu, and the results were Inductively coupled plasma mass spectrometry Instrumental neutron activation analysis comparable to inductively coupled plasma mass spectrometry and instrumental neutron activation analysis Rare-earth elements except Silver-anode X-ray tube X-ray fluorescence. This demonstrates the limitations of the tube-based X-ray Ghana research reactor-1 fluorescence excitation system for rare earth elements analysis, primarily caused by interference between the K- series X-ray emission from the transition metals and the relatively low intensities of L-series lines of the rare earth elements. Data on the precision and accuracy of these methods were based on IAEA SOIL-7 reference material. 1. Introduction detection capacity, ICP-MS is an excellent analytical technique for the investigation of a variety of materials [1,4]. ICP-MS has been utilised by Rare earth elements (REEs) are acknowledged as strategic and vital numerous researchers from across the world over the past three decades minerals [1]. Therefore, many stakeholders are particularly interested in to find REE in a variety of materials [5–9]. In contrast to other regularly the readily available techniques to identify and quantify REEs. The need used analytical techniques like instrumental neutron activation analysis for a cost-effective method of extracting metals of economic relevance, (INAA) and XRF, ICP-MS technology is currently employed rather such as REEs, is one of the primary drivers of research and development extensively for the exact determination of REE in various types of ma- in every country. REEs have remarkably comparable physical and terials [1,10]. The primary limitations of the XRF technique, which chemical characteristics [1]. Because traditional chemical methods for prevent its use for REE purposes are their lower precision and accuracy determining REEs are selective for a single element, they take a long compared to other spectroscopic methods and their insufficient sensi- time to complete and are not cost-effective for several samples of tivity for a few key components [11,12]. However, the Am-241 excita- different elements [2]. The separation of REE using labour-intensive tion-based system-based advance in XRF equipment recently has processes like precipitation, solvent extraction, and ion exchange increased the instrument’s sensitivity, allowing for improvements in before analysis by techniques like X-ray fluorescence (XRF) and induc- both accuracy and productivity [13]. Because of this, XRF has become tively coupled plasma optical emission spectrometry (ICP-OES) made an alternate analytical technique for quantification of REEs. determining the presence of REE in geological samples challenging and Volcanic rock samples from Buem Structural Unit in the Dahomeyide expensive before the development of inductively coupled plasma mass orogenic area of south-eastern Ghana were examined using XRF and spectrometry (ICP-MS) [3,4]. Due to its extremely low detection limits, INAA, and the results were compared with ICP-MS results on the same high sample throughput, the requirement for very tiny quantities, samples at the Analytical Laboratory System laboratory in Vancouver, element flexibility (major, minor, trace, and ultra-trace), and isotopic Canada. ICP-MS has not been widely used in Ghana due to its high cost * Corresponding author. E-mail address: pjadeti@stu.ucc.edu.gh (P.J. Adeti). https://doi.org/10.1016/j.nimb.2023.04.001 Received 15 December 2022; Received in revised form 31 March 2023; Accepted 2 April 2023 Available online 23 April 2023 0168-583X/© 2023 Elsevier B.V. All rights reserved. P.J. Adeti et al. N u c l e a r I n s t . a n d M e t h o d s i n P h y s i c s R e s e a r c h , B 5 40 (2023) 122–128 Table 1 Standards produced by Fluka Chemika with their respective concentrations for sensitivity calibration. Compound Element Concentration Concentration oF Other Elements (%) (%) CaCO3 Ca 39.643 C = 11.880, O = 47.476 CoCO3 Co 49.301 C = 10.048, O = 40.152 NiCO3 Ni 49.396 C = 10.108, O = 40.394 ZnO Zn 79.536 O = 19.463 AS2O3 As 75.588 O = 24.212 Cr2O3 Cr 68.414 O = 31.576 FeCO3 Fe 47.720 C = 10.263, O = 41.015 MnCO3 Mn 47.316 C = 10.344, O = 41.338 MoO3 Mo 66.654 O = 33.346 Sr(NO3)2 Sr 40.988 N = 13.104O = 44.906 CuCO3 Cu 50.919 C = 9.624, O = 38.460 KBr Br 66.474 K = 32.523 KHCO3 K 38.663 H = 0.100, C = 11.877, O = 47.462 – S 32.06 – – Ti 47.8 – NaCl Cl 60.059 Na = 38.946 AgNO Ag 63.373 N 8.229, O 28.199 Fig. 1. The ideal source-sample-detector geometry for the Am-241 excitation- 3 = = BaO Ba 87.772 O = 10.226 based system (measurements in degrees and milliliters). CsNO3 Cs 66.142 N = 6.971, O = 23.886 CdO Cd 87.540 O = 12.460 sample emits after being excited by a primary X-ray. HgCl2 Hg 73.138 Cl = 25.853 HIO3 I 71.781 H = 0.57, O = 27.149 PbO Pb 91.903 O = 7.096 2.3.1. X-ray fluorescence equipment – Sn 99.00 – X-Ray Spectrometer Re-configuration: The benchtop XRF spec- WO3 W 79.297 O = 20.703 trometer at GAEC was attached to a developed Am-241 excitation-based system. The design and the construction of the excitation system focus and accessibility. INAA has been used by several studies to find REE in a on three main parts, namely: variety of earth and environmental materials at very low quantities [14–20]. To better understand the geochemical behaviour of beach (i) An excitation source. sands from Tamil Nadu, India, Ravisankar et al. (2006) used INAA to (ii) A sample measure REE [21]. Two XRF methods were investigated: the first used (iii) A detection system an Am-241 excitation-based system to excite the sample K-series and the second used an Ag-target tube to excite the sample L-series. 2.4. Am-241 excitation-based system irradiation and Measurement 2. Materials and methods The prepared pellets were exposed to radiation using an Am-241 excitation-based source (100 mCi). 2.1. Samples Sensitivity, 25 oxide standards were used for measurements, which were used to create calibration curves. The IAEA-Soil 7′s elemental The rock samples DK4, DK6, and DK9 used in this work have been concentration was established. The element’s content of the IAEA-Soil 7 previously analysed by the ICP-MS method [22]. These samples were reference sample is displayed in Table 3. Results for the reference ma- collected from the Pan-African Dahomeyide belt in southeast Ghana’s terial are represented here as the mean values of three measurements, Buem Structural Unit, primarily from the Asukawkaw, Bowiri-Odumase, with standard deviation (SD). The recommended values and measured and Nkonya localities. The reference material (IAEA-Soil 7) is produced values agree well, and the errors are often between 5% and 10%. by International Atomic Energy Agency and the chemical standards are An X-123 Silicon Drift Detector (Amptek Technologies) with a produced by Fluka Chemika (See Table 1). thickness of 500 µ and a resolution of 125 eV full width at half maximum resolution at 5.9 keV (55Fe) at 11.2 microseconds peaking time had been installed on the Am-241 excitation-based source. The K-shell 2.2. Instrumental neutron activation analysis characteristic X-ray was excited for 3000 s using an Am-241 excitation- based source. At incident angle (φ1) of 50.5 0 and a take-off angle (φ2) of DK4, DK6, DK9, and SOIL 7 samples, were pulverized into a homo- 900 as shown in Fig. 1. Sample-detector separation was measured at geneous powder and 150 mg of each was weighed, wrapped in a thin 28.6 mm. The QXAS programme [24] was used to process the X-ray polyethylene sheet, and heat-sealed. IAEA soil 7 reference material fluorescence data, and the net peak intensities of the K-series lines were sample and REE-containing materials were exposed to thermal neutron determined by fitting the spectra with the AXIL. The AXIL was used to flux of 5.0 × 1011n/cm2s1 using the Ghana Research Reactor-1 (GHARR- calculate the elemental concentration. The concentration values are the 1) with an output of 17 kW. Based on the half-lives of the relevant three measurements’ averages. components, three types of irradiation procedures were adopted: Nuclide half-lives that are short (between 1 min and t (1/2) 2.5 h), 2.4.1. Sample preparation for X-ray fluorescence medium (between 12 h and t (1/2) 3 days), and long (t (1/2) > 3 days) Standards: IAEA-Soil 7 reference material and chemical standards are the three categories [23]. were powdered, and 10 g of each was pressed at 15 tonnes of pressure to produce thick pellets measuring 2 cm in diameter. Before being exposed 2.3. X-ray fluorescence to radiation, the pellets produced were desiccated for 24 h. Rock samples: The rock samples were broken up, pulverised, and XRF is a non-destructive analytical technique used to determine a put through a sieve with a mesh size of 100 µm. At the Ghana Atomic material’s elemental composition. XRF analyzers can detect a sample’s Energy Commission (GAEC), pellets were produced utilising a hydraulic chemistry by measuring the fluorescence (or secondary) X-ray that a pelletizer (Carver Technologies, Auto-CrushIR, Wisconsin, US) and a 123 P.J. Adeti et al. N u c l e a r I n s t . a n d M e t h o d s i n P h y s i c s R e s e a r c h , B 5 40 (2023) 122–128 fluorescent intensity of the element in counts/seconds, Ci is the con- centration of the element i in the element in ppm and a(E) is the average absorption correction factor. 3. Result and discussion 3.1. Calibration For XRF analysis, calibration is the relationship between the atomic number of the element of interest and the number of x-rays emitted mg/ kg of the element of interest per second [25,26]. The levels of REEs found in various samples were determined by comparing them to cali- bration curves made from the oxides of twenty-five standard samples (S, Cl, K, Ca, Ti Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Br, Sr, Mo, Ag, Cd, Sn, I, Cs, Ba, W, Hg, and Pb). The average peak areas obtained from the irradia- Fig. 2. Am-241 Kα-Sensitivity Curve. tion of five 100 g standard pellets prepared in the same manner as the sample pellets were used to construct the concentration calibration stainless-steel pellet die with a 2.5 cm diameter. A pellet is created by curves. A Fig. 2 and Fig. 3 shows a typical calibration curve made from compressing sample powders at a rate of 20 tonnes per square inch the standards for a 6000-second exposure to a 100 mCi Am-241 exci- between two flat, polished discs. Each sample was divided into pellets tation-based system and Ag-anode tube XRF exposure of 120 s respec- having a diameter of 2.5 cm and a thickness of 0.5 cm, each weighing 10 tively. The calibration curves are used to determine an element’s g. The pellets were kept overnight in the 150 mm desiccator (AS ONE, concentration in (mg/kg) in given sample is express as: 240, Vietnam) before being analysed by the XRF. The pellets were exposed to radiation using an Ag-anode X-ray tube and Am-241 exci- IC i (E)a(E) i = tation-based XRF system. Using a silicon drift detector, the distinctive L- Si series X-rays of Cerium (Ce) and Europium (Eu) were measured with Ag- anode X-ray tube. The 120 s irradiation period used a current of 5 µA and a voltage of 45 keV. These systems’ calibration peaks were K-K, Br-K, and Mo-K. IAEA-AXIL evaluated the net intensities of the distinct char- acteristic X-rays based on non-linear least-squares fitting and the Table 2 Elemental Sensitivity method. The concentration values are the three A detection limit of REEs using Am-241 excitation-based XRF, INAA and ICP-MS in mg/kg. measurements’ averages. Element Am-241 Source INAA ICP-MS 2.4.2. Quantification method for XRF La 2.00 0.19 0.06 For a quantitative X-ray fluorescence (XRF) analysis, it is necessary Ce 2.00 0.03 0.05 to convert the recorded intensities of the characteristic radiation to the Nd 2.00 3.03 0.10 Sm 2.00 0.08 0.04 concentrations of the components to be evaluated (analytes). This Eu 5.00 “n/c” 0.02 approach is based on the relationship between the intensity of the Gd 5.00 “n/c” 0.04 excitation source, the concentration of the analyte, the overall compo- Tb “n/c” 0.10 0.01 sition of the sample, and the absorption properties of the sample. Er 2.00 “n/c” 0.03 Yb “n/c” 0.08 0.01 Si(E).Ci Lu “n/c” 0.04 0.01 Ii(E) = a E Y 0.01 0.01 0.30 ( ) Sc 0.1 0.01 4.00 Si(E) is the sensitivity of the element in counts/ppm/second, Ii(E) is the Note: “n/c” - value was not calculated. ICP-MS [22]. Fig. 3. Ag-anode X-ray Tube Lα-sensitivity curve. 124 P.J. Adeti et al. N u c l e a r I n s t . a n d M e t h o d s i n P h y s i c s R e s e a r c h , B 5 40 (2023) 122–128 Fig. 4. Graph displaying the REE concentrations in sample DK4. 3.2. Detection limit The detection limit obtained (Table 2) are comparable to the previ- ous studies. ICP-MS [27] INAA [28] XRF [29] Am-241 excitation-based XRF [21]. 3.3. Matrix effect The standard reference materials are chosen for the proper applica- tion of the XRF method so that they match the matrix type and con- centration of the target measurables, the rock samples were examined in a powdered form. The reference material available that is closers in a matrix to the sample analysed is IAEA-Soil 7 and furthermore, it contains some REE to validate the method. The use of the IAEA-Soil 7 reference material for REE analysis in rock samples in this study is justified by the fact that both the samples and the standards have a geological matrix that is predominately composed of light and heavy elements, making them suitable for use in this study. Experimental research on this assumption was conducted [31]; it Fig. 5. Graph displaying the REE concentrations in sample DK6. was found that the systematic divergence in the calibration due to various matrices were estimated to be less than 10% for majority of the MS is shown in Figs. 4-6. There is still some variance in the concentra- components. tion levels despite the Ag anode X-ray tube spectrometer spectra lines’ Replicate irradiations were used to test the sample homogeneity, and acceptable match. In samples DK4, DK6, and DK9, as well as Er in it was discovered to be within 5%. The discussions mentioned above samples DK4 and DK8, it was unable to detect Y, Nd, Gd, Tb, Yb, and Lu. support the calibrating techniques applied in this investigation. The Y, Nd, and Gd in DK4, DK6, and DK9 could all be detected and A comparison of the outcomes for the Ag anode X-ray Tube spec- quantified by the associated Am-241 excitation-based system, as well as trometer, the attached Am-241 excitation-based system, INAA, and ICP- 125 P.J. Adeti et al. N u c l e a r I n s t . a n d M e t h o d s i n P h y s i c s R e s e a r c h , B 5 40 (2023) 122–128 Fig. 6. Is a graph that displays the ree levels in sample dk9. Table 3 Comparative concentrations (mg/kg) of REE in IAEA-Soil 7 by INAA, ICP-MS, Ag-Tube XRF and Am-241 XRF. Elements Soil 7 Ag-Tube XRF measurements Am-241 XRF Measurement INAA Measurements (REEs Elements) Expected value (mg/kg) Observed value Recovery SD Observed value Recovery SD Observed value Recovery SD (mg/kg) (%) (mg/kg) (%) (mg/kg) (%) Sc 8.3±5.2 7.0±7.2 84.3 5.2 8.1 ± 2.5 98.0 2.5 8.1 ±1.3 97.6 1.3 La 28.0±1 “n/d” “n/c” “n/ 27.5±3.8 98.2 3.8 27.2±1.1 97 .0 1.1 c” Ce 61.0±6.5 “n/d” “n/c” “n/ 60.5 ±2.5 99.2 2.5 60.4±2.3 99.0 2.3 c” Nd 30.0±6 “n/d” “n/c” “n/ 29.4 ±3.0 98.0 3.0 29.7±1.2 99.0 1.2 c” Sm 5.1±0.35 n/d“ “n/c” “n/ 4.6±2.0 90.2 2.0 4.3 ±0.25 84.3 0.25 c” Eu 1.0±0.2 1.2± 0.8 120.0 3.8 0.8 ±0.4 80.0 2.7 0.8±0.05 80.0 0.05 Tb 0.6±0.2 “n/d” “n/c” “n/ “n/d” “n/c” “n/ 0.4 ±0.03 66.7 0.03 c” c” Yb 2.4±0.35 5.1 ±3.4 212.0 13.4 “n/d” “n/c” “n/ 2.3±0.09 95.8 0.09 c” Lu 0.3±0.1 “n/d” “n/c” “n/ “n/d” “n/c” “n/ 0.3 ±1.3 100 1.3 d” c” “n/c” - value was not calculated. “n/d” - element was not determined. Recovery is calculated as (Observed Value / (Expected Value) × 100%. Er in DK4, which EXP-1 was unable to do. 5.9,25.5,16.3,210.0,206.0) The linear regression method and the cor- However, the current REEs findings utilising INAA and ICP-MS show relation analysis method were both used (see Figs. 7-9). Multivariate that the outcomes were comparable to those reported in other works regression robust statistics and equations were used to statistically [1,9,16,18]. With a few exceptions, the current Am-241 excitation- analyse the performance of the attached Am-241 excitation-based sys- based system is the most effective one (see Table 3). This method is tem [30]. For every analyte’s the. effective for REEs within the sensitivity calibration curve’s range. Excited-based system Am-241 = B [INAA (mg/kg)] + A, when the slope (B) and intercept (A) were computed. B = 1 and A = 3.4. Evaluation of Am-241 EDXRF, Ag-Tube EDXRF, INAA, and ICP-MS 0. The intercept (A) should be close to zero (0) and the slope (B) should results be close to 1 as illustrated in Fig. 7 if the connected Am-241 excited- based system concentrations and the INAA concentrations of elements The connected Am-241 excited-based system was compared to the are consistent for REEs. As a result, there was no discernible difference concentration values of the various methods, including the Ag anode-X- between the two methodologies’ results for REEs. On the other hand, ray tube spectrometer, INAA, and ICP-MS, that were employed for this under identical circumstances, statistically significant differences in the study’s validation. The concentration values of Am-241 excitation-based estimated element concentrations of REEs using the Ag anode X-ray tube system (2.9, 7.1,14.1,55.1,91.6, 107) mg/kg was plotted against that of spectrometer were found. When the concentration values of Am-241 the instrumental neutron activation analysis (3.1, excitation-based system (9.4,0.3,27.2) mg/kg was plotted against that 126 P.J. Adeti et al. N u c l e a r I n s t . a n d M e t h o d s i n P h y s i c s R e s e a r c h , B 5 40 (2023) 122–128 (3.8–13.4) of discrepancies for the Ag anode X-tube spectrometer was also very high and significantly different from zero. This demonstrates the limitations of the tube-based XRF excitation system for REE analysis, primarily caused by interference between the K- series X-ray from the transition metals and the relatively small in- tensities of L-series X-ray of the REE. As a result, concentration values are less accurate and the range of REE that can be analysed is constrained. The concentration values of Am-241 excitation-based system (3.1, 7.1, 14.1, 55.1, 91.6, 107.0) mg/kg was plotted against that of the inductively coupled -mass spectrometer (3.4, 6.0, 31.7, 74.6, 133.0, 119.5) as shown in Fig. 9. In Fig. 9 and Fig. 7, the slope, intercept, and linear correlation coefficient (R2) values of the attached Am-241 exci- tation-based system, INAA, and ICP-MS were all close to 1, close to zero (0), and higher than 0.95 in all cases. These findings demonstrated a perfect agreement between the techniques used by the attached Am-241 excitation-based system, INAA, and ICP-MS. However, the current REEs Fig. 7. The graph compares the outcomes for REEs from INAA and Am- findings utilising INAA and ICP-MS show that the outcomes were 241excitation-based EDXRF, together with the correlation and R2 values. equivalent to those reported in other works [1]. As seen in Table 3, the standard deviation (SD) values for this study’s elemental sensitivity method for the Ag-anode X-Tube spectrometer ranged from 8.9 to 10.0 mg/kg and from 2.0 to 3.8 mg/kg for the attached Am-241 excitation-based system. The INAA standard deviation values ranged from 0.09 to 2.3 mg/kg. When the required X-ray fluo- rescence spectrometers’ SD is less than 10% and, occasionally, when elements of a certain concentration are constrained, the SD values are low [31,32]. The findings demonstrate that the elemental sensitivity method for the attached Am-241 excitation-based system is an accurate and comparable method for the determination of REEs. 4. Conclusions The Am-241 excitation-based system was proved to be a trustworthy, accurate alternative for REEs analysis through comparison analysis re- sults with ICP-MS and INAA techniques. Since sample preparation is minimal and analysis time is shorter than with other techniques, the Am- 241 excitation-based system provides opportunities for quick and ac- curate routine analysis of a large number of samples. Fig. 8. The graph compares the REEs results from the Ag anode X-tube spec- trometer and Am-241 excitation-based system to the correlation equation and the R2 value of 0.6129. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Data availability Data will be made available on request. Acknowledgement It is highly appreciated that the Ghana Atomic Energy Commission’s X-ray Fluorescence Laboratory provided the necessary infrastructure. Dr. Joseph Tandoh’s insightful comments on the first draft of the manuscript are much appreciated by the authors. References [1] V. Balaram, Rare earth elements: A review of applications, occurrence, exploration, Fig. 9. The graph compares the REEs in sample DK6 as determined by ICP-MS analysis, recycling, and environmental impact, Geoscience Frontiers 10 (4) (2019) and the associated Am-241excitation-based EDXRF system, as well as the cor- 1285–1303. relation equation and R2 values. [2] J.J. LaBrecque, J.M. Beusen, R.E. 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