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A Microwave Digestion Technique for the Analysis of Rare Earth Elements, Thorium and Uranium in Geochemical Certified Reference Materials and Soils by Inductively Coupled Plasma Mass Spectrometry

https://doi.org/10.3390/molecules25215178

“Two different digestion methods—microwave digestion (Mw) and Savillex digestion (Sx)—were used to evaluate the best quality control for analysis of the rare earth elements, Th and U in the geochemical certified reference material JSd-2, supplied by the Geological Survey of Japan (GSJ). The analysis of trace elements was carried out using inductively coupled plasma mass spectrometry (ICP-MS). The digestion recovery was > 90% for almost all elements by both methods. Mw-4 (four repeats of the microwave digestion) was found to be more effective and faster than Sx. In order to evaluate the efficiency of Mw-4, three other GSJ certified reference materials, JLk-1, JB-1 and JB-3, as well as five different soil samples from Belarus, Japan, Serbia and Ukraine were also analyzed. The Mw-4 method was seen to be promising for complete digestion and recovery of most of the elements. The U/Th ratio showed some heterogeneity for Ukraine and Serbia soils affected by Chernobyl nuclear power plant accident and depleted uranium contamination, respectively. This method can be successfully applied to any type of soils for elemental analyses.”

3.1. Reagents and Chemicals

All chemical procedures and measurements were performed under clean room conditions (class 10000). All solutions were prepared in de-ionized Millipore water (resistivity = 18.2 MΩ × cm) obtained from a Milli-Q water system (Millipore, Moisheim, France).
Ultrapure analytical grade HNO3, HF and HClO4 (TAMA-Pure-AA-100, Tama Chemicals Co. Ltd., Kawasaki, Japan) acids were used for soil decomposition. Other chemicals used were of analytical grade.
The geochemical CRMs JSd-2, JLk-1, JB-1 and JB-3 supplied by GSJ (Tsukuba, Japan) were used in the present work.

3.2. Samples and Sampling Locations

Surface soil samples were collected at (0–5 cm) depth. A stainless-steel scoop was used to collect about 2 kg of soil from five places randomly distributed within a 100 m2 (10 m × 10 m) area. Then these samples were mixed together to form a composite sample. Before collection, grass, litter, roots and shoots were removed from the surface. All soil samples were brought to the laboratory and air dried at room temperature. They were sieved using a 2 mm mesh sieve after manually removing objects like roots, shoots and stones. The sieved samples were oven dried at 110 °C for 24 h. Then, all samples were pulverized using a ball mill to less than 150 μm size prior to chemical decomposition.
Soil samples contaminated with radio cesium activity due to the nuclear accidents at Chernobyl, Fukushima and Gomel were studied in the present work. A soil sample collected from Hiroshima has also been analyzed to extend the analysis of the digestion method. Similarly, a soil sample from an area contaminated with depleted uranium in Bratoselce was analyzed. The details of sampling locations and dates are given in Table 3.
Table 3. Details of sampling locations and dates.
Sample Name (Latitude) °N (Longitude) °E Date of Sampling
Gomel Oblast, Belarus 52.3922 29.5238 September 1998
Chernobyl, Ukraine 51.3731 29.9949 March 2001
Bratoselce, Serbia 42.3447 21.7560 March 2003
Hiroshima, Japan 34.4742 132.4467 April 2009
Fukushima, Japan 37.5401 140.8672 October 2015

3.3. Sample Preparation

All four CRMs were dried in a muffle furnace (Denken Co., Ltd., Kyoto, Japan) at 110 °C for 3 h to remove moisture. The JSd-2 was digested using five different methods, Mw-1, Mw-2, Mw-3, Mw-4 and Sx. Their details are given below.

3.3.1. Microwave Digestion (Mw)

In this method, 0.1 g of dried JSd-2 was transferred separately to closed PTFE vessels for digestion using a microwave digestion system (Milestone Ethos One MA133-003, Sorisole, BG, Italy). Each mixture of concentrated acids was added carefully to avoid drastic reaction.
(A) Mw-1
Microwave digestion was carried out only one time in Mw-1 with a mixture of concentrated acids HNO3 (6 mL), HF (2 mL) and HClO4 (1 mL).
(B) Mw-2
After Mw-1, mixture of concentrated acids HNO3 (2 mL), HF (1 mL) and HClO4 (1 mL) was used for the second digestion Mw-2.
(C) Mw-3
For the third digestion Mw-3, mixture of concentrated acids HNO3 (1 mL) and HF (1 mL) was added after the successive Mw-1 and Mw-2.
(D) Mw-4
In the fourth digestion Mw-4, the mixture of concentrated acids HNO3 (1 mL) and HF (1 mL) was added again. The conditions in the microwave digestion program are given in Table 4. After addition of acid mixtures Mw-1, 2, 3 and 4 the samples were subsequently heated for about 45 min in the microwave digestion system using this program, followed by a cooling cycle for about 60 min or 1 h.
Table 4. Details of the microwave digestion program.
Time (min) Power (W) T1 (°C) T2 (°C)
00:03:00 1000 50 115
00:02:00 0 30 115
00:20:00 1000 210 115
00:20:00 1000 210 115
00:60:00 0 30 30
The microwave-digested sample solutions obtained from Mw-1, 2, 3 and 4 methods were transferred to Teflon beakers for evaporation to dryness. Evaporation was continued till removal of any traces of HF and HClO4 from the solution. Finally, the residue was dissolved completely in 15 mL of 2% HNO3 solution. An experimental blank solution was also processed in a similar way.

3.3.2. Savillex Digestion (Sx)

About 0.1 g of dried JSd-2 was weighed and transferred separately to Savillex PFA vials. A mixture of concentrated acids HNO3 (1.5 mL), HF (3.5 mL) and HCl (0.5 mL) was added slowly to avoid drastic reaction. The vials were closed properly and heated for about 12 h at 100 °C. Then the lids were opened and the solutions were dried. Once again, a mixture of concentrated HNO3 (3 mL) and HCl (1 mL) was added to closed vials and heated for 12 h at 100 °C. This mixture was also dried. The step of addition of mixture of concentrated HNO3 (3 mL) and HCl (1 mL), heating and drying was repeated twice. Finally, the residue was dissolved in 10 mL of 2 M HCl and dried completely. The sample solution was prepared in 20 mL of 2% HNO3. An experimental blank solution was also processed in a similar way. The CRMs JLk-1, JB-1 and JB-3 as well as other soil samples were digested with one of the above methods, after deciding the most efficient one. Before digestion, all soil samples were ashed in a muffle furnace (Denken Co. Ltd.) by increasing temperature step-by-step, 100 °C for 2 h, 200 °C for 3 h and 550 °C for 5 h to decompose organic matter from samples.

3.4. ICP-MS Measurements

ICP-MS (Agilent 8800, Agilent Technologies, Tokyo, Japan) was used for determination of the concentration of U as well as other elements in soil solution. The operating parameters are given in Table 5. All blanks and samples were diluted using 3% HNO3 for ICP-MS measurements. The ICP-MS detection limit was calculated as three times the standard deviation of the calibration blank measurements (n = 10). The experimental blanks were prepared similarly. Detection limits varied from 0.03 to 0.2 ng/L for all elements. The calculated detection limit for 238U was 0.01 ng/L.
Table 5. Operating parameters for ICP-MS.
Plasma
Frequency (MHz) 27.12
RF Power (kW) 1.55
Argon Flow (L/min)
Plasma 15.0
Carrier 1.0
Nebulizer Micro Mist type
Sampling distance (mm) 8.0
Sample uptake rate (mL/min) 0.4
Data Acquisition
Mode Peak jumping mode
Number of points per peak 3
Number of scan sweeps 100
Dwell time per point (s) 0.3
Scan mass range (a.m.u.) 7–238
Multi-element Plasma standards solutions of XSTC-1 as well as XSTC-331 (Spex CertiPrep Inc., Metuchen, NJ, USA) spiked with internal standard 103Rh were prepared to derive calibration curves. In order to correct the signal attenuation due to the presence of various constituents in the sample solution (known as the “matrix effect”) as well as for possible change in instrumental parameters, an internal standard, 103Rh, was used during ICP-MS analysis. The intensities of the abundant stable isotopes of all elements of interest (139La, 140Ce, 141Pr, 146Nd, 147Sm, 153Eu, 157Gd, 159Tb, 163Dy, 165Ho, 166Er, 169Tm, 172Yb, 175Lu, 133Cs, 88Sr, 232Th and 238U) were measured in the present work. From the comparison between the standard and sample intensity ratios, total concentration of that element in the sample was calculated based on its natural abundance of isotopes. The calibration curve was obtained in the analyte concentration range of 0–5000 (ppt or ng/L). The sample solutions were diluted 20-fold for the measurement of REEs, Cs, Th and U whereas 1000-fold for Sr to get a precise working concentration range. Analytical validation of the measurement procedure was carried out using CRMs: JLk-1, JSd-2 (Japanese sediments) and JB-3 (Basalt) supplied by GSJ. The present analysis results were in good agreement with certified values, and their RSD was within 10%.

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