Fast Decomposition Procedure of Solid Samples by Lithium Borates

Article pubs.acs.org/ac

Fast Decomposition Procedure of Solid Samples by Lithium Borates Fusion Employing Salicylic Acid Miha Trdin,* Marijan Nečemer, and Ljudmila Benedik J. Stefan Institute, Jamova cesta 39, 1000 Ljubljana, Slovenia ABSTRACT: A new fast decomposition procedure for solid samples was developed. In this study, we investigated decomposition of samples by lithium borates fusion in combination with salicylic acid. The method described shortens the time required for the sample to be digested and loaded to a resin to up to 2 h, and it was especially suitable for alpha spectrometry measurements in emergency situations. Additionally, the method results in high radiochemical recoveries and when compared to other digestion methods (classical digestion utilizing mineral acids, microwave digestion, and lithium borates fusion in combination with polyethylene glycol (PEG)) gives comparable activity concentration values. The method used was applied to various reference materials with inorganic and organic matrices as well as widely varying amounts of uranium radioisotopes content. The results obtained were compared with reference and literature values and show that the proposed method can be successfully implemented on various types of samples.

A

and the tracer used. As a fast and efficient digestion method, a lithium borates fusion followed by dissolution with salicylic and ascorbic acid for the determination of alpha emitters is proposed in this Article. This method was tested on reference samples, each of a different matrix (tissue, plant, sediment, and soil). To establish the suitability of the method, the results of four different sample digestion/dissolution techniques were compared: (a) lithium borates fusion + PEG (polyethylene glycol), (b) lithium borates fusion + salicylic and ascorbic acid, (c) acid digestion by HNO3, HF, and HClO4 on a hot plate, and (d) microwave digestion by HF and HNO3.

n accurate radiochemical analysis of alpha-emitting radionuclides requires that the solid sample matrix be as decomposed as possible. It is well-known that especially soil samples are often considered to be one of the most difficult matrices to digest, especially in the case of large aluminosilicate content. This is due to the fact that naturally occurring radionuclides such as uranium and thorium are often incorporated into the structure of insoluble minerals, such as zircon, apatite, titanite, allanite, etc.1 On the other hand, most authors presume that the artificial alpha-particle emitting radionuclides such as neptunium, plutonium, and americium are attached to the surface of the sample particles which is probably not strictly correct due to the growth of particles since the beginning of the nuclear era. This can also be said in the case of silicate accumulation in plants.2,3 Current techniques for the determination of radionuclides in solid samples by alpha spectrometry, such as soil, plant, tissue, sediments, food, etc., can take up to several days if acid digestion utilizing mineral acids (HNO3, HClO4, and HF) is used to decompose the samples. Alternatively, thermal fusion and microwave digestion are known as a tool to shorten the digestion process but are limited by the amount. On the other hand, results show that a significant percent of uranium can remain in the leftover solid residue if classical digestion with mineral acids is applied.1,4 The need for a fast, simple, and complete digestion method, in which the analyzed radionuclides and the tracer are equally and homogeneously distributed in the matrix and are then loaded on a separation resin, can therefore not be ignored. Therefore, in our study, we have focused our efforts on finding a fast and efficient way to ensure the equilibrium between the radionuclide(s) of interest © XXXX American Chemical Society

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EXPERIMENTAL SECTION To confirm the suitability of the proposed method for determining the activity concentrations of uranium in solid samples independently of the sample matrix, the experiments were conducted on six samples with significantly different matrices. In the first step, the samples were digested with (a) lithium borates fusion + PEG (polyethylene glycol), (b) lithium borates fusion + salicylic and ascorbic acid, (c) acid digestion by HNO3, HF, and HClO4 on a hot plate, and (d) microwave digestion by HF and HNO3. In the next steps, uranium radioisotopes were separated by extraction chromatography; the measurements sources were prepared by microcoprecipitation, and the measurements were conducted on an alpha spectrometer. The elemental compositions of the samples and Received: December 15, 2016 Accepted: February 9, 2017 Published: February 9, 2017 A

DOI: 10.1021/acs.analchem.6b04980 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

(80 mL) in a Teflon beaker. The content of the Teflon beaker (brownish glass and Milli-Q) was then transferred to a glass beaker. To ensure the quantitative transfer of the sample, the Teflon beaker was washed by the addition of 20 mL of Milli-Q. The dissolution of the brownish glass was performed by the two different methods described below. Lithium Borates Glass Dissolution by Salicylic and Ascorbic Acid (BF (S + A)). In the first method, the glass beaker containing the brownish glass and Milli-Q was put on a hot plate stirrer set at 125 ± 10 °C when salicylic acid (0.5 g) and concentrated nitric acid (5 mL, 65%) were added to prevent silicate precipitation during the remaining steps of the procedure. It took approximately 30 min of heating and intensive stirring for the glass to completely dissolve and for the color of the solution to gradually change to dark purple. After dissolution of the glass, ascorbic acid (0.12 g) was added and the solution became colorless/yellow. The temperature was then lowered to 90 °C, and precipitation of radionuclides with iron hydroxides followed. First, ammonia solution (40−60 mL, 30%) was added to raise the pH over 12. At this stage, a white/ gray precipitate started to form. To ensure as complete a transfer of radionuclides as possible from the solution to the precipitate, FeCl3 solution (5−15 mL, 5 mg/mL) was additionally added until brownish particles appeared. The stirring and heating continued for 10−20 min after which the solution was left to cool down. During the heating and stirring, the color of the precipitate changed to dark gray. Due to the amount and the nature of the precipitate, the entire contents of the beaker was centrifuged (3000 rpm, 3 min) in a 150 mL Teflon bottle. The precipitate bound on the beaker walls was transferred to the Teflon bottle with Milli-Q. Supernatant was then discarded, and the precipitate was washed once with MilliQ (20 mL) and centrifuged (3000 rpm, 3 min); the supernatant was again discarded. What remained was 1−25 mm3 of precipitate that was in the final step dissolved by 3 M HNO3 (15−30 mL) and loaded on the resin. Lithium Borates Glass Dissolution and Silicates Removal by Polyethylene Glycol (PEG) (BF(PEG)). In the second method, the samples were treated by a slightly modified method published by Croudace et al.7 in 1998, in which the glass beaker with fused samples was also put on a hot plate at 125 ± 10 °C and 10 mL of nitric acid (65%) was added to dissolve the lithium borates glass during stirring. After 30−60 min, the glass dissolved but the beaker was left on the hot plate with continuous stirring until the volume of the solution was reduced to 50 ± 5 mL, thus resulting in a 2−3 M nitric acid solution. The entire process took about 4 h. The temperature was then lowered to 90 °C. When the solution cooled, 1 mL (0.2 M) of PEG solution was added dropwise to the solution. The stirring continued for another 3 h after which the beaker was covered and left overnight to allow white precipitate containing the silicates to form and settle. The resulting solution (approx. 50 mL) was filtered before being directly loaded on the column.7 Microwave Dissolution (MW). The procedure employed by Jurečič et al.4 was used for microwave decomposition: After the sample was weighed into a Teflon vessel 232U tracer solution, concentrated HNO3 (5 mL) and HF (10 mL) were added. The microwave heating conditions were: 20 min at 130 °C, 20 min at 200 °C, and 40 min at 200 °C. When the vessel cooled, the contents were transferred to a platinum crucible where HClO4 (5 mL) was added to remove the excess HF. When the HClO4

the precipitates formed in the crucial step of the proposed method were determined by EDXRF. For clarity, the following abbreviations were used in all the figures: BF (S + A), lithium borates glass dissolution by salicylic and ascorbic acid; BF (PEG), lithium borates glass dissolution and silicates removal by PEG; CD, sample digestion by mineral acids; MW, microwave dissolution. Reagents. Chemicals were purchased from Aldrich and were used without further purification. The lithium borates flux (49.75% of Li2B4O7, 49.75% of LiBO, and 0.5% of LiBr) was purchased from Claise and used as purchased. The tracer solution of 232U (0,386 ± 0.007 Bq/g) used in the study was prepared from calibrated solution purchased from Analytics, Inc. (Atlanta, GA, USA). The producer maintains traceability to NIST standards. Samples. Five reference samples and a home prepared soil, all with a significantly different composition, were selected to establish whether the proposed method is appropriate for all types of solid samples independently of the composition. The samples analyzed were (1) IAEA 437, natural and artificial radionuclides in mussel from Mediterranean Sea; (2) NIST 4359, seaweed radionuclide standard; (3) IAEA 300, Baltic Sea sediment; (4) IAEA 375, radionuclides in soil; (5) NIST 4353A, Rocky Flats soil number 2; (6) soil (soil sample from the vicinity of the former uranium mine at Ž irovski vrh, Slovenia). Instrumentation. For thermal fusion, a Claise LeNEO furnace in combination with a platinum crucible and a Teflon beaker was used. For microwave dissolution an ETHOS 1 (MILESTONE SN 130471), microwave system with a maximum power of 1300 W and a Teflon vessel (HPR-1000/ 10S high pressure segment rotor) was used. EDXRF spectra were collected by an energy dispersive X-ray spectrometer composed of a Si(Li) detector (Canberra), a spectroscopy amplifier (Canberra M2024), ADC (Canberra M8075), and PC based MCA (S-100, Canberra). The spectrometer was equipped with a vacuum chamber. The energy resolution of the spectrometer was 175 eV at 5.9 keV. As primary excitation sources, the annular radioisotope excitation sources of Fe-55 (10 mCi) and Cd-109 (25 mCi) from Isotope Products Laboratories U.S.A. were used. The analysis of complex X-ray spectra was performed by the AXIL spectral analysis program.5 Quantification was then performed utilizing QAES (Quantitative Analysis of Environmental Samples) software developed in our laboratory.6 An alpha spectrometer (CANBERRA’s Alpha Analyst) with passivated implanted planar (PIPS) semiconductor detectors with an active area of 450 mm2 and 28% efficiency for 25 mm diameter discs was used for alpha-particle spectrometry measurements. The measured source was placed in a parallel plane, centered at the symmetry axis of the detector at a distance (varying a bit among chambers) of about 5.0 ± 0.5 mm. The calibration of the detectors was made with a standard radionuclide source, containing 238U, 234U, 239Pu, and 241Am (code 67978-121), obtained from Analytics, Inc. Sample Dissolution Procedures. Sample Digestion by Lithium Borates Fusion. The ignited sample (up to 2 g), lithium borates (up to 3 g), and tracer (232U, 0.1−0.2 mL) were weighed in a platinum crucible and dried in a furnace at 160 °C for 20 min before the crucible was transferred to a Claisse LeNeo furnace. The fusion was performed at 1050 °C for 23 min while shaking the crucible. Immediately after the fusion, the glass was poured while still in liquid form in stirred Milli-Q B


a

C

10 10−4 10−5 10−6

1.4 × 10−4

× × × ×

1.3 1.3 2.9 9.0

× × × ×

10 10−3 10−5 10−6

−4

−2

2.4 7.4 7.2 6.1

6.8 × 10−3 5.0 × 10−5

8.0 × 10−3 5.3 × 10−5

1.6 × 10

−1

−1

1.8 × 10

2.7 × 10−3 1.9 × 10−4 7.8 × 10−3

× × × ×

3.4 × 10−4 1.0 × 10−2 1.1 × 10−2

1.1 3.9 1.5 1.3

10−2 10−5 10−3 10−3

× × × ×

10−2 10−4 10−3 10−1

2.5 5.1 4.9 1.4

8.7 3.2 1.5 2.8 1.1 5.1 7.7 3.3 2.8 1.6

4.5 9.3 1.5 3.1 1.6 2.5 2.1 1.1 1.4 4.0 × × × × × × × × × ×

× × × × × × × × × × 10−4 10−5 10−5 10−2 10−2 10−4 10−4 10−6 10−5 10−5

10−2 10−4 10−2 10−2 10−5 10−3 10−2 10−2 10−4 10−2

NIST 4359 × × × ×

10−2 10−4 10−2 10−3

× × × × ×

10−5 10−3 10−2 10−4 10−4

3.2 × 10−5

1.1 3.7 6.7 8.7 6.1

6.9 × 10−4

1.5 × 10−2 8.8 × 10−3 1.4 × 10−2

4.7 4.4 3.7 5.7

NIST 4359P

The superscript letter “P” stands for “precipitate composition”.

Al Br Ca Cl Cu Fe K Mg Mn Na Nb P Pb Rb S Si Sr Ti Y Zn Zr

IAEA 437P

IAEA 437

9.9 1.3 1.7 8.7 2.4 1.2 4.9 7.7 1.7 1.2

9.3 1.2 8.4 3.2 9.7 5.2 2.9 2.4 2.0 2.9 × × × × × × × × × ×

× × × × × × × × × × 10−4 10−4 10−4 10−4 10−1 10−4 10−3 10−5 10−4 10−4

10−2 10−4 10−3 10−2 10−5 10−2 10−2 10−2 10−3 10−2

IAEA 300 × × × ×

10−2 10−5 10−3 10−3

× × × × × ×

10−5 10−4 10−2 10−5 10−4 10−5

5.1 × 10−5

4.2 1.5 5.0 4.4 9.4 4.3

1.4 × 10−5 2.9 × 10−4

6.4 × 10−4

2.6 × 10−2 8.6 × 10−4

2.8 8.7 1.4 3.3

IAEA 300P

× × × × × ×

10−5 10−4 10−1 10−5 10−3 10−5

2.9 × 10−4

3.8 7.7 2.3 6.0 1.6 1.7

7.5 × 10−6 8.1 × 10−4

7.3 × 10−3 9.8 × 10−3

1.0 × 10−2 1.2 × 10−4

5.2 × 10−2

IAEA 375 × × × ×

10−2 10−5 10−3 10−3

× × × × × ×

10−6 10−5 10−2 10−6 10−4 10−6

1.1 × 10−4

5.0 5.9 5.7 9.1 3.8 2.6

2.6 × 10−6 1.5 × 10−4

1.1 × 10−2 3.3 × 10−4

1.5 8.4 1.3 1.8

IAEA 375P × × × ×

10−2 10−5 10−3 10−4

× × × × × ×

10−5 10−4 10−1 10−5 10−3 10−5 2.9 × 10−4

5.6 1.7 3.0 7.2 2.0 3.7

1.7 × 10−5 3.8 × 10−4

2.2 × 10−2 1.4 × 10−2

5.1 1.2 3.3 2.0

NIST 4353A

× × × × × ×

10−5 10−5 10−2 10−5 10−4 10−5 3.6 × 10−5

2.7 9.1 7.4 2.0 5.5 1.1

6.7 × 10−6 1.9 × 10−4

1.7 × 10−2 5.7 × 10−4

6.4 × 10−4 2.3 × 10−3

3.0 × 10−2

NIST 4353AP

Table 1. Elemental Composition (g/g of the Sample) of Reference Materials and the in-House Soil Sample Determined by EDXRFa

× × × × × ×

10−5 10−4 10−1 10−5 10−3 10−5 2.4 × 10−4

8.4 3.7 2.7 4.7 4.4 1.0

8.5 × 10−6 7.5 × 10−4

2.9 × 10−2 1.6 × 10−2

2.8 × 10−3 2.4 × 10−4

9.9 × 10−3

soil × × × ×

10−3 10−4 10−4 10−3

× × × × × ×

10−6 10−5 10−2 10−6 10−4 10−6 7.2 × 10−5

4.7 6.4 3.8 5.8 5.8 9.8

3.0 × 10−6 9.0 × 10−5

9.9 × 10−3 4.2 × 10−4

2.7 1.4 2.9 1.6

soilP

Analytical Chemistry Article


Article


Figure 1. Graphical comparison of silicon content in the sample, the silicon content in the precipitate formed after the precipitation with iron hydroxides, and the volume of the formed precipitate.

Figure 2. Estimated time required for digestion/decomposition of a sample.

Figure 3. Graphical representation of radiochemical recoveries for selected decomposition methods.

Sample Digestion by Mineral Acids (CD). The sample was weighed in a platinum crucible, and 232U tracer and 20 mL of

evaporated, the resulting whitish residue was dissolved in 3 M HNO3 (10 mL). D


Article

Analytical Chemistry Table 2. Radiochemical Recoveriesa

HNO3 were added. The mixture was then left at room temperature overnight to allow a slow initial digestion of organic material. The next day, the crucible was put on a heater at 110 °C. When the HNO3 evaporated, a mixture of HNO3 (10 mL), HF (10 mL), and HClO4 (5 mL) was added, and the crucible was heated to 260 °C and left until only a dry residue was left. This step was repeated three times to ensure as complete a digestion as possible. The final white-yellow residue was dissolved in 3 M HNO3 (10 mL) and loaded on the resin.4,8 Radiochemical Separation of Uranium and Source Preparation. All samples were filtered using 12−15 μm filter paper prior to loading to prevent the column from clogging. The separation of uranium radioisotopes was performed by UTEVA resin according to a published procedure.9,10 The resin was first conditioned to 3 M HNO3 before the sample was loaded. The beakers were then washed twice with 3 M HNO3 (5 mL). After the sample was loaded, the column was consecutively washed with 3 M HNO3 (20 mL), 9 M HCl (5 mL), 5 M HCl with 0.5 M oxalate (25 mL), and 1 M HCl (15 mL). The final 1 M HCl fraction containing uranium isotopes was collected in a plastic centrifuge tube. The microcoprecipitation method with neodymium fluoride was used for thin source preparation for alpha spectrometric determination. The neodymium fluoride suspension was filtered through a 25 mm diameter, 0.1 μm polypropylene filter. The dry filter was mounted on an aluminum disc.11,12 EDXRF Analysis. This technique was used for determination of the elemental composition of the raw material and of the precipitate formed after the iron hydroxides precipitation step. The elemental composition of the reference materials and soil samples was determined by nondestructive energy dispersive Xray fluorescence spectrometry (EDXRF). Samples were excited by radiation sources 55Fe and 109Cd.13

IAEA 300

NIST 4359

IAEA 437

IAEA 375

NIST 4353A

soil

a

BF(S + A) BF (PEG) CD MW BF(S + A) BF (PEG) CD MW BF(S + A) BF (PEG) CD MW BF(S + A) BF (PEG) CD MW BF(S + A) BF (PEG) CD MW BF(S + A) BF (PEG) CD MW

0.76 0.81 0.79 0.64 0.85 0.91 0.69 0.72 0.74 0.88 0.71 0.69 0.80 0.92 0.72 0.92 0.56 1.00 0.92 0.53 0.31 0.87 0.20 0.28

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.08 0.04 0.07 0.07 0.08 0.04 0.07 0.07 0.06 0.04 0.06 0.06 0.07 0.07 0.06 0.08 0.05 0.10 0.08 0.04 0.03 0.08 0.03 0.03

The uncertainty is given as an extended uncertainty at k = 2.

aluminum, iron, and sodium are also present in noticeable quantities in the precipitate. Time Required for the Analysis. The most timeconsuming part of each analysis in radiochemical analysis is certainly the decomposition of the sample. While the radiochemical separation and source preparation, although lengthy, can be done in less than 5 h, sample decomposition can easily exceed 2 days if classical digestion with mineral acids is used. Microwave digestion can shorten this process to several hours, but the analysis cannot be performed in a single day due to the waiting time necessary for the Teflon containers to cool down after the digestion and the time needed to evaporate the nitric and hydrofluoric acid and to dissolve the sample in the proper molarity of the acid before loading on a resin. The entire microwave digestion procedure therefore takes an entire day. If lithium borates fusion in combination with PEG is used, the decomposition and dissolution procedures are shortened to approximately 4 h but the solution still needs to rest overnight after PEG is added to ensure sufficient silicates removal.7 The analysis therefore cannot be done in a single day. On the other hand, the novel presented lithium borates fusion method (including dissolution of the sample with salicylic and ascorbic acid, iron hydroxides precipitation, and centrifugation) takes around 2 h from the start of the decomposition to the time when the sample is ready to be loaded on a resin. The comparison of average times needed for each digestion/ decomposition method is illustrated in Figure 2. Additionally, the time required for measurement of the activity concentration of the selected radionuclide highly depends on the radiochemical recovery, since higher recovery also means shorter counting time before the statistically sufficient number of counts is achieved (counting uncertainty ≤5%). The radiochemical recoveries for the methods described above are graphically presented in Figure 3 and tabulated in Table 2 where it can be seen that the radiochemical recoveries obtained

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RESULTS AND DISCUSSION Elemental Compositions. To establish whether the proposed method employing salicylic and ascorbic acid is appropriate for all types of solid samples independent of the matrix composition (soil, sediment, plant, and tissue), the nondestructive EDXRF analysis for elemental composition determination was applied to all six samples and to the precipitates formed after the iron hydroxides precipitation step (Table 1). The results obtained show that the mass concentration of one of the most problematic elements (silicon) in the final precipitate is in correlation with the volume of the precipitate. As can be seen in Figure 1, the higher (larger) the silicon content in the sample, the higher (larger) is the silicon content in the precipitate and therefore also the larger is the volume of formed precipitate. Since the volume of the precipitate primarily depends on the cumulative mass of the precipitating species, in the case of organic samples, which consist mostly of hydrocarbons, the volume of the precipitate is inevitably smaller than in the case of soil and sediment samples. The analysis can, however, be conducted even when a larger volume of precipitate is formed. In the latter case, a larger volume of acid is required to dissolve the precipitate. The reason that the ratio between the silicon content in the sample/ precipitate and the volume of the precipitate for the NIST 4359 is not consistent with the general trend is by our opinion the fact that the silicon is not the only factor determining the volume of the precipitate. As can be deducted from Table 1, E


Article

Analytical Chemistry Table 3. Activity Concentrations of 234

234

U and

U Determined by Various Decomposition Methodsa

238

238

U A [Bq/g] ± uncertainty [Bq/g]

IAEA 300

NIST 4359

IAEA 437

IAEA 375

NIST 4353A

soil

certified/ reference valuesb

borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion IAEA 300 NIST 4359 IAEA 437 IAEA 375 NIST 4353A

a

U A [Bq/g] ± uncertainty [Bq/g]

7.18 × 10−2 ± 7.3 × 10−3

IAEA 300

7.08 × 10−2 ± 7.5 × 10−3 6.55 × 10−2 ± 5.0 × 10−3 6.52 × 10−2 ± 7.0 × 10−3 1.03 × 10−2 ± 8.6 × 10−4

NIST 4359

9.70 × 10−03 ± 9.2 × 10−04 9.12 × 10−3 ± 1.1 × 10−3 9.46 × 10−3 ± 8.8 × 10−4 2.37 × 10−3 ± 2.4 × 10−4

IAEA 437

2.40 × 10−3 ± 1.0 × 10−4 2.40 × 10−3 ± 3.1 × 10−4 2.40 × 10−3 ± 1.9 × 10−4 2.81 × 10−2 ± 1.8 × 10−3

IAEA 375

2.78 × 10−2 ± 3.2 × 10−3 2.60 × 10−2 ± 1.9 × 10−3 3.04 × 10−2 ± 2.7 × 10−3 4.60 × 10−2 ± 4.5 × 10−3

NIST 4353A

4.22 × 10−2 ± 4.4 × 10−3 3.73 × 10−2 ± 2.6 × 10−3 4.44 × 10−2 ± 3.3 × 10−3 4.98 × 10−2 ± 5.1 × 10−3

soil

5.23 × 10−2 ± 4.9 × 10−3 3.99 × 10−2 ± 5.5 × 10−3 5.84 × 10−2 ± 4.8 × 10−3 6.90 × 10−2 ± 0.5 × 10−2

certified/ reference valuesb

9.50 × 10−3 ± 1.10 × 10−3 2.3 × 10−3 ± 1.00 × 10−4 2.5 × 10−2; 1.7 × 10−2−3.2 × 10−2 4.04 × 10−2; 3.37 × 10−2−4.77 × 10−2

borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion borates fusion (S + A) borates fusion (PEG) classical digestion microwave digestion IAEA 300

7.07 × 10−2 ± 7.20 × 10−3

NIST 4359 IAEA 437 IAEA 375 NIST 4353A

8.67 1.87 2.44 3.96

6.67 × 10−2 ± 7.10 × 10−3 6.44 × 10−2 ± 4.90 × 10−3 6.16 × 10−2 ± 6.70 × 10−3 9.49 × 10−3 ± 8.0 × 10−4 9.62 × 10−3 ± 9.1 × 10−4 8.87 × 10−3 ± 1.1 × 10−3 9.07 × 10−3 ± 8.5 × 10−4 2.09 × 10−3 ± 2.2 × 10−4 2.07 × 10−3 ± 9.3 × 10−5 2.14 × 10−3 ± 2.9 × 10−4 2.09 × 10−3 ± 2.9 × 10−4 2.81 × 10−2 ± 2.8 × 10−3 2.61 × 10−2 ± 3.0 × 10−3 2.63 × 10−2 ± 2.0 × 10−3 3.05 × 10−2 ± 2.7 × 10−3 4.03 × 10−2 ± 4.1 × 10−3 4.18 × 10−2 ± 4.3 × 10−3 3.66 × 10−2 ± 2.6 × 10−3 4.75 × 10−2 ± 3.5 × 10−3 5.56 × 10−2 ± 5.6 × 10−3 5.51 × 10−2 ± 5.1 × 10−3 5.75 × 10−2 ± 7.7 × 10−3 6.24 × 10−2 ± 5.2 × 10−3 6.47 × 10−2; 61−68.7 × × × ×

10−3 ± 5.40 × 10−4 10−3; 1.80 × 10−3−1.92 × 10−3 10−2; 1.9 × 10−2−2.98 × 10−2 10−2 3.19 × 10−2−4.81 × 10−2

The uncertainty is given as an extended uncertainty at k = 2. bConfidence interval (α = 0.05).

by the proposed method vary and are, in the case of analysis of organic materials NIST 4359 and IAEA 437 and the sediment sample IAEA 300, comparable with the radiochemical recoveries of other investigated methods. In the case of analysis of soil samples on the other hand, the radiochemical recoveries obtained by the proposed method are much lower compared to the radiochemical recoveries obtained by the BF (PEG) method but still better compared to the ones obtained by classical or microwave digestion. For the in-house sample “soil”,

the low radiochemical recoveries obtained by CD, MW, and BF (S + A) methods can probably be attributed to the matrix effect. Interestingly, the radiochemical recovery of the BF (PEG) method was for the same sample much higher when compared to the next most efficient method BF (S + A) (87% vs 31% for BF (S + A)). Activity Concentrations. The described decomposition methods also differ in the activity concentrations determined. In the sediment (IAEA 300) and the seaweed (NIST 4359) F


Article


Figure 4. Deviation of activity concentrations of 234U from the certified value. The uncertainty of the measurement and the certified/reference value are given as an extended uncertainty at k = 2. For the in-house sample, the deviation from the average value is shown.

Figure 5. Deviation of activity concentrations of 238U from the certified value. The uncertainty of the measurement and the certified/reference value are given as an extended uncertainty at k = 2. For the in-house sample, the deviation from the average value is shown.

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reference materials, which contain a significant amount of silicon, the results obtained by the two fusion methods were

CONCLUSION

This work presents the development of a novel fast dissolution method employing salicylic acid following thermal fusion utilizing lithium borates as a fusion agent for various solid samples (soil, sediment, plant, and tissue). The developed method was tested by analysis of reference materials and an inhouse soil sample. The results obtained show that the method allows a fast and efficient determination of uranium activity concentrations in all of the analyzed samples. The radiochemical recoveries and activity concentrations determined by the method are consistent with the radiochemical recoveries and activity concentrations determined by the compared methods. The main advantage of the newly developed method, however, is the significantly shorter analysis time needed

higher than when classical or microwave decomposition was employed. In the organic material IAEA 437 on the other hand, all four decomposition methods gave comparable activity concentrations. For soil samples IAEA 375, NIST 4353A, and the in-house soil sample, the highest activity concentrations were determined when microwave digestion was applied. The results, however, are comparable with results obtained by lithium borates fusion with PEG. The results are listed in Table 3, and the deviations between the activity concentrations are graphically shown in Figures 4 and 5. G


Article

Analytical Chemistry (approximately 4−10 times shorter) when compared to the time required if other digestion methods are employed.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Miha Trdin: 0000-0001-9674-0078 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS TThe authors acknowledge the financial support from the Slovenian Research Agency (research core funding No. P-143). The research represent a part of the ERA Chair ISO-FOOD project for isotope techniques in food quality, safety and traceability (Grant Agreement No. 621329).

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REFERENCES

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Fast Decomposition Procedure of Solid Samples by Lithium Borates

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