PIXE ANALYTICAL METHOD APPLIED IN THE STUDY OF ENVIRONMENTAL SAMPLES USED AS BIOINDICATORS E.D. CHELARESCU1, I.D. DULAMA2 , A.I. GHEBOIANU2, I.A. BUCURICA2, D. PACESILA1 1
Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), Tandem Accelerators Department, Reactorului St., P.O. Box MG-6, Bucharest-Magurele, Romania E.D.Chelarescu:
[email protected] Corresponding author: I.D. Dulama:
[email protected] 2 Valahia University of Targoviste, Multidisciplinary Scientific and Technologic Research Institute, 130004 Targoviste, Romania Received December 17, 2015 In this study we apply Proton Induced X-Ray Emission (PIXE) spectrometry to determine the heavy metal content of some environmental certified reference materials and moss samples. The PIXE experiments were performed using a proton beam provided by the 3 MV TandetronTM particle accelerator from the National Institute for R&D in Physics and Nuclear Engineering Horia Hulubei (IFIN-HH) in Magurele-Bucharest. The trace elements in mosses pellets and certified standard samples were measured. The elements identified were: Al, P, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn and As, and quantitatively determined were: K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu and Zn. The concentrations values of heavy metals in the analyzed samples determined by PIXE technique were compared with the results obtained by Flame Atomic Absorption Spectrometry (FAAS) method. Key words: PIXE spectrometry, moss samples, heavy metals.
1. INTRODUCTION
The heavy metals in mosses biomonitoring network was originally established as a Swedish initiative [1]. The idea to use mosses to measure atmospheric heavy metal deposition is based on the fact that carpet formed by ectohydric mosses obtain most of trace elements and nutrients directly from precipitation and dry deposition; there is little uptake of metals from the substrate. The technique of moss analysis as bioindicators provides a measure of heavy metal deposition from the atmosphere to terrestrial systems. In the last decades, mosses have been successfully applied as bioindicators of heavy metal deposition [1] across Europe. It has been shown that at the European scale atmospheric deposition is the main factor determining the accumulation of heavy metals in mosses. Biomonitoring with moss has a number of advantages due to their spatial distribution including parts of southern and eastern Europe [1]. Rom. Journ. Phys., Vol. 61, No. 7–8, P. 1369–1379, 2016
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Different atomic and nuclear methods such as Flame Atomic Absorption Spectrometry (FAAS), Graphite Furnace Atomic Absorption Spectrometry (GFAAS), Inductively Coupled Plasma Atomic Emission Spectrometry (ICPAES), Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), Neutron Activation Analysis (NAA), Wavelength Dispersive X-Ray Fluorescence (WDXRF), Energy Dispersive X-Ray Fluorescence (EDXRF) and Proton-Induced X-Ray Emission (PIXE), has been extensively employed in the life sciences, in particular in the environmental fields, due to their capabilities to measure a wide range of chemical elements [2–12]. The aim of this work was to determine the heavy metals content of some environmental moss samples by PIXE technique. PIXE is a nondestructive method capable to analyze many elements, simultaneously, practically all elements with Z > 12, with concentrations in the range of ppm, in short time and relative low cost [11–24, 30]. The moss samples were collected from different geographical areas of Romania. Analyses were performed using the 3MV Cockroft-Walton TandetronTM accelerator [24–26]. The accelerator is dedicated for Ion Beam Analysis (IBA). The first beam line has all necessary equipment to perform (Proton-Induced X-Ray Emission) PIXE, Particle Induces Gamma Ray Emission (PIGE), Rutherford Backscattering (RBS) [24, 25]. In this work are presented the obtained results by two methods: PIXE at IFIN-HH and AAS at Valahia University of Targoviste, analysis applied to moss samples. Heavy metal concentrations obtained by PIXE method for two standard samples, were compared with the concentrations which are given in the Certificate of Reference Materials (CRM) [28] and for moss samples with values determined by AAS analysis method. The characteristic X-ray spectra were processed off-line, using GUPIX and Leone software [27].
2. MATERIALS AND METHOD
2.1. MOSSES SAMPLES
For this study, different moss species were collected [1]: Pleurozium schreberi is a species of moss that grows in the form of "up" is robust but not very big (a few centimeters) that grows freely, not in the form of bundles. The strain can be anywhere from light green to golden. Young leaves (2–3 years) are green, and when they give the impression of a green wet transparent. Usually prefers acidic soils (sand, clay, loam etc.) in colder areas (mountain areas).
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Hylocomium splendens is known as moss or moss-fern shiny mountain. It is an evergreen moss whose area of distribution is cold forests (at high altitudes). In general, it is an olive-green moss, but can be found with the colors green, yellow and even red. Stems and branches are brown – reddish with length to 20 cm. Each year add green branches, thus allowing estimate age simply by counting the branches. Often develops forest residues. Hypnum cupressiforme is a common species of moss found on all continents except Antarctica. Usually grows on trees, logs, rocks and other surfaces, preferring acidic environments. It is a medium sized moss (2–10) cm, with irregular branches, green leaves thin and sharp, arched towards the top. This species is multiplied by spores contained in brown capsules (1.7–2.4) mm format which is above the carpet of moss. For PIXE, the moss samples were washed, dried at 40 °C and milled. Approximately 0.6 g from each sample were mixed with 0.06 g boric acid and pressed into pellets, with 15 mm diameter, at a pressure of 12–15 tf. For AAS, approximately 0.2 g of moss sample (powder used for PIXE pellets) were digested with 8 mL HNO3 65% (Merck) and 10 mL H2O2 30 % (Merck) in Speedwave MWS-2 Berghof microwave system. The clear solutions were filtered and were bought in 50 mL flask with distilled deionized water. 2.2. PIXE METHOD. EXPERIMENTAL PROCEDURE
Particle-Induced X-Ray (PIXE) is based upon the ejection of inner-shell electrons from target atoms by bombarding with heavy charged particles which lead to the emission of characteristic X-rays during the electronic transitions. A review of the inner-shell vacancy production in ion atom collisions is given by Garcia et al. [29]. By detecting the X-rays spectra, it turns out what elements and how many elements are included. PIXE has been commonly used to analyze the major or minor elements in a wide range of material samples [30]. When the X-rays are detected by a high resolution detector, the well known Z-dependence of the X-ray energies, as well as the intensities of the X-rays lines, allow a straightforward determination of the target elements. PIXE technique has proved to be a sensitive analytical method of the chemical elements, especially convenient for a large spectrum of samples which are, generally, available only in small amounts [11–23]. The detection limit of this analytical technique is in the ppm range for all elements with Z number higher than 12, because intense fluxes of charged particle obtained from tandem accelerator are readily available, the X-ray production yields for particles beam with energies in the MeV range are large and the background associated with the exciting radiation is rather low.
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For our PIXE analyses we used a 3 MeV proton beam, a thick target (standard and moss sample in the form of pellets) was placed in the center of the reaction chamber at an angle of 45° with respect to the incident proton beam and at 90° with respect to the X-ray detector [24, 26]. The beam transport tube of accelerator and reaction chamber was maintained in a high vacuum (10 -6 mbar) during target irradiation. The current value on target was in the range of 1–5 nA, to maintain a low count rate that implies a negligible dead time correction. The characteristic X-ray spectra of samples were detected with a HPGe detector, “IGLET-X-06135-S”, with the following main characteristics: active diameter 6 mm, active depth 6 mm, beryllium absorbing layers 0.0127 mm and energy resolution (FWHM) of 180 eV at XKα 5.9 keV line of Mn, resulting from the radioactive decay of 55Fe, EC decay type. The X-ray spectra were recorded by a spectrometric chain system which includes a multichannel analyzer connected to a computer and processed off-line using the GUPIX and LEONE softwares [21, 22, 27]. PIXE method was applied in relative version – the concentrations of elements from samples were determined using known values of concentrations of the same elements from the standard samples.
3. RESULTS AND DISCUSSION
The results obtained by PIXE were compared with concentrations values given in Certificate of Reference Materials (CRM) [28] and are presented in Table 1 and Figs.1 a,b. Table 1 Comparison of elemental concentrations (in mg/Kg) from standard samples obtained by (a) PIXE method and (b) certified values [28] Elements/ K Ca Ti Mn Fe Cu Zn As Samples SRM2710 a. 21906 ±868 11319±129 3544±42 2070±46 38571±116 3272±61 3778±68 1343±80 Montana soil b. 21700±1300 9640 ±450 3110±70 2140±60 43200±800 3420±50 4180±150 1540±100 – 48±3 91±4 5.39±0.65 12.9±0.8 – SRM1515 a. 16415±523 15739±413 Apple leaves b. 16100±200 15260±15 – 54±3 83±5 5.64±0.24 12.5±0.3 –
Statistically, a good agreement between the concentrations values determined by PIXE method and certified values was observed. The results confirm the correct application of PIXE method.
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In the same experimental conditions, seven moss samples, collected from different geographical areas from Romania (Table 2, Fig. 2), were also analyzed by PIXE method. A typical X-ray spectra of a moss sample is presented in Fig. 3.
a
b Fig. 1 – Comparison diagrams of elemental concentrations in standard samples (SRM 2710 and SRM 1515) determined by PIXE technique and the certified values. Table 2 Sampling points No. 1 2 3 4 5 6 7
Sampling points Baia Sprie Gheorghe Doja Cordareni Ruschita Hurezani Motru Topoloveni
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Fig. 2 – Collection points.
Fig. 3 – Characteristic X-ray spectra of moss sample collected from Gheorghe Doja zone (No. 2, Table 2).
The elements identified in the characteristic X-ray spectra of moss samples were: Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu and Zn. GUPIX analysis program and the relative method using the X-ray spectra of standard samples, with concentrations indicated in the Certified Reference Material (CRM), were used for a quantitative analysis; the concentrations for the elements: K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu and Zn, were determined.
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In Table 3 and Figs. 4 (a _ j), are shown the results obtained by PIXE method compared with the concentrations determined by FAAS method. Table 3 Concentrations values (in mg/kg) of elements from moss samples determined by (a) PIXE method and (b) by FAAS method
Samples
Analysis method
Concentration [mg/kg] K
Ca
Ti
a.
5932 ±273
11733 ±493
547 ±77
Fe
Ni
Cu
Zn
15.2 ±3.5
10.32 201 ±2.5 ±28
4213 ±147
4.3 ±1.8
247 ±34
71.2 ±6.4
b.
6278 ±439
11432 ±621
483 ±39
14.3 ±1.8
9.31 173 ±1.21 ±24
3985 ±198
3.84 ±0.31
243 ±15
68.3 ±5.3
a.
14592 12567 ±466 ±490
1905 ±161
51.7 ±10.8
39.3 ±9.1
492 14651 ±54 ±454
21.7 ±1.7
58.4 ±8.6
76.9 ±6.4
b.
14954 12126 ±628 ±594
1887 ±126
46.2 ±4.5
36.1 ±5.2
481 14412 ±52 ±605
55.3 ±4.8
73.1 ±5.7
a.
13223 11183 ±423 ±480
343 ±53
-
-
b.
12867 10832 ±656 ±552
321 ±35
a.
5032 ±256
6421 ±327
138 ±24
b.
4932 ±394
6127 ±429
164 ±18
a.
16055 ±465
7672 ±391
79.3 ±14.4
b.
15693 ±675
7321 ±411
74.5 ±8.9
a.
12230 ±367
6088 ±398
520 ±73
-
b.
11927 ±667
5911 ±467
462 ±34
a.
11225 10714 ±348 ±493
b.
11468 10387 ±550 ±534
1
2
3
4
5
6
7
V
Cr
Mn
18.3 ±1.8
201 ±28
1283 ±53
-
-
48.1 ±5.3
8.24 5.32 188 ±1.12 ±0.78 ±22
1267 ±101
2.51 ±0.23
-
45.2 ±3.6
134 ±17
1001 ±45
-
-
132 ±12
2.67 6.89 127 ±0.58 ±1.10 ±19
943 ±113
1.34 ±0.17
-
97.4 ±9.8
528 ±73
681 ±46
-
6.02 ±1.9
32.3 ±2.8
2.33 2.56 482 ±0.41 ±0.58 ±43
652 ±78
387 ±69
3780 ±381
-
-
189 ±33
12.7 ±1.3
8.40 346 ±1.51 ±38
3539 ±223
8.01 ±0.64
-
163 ±11
1272 ±194
43.7 ±9.2
37.2 ±7.8
310 14182 ±58 ±496
18.7 ±7.6
33.2 ±4.8
64.3 ±5.6
1386 ±94
34.2 ±4.1
33.7 ±5.3
274 14263 ±36 ±611
17.1 ±1.8
30.4 ±2.7
62.7 ±5.3
-
-
-
-
-
0.50 4.97 31.1 ±0.04 ±0.72 ±3.7
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a
b
c
d
e
f Fig. 4
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h
i
j
Fig. 4 (continued) – Elemental concentrations determined by PIXE and FAAS methods.
It can be seen that, within standard deviations, there is a very good agreement between the results obtained by PIXE method and those obtained by FAAS method.
4. CONCLUSIONS
The results reported in this work, proven the reliability of the 3 MV TandetronTM particle accelerator from the National Institute for R&D in Physics and Nuclear Engineering “Horia Hulubei” (NIPNE) in Magurele-Bucharest, which guarantee the application of PIXE method of high sensitivity and precision, in a wide range of areas, including in especially, the study of environmental pollution with heavy metals. Our experiment confirms, once again, that PIXE technique is very exact and highly reliable with following advantages: is a multielemental analysis method,
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capable to determine in the ppm range all elements with Z number higher than 12; samples do not require any special chemical treatment in advanced and it is directly analyzed; it is a relatively fast analysis (minutes) and it’s completely non-invasive. It is suitable for fragile or very expensive samples [24–26]. Although AAS method has high sensitivity, in comparison with the PIXE analysis method, there are some disadvantages: FAAS is a monoelemental analysis method; involves chemical processing of samples and the successive analysis of elements contained in the sample. So, large experimental errors can occur and the cost per analysis-sample may be higher. Acknowledgements. The authors are grateful to Dr. Dan Gabriel Ghita, head of Tandem Accelerators Department (TAD) of Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH) Bucharest-Magurele, Romania, Dr. Mihai Straticiuc and Dr. Ion Burducea from DFNA-IFIN-HH, for their help and support in conducting experiments and for their useful discussions.
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