Uranium determination using atomic spectrometric techniques: An overview

Analytica Chimica Acta 674 (2010) 143–156

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Review

Uranium determination using atomic spectrometric techniques: An overview Juracir S. Santos a,b , Leonardo S.G. Teixeira a,b , Walter N.L. dos Santos b,c , Valfredo A. Lemos b,d , Jose M. Godoy e,f , Sérgio L.C. Ferreira a,b,∗ a

Instituto de Química, Universidade Federal da Bahia, Campus Universitário de Ondina, Salvador, BA, 40170-290, Brazil INCT de Energia e Ambiente, Universidade Federal da Bahia, Salvador, BA, 40170-290, Brazil c Departamento de Ciências Exatas e da Terra, Universidade do Estado da Bahia, Estrada das Barreiras S/N, Salvador, BA, 41195-001, Brazil d Universidade Estadual do Sudoeste da Bahia, Departamento de Química e Exatas, Campus de Jequié, Jequié, BA, 45206-190, Brazil e Instituto de Radioprotec¸ão e Dosimetria, Comissão Nacional de Energia Nuclear, CP 37750, Barra da Tijuca, 22642-970 Rio de Janeiro, RJ, Brazil f Departamento de Química, Pontifícia Universidade Católica do Rio de Janeiro, Rua Marquês de São Vicente 225, Gávea, 22453-900 Rio de Janeiro, RJ, Brazil b

a r t i c l e

i n f o

Article history: Received 8 December 2009 Received in revised form 8 June 2010 Accepted 9 June 2010 Available online 17 June 2010 Keywords: Uranium determination Spectroanalytical techniques

a b s t r a c t This review focuses on the determination of uranium using spectroanalytical techniques that are aimed at total determination such as flame atomic absorption spectrometry (FAAS), electrothermal atomic absorption spectrometry (ETAAS), inductively coupled plasma optical emission spectrometry (ICP-OES); and inductively coupled plasma mass spectrometry (ICP-MS) that also enables the determination of uranium isotopes. The advantages and shortcomings related to interferences, precision, accuracy, sample type and equipment employed in the analysis are taken into account, as well as the complexity and costs (i.e., acquisition, operation and maintenance) associated with each of the techniques. Strategies to improve their performance that employ separation and/or preconcentration steps are considered, with an emphasis given to solid-phase extraction because of its advantages compared to other preconcentration procedures. © 2010 Elsevier B.V. All rights reserved.

Contents 1. 2.

3. 4.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Uranium determination using spectrometric techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Flame atomic absorption spectrometry (FAAS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Electrothermal atomic absorption spectrometry (ETAAS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Inductively coupled plasma optical emission spectrometry (ICP-OES) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.1. Choice of spectral line for the determination of uranium by ICP-OES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.2. Interference of others elements in the determination of uranium by ICP-OES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.3. Presence of acids in the determination of uranium by ICP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3.4. Determination of uranium in organic phase by ICP-OES . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Inductively coupled plasma mass spectrometry (ICP-MS) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.1. Factors that influence the determination of uranium using ICP-MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.2. ETV-ICP-MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4.3. LA-ICP-MS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Strategies for preconcentration and separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author at: Instituto de Química, Universidade Federal da Bahia, Campus Universitário de Ondina, Salvador, BA, 40170-290, Brazil. Tel.: +55 71 32836800; fax: +55 71 32355166. E-mail address: [email protected] (S.L.C. Ferreira). 0003-2670/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.aca.2010.06.010

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1. Introduction Uranium is an element of the actinide series, has an atomic number of 92 and, in its refined state, it is metallic silver in color, malleable, ductile, slightly paramagnetic, and very dense. In rocks and ore, this element is not found in the metallic state but generally occurs in minerals such as carnotite, uraninite, and pitchblend [1]. Uranium in the environment occurs naturally as three radioactive isotopes: 238 U (99.27%), 235 U (0.72%) and 234 U (0.005%), but other isotopes can be synthesized [2]. Uranium is an element that naturally presents various oxidation states (namely +2, +3, +4, +5 and +6), but uranium appears mostly in its hexavalent form. Usually in nature, uranium is associated with oxygen, forming the uranyl ion UO2 2+ [3]. Uranium is also present often in tetravalent state in the nature on strongly reducing medium, like water with a high amount of organic material [4]. In the terrestrial crust, uranium has an average concentration of 4 ␮g g−1 and, in order of magnitude, is more abundant than other heavy metals, such as mercury and silver. Its concentration in seawater hovers around 3 ␮g L−1 and is distributed uniformly in all the world’s oceans. In surface freshwater (rivers and lakes), the average concentration is as low as 0.5 ␮g L−1 , and depending on the location and contamination of the water, it can reach concentrations as high as 500 ␮g L−1 [4,5]. Uranium is an element of great commercial interest because of its use in the production of nuclear energy, in the manufacture of nuclear weapons, in the shielding of industrial radioactive sources and even as anti-tanks ammunition. Unfortunately, human activities involving mining and milling activities, nuclear weapons and nuclear fuel fabrication have caused widespread environmental contamination. Additionally, contamination may be caused by catalysts, staining pigments, burning of fossil fuel (oil and coal) and the manufacture and use of phosphate fertilizers that contain uranium [2,3,6,7]. The management of these wastes has been a concern in many countries where it causes problems such as groundwater contamination in proximity to populated areas, but a lack of funds and resources make management difficult [8,9]. There is growing concern regarding the presence of uranium in the environment caused by the human activities [7]. This concern stems from the observation that uranium present in the soil can be transferred to water, plants, food, food supplements, and fertilizers and then effect humans [10,11], with studies showing that foods contribute about 15% of the uranium ingested while water represents about 85% [11]. Uranium has both chemical and radiological toxicity but, for natural uranium, the main concern is due to its chemical toxicity. In the human body, uranium tends to be concentrated in specific locations and, because of its radioactivity, can increase the risk of bone cancer, liver cancer, and blood disease [11,12]. The greatest risk to health caused by the uranium toxicity is the likelihood of damage to the structure of the kidneys, which can cause acute renal failure [10]. The amount of available information on the chronic health effects of human exposure to environmental uranium, however, is still relatively small [13]. Radionuclides, such as uranium, represent an important category of inorganic pollutants whose quantitation is necessary now and into the future. Environmental samples (like waters, soils, rocks and sediments), as well as fertilizer and industrial waste are potential samples where an uranium contamination may causes concern [7]. Many regulatory bodies, such as the World Health Organization (WHO) [3], U.S. Environmental Protection Agency (EPA) [14], U.S. Food and Drug Administration (FDA) [15], Canadian Council of Ministers of the Environment [16], and the European legislature [17] are worried about the amount of uranium present in natural water samples and various types of food and are therefore requesting the

Table 1 Guideline value established for maximum concentration level of uranium in drinking water. Regulatory agency

Guideline value (␮g L−1 )

Ref.

World Health Organization Health Canada National Health and Medical Research Council of Australia Brazil Environmental Agency U.S. Environmental Protection Agency

15 20 20

[3] [19] [20]

20 30

[18] [14]

determination of this element in various matrices. In Brazil, the maximum concentration of uranium tolerated in drinking water is 20 ␮g L−1 [18]. The guideline values established for maximum concentration levels of uranium in drinking water for some regulatory bodies in the world are shown in Table 1 [19,20]. The need for sensitive and reliable analytical methods to detect low-level concentrations of uranium is desirable in many fields. Several spectrometric methods have been developed for the determination of trace uranium. Each method has advantages and shortcomings relative to non-spectral or spectral interference, precision, accuracy, cost (acquire, operation and support), sample type (solid or liquid) and apparatus employed in the analysis. The applicability of the method must also take into consideration whether each detection technique is overly complex or requires extensive and laborious separation or preconcentration steps.

2. Uranium determination using spectrometric techniques The determination of uranium concentrations in environmental, biological and radioactive waste samples is extremely important and involves various methods of analysis that depend on the concentration of the metal ion in these samples. A variety of atomic spectrometric techniques have been used for the determination of uranium concentrations, among which are atomic absorption spectrometry (AAS) with flame and graphite tube atomizers, inductively coupled plasma optical emission spectrometry (ICP-OES), inductively coupled plasma mass spectrometry (ICP-MS) and Xray fluorescence spectrometry (XRF). XRF spectrometry was not discussed in this paper because review papers involving this technique were recently published [21,22]. The methods most suitable for routine analysis are laser-induced fluorescence (LIF), XRF and ICP-MS because of their better sensitivities. However, the high cost of operation and equipment for these methods is beyond the reach of many laboratories. Electrothermal AAS (ETAAS) and ICP-OES are alternative techniques that can be easily automated and provide a lower cost benefit; however, these techniques suffer from interference and display low sensitivity. To solve these problems, techniques of separation and preconcentration are required before analysis. The determination of uranium in rocks, sediments, silicates, soils or minerals by spectrometric techniques is usually carried out after conventional dissolution/digestion of samples. In this process is necessary to add reagents to the sample and apply enough energy to break some bonds and the crystalline structure of solids to release the uranium for measurement or prior preconcentration stage [23,24]. The most ordinary way for decomposition of solid samples for uranium determination is employing acids such as nitric acid, hydrochloric acid, hydrofluoric acid and hydrogen peroxide, as well as mixtures of such reagents, and microwave or conventional heating [25–29]. Other methods like fusion [30] and electro-oxidative leaching [23] also can be used for digestions of these samples.

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However, it should be clear that spectrometric techniques, with the exception of the mass spectrometric ones, are able to determine only the total amount of uranium present that, for naturally occurring uranium, represents mainly 238 U. If isotopic information is needed radiochemical methods must be taken into account as alternative for the mass spectrometry. However, the discussion of radiochemical methods like alpha-spectrometry is out of the scope of the present work. It is beyond the scope of this review to describe the basic principles of these analytical techniques, and the reader is referred to several excellent textbooks and review articles for detailed descriptions [31–35].

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800 ␮g mL−1 of aluminum resulted in a four-fold enhancement of the signal [31]. Another possibility that has been poorly explored, which can increase the sensitivity, is the indirect determination of uranium by FAAS. Alder and Das [36] have developed an indirect method for the determination of uranium by FAAS using an air–acetylene flame. The method involves the reduction of copper(II) by uranium(IV), followed by complex formation of the copper(I) ions produced with neocuproine (2,9-dimethyl-1,10-phenanthroline). Uranium is then determined by measurement of the copper by FAAS after back-extraction into hydrochloric acid. The sensitivity achieved was 4.9 ␮g of uranium with an upper limit of 500 ␮g without dilution.

2.1. Flame atomic absorption spectrometry (FAAS) 2.2. Electrothermal atomic absorption spectrometry (ETAAS) Flame atomic absorption spectroscopy (FAAS) is a wellestablished and popular technique because of its relatively high sensitivity, selectivity, speed and lower cost for the determination of a large number of elements in various kinds of matrices [36]. In flame methods, memory effects are insignificant as no sample is retained in the flame [37]. However, there are few analytical descriptions involving uranium concentration determination by FAAS. The infrequent application of this technique can be explained by the weakness of the lines in the complex spectrum of uranium and the fact that its complexes are not completely dissociated in the flame [38]. One of the few disadvantages of FAAS is the lack of sensitivity for certain refractory elements, particularly uranium, because of high stability of its oxides formed in the flame, namely U3 O8 , UO2 and UO [36]. The most sensitive absorption lines for uranium are found at 358.5 and 356.7 nm, with 358.5 nm being the more sensitive of the two. This is also a region where CN radical absorption bands occur from the flame, however, causing a high level of background. Thus, to improve the instrumental sensitivity, the 356.7 nm line can be used [39]. In addition, uranium can be determined at 351.5 nm with a nitrous oxide–acetylene flame [31]. Although there are other distant lines of the CN absorption region, the sensitivity is so low that it can be difficult to determine uranium for analytical purposes by FAAS [39]. The determination of uranium by FAAS requires higher temperatures than those supplied by an air–acetylene flame. Under these conditions, to provide a greater population of atoms of uranium atomized in the path of the radiation of the hollow cathode lamp, it is necessary to use a nitrous oxide–acetylene reducing flame [40]. Even with the use of high temperatures and a nitrous oxide–acetylene flame, a poor sensitivity is achieved for uranium, approximately 50 ␮g mL−1 for 1% absorbance, while the nitrous oxide–acetylene flame gives an absorbance reading of about 0.09 for 1000 ␮g mL−1 uranium if sufficient alkali has been added to suppress ionization [36]. The ionization of uranium in nitrous oxide–acetylene is only about 45%. The addition of an ionization suppressor, such as alkali metals, does not completely suppress the ionization of uranium to provide a considerable increase in the absorption [39]. Thus, Welz and Sperling [31] state that 1 g L−1 potassium chloride should be added to all measurement solutions to suppress this effect. The presence of metals that form stable oxides in the flame, such as aluminum, titanium and gallium, provides an effective increase in signal absorption. This effect is due to competition for available oxygen in the flame, shifting the chemical equilibrium towards the atomization of uranium oxides and providing a better sensitivity [39]. Gallium displays the most pronounced effect, where the addition of 10 g L−1 gallium produced a characteristic concentration of 16 ␮g mL−1 at the 358.5 nm line and a characteristic concentration of 26 mg L−1 at the 351.5 nm analytical line. The presence of

Electrothermal AAS using a graphite tube atomizer is one of the most commonly used techniques for the determination of trace elements in complex matrices and quality control due to its high sensitivity. In addition, ETAAS displays a series of advantages, such as a small sample size requirement (2–50 ␮L), a significant decrease of sample preparation time, a reduction in spectral interferences and the possibility of matrix removal from the analyte of interest using drying and pyrolysis steps prior to the high temperature vaporization step [41]. Although uranium has been determined by ETAAS, this element possesses very poor detection limits in this method. Norval [42] found a detection limit of 0.6 ␮g mL−1 at an atomizing temperature of 2750 ◦ C employing pyrolytically coated graphite tubes. The atomization of uranium in graphite furnaces appears to be a complex process. The sensitivity of uranium is adversely affected both by carbide formation and by the formation of a number of oxides, particularly U3 O8 , which is thermally very stable and is obtained between 800 and 900 ◦ C [41]. According to Welz [31], uranium is one of the elements with the lowest sensitivity in this method. In a longitudinally heated atomizer at an atomization temperature of 2650 ◦ C, a characteristic mass of 12 ng has been achieved, and in a transversely heated isothermal atomizer under STPF (stabilized temperature platform furnace) condition, 40 ng has been attained as a characteristic mass at 2550 ◦ C [31]. Goltz et al. [41] proposed the use of ETAAS and ETV-ICPMS (electrothermal vaporization inductively coupled plasma mass spectrometry) to elucidate the mechanisms of vaporization and atomization of uranium in a pyrolytically coated graphite tube. The ETAAS provides information about the temporal behavior of atomic species, while the ETV-ICP-MS signal arises from both atomic and molecular analyte species. In ETAAS at temperatures below 2400 ◦ C, there was no atomic absorbance signal, indicating that uranium atoms were not formed. Unlike that observed for more volatile elements, the atomization curve for uranium did not plateau at higher atomization temperatures, indicating that a relatively involatile complex of uranium was produced and was not completely vaporized at the highest atomization temperature (2750 ◦ C) available. Through the use of ETV-ICP-MS, it was possible to prove that the signals for uranium between 1100 and 2400 ◦ C were due to volatile molecular uranium species and not uranium atoms. A number of volatile uranium oxide species, such as U3 O8 , U3 O5 and U2 O4 , are formed and thermally decompose to form UO2. The appearance of molecular uranium species below 2400 ◦ C would at least partially account for the poor sensitivity of uranium in ETAAS. In addition, pre-atomization loss of uranium as the molecular species would occur during the pyrolysis step at temperatures above 1100 ◦ C or during the atomization step at temperatures below 2400 ◦ C [41].

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Another reason for a low sensitivity of uranium with ETAAS can be explained by the fact that not all uranium is in the atomic form. At temperatures above 2400 ◦ C, the formation of stable refractory carbides resulting in incomplete atomization of uranium also occurs. The presence of carbide (namely: UC and UC2 ) degrades both the detection limit and precision, and serious memory effects are also observed in subsequent determinations [43]. In addition to uranium, several elements, such as V, Cr, As, Ba, Ta, W, B, Si, La, Mo and Nb, are able to form refractory carbides with the carbon of the graphite tube used in electrothermal vaporization or atomization. The extent of the carbide formation by these elements depends mainly on the temperature of the graphite tube and on the matrix. The temperature required for decomposition of the carbides is about 3000–4000 ◦ C, higher than the sublimation temperature of graphite [44,45]. To reduce the effects caused by the formation of uranium carbide and increase the sensitivity in ETAAS, it is necessary to use higher atomization temperatures, above 2600 ◦ C. This severe condition is inadequate for tube life because elevated temperatures in the graphite tube decrease their lifetimes due to oxidation of outer tube surface. This oxidation promotes the interaction of the uranium with carbon of the graphite tube and forming and dissociating UC and UC2 , which causes further degradation of the inner surface [42]. In this condition, the firing number should also be taken into consideration. For example, after about the first 25 firings, one to two extra firings were sufficient to clean the tube, a number that increased to four or sometimes five at the end of the lifetime of a tube. The blank values were found to increase steadily with increase in the number of firings [42]. To reduce carbide formation and improve the sensitivity, reproducibility and lifetime of the tube, several chemical modifiers have been studied for uranium atomization in ETV and quantification by ICP-MS. A suppression of uranium signal in the presence of NaCl, MgCl2 and Pd has been reported [46]. The presence of NaCl in the uranium determination was observed to cause considerable memory effects, possibly due to NaCl favoring the intercalation of uranium in the graphite tube. This effect can be observed even at atomization temperatures as high as 2700 ◦ C. The memory effect is also strongly increased in the presence of seawater due to its high NaCl content [44]. The presence of a plutonium matrix in the graphite furnace also affects the absorbance signal in ETAAS. The absorbance signal was observed to continuously decrease as a function of the Pu concentration. In temperatures above 1600 ◦ C, plutonium oxide loses oxygen and forms either lower oxides or becomes dissociated. The oxygen released during these processes may be responsible for suppression of the atomization of uranium, probably by the formation of uranium oxides [37]. In an attempt to increase the sensitivity of the graphite furnace, organic solvents were employed for the complexation of uranium. The presence of organic solvents (such as tetrahydrofuran and dioxin) in the tube promotes formation of a thin carbon layer, which apparently protected the tube against carbide formation to a certain extent. The use of such solvents provides an increase of 40–50% in the uranium absorbance value. The blank values were lower, fewer cleanings were required and the lifetime of the tube increased. Limitations of the application of these solvents are difficulties in the sampling process due to lower surface tension, formation of air bubbles in the sampling capillary and replacement of the sampling cup to avoid concentration of analyte through evaporation [42]. To improve the reproducibility of the absorbance signals, experiments were carried out to coat the inner surface of the graphite tube atomizer with other refractory elements, such as Ta, Ti and W. However, these studies did not result in any significant improvement in

reproducibility of absorbance values for uranium, probably due to the instability of the respective carbides at the uranium atomization temperature. This would again expose uranium atoms to the graphite surface for the formation of stable UC complexes. Stability of carbides of Ta, Ti and W were found to be in the range of 1400–1700 K (i.e., 1130–1430 ◦ C) [37]. Norval [42] reported coating the inner surface with zirconium by sputtering, and then the coating metal was transferred to the carbide during the pyrolytic coating process to achieve an increase of about to 50% in the absorbance value, along with good reproducibility over a large number of firings. However, these tubes are not recommended for routine use because the increase in the absorbance does not justify the cost and time required to obtain the zirconium carbide coating. Goyal et al. [37] developed the method to determine trace concentrations of uranium in the presence of plutonium as the major matrix, in spite of the problems associated with the dissociation of plutonium oxide and uranium carbide formation. This method showed good precision, sensitivity and the possibility to determine trace concentrations of uranium when more than 5 mg mL−1 of plutonium was present in the matrix. In addition, the method required small sample sizes and offers a direct determination of uranium approach in the presence of plutonium. It is important to report that, in uranium concentration determination in a plutonium matrix, the absorbance signal for the standard and samples initially showed a continuous increase of the values for the first ten to twelve atomization cycles after which the absorbance values showed a constant value. This effect probably occurs due to formation of stable carbide between uranium and carbon material of the atomizer. In successive atomization cycles, the rate of carbide formation is reduced due to the availability of less carbon in the sample zone. Then constant absorbance signal are achieved when the fraction of free uranium is comparable to the fraction forming uranium carbide. Due to problems associated with the atomization of uranium and the memory effects caused by carbide formation, the determination of uranium by ETAAS still has little application in analytical chemistry. However, with the use of techniques of separation and preconcentration, studies of new platforms and modifiers the performance of this technique can be improved yet. 2.3. Inductively coupled plasma optical emission spectrometry (ICP-OES) Inductively coupled plasma optical emission spectrometry (ICP-OES) is widely recognized as a suitable technique for the determination of metals because of its high sensitivity and relative freedom from matrix effects [47]. ICP-OES has attracted the attention of the analytical community since the 1970s due to its simultaneous and multi-element determination capability [48]. Unlike the determination of uranium by FAAS and ETAAS, the determination of uranium by ICP-OES is not affected by interferences caused by the formation of carbides and uranium oxides. The plasma temperature of 6000 K is presumably sufficiently high to dissociate the UO molecule [49]. This effect can explain the good sensitivity of ICP-OES for uranium concentration determination, however problems with spectral interferences and matrix effects occur in the determination of trace amounts of uranium in complex matrices [47]. The ICP-OES sometimes suffers from signal suppression and clogging of the nebulizer when the sample contains dissolved salts at concentrations above 0.2% (w/v) [50]. 2.3.1. Choice of spectral line for the determination of uranium by ICP-OES Uranium has several spectral lines in the 200–450 nm wavelength region; however, none of them present a high emission

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intensity. Scott et al. [49] stated that, despite the elevated number of lines, no atomic line of uranium shows any appreciable relative intensity for the determination of uranium by ICP-OES. It was observed that only the emission from ionic lines was predominant in the spectrum. Operational parameters, such as radiofrequency applied power, argon coolant/support gas, nebulizer gas flow-rate, and observation height, must be optimized to obtain the best signal-to-background ratio and the lowest possible limit of detection for uranium. To obtain correct emission measurements for uranium by ICP-OES, it is necessary to find a suitable wavelength with a high sensitivity and low interferences caused by matrix elements [51,52]. The choice of spectral line should be made taking into consideration the signal-to-background intensity ratio, limit of detection, and spectral interferences by matrix elements. Previous papers have reported the main interferences in the determination of uranium by ICP-OES [49,52,53]. It is important to emphasize that interference effects caused by concomitants present in the matrix will depend on the type of sample employed, concentration of this concomitant element in the matrix and the concentration of uranium in the sample.

2.3.2. Interference of others elements in the determination of uranium by ICP-OES The determination of uranium by ICP-OES is difficult due to spectral interferences caused by other elements. Most rare earth elements and thorium can interfere in the determination of uranium when present in a 10-fold excess [52]. Calcium, iron, vanadium, and zirconium strongly affect the emission of uranium. The introduction of easily ionized elements, mainly calcium, can cause matrix effects in ICP-OES, which are generally found in great amounts in various types of samples, such as seawater, sediment and rock samples. Elements with high ionization potentials can also cause matrix effects, but their concentrations should purposely be high to produce appreciable effects [48]. Fugino et al. studied in detail the interference caused by calcium [51,52]. The emission intensities of several uranium lines increased when synthetic hydroxyapatite was added, even without the presence of uranium. The emission intensity of uranium decreased when hydroxyapatite was added and started to increase again around 10 mg mL−1 of the mineral. The spectral interferences caused by calcium and the interferences caused by calcium phosphate seem to occur simultaneously. This interference affected all uranium and thorium wavelengths and can be explained by the presence of large quantities of interfering species that cause changes in the plasma thermal characteristics, in the analyte excitation efficiency and/or in the distribution of the metals in the plasma [48]. Scott et al. used a correction system for uranium to overcome interferences of calcium and iron by simultaneous monitoring the concentration of these elements in the sample solution employing a multi-channel spectrometer [49]. However, the most common strategy to overcome the interferences caused by calcium, iron and others metals is to employ preconcentration and separation methods. Zaror et al. overcame the interference of calcium on uranium determination by ICP-OES employing anion exchange chromatography and obtained good agreement with the results obtained by a standard method [53]. Lee et al. proposed a cloud point extraction procedure as a preconcentration step for simultaneous extraction and determination of low concentration of uranium, thorium, zirconium and hafnium in aqueous samples. This method provides high sensitivity with good precision and low consumption of organic solvents. The accuracy was verified by addition and recovery experiments using real samples [54].

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2.3.3. Presence of acids in the determination of uranium by ICP Most of spectroanalytical methods require introduction of samples as solution. Thus, many analytical procedures have been developed to solubilization and/or decomposition of samples employing concentrated acids and acid mixtures. The addition of acids can also be used for analyte stabilization. ICP-MS, for example, is applicable to diluted aqueous solutions containing acid concentrations lower than 5% [55]. The main effects caused by the presence of high concentrations of acids in the ICP-OES or ICP-MS are related to the physical effects of the acids on the solution properties. The reduction in the nebulizer aspiration rate when a free aspiration is used, the change in the primary and tertiary drop size distributions of the aerosol, the modification of mass of the solution transported to the plasma, and the element concentrations as a function of the drop size are all among these acid effects. Interferences caused by presence of acid in the samples through the processes that occur inside the plasma must also be considered [48,56]. These interferences may occur due to the change of analyte atomization and excitation conditions caused by changes in electronic density and temperature when the acid reaches the plasma. However, these effects can be minimized by the optimization of plasma operating condition (power, carrier gas flow-rate, type of nebulizer and nubulizer flow-rate) [48,57]. Generally, the main way to correct physical acid effect in ICP-OES or ICP-MS is the use of internal standardization. 2.3.4. Determination of uranium in organic phase by ICP-OES The back-extraction of the analyte from an organic phase to an aqueous one provides the possibility of loss of the analyte, introduction of interferences due to the addition of reagents, and an increase in the time of analysis. With the purpose of eliminating the step of back-extraction of analyte to an aqueous phase, the possibility of direct introduction of the organic phase in ICP-OES has been studied. The introduction of organic solvents or samples with high levels of organic matter can cause serious problems to the performance of ICP. Besides changing drastically its characteristics and generating argon plasma fluctuation due to the energy spent for the dissociation of the organic molecules, it also leads to plasma instability and can even extinguish the plasma. Organic solvents may also lead to the formation of carbon deposits on the plasma torch, interface cones and lenses. On the other hand, the presence of organics in the plasma may increase or decrease the signal intensities of some elements for ICP-OES and also may lead to formation of some undesirable polyatomic species in the ICP-MS methods causing interference in the measurements [58,59]. Despite drawbacks, organic solvents with 8–9 or more carbons can be suitable for direct injection in the ICP. In this case, the incomplete combustion of the organic solvent can be solved by adding some oxygen to the carrier gas to ensure that the organic solvent burned completely in the plasma. When the number of carbons is too high, the viscosity becomes higher and the sample flow-rate through the nebulizer decreases. Organic solvents, such as methyl isobutyl ketone (MIBK, C6 H12 O), diisobutyl ketone (DIBK, C9 H18 O), dibutyl ether (C8 H18 O), n-decane (C10 H22 ), cyclooctane (C8 H16 ) and hexylacetate (C8 H13 O2 ), can all be used without extinguishing the plasma or causing carbon deposition at the plasma torch and pre-optics interface [51,52] when oxygen is used as auxiliary carrier gas [60]. The down side to the direct introduction of organic solvents into the argon plasma is an increase in the background intensity caused by the Swan bands [47]. Beyond the use of oxygen to the plasma gas, the effects induced by organic solvents may be minimized or eliminated by increasing

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the applied radiofrequency power, using miniaturized or refrigerated nebulization chambers, or an electrothermal vaporizer to introduce the sample [58]. Kamata et al. proposed a method of sample preparation and liquid–liquid extraction for determination of thorium and uranium in coal ash samples. The elements were separated from the matrix by 2-thenoyltrifluoroacetone (TTA), and then uranium and thorium were back-extracted with diluted acid solution and determined by ICP-OES. The back-extraction was essential for attaining proper accuracy [47]. Agrawal et al. developed a procedure for solvent extraction, separation and recovery of uranium in the presence of cerium, thorium and lanthanides where the extract, in dichloromethane solution, was directly introduced into the plasma. This procedure provides good accuracy and precision, and allowed the determination of uranium in environmental samples without a back-extraction step by ICP-OES [61]. 2.4. Inductively coupled plasma mass spectrometry (ICP-MS) Inductively coupled plasma mass spectrometry (ICP-MS) is currently considered one of the most powerful analytical techniques for the determination of the total concentration and isotopic composition of the actinide elements. The principal advantages of ICP-MS are minimal analysis time, superb sensitivity and high accuracy. In addition, ICP-MS is a multi-element technique and can be used to determine all the actinide elements within a minute, at concentrations smaller than 1 pg mL−1 in liquid samples by sector field instruments. Additionally, due to separation in the mass spectrometer according to the mass/charge ratio, the number of sample pre-treatment stages can be greatly reduced [45,62,63]. The determination of the isotopes concentration of uranium at trace and ultra-trace levels is of increasing interest for environmental monitoring, nuclear safeguards and nuclear forensic studies [64]. Consequently, the development of methods and techniques increasingly sensitive to the determination of isotopes of uranium in different materials are being proposed [65–72]. Inductively coupled plasma-MS has been used for measurements of isotopes uranium concentrations and uranium isotope ratios in many environmental, biological and medical samples [73]. Taking into consideration the lower detection limit, smaller samples volumes, multi-isotope capability and shorter measurement times; ICP-MS is one of the most suitable methods for routine determination of each uranium isotope (234 U, 235 U, 236 U, 238 U) and the isotopic ratio. It is important to emphasize that the 238 U is the isotope more easily measured by ICP-MS in most of environmental samples. However, due to lower natural abundance of others isotopes of uranium the measures cannot be achieved by ICP-MS in an easy way. Measurement of 233 U, 235 U and 236 U using ICP-MS is very difficult without preconcentration step. The ICP-MS has been also widely used for routine isotopes other than 238 U determination in environmental samples. However, up to now the abundance sensitivity provided by common ICP-MS instruments as well as the uranium hydride ion formation (235 U1 H+ ) are the major interference [74]. One possible way to improve the abundance sensitivity in ICP-MS instruments is by increasing the resolution of the system [75,76]. More detail about improvement in the abundance sensitivity for uranium can be found in the Refs. [74,75]. Isotope dilution is an alternative mode of calibration which uses an artificially enriched isotope (called a “spike”) as an internal standard in order to quantify elements. One pre-condition in the application of IDMS in element analyses is that the element to be determined has at least two stable or long-lived radioactive isotopes. Isotope dilution mass spectrometry (IDMS) is a well known primary ratio method that potentially provides very good accuracy

for elemental analysis. In IDMS method an isotopic analogue of the analyte is used in combination with a calibration standard. This isotopic analogue would ideally be a 13 C labeled analogue for an organic analyte or an isotopically enriched analogue of a trace element analyte. This analogue is commonly referred to as the “spike” and is added to both the sample and the calibration standard. The IDMS method has been applied to a wide range of areas of analytical chemistry to provide high accuracy reference measurements [77–79]. The advantages of IDMS have been recently established for ICP-MS [80]. Isotope dilution ICP-MS procedures have been developed for uranium quantification in urine [81], coal [82] and fuel [83] samples. The use of one artificial isotope as spike simplify and reduces the combined uncertainty since it is not necessary to take into account the original isotopic composition during sample calculation. Uranium has three long-lived artificial isotopes, 232 U (68.9 years), 233 U (2.45 × 105 years) and 236 U (4.47 × 109 years), which could be applied as spikes. In fact, 233 U and 236 U are available at the market as spike solutions and the chose between both depends on the application. For example, 232 Th is an usual contaminant of 233 U, therefore, its use is avoided when a simultaneous 232 Th determination is needed. On the other hand, 236 U is present on spend fuels and its presence gives important information about the uranium origin, in particular, for safeguards purposes. It is also important to report the use of 233 U/236 U “double spike”, the use of double spike allows internal mass fractionation correction, and the 235 U/238 U isotopic ratio can be determined with the highest possible precision and accuracy [84,85]. The development of new ICP-MS instruments based on double focusing sector field mass spectrometers (ICP-SFMS), and their commercial availability after 1988, allowed a significant increase in sensitivity in comparison to quadrupole ICP-MS [86]. Until little time ago, the ICP-SFMS has not found worldwide use in laboratories because of their relatively high cost [34,87]. For both instruments, the analytical figures of merit can be limited by spectroscopic and non-spectroscopic interferences. Spectroscopic interferences are caused by atomic ions, multiply charged ions and molecular ions of various sources having the same nominal mass as the analyte isotope of interest [34]. Some of the interference problems can be solved by the use of high-resolution spectrometers or by previously separating the analyte from the matrix. The latter approach is frequently used, since it may also provide preconcentration of the analytes, lowering their detection limits [58]. Non-spectral interferences can be caused by difference in viscosity, surface tension, density, and in the concentrations of the matrix elements in the individual samples, which can lead to significant effects on the nebulization and ionization conditions in the ICP source. This type of interference can be corrected by using a reference element [86], such as Ir, Bi, Th, or Ru, that have been used as reference elements for the determination of uranium using ICP-MS [63,73,86]. Currently, ICP-SFMS arrangements are becoming increasingly popular owing to low instrumental background, high sensitivity (in low-mass resolution operation, m/m ≈ 300), reduction of cost and especially the ability to separate isobaric interferences caused by molecular ions of analytes in higher mass resolution mode [88]. However, the isobaric overlap of isotopes of different elements that coincide at the same nominal mass is hardly eliminated, and even commercial ICP-SFMS instruments operated in the maximum mass resolution are not sufficient to overcome these interferences [34]. For example, when Pb is present in the sample, different leadbased molecule ions can interfere with 236 U. Boulyga and Becker studied the determination of uranium isotopic ratios (236 U/238 U) in soil samples without matrix separation, evaluating isobaric interferences caused by molecular ions of lead [89]. The presence of spectral interferences can deteriorate the accuracy of the determi-

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149

Table 2 Propriety of uranium isotope [64,72,89–92]. Uranium isotope Atomic mass (U)

Half-life (years)

233

233.0396

1.59 × 105

236.0456

2.34 × 10

U

Isotope abundance (%)

Decay mode Atomic isobar interference

Polyatomic interference m/z

␣ 235

236

U

7

10−8 in U ore 10−11 –10−12 in Earth’s crust

␣

UH+ Pb14 N2 + 207 Pb14 N2 1 H+ 208 Pb28 Si+ 208

236

Np

232

Th2 H Tl31 P 202 Hg32 S 201 Hg33 S 200 Hg34 S 199 Hg35 Cl 198 Hg36 Ar 198 Pt36 Ar 197 Au37 Cl 196 Pt38 Ar 194 Pt40 Ar

234.0521 233.9461 233.9427 233.9417 233.9362 233.9371 233.9343 233.9354 233.9325 233.9277 233.9251

204

Pb31 P Hg31 P 203 32 Tl S 202 Hg33 S 201 Hg34 S 200 Hg35 Cl 199 Hg36 Ar 198 Hg37 Cl 198 Pt37 Cl 197 Au38 Ar 195 Pt40 Ar

234.9468 234.9472 234.9444 234.9421 234.9382 234.9372 234.9358 234.9327 234.9338 234.9293 234.9272

207

237.9497 237.9466 237.9465 237.9409 237.9413 237.9412 237.9382 237.9362 237.9310 237.9291 237.9303

203

234

U

234.0410

2.45 × 105

0.005

␣

204

235

U

235.0439

7.04 × 108

0.711

␣

235

Np

Pb31 P Pb32 S 206 Pb32 S 204 Pb34 S 204 Hg34 S 203 35 Tl Cl 202 Hg36 Ar 201 Hg37 Cl 200 Hg38 Ar 198 Hg40 Ar 198 Pt40 Ar 206

238

U

238.0508

4.47 × 109

99.283

␣

238

Pu

nation, as well as the detection limits for the elements investigated [34]. The possible spectral interferences in the determination of uranium by ICP are summarized in Table 2 [64,72,89–92]. Although 238 U is the isotope of greater interest in the study of environment samples the atomic mass, isotope abundance, decay mode, atomic isobar and polyatomic interferences were also reported for the other isotopes of uranium. In addition, the data compiled in the table can serve as a quick reference to aid researchers who are developing methods for uranium determination using ICP-MS.

The physical acid effects were reported in Section 2.3.3. Another effect caused by the presence of acids that can be observed in ICPMS is the spectral interferences due to the formation of polyatomic ions [56]. In the uranium concentration determination by ICP-MS, mainly when using a quadrupole MS, HCl must be avoided because chloride forms complexes with several elements, such as Au and Hg, forming the interfering species 201 Hg37 Cl, 203 Tl35 Cl, 197 Au37 Cl, 198 Hg37 Cl and 200 Hg35 Cl [95]. The problems involved with the introduction of organic solvents in ICP-MS are the same for the discussed in Section 2.3.4.

2.4.1. Factors that influence the determination of uranium using ICP-MS Beyond the spectral and non-spectral interferences, many factors may influence the determination of 238 U using ICP-MS. Among them, the presence of organic compounds, salts and dissolved solids, acids, as well as memory effects and sensitivity can influence the ICP-MS. Special attention must be taken with contamination problems when uranium isotopes concentration is very low, e.g. at fg g−1 . For example, ICP-MS must be located in a clean room and laminar flow benches and more purified chemicals (e.g. suprapure reagents) must be used to avoid contamination when handling of the samples. As a rule, the glassware, the sampler and skimmer cones must be regularly and carefully cleaned to ensure the lowest background and the best sensitivity and stability [93,94].

2.4.1.1. Presence of salts and dissolved solid. As ICP-OES, the concentration of uranium cannot be accurately measured in high saline samples by direct aspiration into ICP-MS because the high content of total dissolved solids (TDS) leads to suppression of ionization of the analyte. Moreover, environmental samples with high levels of matrix components, such as Na, K, Ca, Mg, can accumulate on the nebulizer, cones and torch of the ICP, causing the blockage of the nebulizer or torch due to the deposition of dissolved solids from the sample solution [87,95,96]. The deposition of matrix constituents on the sampler and skimmer cones of the spectrometer can seriously affect the precision and accuracy of analytical results. These effects can occur even after introduction of even a few milliliters of sample [97,98]. In addition, can also cause serious damage to the mass spectrometer. The presence of elements reported above, can still interfere in the determination of trace

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concentrations of uranium and also damage the detector of the ICP-MS. Magara et al. reported that the 238 U signals began to increase when the concentrations of alkali metals were about 1 ␮g g−1 , and reached a maximum at around 10 ␮g g−1 , then the intensity decreased with increasing concentration of alkali metals [99]. The mechanisms for signal enhancement is based on space-charge effects created in the interface zone by co-existing elements. The radial diffusion of analyte ions in the plasma causes the analyte ions to shift into the zone just outside the central channel. Since the temperature in this zone is higher than in the central channel zone, a relative increase in the degree of ionization may occur, which would result in signal enhancement. The reduction of signal occurs when the concentration of alkali is high (≈10 ␮g g−1 ) because other mechanisms may be dominant, provoking a shift in the ionization equilibrium, space-charge effects and ambipolar diffusion. These effects all lead to signal suppression of uranium [99]. The authors stated that the measurement of samples containing more than 1 ␮g g−1 of co-existing elements brings about incorrect results not only for quantitative analysis, but also for isotopic analysis. Samples with high levels of dissolved solids require some form of sample pre-treatment. The simplest way to circumvent this situation is to dilute the samples, decreasing the matrix effects but also decreasing the concentrations of uranium. Although low detection limits have been obtained for diluted samples using ICPSFMS, problems have been reported using ICP-QMS, even with diluted samples [96]. Therefore, for this type of matrix, separation is often required before the determination of uranium by ICP-MS [87]. In addition, the preconcentration procedure is necessary when uranium is at extremely low levels to achieve more accurate measurements [97,98]. Torgov et al. stated that the standard version of this technique is applicable to only diluted aqueous solutions with the concentration of salts no higher than 0.1% [55]. For example, the high dissolved salt content (typically about 3.5%) in seawater precludes direct analysis by ICP-MS [100], however, due to the high concentration of uranium in seawater (∼3 ␮g L−1 ), a simple dilution is enough to overcome the problem. Unsworth et al. proposed an on-line solid-phase extraction method coupled to ICP-QMS for the determination of low levels of uranium and thorium in aqueous samples with a high matrix concentration and significant levels of dissolved organic carbon. The samples were injected directly into the column without any dilution. This procedure demonstrated good accuracy and precision [96]. Tagami and Uchida have shown that the presence of Na+ and NH4 + from NaOH and NH4 OH used as eluents in TRU resin (octylphenyl-N,N-diisobutyl carbamoylphosphine oxide dissolved in tri-n-butyl phosphate) degrades the sensitivity of ICP. They attributed this effect to decreased ICP ionization ability [73]. 2.4.1.2. Memory effect. The memory effect does not cause many problems in uranium determination by ICP-MS measurement. Usually, the memory effect is minimized or even eliminated by careful rinsing of the nebulizer and nebulization chamber between each consecutive sample [87]. 2.4.1.3. Sensitivity of ICP-MS. The direct measurement of uranium concentrations in terrestrial water samples using 238 U is possible by ICP-MS because the detection limit is usually lower than 0.1 pg mL−1 . However, the concentration of uranium in some water samples like tap water is generally several times below 0.1 pg mL−1 , which is very difficult to be measured [63]. Prior sample preparation for determination of uranium by ICPMS in water samples is necessary to separate the analyte in samples with high matrix contents [73]. To investigate the behavior of

uranium in aquatic environment samples, separation and preconcentration methods are prerequisite when uranium is present at part per trillion levels [97,98]. The extremely low concentrations of 234 U, 235 U and 236 U in several environmental materials demand a separation of the uranium from the matrices to remove interfering elements from the sample solution for measurement [63,73,89,95]. In addition, for measuring these uranium isotopes mass by ICP-QMS, it is necessary to concentrate uranium from the environmental samples to a suitable concentration for analysis, because the increase of uranium concentration decreases the errors in the measurement (mainly counting errors) [63]. Despite the great initial sensitivity of the ICPMS, the use of preconcentration and separation procedures further improves the precision, increases sample throughput, simplifies the handling of samples, and facilitates the measurement of sample replicates [101]. Tagami and Uchida applied a simple separation method using TEVA resin, which was adapted to measure the U isotopic mass ratio of 235 U/238 U in water and soil by ICP-QMS [63]. 2.4.2. ETV-ICP-MS The use of electrothermal vaporization (ETV) coupled to ICP-MS offers several advantages, including improved transport efficiency (20–80% in contrast to 2–5% using pneumatic nebulization), reduced oxide and hydride formation, because the solvent is largely removed, minimal sample consumption (10–100 ␮L), no waste production and accommodation of complex sample matrices (such as salts, organic matrices, solids, strong acids and slurries) with very limited sample preparation. In addition, the ability to use thermal programming allows selective removal of sample matrix constituents, thereby eliminating or reducing spectroscopic interferences that would otherwise arise from the matrix [45]. However, the use of a graphite tube as the vaporizer requires the use of chemical modifiers to reduce memory effects caused by carbide formation. When using NH4 F as a modifier, the linearity of the analytical curve and the sensitivity for uranium improves because the signal intensities of uranium are high at temperatures above 1700 ◦ C. This facilitates the removal of uranium from the graphite tube, probably due to formation of UF6 . However, it does not completely avoid the memory effect and the carbide formation continued to occur [44]. Dressler et al. stated that uranium could not be directly measured in seawater by ETV coupled to ICP-MS, using NH4 F as chemical modifier, due to strong memory effects in the presence of NaCl [58]. Other strategies to minimize carbide formation include the use of trifluoromethane (Freon® ) as a chemical modifier [45]. The presence of freon gas in ETV can prevent the formation of refractory carbides on the surface of the graphite tube, but it is advisable to only introduce the gas during the ashing stage. If Freon® is introduced during the vaporization stage, tube lifetimes are reduced substantially [62]. Grinberg et al. stated that, in the presence of Freon® , the sensitivity, based on integrated response, increased substantially (about 100-fold) compared to that obtained using the untreated graphite furnace, and well-defined signals were obtained with greatly reduced memory effects [45]. Truscott et al. studied the vaporization profiles for 238 U with and without Freon® added during the ashing. In the absence of Freon® , the peaks were approximately 2.5 s wider. However, when Freon® was added, peak height and peak area signals increased by about 10 times for 238 U, resulting in much improved detection limits [62]. Despite the advantages offered, the use of ETV requires a more experienced analyst, besides being more time consuming and expensive [58]. 2.4.3. LA-ICP-MS Laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) is a very useful technique for the isotopic analysis of

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solid samples, which is much simple and allows direct measurements. Actually it is one major advantage for the measurement of uranium in solid samples, and surface analysis using ICP-MS. The interest in this technique was mainly based on low sample preparation needs, fast sample throughput and in situ isotope measurement capabilities in combination with high spatial resolution [102]. In the technique, the laser ablation allows the direct introduction of the sample into the mass spectrometer by evaporating and ablating the material using high-energy density laser beam [103]. The accurate determination with laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) of solid samples composition, which includes major, minor and trace elements, implies that the material under investigation is reasonably homogeneous [104]. However, remaining difficulties of performing “fractionation-free” analyses using non-matrix matched calibration have been found to restrict its accuracy. At the present stage, LA-ICP-MS can, therefore, be regarded as a method of choice only for certain matrices such as steel, minerals and silicate glasses which can either be analyzed using matrix matched calibration standards or whose analysis is coincidentally less affected by elemental fractionation [105]. Recently, femtosecond ablation was shown to lead to stoichiometric sampling and studies suggest that non-matrix matched calibration is feasible with it [106]. It should be pointed out the development of the laser technology during the last years has allowed its application to several fields including nano-technology, historical heritage, imaging of tissue samples [107,108]. Also, the development of the sample introduction interface associated with the femtosecond lasers will also contribute to overcome memory effects due to the introduction of large amounts of sample on the ICP-MS [106]. Detailed descriptions about achievements, problems and prospects of the LA-ICP-MS can be found in a previously review [109]. The application fields of uranium isotopes analysis by LA-ICPMS include geological, environmental [110], biological and single particles [111], and medical samples [112]. In the environmental area, LA-ICP-MS has been used to directly investigate mass loading of metal and radionuclide contaminated soils onto plant surfaces. It was observed that uranium is unavailable for plant uptake, but elevated U and Ni concentrations associated with foliage of understory plants suggested mass loading. The application of the technique required very little sample preparation and was able to distinguish between metals located inside biological tissues with those that are adhering to the surface [110]. Varga [113] developed a LA-ICP-MS method for the isotopic analysis of individual uranium oxide particles for determination of 234 U, 235 U, 236 U and 238 U isotopes in single actinide particles with different isotopic composition (depleted, natural, low and highly enriched uranium samples). The effect of the interference of molecular ions was found to be negligible for the major isotopes; however, for the minor isotopes (below 10−4 isotope abundance) higher mass resolution was required to avoid spectral interference. The use of the higher mass resolution also is necessary because the abundance sensitivity (i.e. the overlapping of the analyte signals to the adjacent peaks) is improved with a factor of 5–10, which is of high importance for the measurement of low-abundant isotopes, especially 236 U. Elish et al. [114] presented the determination of uranium in a single hair strand by LA-ICP-MS. Two techniques were applied: continuous laser ablation and a single pulse laser ablation. During application of the technique was checked that continuous ablation is preferable, but when the amount of material is limited, the single pulse should be applied. The procedure of calibration was based on the analysis of digested hair samples and the LA-ICP-MS presented a limit of detection for uranium of 19 ng g−1 for the continuous abla-

151

tion mode, and 700 ng g−1 for a single laser pulse. The authors also showed that image processing of the laser trace for single pulse mode can be applied for improving the precision of results and understanding intensity changes.

3. Strategies for preconcentration and separation Generally, when it is necessary to analyze complex matrices for uranium present at trace levels in a sample, a prior separation and/or preconcentration procedure is required for the use of spectrometric techniques. This occurs because it can be difficult to determine uranium using molecular spectrophotometry, ICP-OES and ICP-MS, mainly when thorium, calcium, zirconium and actinides are presents, as they can cause interferences in the determination. A variety of methods for preconcentration and separation, such as liquid–liquid extraction [115,116], ion-exchange [117,118], solidphase extraction [8,12], ion imprinting polymers [119], evaporation [118,120] and cloud point extraction [121,122], were proposed to aid in the determination of uranium by spectroanalytical techniques. Rao et al. [123], in a recent review, critically described many different ways for preconcentration and separation of uranium and thorium prior to their analytical determination. This review took into account relative merits and drawbacks, enrichment factor, retention/absorption capacity, validation using certified reference materials and application to complex real samples. In many cases, the application of separation and preconcentration techniques can be used to improve the sensitivity and minimize effects of the matrix in the determination of uranium by ETAAS, ICP-OES and ICP-MS. Agrawal et al. [40] employed liquid–liquid extraction for preconcentration, separation and simultaneous determination of uranium in environment samples. This method was used to determine uranium by the graphite furnace technique in aqueous or organic phase and increased the sensitivity by fifty times. Good precision and accuracy were also obtained using this method. Jain et al. [124] achieved a better limit of detection of uranium concentration determination by ETAAS by employing calix[4]resorcinarene-hydroxamic acid as a complexation agent and extraction with ethyl acetate solution on the same matrices. More details about the uranium determination by ETAAS are found in Table 3 [37,125,40,124]. Separation of uranium from the matrix is required by ICP-OES and ICP-MS when the sample shows high levels of dissolved salts and organic compounds [126]. Apart from overcoming problems in the sensitivity of the ICP-OES, the processes of separation and preconcentration are still necessary to separate the uranium from matrices which may contain elements that will produce polyatomic and/or isobaric interferences in ICP-MS [90]. Solid-phase extraction (SPE), also called extraction chromatography, is one of the more important preconcentration–separation procedures for trace heavy metals ions, due to its simplicity and limited usage of organic solvents [127]. Additionally, airborne contamination can be easily eliminated using a closed system [100]. SPE separates the sample from the matrix without reducing the analyte concentration, requiring a small volume of sample, and can be easily performed in-line [96]. Application of SPE for uranium concentration determination prior to ICP-OES and ICP-MS measurement overcomes the problem of signal suppression due to contaminant matrix elements and, thus, enhances the sensitivity and detection limit of the ICP-OES and ICP-MS. When it is necessary to analyze samples with high matrix concentrations or samples of restricted quantity [128], the

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Table 3 Preconcentration and/or separation procedures for uranium determination by ETAAS. Sample

LLE complexant/extractor

Plutonium matrix Natural water, monazite sand and standard geological materials

– –

Rock and seawater Rock and seawater

NHDTAHA/chloroform C4RAHA/ethyl acetate

SPE sorbent/chelating Merrifield chloromethylated resin/calix[4]areneovanillinsemicarbazone – –

Extraction pH

6.0–7.0

6.0–7.0 8.0–8.3

Linear range (␮g mL−1 ) 0.5–2 0.1–15.0

– –

EF

Detection limit

Ref.

– 143

2.5 ng 6.14 ␮g mL−1

[37] [125]

50 142

0.01 ␮g mL−1 2.87 ␮g L−1

[40] [124]

LLE = Liquid–liquid extraction. SPE = solid-phase extraction. EF = enrichment factor. NHDTAHA = 5, 14-N,N -hydroxyphenyl-4, 15-dioxo-1,5,14,18-tetraaza hexacosane. C4RAHA = [4]resorcinarene-hydroxamic acid.

use of column extraction is more convenient for pre-treatment of the samples. Taking into account the UO2 +2 complexation ability with different ions like Cl− , F− or CO3 −2 and the existence of two ionization states in aqueous solution, U(IV) and U(VI), it is possible to design several separation/preconcentration scheme for U accordingly to the sample matrix and the measurement method. A large experience was obtained from the uranium industry which has developed mainly solvent extraction and ion-exchange based methods applied to the milling and to the reprocessing steps of the uranium fuel cycle as, for example, ternary amines (Aliquat 336), organic phosphates (Tri-butyl phosphate, TBP) or phosphine oxides (Tri-octyl phosphine oxide, TOPO). Due to the formation of anionic complex with nitrate and chloride, there are a large number of publications with separation methods based on these species. Separation methods applying anionic ion-exchange can be observed as ion-exchange resin on columns, solvent extraction or extraction chromatography columns (UTEVA). Extraction chromatography can be though as a natural evolution of the multiples mixer-settler units found in industrial facilities. Actually, commercial available extraction chromatography columns have dominated the scene as TRU, UTEVA and TEVA, all produced by Eichrom® . Particularly, interesting is their use for Flow Injection (FIA) ICP-MS. Coupling both techniques it is possible to speed up the analytical methods and, at the same time, increase precision and accuracy. The main characteristics, as well the applications of these resins for preconcentration and separation of uranium, followed by determination have been discussed by many authors [63,73,95,110,129]. FIA is a powerful tool when combined with SPE, performing and automating preconcentration and/or separation procedures for uranium concentration determination by ICP-MS. This method is particularly advantageous when dealing with samples with extremely low concentrations of analytes by minimizing the risks of sample contamination and significantly reducing the analysis time [87,128]. Aldstadt et al. developed, characterized and tested a flow injection (FI) system for the on-line determination of 238 U in groundwater samples by ICP-MS using a TRU resin [130]. The system preconcentrates and removes the potential interfering species of the environmental matrix, as well as the total dissolved solids (TDS), reducing the concentration of these species in the plasma below 1%. The method showed good sample throughput, precision and accuracy when tested. Depend on the sample matrix complexity and the uranium concentration, the applied separation method is a combination of two or more separation principles as, for example, co-precipitation and ion-exchange. Other resins have also been applied in sample preparation for the determination of uranium using ICP-MS. Sohrin et al. developed an extraction method using a fluoride-containing metal alkoxide

glass immobilized 8-hydroxyquinoline column to the separation of uranium and other elements (namely: V, Co, Ni, Ga, Y, Mo, Cd, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu and W) from a seawater matrix. Determination by high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS) obtained good precision and accuracy [100] from this method. The equipment employed by Sohrin et al. allowed the elimination of most of the molecular interferences due to the high-resolution power of the ICP-MS (1000 ± 10). In spite of the high quality of the instrumentation in eliminating spectral interferences, the presence of dissolved salts in the matrix demanded the use of a separation technique [100]. Aydin and Soylak proposed a solid-phase extraction procedure for the separation-preconcentration of ultra-trace U(VI) and Th(IV) samples as 9-phenyl-3-fluorone chelates on Duolite XAD761 resin prior to their determination by ICP-MS [127]. The developed procedure provides good precision with low relative standard deviations and high accuracy for U(VI) and Th(IV). Solvent extraction and co-precipitation have both been used for sample pre-treatment, but they are laborious and carry a risk of contamination for the determination of uranium [100]. Generally, co-precipitation can increase the level of salts or introduce interfering elements. Thus, a new step is necessary to reduce or eliminate them. Chou and Moffatt developed a method for determination of 238 U in seawater after concentration by co-precipitation with iron hydroxide by ICP-MS [101]. However, a step of separation using a Bio-Rad AG1 X8 Anion exchange column was required to remove the chloride from the FeCl3 , HCl and other elements like iron. When solvent extraction is applied, a backextraction is a necessary procedure because the ICP-MS technique is almost never used for the analysis of organic solutions. The concentration of acids in the back-extraction solution must also be kept low (below 5%). Torgov et al. developed a solvent extraction procedure for preconcentration, separation and determination of trace amounts of uranium and thorium in bottom sediments [55]. Their proposed procedure involves sample decomposition, coextraction of uranium and thorium with trioctylphosphine oxide in toluene, quantitative back-extraction with sulfuric acid after diluting the extract with caprylic acid, and ICP-MS analysis of the back extract. Table 4 shows in detail some parameters, such as type of sample, preconcentration and separation method, emission line, and detection limits typically employed for the determination of uranium by ICP-OES [131,4,50,12,47,132–136,61,126,49,51–54]. Through the datas compiled in Table 4, it is possible to verify that the use of separation and preconcentration techniques enables the elimination of interfering allowing the use of ICP-OES for the determination of total uranium concentration in parts per billion level. Table 5 shows the main strategies for preconcentration and separation, limits of detection, preconcentration factor, instrumentation used and type of sample employed in the determination of uranium by ICP-MS [100,98,130,127,62,136,55,96,128,87,137,58].

Table 4 Preconcentration and/or separation procedures for uranium determination by ICP-OES. Batch/FIA

Preconcentration and separation, SPE – sorbent/chelating; LLE – extractor/solvent; CPE – chelating/surfactant

Extraction pH

Wavelength of emission (nm)

Linear range

Volume of sample solution (mL)

EF

Detection limit

Ref.

Seawater and spring water Natural waters Natural waters Natural waters Coal ash

B B F B B

MCM-41/5-nitro-2-furaldehyde (fural) SPE – ion-exchanger Hyphan SPE – C18/ PAN SPE – mesoporous silica (MCM-41)/salicylaldehyde LLE – 2-thenoyltrifluoroacetone (lTA)/benzene

5.5 6.0–7.0 8.0 5.0 3.0–4.5

385.958 – – 385.958 409.014

100 10000 25 100 10

100 – 382 100 –

0.3 ␮g L−1 2 ␮g L−1 69 ng L−1 0.5 ng mL−1 . 29 ␮g L−1

[131] [4] [50] [12] [47]

Coal and fly ash Geochemical and Environmental Aqueous samples. Apatite mineral

B B

– –

– –

367.01 386.0

– –

– –

63 ␮g L−1 6 ␮g g−1

[132] [133]

– B

6.0 2.0

367.007 409.01

50 –

37.0 –

1.00 ␮g L−1 –

[134] [51]

Nuclear fuel, phosphate rocks, sea water and monazite sand Seawater

F

CPE – dibenzoylmethane (DBM)/Triton X-114 LLE – 1-phenyl-3-methyl-4-trifluoroacetyl-5-pyrazolone (HPMTFP)/diisobutyl ketone LLE – N-phenylbenzo-18-crown-6-hydroxamic (PBCHA)/dichloromethane

1–1500 ␮g L−1 – 0.5–200 ␮g L−1 2–1000 ␮g L−1 Two orders of magnitude 0.063–50 ␮g mL−1 Four orders of magnitude 2.5–1240 ␮g L−1 0.1–30 ␮g mL−1

5.8–6.5

385.96

–

1000

110

0.5 ␮g L−1

[61]

SPE – phenylarsonic acid-type chitosan resin

5.0

1–10 ng mL−1

50

25

0.1 ng mL−1

[126]

–

0.6 mg L−1 0.6 mg L−1 0.3 mg L−1 0.4 mg L−1 0.5 mg L−1 0.2 mg L−1 0.3 mg L−1 0.3 mg L−1 0.2 mg L−1 0.8 mg L−1 0.2 mg L−1 0.4 mg L−1 0.1 mg L−1 0.5 mg L−1 0.3 mg L−1 4.1 mg L−1 0.1 mg L−1 10 mg L−1 1.3 mg L−1

[49]

[53]

Rocks

F

B

–

–

434.17 424.44 424.17 417.16 411.61 409.01 405.00 398.58 393.20 386.59 385.96 383.15 378.28* 374.64 370.15 367.26 367.01 355.22 288.27

–

–

–

24 ␮g L−1 20.6 ␮g L−1 4.2 ␮g L−1 4.7 ␮g L−1 10.9 ␮g L−1 8.5 ␮g L−1

–

–

Calcium phosphate samples (ore and fertilizer)

B

SPE – Bio-Rad AG 1-X-8

–

256.541 263.553 367.007 385.958 389.036* 409.014*

Waste solutions Waste water

B B

SPE – chelating/TAR-XVI SPE – polyacrylamidoxime-carboxylic acid chelate fiber (PAN-3) SPE – chitosan resin/3,4-dihydroxy benzoic acid (CCTS-DHBA resin) Without preconcentration

4.3–4.5 5.5–8.0

– 263.553

– –

1000 200–1000

100 –

– 5 ␮g L−1

[54] [135]

3.0–9.0

385.957

–

–

8.4

0.9 ng L−1

[136]

River and sea water

F B

Apatite mineral

B

LLE – 3-phenyl-4benzoyl-5-isoxazolone (HPBI)/Diisobutyl ketone (DIBK)

–

0.3

367.01* 263.55 385.96 393.20 409.01

5

–

–

−1

–

10 ng L

–

0.02 mg L−1 0.05 mg L−1 0.75 mg L−1 0.07 mg L−1 0.18 mg L−1

[52]

153

SPE = Solid-phase extraction; LLE = liquid–liquid extraction; CPE = cloud point extraction; EF = enrichment factor; PAN = l-(2-pyridylazo) 2-naphtol.

0.5–50 mg L−1

J.S. Santos et al. / Analytica Chimica Acta 674 (2010) 143–156

Sample

154

Table 5 Preconcentration and/or separation procedures developed for uranium determination ICP-MS. Isotope

Sample

Preconcentration and separation, SPE – sorbent/chelating; LLE – extractor/solvent

Volume of sample solution (mL)

High-resolution inductively coupled plasma mass spectrometer with an ultrasonic nebulizer ICP-MS with a glass concentric nebulizer ICP-QMS with a Meinhard-type pneumatic nebulizer ICP-QMS with a Babington nebulizer ICP-QMS with a pneumatic nebulizer ICP-QMS with an electrothermal vaporization (ETV-ICP-MS) ICP-QMS with a glass concentric nebulizer

238

U

Seawater

250

–

238

U

100

100

238

U

Natural river, sea and tap waters Groundwater samples.

SPE – 8-hydroxyquinoline immobilized on fluorinated metal alkoxide glass (MAF-8HQ) SPE – serine-type chitosan resin TRU resin

238

U

Tap water, river water and seawater samples

DF-ICP-MS

234

U

Bottom sediments

ICP-QMS with a V-groov nebulizer

U

FI-ICP-QMS with a ultrasonic nebulizer

235

U

Seawater CRM, ground water and synthetic sample with dissolved organic carbon Urine and blood

238

U

238

U

SPE – Duolite XAD761 adsorption resin SPE – TRU resin

Ref.

<10

0.49 ng L−1

[100]

2.63

–

[98]

1.1

030 ng L−1 −1

[130]

2.3

4.5 ng L

[127]

10–50

10

–

48 pg L−1

[62]

–

–

21 pg L−1

a

0.01 ng L−1

SPE – chitosan resin derivatized with 3,4-dihydroxybenzoic acid moiety (CCTS-DHBA resin) LLE – TOPO in toluene and caprylic acid/back-extraction with sulfuric acid SPE – TRU

–

5–10

2–6%

0.3 pg mL−1

[55]

0.5

–

4.3

0.0015 ng mL−1

[96]

SPE – UTEVA

Urine – 5

–

4.9

0.0014 ng L−1

[128]

100

Urine

SPE – UTEVA

Blood – 1 10

234

U

Soil

SPE – TRU

10

238

U

Standard reference enriched water, sea and riverine waters, milk powder, apple leaves and urine

SPE – C18 immobilized on silica and retention of complexes formed with O,O-diethyl dithiophosphoric acid

2.3

SPE = Solid-phase extraction. LLE = liquid–liquid. EF = enrichment factor. a Quantification limit.

Detection limit

30

U U

238

30

Precision (%)

150

–

238

ICP-SFMS with an Apex-Q high-sensitivity desolvation system ICP-QMS with a concentric glass nebulizer ICP-QMS with a cross-flow pneumatic nebulizer

Tap water and a mineral water Several certified reference material

5

EF

10

[136]

−1

4

– 2

0.05 ng L 20 pg L−1

[87]

10

–

0.03 ng L−1

[137]

0.05 pg mL−1

[58]

2.6

<10

J.S. Santos et al. / Analytica Chimica Acta 674 (2010) 143–156

Instrumentation

J.S. Santos et al. / Analytica Chimica Acta 674 (2010) 143–156

4. Conclusions The growing exploitation and use of uranium in recent decades has attracted the concern of the analytical community because it is an inorganic pollutant that spreads easily in the environment and presents both chemical and radiological effects to living beings. Thus, there is an increasing need for the development of simple and quick methods for the monitoring of this element in the environment. Spectrometric techniques, such as, ETAAS, ICP-OES (when associated with techniques of preconcentration and separation), and ICP-MS, described in this review allow the determination of uranium in trace and sub-trace levels in several types of samples simply and quickly. The choice of one spectrometric method must take into consideration many factors, such as the costs of analysis, the nature of the sample being analyzed, the required sensitivity, the quantity of sample, and the need for separation and/or preconcentration. Among the spectroanalytical techniques described in this review, FAAS and ETAAS are not very suitable for the determination of uranium at trace levels. The lack of sensitivity of FAAS is attributed to the high stability of uranium oxides even with the use of a nitrous oxide–acetylene flame. However, FAAS technique allows uranium determination at mg L−1 level. The major problem with the determination of uranium by ETAAS is low pyrolysis temperature and the formation of uranium carbides in the graphite tube. The sensitivity of ETAAS can still be improved with the use of modifiers, changing the metallic coating of the tube, and using a separation and preconcentration step before analysis. Unlike of the uranium determination by FAAS and ETAAS, the determination by ICP-OES does not suffer from interferences caused by the formation of oxides and carbide, however suffer from severe spectral and non-spectral interferences due to presence of Na, K, Mg, Ca and other elements. Many papers have been published using ICP-OES, but most of them were associated with a technique of preconcentration and separation making this technique useful for the determination of total uranium level in part per billion. ICP-MS is one of the most powerful methods for the determination of uranium and, among the methods reviewed, is the only one that can measure the uranium isotope ratio. ICP-MS allows for the determination of isotopes of uranium at levels as low as fg mL−1 with good precision and accuracy. The development of more sensitive and inexpensive instruments, more efficient nebulizers and the coupling of preconcentration and/or separation systems has enabled the fast determination of uranium at extremely low concentrations. Surely, this superb analytical capability helps to increase the popularity and spread of ICP-MS for routine analysis of uranium. In comparison with radiometric measurement, spectroanalytical techniques as ICP-MS have similar or even lower detection limits, smaller sample volumes, shorter measurement cycles, and depending of kind and amount of sample the preparation procedures are generally simpler. Acknowledgement The authors acknowledge the financial support from CNEN (Comissão Nacional de Energia Nuclear from Brazil) and CNPq. References [1] Agency for Toxic Substances and Disease Registry (ATSDR), Toxicological Profile for Uranium, U.S. Department of Health and Human Services, Public Health Service, Atlanta, GA, 1999. [2] J.D. Wall, L.R. Krumholz, Annu. Rev. Microbiol. 60 (2006) 149–166. [3] Uranium in drinking-water background document for development of WHO Guidelines for drinking-water Quality (2005).

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