Determination of Transuranium Isotopes (Pu, Np, Am) by Radiometric

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Determination of Transuranium Isotopes (Pu, Np, Am) by Radiometric Techniques: A Review of Analytical Methodology N
ora Vajda*,† and Chang-Kyu Kim‡ † ‡

RadAnal Ltd., Konkoly-Thege M.
ut 29-33, Budapest, 1121-Hungary Terrestrial Environmental Laboratory, Seibersdorf, International Atomic Energy Agency, A-1400 Vienna, Austria

’ CONTENTS Basic Chemical Properties of Pu, Np, and Am Development of Separation Procedures for the Determination of Pu, Np, and Am Isotopes Sample Preparation/Preconcentration Chemical Separation of Actinides Liquid
Liquid Extraction (LLE) Ion Exchange Extraction Chromatography (EC) Combined Procedures for the Determination of Various Actinides Application of FI/SI System to Separation of Actinides R Source Preparation Comparative Evaluation of Different Separation Procedures Procedures for Pu Determination Procedures for Np Determination Procedures for Am Determination Procedures for Determination of Various Actinides Radiometric Measurement Techniques Radioactive Tracers R Spectrometry Liquid Scintillation Counting β and Electron Counting R Spectrometry Using LSC γ Spectrometry NAA Comparison of Radiometric and Mass Spectrometric Measurement Techniques Radiometric Methods ICPMS Other Mass Spectrometric Techniques Conclusions Author Information Biographies Acknowledgment References r 2011 American Chemical Society

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A

great number of analytical methods have been developed and applied for the determination of plutonium, neptunium, and americium isotopes in environmental and nuclear samples using radiometric techniques. This paper is intended to give an overview about the development of the radiochemical procedures starting with a brief description of the “historic” procedures from the early times of nuclear industry followed by a more detailed discussion of the recent developments from the 1990s until 2011. Nuclear measuring techniques, i.e., R, β, γ, and X-ray spectrometry, and neutron activation analysis are critically discussed. Capabilities of these techniques are compared with those of mass spectrometric techniques. Transuranium isotopes are present in the environment on a global scale from weapons testing and satellite failure and on a local scale from nuclear operations and accidents. Since the present levels of contamination are typically low from the points of determination and radiological safety, sensitive analytical techniques are needed for the exact measurement. Plutonium has three long-lived R-isotopes and one β-emitting isotope in the environment, i.e., 238Pu (t1/2 = 87.74 years), 239Pu (t1/2 = 24 110 years), 240Pu (t1/2 = 6561 years), and 241Pu (t1/2 = 14.4 years), respectively, that are produced in nuclear reaction and decay processes according to the following equations:

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238

ðn, γÞ ðβÞ ðn, γÞ Us f 239 U s f 239 Np s f 239 Pu

239

237

ðn, γÞ ðn, γÞ Pu s f 240 Pu s f 241 Pu

ðn, γÞ β 238 Np s f 238 Np s f Pu 242

R

Cm s f 238 Pu

Because of their very small amounts in the environment, R emitting 242Pu (t1/2 = 3.7  105 years) and 244Pu (t1/2 = 8.3  107 years) are not discussed in the present review. Neptunium has one long-lived (t1/2 = 2.14  106 years), the R-emitting isotope that is present in the environment. 237Np is produced both from 238U by fast neutron reaction and from 235U Special Issue: Fundamental and Applied Reviews in Analytical Chemistry Published: April 05, 2011 4688

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Analytical Chemistry

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by successive neutron capture and β-decay: 238

235

ðn, 2nÞ β 237 Us f 237 U s f Np

ðn, γÞ ðn, γÞ β 237 Us f 236 U s f 237 U s f Np

Np is also produced by R-decay of 241Am. Americium has one isotope that is present in the environment. 241Am that is a long-lived nuclide of a half-life of 432 years is produced basically in neutron caption reactions as an activation and decay product of plutonium: 237

241

Pu

β, t ¼ 14 years

s f

241

ðn, γÞ Am s f 242 Am

It is the daughter of the 14 year-long half-life β decaying 241Pu isotope giving rise to the increase of the level of 241Am in the environment contaminated by Pu from nuclear explosions, authorized or accidental releases. 238 Pu, 239Pu, 240Pu, and 237Np emit R particles and low energy γ radiation of low abundance. Their sensitive determination can be performed via detection of the R particles. Direct measurement of the γ photons requires low energy photon detectors and is commonly used for sources of high activities, e.g., in case of spent fuel analysis. 241Pu is a weak β particle emitter (Eβ,Max = 20.8 keV, 100% yield) that can be detected by liquid scintillation counting. 241Am emits both R particles and γ radiation (59.5 keV, 36% abundance, etc.).1 The determination of becquerel/gram or higher levels of 241Am can be performed straightforward by high-resolution γ spectrometry using highpurity Ge detectors most likely with thin detector windows, e.g., detectors with Be or carbonepoxy windows. The n-type Ge detectors usually have thin windows, and the well-type geometry provides both high sensitivity and high counting efficiency. Because of the relatively low energy of the γ radiation, selfabsorption correction has to be performed frequently to obtain accurate analytical results. Nonetheless, γ spectrometry is not adequate for the detection of low levels of 241Am. There is a limited chance to directly detect the γ radiation (312 keV, 38.6% yield) of 233Pa (t1/2 = 24 days), the daughter of 237Np. R spectrometry is one of the oldest, most frequently used, and most sensitive nuclear measuring techniques for the determination of R-emitting 238Pu, 239þ240Pu, 237Np, and 241Am. Because of similar energies of the R particles from 239Pu (5.16 MeV, 71%) and 240Pu (5.17 MeV, 73%)1 the isotopes cannot be distinguished by standard R spectrometry using Si detectors, thus R spectrometry provides the sum of the activities. Significant overlapping occurs between the R peaks of 238Pu (5.46 MeV, 29.0% and 5.50 MeV, 71.0%) and 241Am (5.44 MeV, 12.8% yield and 5.49 MeV, 85.2%) that usually cannot be spectrometrically resolved. The major advantages of R spectrometry are the relatively low price of the equipment and the high sensitivity due to the low background and the high selectivity for R particles against other types of radiation. Detection limits as low as 1 mBq/sample can be achieved by standard R spectrometry using Si detectors if counting times are around a couple of days, R chambers are kept free of contamination, and close sample
detector geometry is assured. The unique advantage of R spectrometry is that the relatively short half-life (87.7 years) nuclide 238Pu can be measured straightforward.

The major disadvantage of R spectrometry originates from the necessity of the complete separation of the analytes from the sample components in order to obtain “infinitely” thin R sources. Radiochemical procedures, often fairly sophisticated ones, have to be applied to the samples to remove major and minor components. Because of the short-range of R radiation in matter, the source thickness should not exceed a few micrometers, otherwise the R spectrum becomes degraded and peak resolution diminished and in the worst case can prohibit the evaluation of the spectra. The basic spectrometric interferences in Pu analysis by R spectrometry are 241Am overlapping with 238Pu and 210Po and 228Th partially overlapping with the 238Pu peak. Occasionally other interferences, e.g., 224Ra, 229Th, 231Pa, 232U, and 243Am can be identified in Pu spectra. Traces of U, Th, and Np can contaminate the Pu source if chemical separations are not effective. The major interferences in R spectrometric determination of 237Np (4.64 MeV, 6.2% yield, 4.66 MeV 3.3% yield, 4.77 MeV, 33.0% yield, 4.79 MeV, 48.6% yield, 4.82 MeV, 2.5% yield) are 234U, 230Th, and 226Ra. The major interference for 241Am determination using 243Am tracer has already been mentioned but besides 238Pu the presence of other nuclides, e.g., 210Po, 228 Th, can encumber the analysis. A good resolution of the R spectrum is needed to separate the peak of 241Am from that of 243 Am (5.23 MeV, 11% yield, 5.28 MeV, 87.4% yield) that is often used as a radioactive tracer. Occasionally, other interferences, e.g., 232U can be identified in the spectra. R spectra of Am often contain the peaks of curium isotopes, i.e., those of 244Cm and 242Cm, that do not influence the determination of 241Am; moreover, they can be determined simultaneously if the chemical recovery is known. The recovery of Cm is often taken as equal with that of Am if it is assured that Am
Cm separation has not occurred. Radiochemical separations and good spectral resolution can ensure together the quality of the R spectrometric analysis. Several monographs and review papers have been published about the physical, chemical, and nuclear properties of the transuranium isotopes. Reviews on Pu were prepared by Coleman2 in 1965, Miljukov et al.3 in 1969, and Cleveland4 in 1970 followed by the works of Katz et al.5 and Hoffman.6 The behavior of Np was discussed in the papers of Burney and Harbour7 in 1974 and Roberts et al. in 1986.8 The physical and chemical properties of Am isotopes were described in the early reviews of Penneman et al.9 from 1960, Myasoedov et al.10 from 1974, and Petrzilova11 from 1976 followed by the excellent work of Warwick et al.12 from 1996. Two chapters have been devoted to actinide separations and actinide trace analysis written by Nash et al.13 and Wolf,14 respectively, in the series on actinides and transactinides from 2006 edited by Morss, Edelstein, and Fuger. The purpose of this paper is to review and assess the radiochemical procedures that are currently the state-of-the-art for the determination of Pu, Np, and Am nuclides in various matrixes focusing on environmental samples using R spectrometry. A summary about development of radiochemical separations will be given including a brief description of the “old” procedures to show major trends. Nuclear measuring techniques, i.e., semiconductor R, γ, and X-ray spectrometry and liquid scintillation counting (LSC) will be briefly discussed. Capabilities of these techniques will be compared with those of various mass spectrometric (MS) techniques. The literature survey is based on the publications of the last 20 years referenced in the International Nuclear Information System (INIS) database. 4689

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’ BASIC CHEMICAL PROPERTIES OF PU, NP, AND AM Pu and Np are the sixth and fifth members of the actinide group, respectively. In aqueous solutions they exist in the þ3, þ4, þ5, and þ6 oxidation states, although the most stable forms of Pu(IV) is the tetra and that of Np(V) is the pentavalent state. In alkaline solutions Np(VII) is also stable. Because the standard electrode potentials of the various Pu and Np couples in acidic solutions are only slightly different (0.9
1.1 V in the case of Pu and 0.7
1.1 V in the case of Np), the various forms can coexist in solutions or can be turned to a given state by slightly modifying the redox conditions. The Np3þ = Np4þ þ e
couple with the standard electrode potential of 0.15 V is basically different and is what explains the instability of Np(III) in solutions. The possibility to change the oxidation states is of supreme importance in Pu and Np radiochemistry, since they can be selectively maintained in single oxidation states basically influencing the separation processes. According to the actinide theory of Choppin et al.,15 differences among the various oxidation states of a single actinide are bigger than those of different actinides in the same oxidation state. The formation of complex ions in aqueous solutions with inorganic ions or organic compounds is an important feature of the chemistry of Pu and Np. The ability of the various species to form complexes is dependent on the charge density. The relative complex forming tendency of Pu species and analogously that of Np species are the following: Pu4þ ðIVÞ > Pu3þ ðIIIÞ  PuðVIÞO2 2þ > PuðVÞO2 þ The stability series of the inorganic complexes of tri-, tetraand hexavalent actinides are the same: OH
> carbonate > oxalate > F
 acetate
> SO4 2
> NO3
> Cl
> ClO4
There are small differences among the stability constants of the different actinides in the same oxidation states. Oxidation state adjustment and complex forming conditions have been discussed in many papers from the discovery of Pu and Np until the recent times. Conditions of oxidation
reduction reactions of Pu and Np ions as well as stability constants of a great number of complexes were measured in the 1950s and 1960s.2,5,7 Formation of the given species is influenced by many parameters (composition of the sample, presence of oxidative/reductive agents, amount and stability of the reagents, kinetics of the process, etc.). Up-to-date analytical techniques, e.g., inductively coupled plasma mass spectrometry (ICPMS), helped understand the mechanism of the reactions. The sensitive multielement analytical technique can provide information not only about the chromatographic behavior but also about the effect of other actinides and sample components; furthermore, the interaction between the resin and the ionic species can be studied under oncolumn conditions. ICPMS measurements proved to be good tools to develop procedures for R spectrometric determinations. Grate et al.16 determined the conditions for on-column oxidation of Pu(III) to Pu(IV) using NaNO2, and studies of several reducing agents revealed that Ti(III) enabled the rapid quantitative on-column reduction of Pu. Lee et al.17 investigated the oxidation states of Pu in hydrochloric acid media by UV
visibleNIR absorption spectroscopy and found that oxidized Pu(VI) was reduced on-column by the anion exchange resin. Similar conclusions were drawn about the interaction of Pu(VI) and UTEVA extraction chromatographic material.18

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Since tetravalent actinides have the highest complexing power, the adjustment of this valence state has formed the crucial part of many separation procedures. Np(IV) is easily formed by the addition of various reducing agents, e.g. ferrous ions, iodide ions to hydrochloric or dilute nitric acid solutions. Strong reducing agents generating Fe2þ in situ by the reduction of Fe3þ, e.g., hydrazine, hydroxylamine, are also adequate for the adjustment of the Np(IV) oxidation state.7 These agents reduce Pu to Pu(III). Trivalent Pu can be oxidized back to tetravalent species by various oxidizing agents. To stabilize the Pu(IV) state without further oxidizing it to Pu(V) or Pu(VI), only a few moderately strong oxidizing agents can be used, e.g., NaNO2 can be added to nitric acid solutions to oxidize Fe2þ and consequently Pu to Pu(IV) or H2O2 can be added to hydrochloric acid solutions,2 although the latter procedure is regarded slow. NaNO2 and H2O2 are much less effective if Pu in higher oxidation states has to be reduced to the tetravalent form. To keep both Pu and Np in the tetravalent states is an especially challenging job that has been studied for many years. Pu(III) and Np(IV) obtained by reduction with Fe2þ using, e.g., ferrous sulfamate or Fe2þ and hydrazine can be gently oxidized back by adding nitric acid 6 M
8 M concentration and heating the solution to obtain Fe3þ or by adding an excess amount of NaNO2 with or without warming.7,19
21 The role of NaNO2 in the stabilization of the oxidation state of Np was studied by Drake et al.22 and recently by Precek et al.23 who found that small amounts of HNO2 as catalyst oxidize Np(IV) to Np(VI) while HNO2 in bigger concentrations reduces Np(VI) to Np(V). Koyoma et al.24 and Perna20 added H2O2 to the reduced Pu and Np species to turn them tetravalent. Morgenstern et al.25 studied oxidation state adjustment processes when H2O2 was added to 6 M HNO3 solution and found that Pu was reduced rapidly to Pu(III) and Np to Np(IV). In the presence of bigger amounts of U as U(VI) species, the reduction was less effective. It was also shown that the presence of organic material (UTEVA resin) changes the redox processes. Hexavalent states of Pu as well as Np can be obtained using strong oxidizing agents, e.g., HBrO3, KMnO4, and K2S2O8. Oxidization of actinides has recently been comparatively evaluated by Osv
ath et al.26 and Guerin et al.27 Am is the seventh member of the actinide group, the chemical “analogue” of the lanthanide Eu with similar electronic configurations (Am, 5f27s2) and ionic radii (Am3þ, 98 nm).12 In aqueous solutions it can exist in the þ3, þ4, þ5, and þ6 oxidation states. Although Am(III) can be oxidized theoretically to Am(IV), the standard electrode potential of the Am4þ/Am3þ couple in acidic solution is very high (2.44 V), in basic ones significantly lower (0.4
0.5 V). Hence, the formation of Am(IV) is possible in basic solutions, in the presence of strong complexing agents, e.g., phosphates, but the species is unstable and will be reduced back to the tervalent state easily.12 In aqueous solutions Am can be oxidized to penta- and hexavalent states forming oxo cations Am(VI)O22þ and Am (V)O2þ. In acidic solutions, the standard electrode potentials of the Am(V)/Am(III) and the Am(VI)/Am(III) couples are 1.74 and 1.69 V, respectively, that allows the formation of both species, but Am(V) disproportionates in acidic solutions: 3AmO2 þ þ 4Hþ ¼ 2AmO2 2þ þ Am3þ þ 2H2 O Thus, the most stable form of Am is the tervalent state. In acidic solutions, Am(III) and Am(VI) can coexist, and oxidized 4690

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Analytical Chemistry Am(V) and Am(VI) species are more stable in basic solutions. To oxidize Am to Am(VI), very strong oxidizing agent, e.g., Ag catalyzed K2S2O8, is needed.28 The formation of complex ions in aqueous solutions with inorganic ions or organic compounds is an important property of Am. The relative complex forming tendency of Am species depending on the charge density is the following: AmðVIÞO2 2þ ∼ Am3þ ðIIIÞ > AmðVÞO2 þ The lack of the tetravalent Am species of the highest complex forming power and the chemical similarity of Am with the lanthanides badly influence the separation possibilities of Am; very stable Am complexes, separations of extremely high distribution coefficients and of high selectivity toward lanthanides are not available. This is the main reason why Am chemistry is still the most problematic part of actinide analysis and in actinide technology of spent fuel reprocessing/partitioning and radioactive waste management. The stability constants of various complexes of Am(III) with inorganic and simple organic complexing agents are given in the paper of Myasoedov et al.10 The stability series of the inorganic complexes of Am(III) is the same as that of other actinides: OH
, CO3 2
> F
, HPO4 2
, SO4 2
> Cl
, NO3
> ClO4
Thiocianate (SCN) complexes of Am do not have high stability but, because of the relatively high separation factors toward lanthanides, play an important role in Am chemistry.

’ DEVELOPMENT OF SEPARATION PROCEDURES FOR THE DETERMINATION OF PU, NP, AND AM ISOTOPES Sample Preparation/Preconcentration. Objectives of sample pretreatment are basically the same in case of different actinides: to obtain a homogeneous sample solution, free of any insoluble residues that would interfere during the subsequent chemical processing and could retain the analyte and free of any organic material that could form complexes with the analyte or the reagents, and contains the analyte without significant losses. For soil, sediment, and other mineral samples, pretreatment usually means drying in an oven, homogenizing, and sieving (optional) and removal of organic material by ashing followed by wet chemical destruction. In the case of actinide determination, samples are usually ashed at 450
600 °C for a couple of hours or days. Elevated temperatures are not desirable because of the formation of refractory particles. For sample dissolution, several methods have been used including acid leaching with mineral acids (most frequently with 8 M HNO3 in the case of actinide determination,29
35 or aqua regia,36
40 complete destruction by a mixture of mineral acids typically including nitric, hydrochloric, and hydrofluoric acids and hydrogen peroxide).41,42 Kenna43 studied the leachability of Pu and Np from sediment samples by applying a series of extractants. If hydrofluoric acid is used to volatilize silica as SiF4, then boric acid is added afterward to the final solubilization in order to complex any fluorides. Wet acid decomposition can be carried out in open systems, e.g., in Teflon beakers on a hot plate or in closed systems, e.g., in a microwave oven or pressure bombs. The higher the pressure and the temperature the faster the chemical reaction. Significantly higher temperatures than the boiling points of the aqueous systems can be attained by fusion with a variety of salts, thus speeding up the destruction process. Several good monographs and papers have

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been published about sample decomposition methods44
47 that can be applied to the analysis of actinides. Fusion is a very effective way of rapid and complete sample decomposition, but big amounts of salts often together with impurities are transferred to the sample solution that may create problems in the course of subsequent chemical separations. The other disadvantage of fusion is that usually relatively small samples can be treated. For rapid methods of actinide determination, the only appropriate destruction technique is fusion. Burnett et al.48 and Maxwell et al.49 used NaOH fusion, Li borate fusion was used by Nuygren,231 Ayranov et al.,50 Guerin et al.,27 and Vajda et al.,21 and a carbonate-borate fusion was used in the rapid method by Ohtsuka et al.51 Sill et al.52,53 proposed a very efficient potassium fluoride/pyrosulfate fusion to treat refractory material. Several standards are available about sample decomposition. The ASTM C1342-96 standard deals with fusion decomposition of solid materials, the ASTM D1971-95 standard describes hot plate and convection-oven digestion of water and solid samples with nitric and hydrochloric acids, the ASTM D4309-91 standard is about microwave digestion with hydrochloric and nitric acids for determination of metals and groundwater, and the U.S. EPA Method 3052 (1996) discusses microwave-heated, closed-vessel decomposition of ashes, biological tissues, oils, oil-contaminated soils, sediments, sludges, and soils. In the case of biological samples, complete destruction is preferred and carried out usually by the combination of dry and wet chemical ashing. Because of the lack of silicates and refractory components, the use of HF can be omitted. Microwave digestion is a preferred way of decomposition of smaller samples.54
57 An efficient way of decomposition of organic materials is the Fenton’s reaction using hydrogen peroxide and Fe2þ catalyst. Tavcar et al.58 applied this procedure to destroy spent organic resin. Preconcentration of actinides by coprecipitation is a frequently used method for big volumes of water samples as well as for sample solutions obtained after destruction of solid samples. A well-selected coprecipitant concentrates the analyte in a small volume, removes most of the cationic and anionic interferences, and helps increase the decontamination factor (DF) for the whole radiochemical procedure. Bismuth phosphate (BiP) was the first precipitate applied in large scale processing of Pu, and BiP carries Pu(III) and Pu(IV) and was used to separate Pu from U and fission products both in technology and analyses.59 The coprecipitation of Am with BiP was studied by Mathew et al.60 who found that below 0.3 M acid concentration, Am is carried with the precipitate. The recovery is reduced by increasing amounts of iron in the solution. The most frequently used carriers of actinides are the following: ferric and ferrous hydroxides, lanthanide fluorides and hydroxides, alkaline earth oxalates, phosphates, and fluorides, some other phosphates and oxalates, barium sulfate, and manganese dioxide. The coprecipitation behavior of actinides is discussed in detail in ref 5. Hydroxide precipitates, e.g., ferrous and ferric hydroxides, lanthanide hydroxides, calcium phosphate, and calcium/magnesium carbonates are formed in neutral or basic solutions. Hydroxide, phosphate, and carbonate precipitates are usually easily dissolved with acids. Ferric hydroxide is known to coprecipitate actinides in all oxidation states, but sometimes recoveries are low and that’s why ferrous hydroxide that acts as a reducing agent itself is preferred. Chen et al.40 uses sodium sulfite to reduce Np and Pu and adjusts the pH to 9 with NH3 to form a 4691

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Analytical Chemistry mixed Fe(OH)2
Fe(OH)3 precipitate. The precipitation is repeated again in the presence of K2S2O5 reducing agent by adjusting the pH to 10 with NaOH to dissolve and separate the amphoteric interferences, e.g., Al. Coprecipitations taking place in acidic media, e.g., fluorides are usually more selective than those formed in basic solutions, e.g., hydroxides. Calcium fluoride and especially lanthanide fluorides of low solubility form precipitates in 4
5 M HF solution. Fluoride precipitates can be dissolved as borate complexes and carry only tri- and tetravalent actinides. The separation of actinides by coprecipitations usually takes advantage of the oxidation and reduction cycles. Cooper61 developed a streamlined method for the analysis of actinides based on selective oxidation/reduction processes followed by coprecipitation with NdF3. Pu, Np, and U are oxidized to the hexavalent state with bromate, and Am is coprecipitated with NdF3, then Pu is reduced to Pu(IV) with NaNO2 while Np and U are kept oxidized and Pu(IV) is coprecipitated with a subsequent NdF3 source. Barium sulfate carries also only tri- and tetravalent actinides. It can be dissolved either with EDTA or with HClO4. The use of EDTA is beneficial if samples have Ca, Mg, and Al content that forms strong complexes and can be separated from actinides during a succeeding lanthanide hydroxide precipitation. Sill et al.62 developed a method for the separation of Np from Ce, Ba, and La based on its coprecipitation with barium sulfate under strict oxidation state control. The oxidation state of Np is adjusted with H2O2 to Np(IV) that is coprecipitated with barium sulfate from a sulfuric acid solution containing potassium sulfate. Then Np is oxidized with chromic acid to Np(VI) and separated from fission products that cannot be oxidized and form a precipitate with the next barium sulfate. The Np R source is also prepared by coprecipitation with microcrystals of barium sulfate. Sill et al.52 adopted this method for the simultaneous determination of Np, Pu, and U based on a series of barium sulfate coprecipitations. Moore63 developed a procedure that was later improved by Sill64 for the simple separation of Am from many elements including actinides and lanthanides. Am was oxidized to its hexavalent state with ammonium peroxydisulfate in a dilute nitric acid solution. The nonoxidizable lanthanides and actinides were than precipitated as fluorides and filtered off. The Am was than reduced and coprecipitated with LaF3. In the improved method of Sill, the nonoxidized rare earths were coprecipitated with BaSO4 or NdF3 while oxidized Am(VI) was kept in solution. After reduction with H2O2, Am(III) was coprecipitated with a small amount of NdF3 allowing direct detection of the R source (see later in R Source Preparation). By this method, Am can be separated from the nonoxidized Cm isotopes too. MnO2 precipitated from KMnO4 and MnCl2 at pH 8
9 is used for the preconcentration of Am, Pu, and Np from seawater samples at the IAEA’s Marine Environmental Laboratory.65 MnO2 is dissolved with HNO3
H2O2. The pH dependence of metal oxalates precipitation was studied by Yamato66 who found that calcium oxalate formed at pH g 1.5 carries Am almost quantitatively. Oxalate precipitates are easily decomposed by ashing or wet chemical oxidation. The following precipitates have been recently used to preconcentrate actinides from various samples: ferric hydroxide,29,67
71 ferrous hydroxide,72
76 calcium oxalate,35,77 ferric hydroxide and calcium oxalate,55,66,78
80 calcium phosphate,48,81
85 calcium fluoride,21,86
88 lanthanum fluoride,39,89 manganese dioxide,65,90 manganese dioxide, and ferric hydroxide.91
93 For concentration

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of actinides from large volume of water or solutions of biological origin, special extraction chromatographic materials of extremely high distribution coefficients have also been developed and applied, e.g., TEVA,94 TRU,95,96 DIPHONIX,97 and DIPEX resins48,98 (see in detail later). Smith et al.99 developed a method for the preconcentration of Am and Pu from waste waters using a synthetic water-soluble metal-binding, for example, phosphonic acid polymer that was removed from the waste by ultrafiltration. Chemical yields over 90% have been achieved. Chemical Separation of Actinides. Actinides are amenable to a variety of chemical separation procedures including precipitation/coprecipitation (see above), liquid
liquid extraction, ion-exchange chromatography, extraction chromatography, and a combination of two or more of these methods. Procedures for the trace analysis of actinides have been compiled recently by Wolf14 and Hou et al.100 A review of analytical methodology on the determination of Pu isotope concentrations and isotope ratio by ICPMS was prepared by Kim et al.,101 and another review on the determination of transuranic elements by ICPMS was prepared by Ketterer et al.102 focusing on environmental applications. Potentially, 239Pu, 240Pu, 242Pu, 238Pu, 237Np, and 241Am are all amenable to determination by mass spectrometry, and separation procedures of Pu, Np, and Am for ICPMS measurements are basically the same as for R spectrometry, but the major interferences are different. At the mass numbers 239 (Pu) and 237 (Np), 238U is the most important interference due to the formation of the polyatomic UHþ species and the spectral interference (tailing), respectively. The determination of 238Pu is encumbered by serious isobaric interference from 238U and the typically very low atom concentration of the short-lived 238Pu. The determination of 241Am requires also very efficient separation from parent 241Pu and treatment of bigger sample masses to compensate for the typically low atom concentration. The requirements for purity are generally lower for many elements in the case of ICPMS compared to R spectrometry, but extremely high DFs are required against the interferences, e.g., U, Pb, Hg, Tl, rare earth elements, or other actinide traces, to avoid isobaric or polyatomic interferences and reduce abundance sensitivity that may appear in the mass range 230
245. To meet the demands of high purity, chemical separation steps are often repeated or various steps are used in combinations resulting in sometimes more complicated procedures (see later). The principles of the separation processes of Pu and Np will be discussed parallel due to the chemical similarities. Americium separations have triple objectives: (i) to extract/retain Am together with Cm and probably with lanthanides from other actinides and sample components, (ii) to separate Am together with Cm from the lanthanides of similar properties, and (iii) occasionally to separate Am from Cm. Since different separation systems can meet these requirements, they will be discussed separately. Liquid
Liquid Extraction (LLE). A great deal of research on solvent extraction of Pu and U was carried out in order to develop large-scale technological processes, and less attention was paid to the minor actinides, Np and Am. The basic data on extractive properties of the solvents are equally applicable in the laboratory and the factory. Solvent extraction offers moderate selectivity that can be multiplied by performing the process in a continuous technological system. Three types of extractants can be distinguished: (i) neutral extractants form neutral (ion association) complexes with the actinide ions involving the counterions, very often nitrates or 4692

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Analytical Chemistry chlorides and the organic molecule forming chemical bonding via electron donor atoms of the organic molecules (oxygen, phosphoryloxygen, sulfur, nitrogen); (ii) amine extractants where positively charged organic amines form complexes with anions via Coulomb forces and anions in the complexes can be replaced by anionic actinide complexes in a manner analogous to anion exchange, amine extractants are long-chain alkyl or aryl quaternary amines or primary, secondary, tertiary amines that form organic cations with acidic hydrogen; and (iii) acidic extractants form chelate complexes with the actinide cations replacing the Hþ in the organic compound. Groups i and ii can be regarded together simply as ion association complexes. Ion association complexes have the advantage of being applied in acidic media; in general, distribution coefficients are increased when acid concentration increases. On the contrary, chelates are used in media of low acidity because Hþ ions are competitors of the cations for the binding sites. Traditional Extractants. Originally, the LLE of Pu was accomplished with a wide range of organic acids, ketones, esters, alcohols, and ethers for reprocessing Pu and U in the nuclear industry. In the REDOX process, Pu(VI) and U(VI) were extracted with methyl-i-butyl ketone (MIBK called also as hexone). In the TRIGLY process, Pu(VI) was extracted with triglycol dichloride and in the BUTEX process with dibutyl carbitol, both being ether derivatives. Later these processes were uniformly replaced by the PUREX process based on the extraction of Pu(IV) and U(VI) with tributyl phosphate, a much less explosive compound that also has a higher (but still relatively low) distribution coefficient for Pu and U. Neptunium follows Pu in the extraction processes whenever the oxidation states are the same but adjustment often fails (see before) and Np is found both in the organic and the aqueous phases. The extraction chemistry of Np by TBP was summarized in the review of Drake.22 In the advanced PUREX process, Np is oxidized to Np(VI) using 0.01 M K2Cr2O7 that is extracted by TBP. There have been various attempts to selectively strip Np as Np(V) using a 0.1 M ascorbic acid/0.1 M H2O2/2 M HNO3 mixture or hydroxamic acid or butyraldehide (Nash et al.13). MIBK and TBP were used for analytical purposes for the determination of Pu in the 1950s and 1960s (see ref 447, ref 167 by Hart et al., and ref 144 by Geiger in ref 2). At present, TBP is used in a mixture with CMPO for extraction chromatography (see later). While tetra and hexavalent actinides are readily extracted by organic acids, ketones, esters, alcohols, and ethers, none of these organic compounds nor tributyl phosphate can extract the trivalent Am. Trivalent Am3þ cations do not form stable ion association complexes with simple monofunctional organic compounds, can be complexed with bifunctional compounds, do not have the affinity to form anions but can be extracted/ retained by chelates. Chelate Extractants. Among the ketones tenoyl-trifluor acetone (TTA) has got quite a significant analytical application. It is a diketone with a sulfur containing aromatic ring and a fluorine substituted methyl group that exists mainly in enol form, thus acting as a chelate. TTA has been used to separate Pu from water and urine samples (see ref 352 by Messanguiral et al., ref 125 by Everett, and ref 316 by Perkins in ref 2), and in combination with other extractants for separation of Pu from fission product mixtures (see ref 332 by Rider, ref 255 by Lingjaerde, ref 341 by Rydberg, and ref 261 by Maeck et al. in ref 2). TTA has been used recently for the separation and purification of Pu in

REVIEW

combination with anion exchangers from marine sediment samples, and extremely high DFs were achieved while Pu recovery was 56%, Pu was measured by ICPMS (Donard et al.57). TTA proved to be one of the most selective reagent for the separation of Np(IV) when Pu is kept in trivalent Pu(III) oxidation state. TTA was used as early as 1947 by Magnusson.102 Neptunium was reduced with ferrous sulfamate and hydroxylamine in 1 M HCl,103 ferrous sulfamate in 1 M HNO3,104 hydroxylamine in 1 M HNO3,105 or ferrous ions and hydroxylamine in 1 M HNO3.106 Neptunium retained from 1 M HNO3 or HCl on about 0.5 M TTA in xylene was later stripped with 8 to 10 M HNO3. TTA has also been used for speciation studies where “TTA extractable” species are regarded as tetravalent ones (Wolf14). The trivalent Am cation can form stable chelate with TTA at a relatively high pH. Sekine et al.107 used 0.5 M TTA in xylene to purify Am from a solution at pH g 4. 1-Phenyl-3-methyl-4-benzoyl-5-pyrazolone (PMBP) is a N-containing ketone that also forms chelates in the enol form and has gained application in Am separations. It extracts several elements from more acidic media than TTA does. Distribution coefficients for Am and the separation factor of lanthanides are slightly higher than in TTA.10 Jia et al.108 extracted Am from a 0.1 M HNO3 solution into a mixture of 0.05 M PMBP and 0.025 M t-octylphosphine oxide (TOPO) in cyclohexane. Americium was back-extracted with 5 M HNO3. Guogang et al.109 used PMBP/TOPO extraction to purify Am before preparation of R sources, and Am was extracted from a DTPA/ lactic acid solution directly into 0.05 M PMBP þ 0.025 M TOPO in cyclohexane. Extraction maximum (about 90%) was in the pH range 1
2. Bis-2-etylhexyl-phosphoric acid (HDEHP) as a chelate shows selectivity for cations of higher charges against those of lower charges, thus HDEHP is a frequently used extractant in Am (Cm) separations and is a good choice for the separation of tetravalent actinides. In the TALSPEAK process, the nuclear technology of radioactive wastes, HDEHP is used both for the separation of americium together with curium and the lanthanides from waste solutions and for the separation of Am and curium (trivalent actinides) from lanthanides. The designation “TALSPEAK” is derived from the initial letters of the phrase “Trivalent actinide
lanthanide separation by phosphorous reagent extraction from aqueous complexes”. First, Am, Cm, and the lanthanides are extracted together from a lactic acid solution (pH 2.5
3) into the HDEHP, then actinides and lanthanides are separated using DTPA complexing agent. The distribution coefficients (D) for Am in HDEHP solution strongly depend on the acidity of the aqueous phase, e.g., lg(D) = 1 for Am extraction by 0.5 M HDEHP in i-octane if the HNO3 concentration is 0.1, and lg(D) = 4 in 0.01 M HNO3 solution.10 The separation of trivalent actinides from lanthanides is only possible if strong complexing agents, e.g., lactic acid, DTPA, are added to the aqueous phase. HDEHP has been used for the separation of Am together with Cm and the lanthanides in the analytical practice, as well. It was observed that Fe severely interferes with the extraction. Attempts have been made to apply an improved procedure based on HDEHP extraction for the separation of Am from lanthanides,12 but procedures based on anion exchangers proved to be superior (see later). Bernabee et al.110 extracted Am from 72% HClO4 solution into 15% HDEHP in n-heptane. Americium was backextracted with 4 M HNO3. Holm et al.111 extracted Am from 4693

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Analytical Chemistry 0.01 to 0.001 M HNO3 and back-extracted it with 4 M HCl. Lanthanides were not separated from Am in the procedures. In the standard test method ASTM C 1205-07 for “The Radiochemical Determination of Americium-241 in Soil by Alpha Spectrometry”, 241Am is determined in soil samples up to 10 g. The soil is completely dissolved by the use of pyrosulfate fusion. After the initial separation on barium sulfate, Am together with other actinides and lanthanides is extracted from concentrated HClO4 solution with 15% HDEHP in n-heptane and back-extracted with 5 M HNO3. Americium is separated from other trivalent actinides and lanthanides by oxidation of Am and precipitation of the interferences with LaF3. The standard test method is based on the procedure developed by Sill.64 In the procedure of Guogang et al.,109 Am was extracted from 0.1 M HNO3 on a supported KF-HDEHP column, followed by its stripping with 6 M HNO3. Am was separated from lanthanides on a second KF-HDEHP column. The pH of the load solution was 2, the column was eluted with 0.1 M HNO3, and Am was stripped with a 07 M DTPA
1 M lactic acid solution. The average chemical yield for the whole procedure including Pu and Sr removal and purification of the Am fraction with PMBP/ TOPO extraction (see above) was 58%. Rameback et al.112 extracted Pu and Np after oxidation with bromate as Pu(IV), Pu(VI), and Np(VI) together with U, Am, and Cm from 0.1 M HNO3 solution using HDEHP, then stripped Am and Cm with 5 M HNO3, followed by the backextraction of Np with 1 M HNO3/hydroxil amine and Pu(III) with 3 M HCl/TiCl3. Chemical recoveries for Pu, Np, and Am were high, 91%, 82%, and 97%, respectively. Ion Association Complexes. Later attention of scientists was turned to other organo phosphorus compounds than phosphates including phosphonates, phosphinates, and phosphine oxides, which showed higher selectivities for many actinides from acidic media. Beside TBP, trioctyl-phosphine oxide (TOPO) is the most well-known representative of the neutral organo phosphorus compounds. It has been used for the separation and determination of actinides from the 1950s and is still in use although for analytical purposes supported extraction chromatographic materials have been prepared. TOPO has been used in the TALSPEAK process for the separation of minor actinides in nuclear technology. Pimpl et al.,32 Ayranov et al.,50 and Lujaniene et al.113 used TOPO as the solvent extractant for preconcentration of Pu and Am from acidic leach solutions. By reduction of the acidity, TOPO can also extract trivalent actinides. Pimpl et al.32 and Ayranov et al.50 used TOPO as the solvent extractant for preconcentration of both Pu and Am from acidic leach solutions. Pu together with Th, U was extracted from 4 M
6 M HNO3 to TOPO in cyclohexane. Then, the acidity was reduced, the pH of the solution was adjusted to 1, and Am was also extracted with TOPO and back-extracted with 2 M HNO3. Kalmykov et al.114 extracted Pu and Np together from 5 M HCl with TOPO and selectively back-extracted Pu as Pu(III) with 1 M HCl and ascorbic acid. Np(IV) remaining in the organic phase was irradiated after evaporation in order to determine 237 Np by NAA. Dibutyl-N,N-diethylcarbamylphosphonate (DDCP) as a bifunctional organophosphorous compound is one of the first representatives of a new group of extractants that was applied in acidic solutions. DDCP has been used to extract Am from 12 M HNO3 by Ballestra et al.80 Lanthanides followed Am in the procedure. Americium was back-extracted into 2 M HNO3.

REVIEW

Significant improvement was recorded in the Am recovery (69
79% for the overall procedure including coprecipitations, anion exchange separations, and source preparation) compared to the procedure using HDEHP. DDCP extraction became the basis of the method for Am determination in seawater at the IAEA’s Marine Environmental Laboratory.115 Actinides were preconcentrated by coprecipitation with MnO2 and ferric hydroxide, Pu, Np, and Th were separated by anion exchange chromatography, and Am was coprecipitated with Ca oxalate, passed through an anion, and a cation exchanger. Americium was extracted with DDCP. Finally Am
lanthanide separation was performed on an anion exchange resin column. Later DDCP was replaced by a new and more effective bifunctional organophosphorous extractant, CMPO. CMPO is octyl(phenyl)-N,N-di-i-butylcarbamoylmethylphosphine oxide, a neutral organophosphorous extractant, used in conjunction with t-butyl phosphate as a process solvent.116 The TRUEX (transuranium extraction) process is based on the extraction of transuranium elements from acidic nuclear wastes by CMPO combined with TBP and an organic diluent. The attractive feature of the process is that it allows the extraction of Am and its separation from tetra and hexavalent actinides by using different strip solutions. The pitfall of the procedure is that Am is not separated from Cm and lanthanides. The distribution ratios of tri-, tetra-, and hexavalent actinides and nonactinide elements and the mechanism of extraction were determined in nitric and hydrochloric acids by Horwitz et al.,117 e.g., in the TRUEX
chloride system the different actinide species were retained from 6 M HCl containing AlCl3 followed by adjustment of the oxidation state of Pu(IV) with NaClO2. Actinides were stripped sequentially according to the following sequence: Am with 2 M HCl, Pu(III) together with Np(IV) and Th with 0.2 M HCl
0.1 M ascorbic acid, and U with 0.1 M oxalic acid. In the analytical practice, the extraction chromatographic version of the CMPO/TBP extractant called TRU resin has made an imposing career (see it later in the paragraph about Extraction Chromatography (EC)). The higher oxidation states of Am (preferably the complex forming hexavalent one) are not stable in the presence of organic materials, thus extraction of Am in higher oxidation states has been reported only in a few cases.118,119 Amine Extractants. Tertiary amines act as strong base anion exchangers and strongly retain the nitrate or chloride complexes of Pu (and Np analogously), e.g., [Pu(NO3)6]2
, [Pu(Cl)6]2
, formed in concentrated acidic solutions. Trioctyl amine (TOA) is the most frequently used liquid amine extractant. Alamine 336 also called tricapryl amine is a mixture of n-octyl and n-decyl amines. Quaternary amines are used in the TRAMEX process for the separation of americium from lanthanides. The use of amine extractants for Pu analysis has a long tradition (see ref 61 by F. W. Buenger et al. in ref 2), and TOA still has a widespread analytical application, especially as supported liquid extractant (see later under Extraction Chromatography (EC)). Guogang et al.109 separated Pu as Pu(IV) from the leachate of soil samples by extraction with TOA and got a Pu recovery of 85%. In the experiment, TOA was fixed to microthene particles. Schneider103 extracted Np(IV) from 5 M HNO3 in 10% TOA in xylene after reduction with ferrous sulfamate and hydrazine and stripped Np with 1 M HCl in order to analyze 237Np in spent fuel solution by R spectrometry. Sill et al.62 used Aliquat 336 to extract Np(IV) from environmental samples. Vance et al.120 used amine extraction from 0.25 M H2SO4 to separate U from Np. Neptunium was 4694

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Analytical Chemistry not extracted and could be detected directly from the aqueous phase using ICPMS. Chen et al.73 developed a method for the determination of 237 Np in soil and sediment samples up to 10 g and seawater up to 100 L using both R spectrometry and ICPMS. Neptunium separation consists of a preconcentration with mixed Fe oxides precipitated in the presence of K2S2O5 reducing agent, extraction with TOA from 8 to 10 M HCl, repeated coprecipitation with Fe(OH)2, and purification of Np(IV) by anion exchange from 8 M HNO3. An overall recovery of 74
93% was achieved while DFs for U, Th, and Pu were also high (>104). The method was extended for the simultaneous determination of 239,240Pu, 241Am, 237 Np, 234U, 238U, 228Th, 230Th, 232Th, 99Tc, and 210Pb-210Po in environmental materials.40 Trivalent actinides are not well retained by amine extractants from acidic solutions. Amine extractants as well as anion exchangers have found another application field in americium chemistry, and they have been used for the separation of trivalent actinides from lanthanides. Separation of Am from Lanthanides. For the separation of trivalent actinides from lanthanides, new extraction systems containing soft donor atom (N, S, Cl) have been proposed based on the experience that actinides form slightly stronger complexes with ligands containing soft donor atoms than lanthanides. The soft donor atom may be present as free ion (SCN
or Cl
), in a water-soluble complexant, or in a lipophilic organic extractant. This idea formed the basis of the separations from the early 1940s. Solvent extraction, ion exchange, and extraction chromatographic techniques fit prominently in these processes. The extractability of tervalent actinides is increased by adding special complexants or high amounts of salts to the aqueous phase. In the TALSPEAK process, DTPA containing the soft donor N atom and lactic acid were added to the aqueous solution to form stronger complexes with Am than with lanthanides, thus allowing their separation using HDEHP (see above). The procedure of Guogang109 is based on the same principle. SCN
complexant containing the soft donor S and N atoms can help increase the separation factor between Am and lanthanides. Moore121 first applied quaternary amines to lanthanide/ actinide group separations examining the system Aliquat 336/ xylene/H2SO4
NH4SCN. Quaternary amines (0.6 M Alamine 336 in diethylbenzene) are used in the TRAMEX process for the separation of americium from lanthanides by selectively extracting Am from 11 M LiCl, 0.2 M HCl solution. Trivalent actinides are extracted while lanthanides remain in the raffinate. The actinides are subsequently back extracted with 5 M HCl. The separation factor between Am and Eu was 108.13 In the SANEX process, bistriazinylpyridine (BTP) is used as a soft donor extractant for the separation of trivalent actinides from lanthanides. In this ligand, triazine rings are bound to pyridine in a geometry that favors at least tridentate coordination of the actinide ion. Selective extraction of actinides from 1.9 M HNO3/ NH4NO3 has been reported. Actinide/lanthanide separation factors as high as 100
120 have been achieved.13 High separation factors have been achieved by the application of sulfur donor extractant, e.g., thiophosphinic acid derivatives as CYANEX-301. In a countercurrent fractional process having 3 extraction and 2 scrubbing stages, more than 99.99% of Am was separated from trace amounts of Eu.13 Research for the development of new extraction systems to achieve better extractability of Am and higher separation factors

REVIEW

for americium
lanthanide separations has been continued. Many new candidate molecules for the selective extraction of actinides especially minor actinides, e.g., cobalt dicarbollide, crown ethers, calixarenes122 have been studied but have not been discussed in the present paper because neither technological nor analytical applications have been mentioned so far. In the analytical practice, anion exchange resins or EC materials are favored to solvent extraction. Comparative Evaluation of LLEs. Acid dependence of the distribution coefficients for all elements including Pu, Np, and Am were determined by many authors, and data were compiled in the form of periodic tables. One of the data collections was published as the Table of Chromatographers.123 Among the extractants discussed above, the distribution coefficients (D) for Pu(IV) are increasing in the following order: TBP/7 M HNO3 < TTA/0.5 M HNO3 < TOPO/1 M HNO3 < TOA/4 M HNO3 < HDEHP/1 M HNO3 starting from about 20 for the TBP and ending at about 2000 for the HDEHP system.10 The retention sequence of the various extractants is the same for Np(IV) species. Bigger differences in D values exist for different oxidation states. Among the extractants discussed above, the distribution coefficients (D) for Am(III) are increasing in the following order: < TTA/pH g 4 < PMBP/0.1 M HNO3 < TOPO/0.1 M HNO3 < HDEHP/0.1 M HNO3 < DDCP/12 M HNO3 < CMPO-TBP/2 M HNO3 starting from about 1 for the TTA and ending at about 100 for the CMPO system.10 In the case of ion association complexes, D values can be increased by the addition of salting-out agents. Although in many analytical applications LLE has been replaced by more efficient chromatographic techniques, solvent extraction fits perfectly for the purpose of measurements by liquid scintillation (LS). Extractive scintillators containing solvent extractants discussed above and scintillation cocktails have been developed for the PERALS R liquid scintillation spectrometers.50,61,124 In the ETRAC procedure, Pu is extracted by a high molecular weight tertiary amine converted to the nitrate form.125 The ALPHAEX system based on HDEHP extractant can be applied for the separation of Am. Yang Dazhu et al. used the extractive scintillator containing TOPO extractant in the PPO-naphthalene-toluene scintillator cocktail for the rapid determination of U, Pu, Am, and Cm in the nuclear fuel cycle and in environmental samples.126 New Extractants. New generations of extractants with higher distribution coefficients or special selectivity are still under investigation. Nuclear industry needs special chemicals especially for the more efficient separation of minor actinides including Am and Np. New extractants with high distribution coefficients for trivalent actinides and lanthanides, e.g., diamides, are under investigation. Malonamides are the nonphosphorus-containing alternatives of CMPO.13 D values have a steeper dependence on nitric acid concentration than CMPO, offering extraction from nitric acid solutions of higher concentrations (>3 M) and easier stripping of the retained actinides with dilute solutions. Trivalent actinides can be extracted as nitrate or thiocyanate complexes. The process built around malonamide extractants is referred to as the DIAMEX process and is used to separate Am, Cm, and lanthanides from process solutions. Malonamides have not been applied to analytical scale separations. Diglycolamides have been synthesized because of the similar sterical configuration to malonamides and two derivatives, i.e., N, N,N0 ,N0 -tetradecyl-3-oxapentanediamide (TODGA) and N,N, N0 ,N0 -tetradecyl-3-oxapentanediamide (TDDGA), were found 4695

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Analytical Chemistry to have extremely high distribution coefficients for Am nitrate complexes from concentrated nitric acid solutions. At present, these extractants exhibit the greatest affinity for trivalent actinides. Alkyl-dithio-phosphine acids, crown ethers, and calixarene compounds are under investigation. Ion Exchange. Ion exchange resins are high-molecular weight organic polymers, e.g., styrene and divinylbenzene copolymers, containing various functional groups covalently bound to the polymer network. The most frequently used cation exchangers are the strongly acidic sulfonated resins containing
SO3
groups. Less acidic phosphorylated and carboxylated resins are also available. The positively charged counterions can be replaced by any other cation. The ion exchange affinity of the cations increases with increasing charge and decreasing hydrated ion radius. All Pu species, i.e., Pu4þ, Pu3þ, PuO22þ, the tetra and hexavalent Np species, i.e., Np4þ and NpO22þ, and Am3þ are retained well (high D) at low acidities, and they are removed from the resin by concentrated acids, but no special selectivity against other cations was observed. Ion exchangers are typically used in the form of chromatographic columns assuring much better separation than batch techniques. Cation exchangers have been used for the preconcentration of actinides from high volume dilute acidic solutions. Fjeld et al. used cation resin column for the preconcentration of actinides from groundwater and liquid radioactive wastes.127 Cation exchangers have also been used for the chromatographic separation of actinides in ion chromatographs (IC) either alone or together with anion exchangers as mixed bed resins. IC separation fits well to the purposes of separation of actinides prior to ICPMS but is less adequate for R spectrometric determination. R€ollin et al.128,129 used CS10 cation exchange chromatography to separate actinides and analyze spent fuel leaching solution. The HPLC unit was attached to ICPMS equipment. Pu was oxidized to Pu(VI) with KMnO4 and K2Cr2O7. It was observed that oxidized species were partially reduced on-column. The elution sequence of actinides depending on the oxidation state (ionic charge) in HCl was found to be V, VI, III, and IV and in H2SO4 the order was V, VI, IV, and III. Pu(III) and Am(III) were successfully separated on a cation exchange column TCC-II by Jernstr€om77 using different complex forming eluents and detecting the sources by R spectrometry. Samples were loaded from dilute HCl solution, Pu was stripped with dipicolinic acid, and Am was eluted with oxalic acid. The eluted fractions were found to be radiochemically clean using R spectrometry. Jernstr€om also used a mixed bed ion exchanger CSA5-CGA5 column for IC of Pu and Am.77 Americium. The cation exchange behavior of Am was studied in detail in various solutions of acids, i.e., HClO4, HBr, HCl, HNO3, etc. Trivalent Am3þ cations are usually retained well (high D) at low acidities. Several other elements coadsorb with Am including lanthanides. It was observed that Am (and transplutonium elements) show an anomalous strong retention on cation exchangers from concentrated HClO4 (>4.5 M). There is no unambiguous explanation for this phenomenon. As the concentration of HCl increases, the distribution coefficient for Am declines whereas those for lanthanides is smaller than what is revealed by increasing separation factors130 and this property has been advantageously used for the separation of Am from lanthanides. The role of alcohol in the sorption processes was also studied, and an increase in the distribution coefficients both for Am and lanthanides has been observed when 2
6 M HCl solutions containing >40% of alcohol was used. Cation

REVIEW

exchangers have been used to preconcentrate Am. Lee et al.131 used cation column to separate Am from interfering Ca, Mg, etc. to determine Sr and actinides in bioassay samples. Separation of Am from Lanthanides Using Cation Exchangers. One of the oldest techniques for the separation of Am from lanthanides is based on cation exchange chromatography using complexing agents, e.g., hydroxycarboxylic acids. Ammonium citrate and ammonium lactate were used, and R-hydroxy-ibutyric acid (R-HIBA) proved to be the most effective eluent for the selective separation of Am. Choppin et al.132 separated Am from the lanthanides on a jacketed (temperature controlled) cation exchange column using R-HIBA, but the Am fraction contained Pm. R-HIBA has been used for the separation of individual transplutonium elements too,10 and some success has been achieved in the separation of Am from Cm. Aminopolyacetic acids, e.g., EDTA, DTPA have also been used as eluents. The disadvantages of their use are the low solubility and the slow establishment of the equilibrium. Agayev et al.79 separated Am and Cm from lanthanides by cation exchange chromatography with gradient elution using 0.16
0.4 M R-HIBA after removal of Pu with anion exchange. Yields of Pu and Am were in the ranges of 60
80% and 50
70%, respectively, when 100
200 g of environmental samples exposed to the emissions from the Chernobyl accident were analyzed. A similar technique has been used for the purification of the Am fraction by Berlioz et al.133 Ermakov et al.134 used 0.2 M ammonium-hydroximethylbutirate to separate Am from lanthanides. 143Pm tracer was applied to indicate the separation front between Cm and Am. In anion exchangers, the functional groups are various amines. Resins with substituted quaternary amines are strong basic anion exchangers. Pu and Np in tetra- and hexavalent forms are strongly retained from concentrated nitric and hydrochloric acid solutions. The selectivity of the anion resin for Pu and Np originates from the situation that only a few cations (other actinides, Fe, etc.) are able to form strong anionic complexes in acids. D for Pu(IV) is as high as 104. Plutonium. A convenient way to separate Pu selectively from other ions is to load tetravalent Pu(IV) from 8 M HNO3 on an anion resin, and remove the possible interferences, primarily Th, by washing the column with 9 M HCl followed by stripping of Pu as Pu(III) with 9 M HCl using a reducing agent, e.g., iodide. This is the procedure that became the basis of many methods using different measuring techniques, including R spectrometry and mass spectrometry. Because of the high selectivity of the resin for Pu(IV), very often a single chromatographic separation provides sufficient purity for the analysis. Basically the same procedure was already used in the 1960s, e.g., by Morrow (ref 293 in ref 2), Hart (ref 167 in ref 2), Campbell et al. (ref 74 in ref 2), and has been used by many authors recently, e.g., Moreno et al.,135 Qu et al.,98 Pilvi€o,95 Michel et al.,29,78 Komosa et al.,136 Solatie et al.,137,55 Hrnecek et al.,34,138 Viogue et al.,139 Berlioz et al.,140 LaRosa et al.,65 Ageyev et al.,79,89 Giardina et al.,69 Lee,131 Jakopic et al.,35 and Michel et al.78 Anion exchange procedures for Pu separation have received wide application not only in R spectrometry but in ICPMS methods (see in the monograph of Kim et al.101). The standard procedures for Pu analysis by R-spectrometry are usually also based on anion exchange separation. According to the ASTM C 1001-05 (2005) standard (Standard Test Method for Radiochemical Determination of Plutonium in Soil by Alpha Spectroscopy), 10
50 g of soil are destructed with 4696

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Analytical Chemistry a mixture of nitric, hydrofluoric, and hydrochloric acids. Pu is isolated by anion exchange, followed by R source preparation. According to the standard ASTM D3865-02 (2002) “Test Method for Pu in Water”, Pu is coprecipitated with iron as ferric hydroxide, the coprecipitated Pu is dissolved, and the solution is adjusted to 8 M in HNO3 for anion exchange separation and then the R source is prepared. The recommended procedures of the International Atomic Energy Agency (IAEA) described in the Technical Report Series 295 from 1989 and in the most recent publication from 2010141 are also based on the separation of Pu by anion exchange chromatography. The latest ISO standard, ISO 18589-4 and BS 07/30047016 DC (April 2007) on “Measurement of radioactivity in the environment. Soil. Part 4. Measurement of Pu isotopes by R-spectrometry” recommends the optional use of HDEHP extraction, strong basic anion resin chromatography, and extraction chromatography using TRU resin (see later) for Pu determination. Many examples exist on the use of anion exchange resins for Pu separation from HCl solution. Samples containing Pu(IV) are loaded from 9 M HCl on the column and after washing Pu is stripped using a reducing agent. This procedure has the disadvantage that the anionic ferric chloride complex is also retained by the anion resin. Similar procedures were used by Hoffman (ref 185 in ref 2) in the 1960s and are still in use today. Lee et al.17 studied the oxidation states of Pu in HCl solutions and carried out Pu separation from 9 M HCl solution in the presence of NaNO2. Pu was stripped with 0.36 M HCl þ 0.01 M HF, and the Pu recovery as a mixture of Pu(IV) and Pu(VI) was >75% . Neptunium. Anion resins both from nitric and hydrochloric acid solutions are used for the selective separation of Np. In reducing media, when Np is turned to the tetravalent form and Pu is reduced to trivalent state the separation of Np from Pu is easily accomplished. This is the most common method of selective 237 Np separation. Np was retained as Np(IV) together with Th on anion resin from 8 M HNO3 in reducing media in the procedures of Niese et al.,142 Germian et al.90 According to the procedures of Koyoma et al.,24 Michel et al.,29 Moreno et al.,135 La Rosa et al.,65,42 Np(IV) together with Pu(IV) and Th were retained also from 8 M HNO3 followed by the selective stripping of the actinides. The tetravalent oxidation states were adjusted by applying reduction/oxidation cycles with or without addition of NaNO2 or H2O2. Np(IV) and Pu(IV) together with U can be retained on anion resin from 9 to 10 M HCl load solution. Then Pu is stripped selectively with 9
10 M HCl/0.1 M NH4I followed by stripping of Np with more dilute 1
4 M HCl. In the procedure of Rosner et al.143 fractionation of Np between the wash and the Np strip solutions was observed. Sumiya et al.68 and recently Joe et al.144 successfully used similar procedures. To achieve higher DFs the anion exchange procedures are sometimes repeated142,145 or additional purification steps using TTA,39,68,74,146,147 TEVA,42,65 TOPO148 extraction are included. Anion exchange resin has been used to separate Np from U from 4 M acetic acid solution when Np is released while U is retained.149 If Np and Pu are to be detected in the same fraction, they can be loaded as tetravalent species and stripped also together. This procedure was used by Kim et al.150 and recently by Qiao et al.38 to prepare sources for ICPMS measurement from solid environmental samples. In the first procedure actinides were retained from 10 M HCl, in the latter procedure from 8 M HNO3 following a reduction with K2S2O5 and equilibration with the 8 M HNO3 solution.

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Americium. Am(III) does not form strong complexes with nitrate or chloride ions and is not well retained on anion resins. In concentrated HCl solutions (>11 M) there is an increase in the retention of Am on anion exchangers while lanthanides are sorbed less. However, the difference is not as high as in case of cation exchangers, thus the separation of Am from lanthanides is less successful under these conditions. Separation of Am from Lanthanides Using Anion Exchangers. The presence of alcohol in the media enhances the anion exchange of Am especially in nitric and sulfuric acids. Guseva et al.151 measured distribution coefficients as high as 5000 from 0.5 M HNO3 in 95% methanol. Lighter lanthanides also have high distribution coefficients while heavy lanthanides do not. In HCl - methanol system the anionic complexes of Am together with the heavy lanthanides are well retained by the resin while light lanthanides are less retained. Both the HNO3
methanol and the HCl
methanol systems, and preferably, the combination of the two systems can be used for Am
lanthanide separation. A mixture of 0.5
1.0 M HNO3 and 90
96% methanol was reported as the load and wash solutions by many authors (Holm and Fukai,67 Ballestra and Fukai,80 etc.) followed by stripping of the best retained Am with a mixture of 0.5
1.5 M HNO3113,152 and 70
86% methanol or simply with HCl153 or HNO3.154 Methanol has been replaced with ethanol with similarly good results. Kraus et al.155 found that many elements have higher distribution coefficients in LiCl solutions than in HCl. The sorption of Am on anion exchangers increases with increase in LiCl concentration, whereas the sorption of lanthanides varies only slightly favoring Am-lanthanide separation. The separation is further improved if alcohol is added to the LiCl solution, e.g. a good separation was achieved with 8 M LiCl and 40% methanol on Dowex 1  8 anion exchange resin by Guseva et al.151 The group separation of trivalent actinides and lanthanides can be carried out with concentrated (>8 M) NH4SCN solution as eluent (Surls and Choppin156). Lanthanides are less strongly retained on an anion column in the SCN form than Am. Am can be stripped with 4 M HCl. Bojanowski157 and Lovett76 applied this method successfully for the analysis of marine environmental samples. Guseva et al.158 showed that the best separation can be achieved with 1
2 M NH4SCN solution containing 50
70% methanol, ethanol or propanol. This procedure became the basis of many methods where Am
lanthanide separation was necessary. Holm et al.111 used this procedure for the final purification of the Am fraction from large amounts of environmental samples (200 L seawater, 100 g sediment, 500 g biological material) after the removal of the interfering components. The final residue containing Am and lanthanides was dissolved in a few milliliters of 1 M HNO3
93% methanol and was loaded on an anion exchange resin column. The column was washed with dilute HCl
NH4SCN
methanol solution to remove lanthanides, and Am together with Cm was stripped using 1.5 M HCl
80% methanol. The overall chemical recovery varied between 40 and 80%. A similar procedure was followed by Yamato65 to determine Pu and Am from 50 g of soil. In the final purification of Am 1 M HNO3
93% methanol, 0.1 M HCl
0.5 M NH4SCN
80% methanol and 1.5 M HCl
86% methanol solutions were used as load, wash and strip solutions, 75
92% chemical recoveries for Am were reported for the whole combined procedure. The procedure has been successfully applied by many laboratories for Am
lanthanide separation, it is the recommended 4697

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Analytical Chemistry procedure in the laboratories of the IAEA, both in Monaco and Seibersdorf (LaRosa65,115). It has been used by Jakopic35 for the analysis of environmental samples. Extraction Chromatography (EC). In EC, also called solid state extraction and reversed-phase partition chromatography, liquid extractants are sorbed on the surface of inert solid support material. The theory of EC was described already more than 30 years ago in the book of Braun and Gherseni.159 EC has the following advantages compared to solvent extraction and ion exchange (IE): (i) the extraction process takes place in the thin surface layer allowing good contact of the reagents and fast exchange kinetics compared to ion exchangers, (ii) as a result of the chromatographic technique more effective separation is achieved than in the batch technique in which LLE is performed, (iii) less reagents and chemicals are used than in IE or LLC, (iv) less hazardous waste is produced, and (v) the whole process is more economical. Attempts have been made to replace solvent extraction by extraction chromatography from the end of the 1960s. The chelating extractant, HDEHP, showing high distribution coefficient for Am from dilute acid solutions, has been prepared as a solid EC material on various supports to serve as a stationary phase. Moore et al.160 loaded HDEHP onto the PTFE support and separated Cm from californium. HDEHP was prepared on microporous polythene support by Jia et al.108 The 0.01 M HNO3 load solution containing Am was passed through the column and washed with 0.1 M HNO3, and Am was stripped with 0.07 M DTPA
1 M lactic acid. Pu and Po were separated before the EC by extraction using supported TOA columns. Guogang et al.109 determined Pu, Sr, and Am in a sequential separation procedure. First Pu and Sr were separated by EC, next Am was concentrated on a HDEHP column. Americium was eluted with a 0.07 M DTPA
1 M lactic acid solution to separate it from lanthanides. The Am fraction was purified by an extraction with PMBP/TOPO. The chemical recovery for Am was 58%, and the source was free from Fe, Po, and lanthanides contamination. Desideri et al.161 determined actinides in depleted uranium. After EC separation of the actinides using supported TOA columns, the Am fraction was cleaned using a supported HDEHP column (pH 2.4). Arginelli et al.84 prepared HDEHP on polythene support and loaded the pretreated urine sample from dilute acidic solution (pH 2) on the column, and Am was stripped with 4 M HCl. Supported HDEHP has become commercially available as Ln resin produced by Eichrom Co. Gleisberg et al.162 separated Np from uranyl nitrate solution using a Ln resin column. It is not clear which Np and Pu species were retained from the 10 M HCl load while U got into the effluent. Meanwhile, the interest for Am separation by HDEHP has declined due to the development of more efficient extraction chromatographic materials (see TEVA resin later). Supported TOPO on Microthene (microporous polyethylene) and TOA on Icorene (microporous polyethylene) were prepared by Jia et al.163 from the 1970s. Supported TOPO on Kieselgur was used by Afsar and Sch€uttelkopt.164 Americium was loaded onto the column from 0.1 M HNO3, washed with 0.1 M HNO3, and stripped with 8 M HNO3. Delle Site et al.165 separated actinides from urine samples on Microthene supported TOPO loaded from 4 to 6 M HNO3. Actinides were stripped consecutively with 0.3 M H2SO4 (Th), 6 M HCl
0.2 M HF (Pa), 1 M HF (U), 6 M HCl þ Cl2 (Np), and 6 M HCl þ 0.1 M HI (Pu). Americium was separated on a Microthene supported HDEHP column. Acceptable high

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recoveries (>70%) were obtained with the exception of Am even in the presence of DTPA complexing agent. EC using supported TOPO was successfully applied for Pu determination by Cozzella et al.166 Supported liquid anion exchangers were used for the separation of Np from environmental samples in the procedure of Ji et al.167 Neptunium was retained on a Teflon coated TOA from 2 M HNO3 and stripped with hot 0.02 M oxalic acid/0.16 M HNO3. High recovery (>80%) and high DF against U (>104) was reported allowing the sensitive determination of 237Np by ICPMS. In the most recent work of Desideri et al.,168 Am, Cm, Pu, Np, and U were separated from small volume of water samples and determined by R spectrometry. Samples were loaded onto macroporous PE supported TOA from 9 M HCl solution and Am, Pu, Np, and U were sequentially stripped. Recoveries higher than 74% were reported for each actinide. Horwitz et al.169 reported about a method that can separate Am from Cm on a supported (hydrophobic diatomaceous earth) Aliquat 336 column using 8 M LiNO3
0.01 M HNO3 for loading and 3.5 M LiNO3
0.01 M HNO3 solution to separate Am from Cm. More than 99% of Am and Cm were recovered in a radiochemically pure state (80%), and DFs were acceptably high for naturally occurring radionuclides. Smaller DF was obtained by TRU resin (see later). From the 1990s, a new family of EC materials has been developed for the separation of actinides by Horwitz and colleagues at Argonne National Laboratory, and later these materials became commercially available from EiChrom Co. and Triskem SAS. The EC materials have been carefully characterized by measuring physical and chemical properties, distribution coefficients, extraction kinetics, elution behavior, and by developing separation procedures for special purposes. If a single resin is not capable for the separation of all the desired radionuclides because of limited selectivity, the use of tandem column arrangements is recommended, in which the effluent of one column serves as the load solution for the subsequent one. Because kinetics of the extraction is fast, the columns can be operated at higher flow rates than gravity flow and for this purpose vacuum boxes have been proposed. TEVA resin171 dedicated to separate TEtraVAlent actinides is a supported quaternary amine-based (Aliquat 336) liquid anion exchanger where the support material is Amberchrom CG-71 ms. TEVA as an analogue of strong basic anion exchangers is expected to be an excellent chromatographic material for the retention of Pu(IV) and Np(IV). The resin capacity factors (k0 ) that are directly proportional to the distribution coefficients for Pu(IV) and Np(IV) in 6 M HNO3 are 3  104 and 4  103, respectively, and are superior to other actinides. This is the reason why the TEVA resin has got widespread application in Pu and Np analysis since the first report of Horwitz in 1995. Plutonium. Varga et al.86 used TEVA resin for the separation of Pu from environmental samples of 1
5 g after a CaF2 coprecipitation. NaNO2 was used to adjust the oxidation state of Pu(IV). The sample was loaded in 3 M HNO3 and eluted with 6 M HCl, and Pu was stripped with 0.1 M HNO3/0.1 M HF. 4698

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Analytical Chemistry (Note: Reagents compared with those used in the standard anion exchange procedure are somewhat diluted.) The Pu recovery for the whole procedure including R source preparation was 72
93%, and the DF for U was >105, which was also sufficient for ICPMS measurement. Chamizo et al.172 determined Pu in environmental samples without preconcentration using TEVA chromatography. Pu(IV) was loaded on the column in 3 M HNO3/Al(NO3)3, followed by washing with 6 M HCl, and stripping with 0.5 M HCl. Recovery of Pu (>60%) and DF for U (>100) was somewhat poorer than in the previous method, probably due to the lack of a selective preconcentration. Several authors used TEVA successfully for the separation of Pu for ICPMS measurements.30,56,81,173
176 The standard procedures for the determination of actinides in water (ACW13VBS, ACW16, and ACW17) developed by EiChrom Technologies, Inc. are based on the use of different column sets where Pu is separated by TEVA extraction chromatography. Neptunium. For the individual determination of 237Np, samples are usually loaded on a TEVA column from 2 to 3 M HNO3 after reduction of Np to Np(IV) while Pu is in the nonretainable Pu(III) oxidation state. Neptunium can be stripped with a dilute HNO3
HF mixture. High Np yields were obtained in the procedures of Anton et al.,177 Ayranov et al.,50 and Maxwell et al.19 in analyzing soil, sediment, and nuclear materials, respectively. If Np and Pu are to be determined together, the sample has to be loaded on TEVA after adjusting the Np(IV) and Pu(IV) oxidation states (see earlier). Kenna41,43 got high yields for both elements analyzing sediment samples, while Np recoveries varied between 19 and 54% in the procedure of LaRosa et al.178 In the rapid separation procedure for determination of 237Np and Pu isotopes in large soil samples, Maxwell et al.179 also found that Np recoveries were reduced when the matrix content increased (sample masses were higher than 30 g). Plutonium recoveries were high (>82%) and independent of the matrix effect. In the standard procedure for the determination of Th and Np in water (ACW08) developed by EiChrom Technologies,180 quantitative recoveries have been attained using the simple TEVA procedure. The standard procedure for the determination of 237Np in soil (ASTM C 1475-05) is based on the use of the TEVA column. The Np(IV) oxidation state is adjusted using ferrous sulfamate and NaNO2. Sample is loaded from 2.5 M HNO3/0.5 M Al(NO3)3, and Np is stripped with 0.02 M HNO3/0.02 M HF. Americium. Am(III) is only slightly retained on TEVA resin from nitric or hydrochloric acid solutions. The maximum of the distribution coefficient is 0.1. However, TEVA resin, an analogue to anion exchange resins, can be used for the separation of Am from lanthanides that was performed by loading the sample in a 2 M NH4SCN
0.1 M formic acid solution followed by the elution of lanthanides with 1 M NH4SCN
0.1 M HCOOH and stripping Am (together with Cm) with 0.25
2 M HCl (Horwitz et al.171). Good separation was achieved. A pitfall of the procedure is the same as in case of the classical Am
lanthanide separation using anion exchanger, it is difficult to dissolve bigger sample amounts in a small volume of load solution. The method is applied after the separation of Am from most of the sample components. The same method was applied by Berne181 in the standard procedure of the Environmental Measurement Laboratory (EML) for the determination of Am in soil after leaching the actinides, removal of Pu and Th with anion exchange resin, concentrating Am from the effluent with Ca oxalate coprecipitation, and separating Am from most of the interferences on a TRU

REVIEW

column (see later). The procedure was applied for purification of the Am fraction by Michel et al.,29,78 and Maxwell.182 Jernstr€om77 separated Pu(III) and Am(III) on TEVA resin, and plutonium was sorbed quantitatively (98.5
99.1%) by the resin when loaded and washed with a mixture of 2 M ammonium thiocyanate in 0.1 M formic acid. Quantitative elution of Pu(III) from the resin was achieved with 0.25 M HCl. Sorption of Am(III) on TEVA resin was quantitative in the same mixture, and americium was eluted from the resin with 0.25 M HCl. UTEVA resin183 is a dipentyl
pentyl phosphonate impregnated Amberlite XAD-7 EC material that was designed to separate U and TEtraVAlent actinides. The resin contains a phosphonate compound, and actinides are retained at higher distribution coefficients than on phosphates, e.g., TBP. The k0 capacity factor for Pu(IV) and Np(IV) are about 103 and 4  102 in 8 M HNO3 that assures acceptable retention and easy removal of the tetra- and also for the hexavalent actinides. Apostolidis et al.184 developed a procedure for the determination of Pu and U from a reprocessing solution using a single UTEVA column. The oxidation state of Pu was adjusted with H2O2, the sample in 4 M HNO3 was loaded on the column, Pu was subsequently eluted with dilute HNO3 containing hydroxil amine and ascorbic acid as reducing agents, and finally U was stripped with bioxalate solution. High recovery (>95%) was achieved for Pu with a DF for U of about 104. Pu and Np were determined by UTEVA EC in a combined procedure with the analysis of Th and U from environmental and nuclear waste samples by Vajda et al.72 after preconcentration of the actinides in reduced form with ferrous hydroxide. Sample solution was loaded in 8 M HNO3 that contained ferric nitrate as a salting-out agent. Pu was stripped with 9 M HCl/0.1 M NH4I as Pu(III), Th and Np were stripped together with 4 M HCl, and U was eluted finally with dilute acid. Pu recoveries varied in the range of 69
92%, and the DF was high enough to obtain Pu R sources without interferences. Osvath et al.26,28 improved the procedure by using a strong oxidizing agent to form Np(VI) and Pu(VI) that are retained by the resin and by purifying the Np
Th strip solution by EC separation on a second UTEVA column from the 9 M HCl solution. In the modified procedure, Pu and Np recoveries were 65
82% and 66
93%, respectively, and the Np source could be analyzed by ICPMS. UTEVA as a stationary phase for chromatographic separation of actinides was studied using an online flow injection inductively coupled plasma mass spectrometer by Perna et al.20 The sample was loaded from 3 M HNO3 containing Fe2þ reducing agent and NaNO2. Th, Np, and Pu were stripped together with 2 M HCl/ 0.1 M oxalic acid well separated from U. Guerin et al.27 determined the k0 values of Np(VI) on various EC materials including TEVA, UTEVA, TRU, and DGA from HNO3 and HCl solutions. The Np oxidation state was adjusted with KBrO3, KMnO4, K2S2O8, Ag2O, and O3. A simple method using two UTEVA columns was used to detect Np in soil and sediment samples. Np recovery was 70%. Lujaniene et al.113 used UTEVA for the separation of Pu from environmental samples after preconcentration by solvent extraction with TOPO. Experimental conditions are unfortunately not detailed. In the rapid procedure of Ohtsuka et al.,51 UTEVA is used in an ion chromatograph directly connected to ICPMS. The authors pretend to say that the complete analysis of an environmental sample (1 g) including destruction by fusion is performed within 60 min. UTEVA has been used for Pu separation by other authors in connection with ICPMS detection.100 4699

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Analytical Chemistry TRU resin185,186 is a chromatographic material comprised of a tri-n-butyl phosphate (TBP) solution of the bifunctional organophosphorous extractant octyl(phenyl)-N,N-di(iso-butyl)carbamoyl-methyl-phosphine oxide (CMPO) sorbed on an inert polymeric substrate (Amberlite XAD-7). It was intended to separate all TransUranium nuclides including trivalent Am and Cm besides the tetra- and hexavalent species. Extremely high capacity factors were measured on TRU resin in concentrated nitric acid solutions, k0 is higher than 105 and 5  104 for Pu (IV) and Np(IV), respectively, at HNO3 concentrations above 1 M, and for Am k0 is about 100. Unfortunately, Fe3þ present in the sample is also retained and competes for the bonding sites with the actinides, especially for those occupied by the less strongly retained Am (Cm). Reduction of Fe to Fe2þ helps reduce the interference. TRU resin has recently got a wide application for the separation of Am, but it can be used for Pu retention either for preconcentration or also for analytical separations. It seems to be an ideal material for the simultaneous separation of all actinides. TRU resin replaced almost exclusively all the methods used for concentration of Am in the past, i.e., chelating extractants, cation exchangers. Recently several new molecules have been synthesized and proved to be superior to TRU material regarding the distribution coefficient for Am or even separation factors, but the development of analytical procedures has a timelag and application examples are scarce yet. Because of the moderately high distribution coefficient for Am on TRU and the various interferences from trivalent ions, e.g., Fe, lanthanides and from matrix components, the separation procedures cannot miss adequate preconcentration of Am. Berne186 determined Am, Pu, and U in air filter and water samples. The sample was loaded in 8 M HNO3/NaNO2, Am was eluted with 0.025 M HNO3, Pu and U together with bioxalate. Because of cross-contamination, it was recommended to separate Pu first on an anion exchange resin and use the TRU resin for the purification of Am only. This procedure was followed by many laboratories later, including the Environmental Measurements Laboratory and the IAEA’s Laboratories at Seibersdorf135,141,187 and at Monaco.65,91,115 Horwitz et al.185 recommended a procedure for the sequential separation of actinides using a single TRU column. Actinides were loaded in 2 M HNO3 after reduction when Pu was present as Pu(III). Am was eluted with 4 M HCl and Pu was eluted with 4 M HCl/0.1 M hydroquinone, followed by stripping of Th, Np, and U with 1.5 M HCl, 1 M HCl/0.03 M oxalic acid, and 0.1 M ammonium bioxalate, respectively. This procedure was slightly modified and applied for the rapid determination of Am, Pu, Th, and U in small environmental samples (1 g) by Vajda et al.21 After LiBO2 fusion and CaF2 coprecipitation, the oxidation state was adjusted with NaNO2 turning Pu to Pu(IV). The sample was loaded in 2 M HNO2. Am was stripped with 4 M HCl. Pu was eluted after fast on-column reduction with 4 M HCl/0.1 M TiCl3. High recoveries were obtained both for Am (96%) and Pu (85%), and no traces of contamination were detected in the Am and Pu R sources. The behavior of Np on TRU resin was also tested, but a good separation from U was not attained. A sequential separation procedure for the determination of the actinides in power plant effluents was developed by Spry et al.188 In the 3 M HNO3 load solution, Pu was present as Pu(III) and it was oxidized after retention on-column with NaNO2. Am was stripped as above, and Th was eluted with 4 M HCl/0.01 M HF and Pu with 0.1 M bioxalate. Am and Pu recoveries were

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acceptably high (52 and 56%, respectively). The robust procedure for the determination of plutonium and americium in seawater developed by Sidhu93 is based on MnO2/Fe(OH)3 preconcentration of the actinides and their separation using TRU column. Pu as trivalent ion was loaded together with other actinides from 3 M HNO3, and it was stripped after Am with 4 M HCl/0.02 M TiCl3. High recoveries were achieved for both nuclides (Pu, 78%; Am, 85%). Similar procedures were applied by Olahova et al.92 and Gogorova et al.189 A single TRU resin column has been used for Pu separation and determination by ICPMS by many authors.17,54,101,190
192 According to the procedure of Crain et al.,193 long-lived actinides in soil leachates were determined by ICPMS. Pu, Np, and Th were stripped together from the TRU column with 0.1 M tetrahydrofuran-2,3,4,5-tetracarboxylic acid followed by stripping of U with 0.1 M ammonium bioxalate. Pu, Np, and Th were analyzed directly from the diluted strip solution. Unfortunately, the performance parameters have not been reported. DIPEX resin,194 also called ACTINIDE resin, is a neutral, bifunctional organophosphorous extractant, consisting of bis(2ethylhexyl)methanediphosphonic acid supported on an inert polymeric substrate. It has extraordinary affinity for actinides from dilute acids. The capacity factor for Pu(IV) and Np(IV) from 0.1 M HCl solution is about 2  107 and 107, respectively, and the same for Am, the traditionally least extractable actinide is even 1 order of magnitude higher (2  108). Actinides together with the extractant can be recovered from the resin by alcohol stripping and wet oxidation. These properties can be exploited in the preconcentration of actinides from large volume samples or complex matrixes. The direct analytical application of the resin may be limited by the incomplete removal of the actinides from the resin. Burnett et al.48 used DIPEX resin for the preconcentration of actinides from soil solutions and large volume water samples. Actinides from soil samples after NaOH fusion, dissolution, and ferrous hydroxide scavenging were concentrated on a DIPEX column. Isopropanol was used to solubilize the extractant that retained the actinides followed by the oxidation of the organic component. Basic calcium phosphate was used to scavenge the actinides repeatedly, and the final separation was performed on a UTEVA-TRU tandem column setup as described below. Recoveries of Pu, Am, and U varied in a wide range (31
91%, 22
99%, and 60
85%, respectively). ACTINIDE resin was used for the preconcentration of Pu and Am from human tissue followed by the separation of Pu with anion exchange resin by Qu et al.98 The overall recovery for Pu was high (>80%) assuring the adequacy of the preconcentration as well. DIPHONIX resin71 can be regarded as the chelating ionexchange resin analogue of the DIPEX extraction chromatographic material. It contains geminally substituted diphosphonic acid groups chemically bound to a styrenic-based polymer matrix. It also contains strongly hydrophilic sulfonic acid groups in the same polymer network. Also extremely high capacity factors were observed for the retention of all actinides, close to those of DIPEX from dilute acid, but several transition metals (e.g., Cr, Ni, Cu, Zn) are also retained. The removal of the actinides is more problematic due to the massive resin structure. Strong complexing agents can remove the actinides that have to be decomposed, or the resin itself has to be destroyed. Analytical procedures have been proposed by the authors, but because of the difficulties in removing the actinides from the resin they have not been applied in the practice. Rosskopfova et al.97 reported 4700

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Analytical Chemistry about the use of DIPHONIX resin for the preconcentration of actinides from large soil samples followed by their separation using different eluents, i.e., Am was stripped with 6 M HNO3 and Pu with 4 M HCl/0.015 M TiCl3. Recoveries for Am were high (70, 78%) but for Pu were varying (37, 93%). DGA resin195 consists of N,N,N0 ,N0 -tetraoctyldiglycolamide sorbed onto Amberchrom CG-71 resin. Like the DIPEX resin, DGA retains Am strongly, in concentrated HNO3 solutions (>1 M) and the capacity factor of Am is higher than 104. High capacity factors were found also for Pu (>103) at high HNO3 concentrations, but they were not significantly smaller at low acidities. DGA resin can be used advantageously for the separation and purification of Am without losses from various matrixes, e.g., soil, water. Maxwell et al.49,182 developed a very promising rapid method for the determination of actinides in large soil samples up to 200 g. Samples are fused with NaOH, and actinides are preconcentrated with Ce(OH)3 and CeF3 coprecipitation. Stacked TEVA
TRU
DGA cartridges are loaded with 8 M HNO3. TEVA serves for separation of Th and Pu(IV), while Am (and Cm) are separated with the TRU
DGA multistage column. Americium originally retained on TRU was eluted with 4 M HCl and transferred to the DGA column. Americium was stripped from DGA with a dilute (0.25 M) HCl solution. Am and Pu recoveries for the whole procedure were high, varied between 76 and 98% and 61
92%, respectively. The method was adopted for the analysis of water samples196 up to 1 L and extended for the determination of Sr radionuclides using Sr resin. The procedure combined the advantages offered by the rapid and effective destruction with NaOH fusion, the selective preconcentration of actinides with Ce(OH)3 and CeF3, and the EC separation using highly specific resins, i.e., TEVA for Pu and TRU-DGA for Am. Recently Maxwell197 developed a rapid simplified method for the determination of Pu and Np in water samples using a stacked column set of TEVA and DGA resins. In the combined procedure, Pu(IV) and Np(IV) were retained on a TEVA column first, then Pu was reduced as Pu(III) was retained on the DGA column together with U. From the DGA, U was eluted with 0.1 M HNO3 and Pu(III) was stripped with 0.02 M HCl/0.005 M HF/0.0005 M Ti3þ. High recoveries and DFs for U (higher than 106 and 104 in case of Pu and Np, respectively) were reported. Source quality met the requirements of both R spectrometry and ICPMS. Some other EC materials of potential application in actinide chemistry are also known, e.g., the LN resin of Eichrom Co. that contains HDEHP or the supported calixarenes,198 but their selectivities for Pu have not been proved or they have not been well characterized yet. Combined Procedures for the Determination of Various Actinides. Radiochemical procedures for the determination of individual actinides do not always consist of a single separation step. In the case of small sample sizes and chemical procedures of high selectivity, e.g., Pu separation by anion exchange chromatography, a single step separation may be adequate to prepare R sources. The recommended procedure of the IAEA, the ASTM standard procedures D3865-02, C1001-05, and ISO/DIS 185894 for Pu determination, are all based on a single step anion exchange separation of Pu followed by source preparation using microcoprecipitation or electrodeposition (see above). Procedures where bigger sample sizes are treated are typically preceded by an adequate preconcentration step (see, e.g., the ASRM D3865-02 procedure).

REVIEW

A single step separation does not always provide sufficient selectivity/DF, thus a further purification step is required to remove traces of impurities. Multistage separation procedures have been favored in various applications, e.g., for separation of high concentrations of U from trace amounts of Pu and Np in order to determine 239Pu and 237Np in environmental samples by ICPMS. It is often recommended to repeat the separation steps or to combine various steps. Combined procedures can be applied advantageously for the simultaneous determination of various actinides. The demand for the determination of two or more radionuclides from a single sample aliquot can be met by the development of such sequential separation procedures. Out of the great variety of combined procedures, only some typical examples are given below. A few combinations have been mentioned above in the section on Chemical Separation of Actinides. Separations Using Anion-Anion Resins. Zheng et al.70 separated Pu using anion exchange resin repeatedly and got a high DF of 5  105 for U and an acceptably high Pu recovery (60
70%) when Pu isotopes were analyzed in 1 L of seawater by ICP-SFMS. In the combined procedures for the determination of 237Np and Pu isotopes, Beasley et al.145 and Rosner et al.143 also used anion exchange resins repeatedly to increase the purity of the analytes (see also earlier under anion exchangers). Separations Using Anion-TEVA Resins. Muramatsu et al.30 used TEVA resin for purification of the Pu strip solution from an anion exchange column when 2
50 g soil samples were processed and got 89
97% recovery for Pu and better than 104 DF against U. A similar chemical approach has been followed recently by Lee et al.199 and Godoy et al.176 obtaining also good separation for R spectrometric and ICPMS purposes, respectively. LaRosa et al. purified both Np42,65 and Pu65 sources after anion exchange separation using the TEVA resin. Separations Using Anion-UTEVA Resins. In the procedure of Michel et al.,78 U and Th were removed from the sample solution using UTEVA followed by separation of Pu with an anion exchange resin column. Recoveries for Pu were higher than 75%. Separations Using Anion-TRU-TEVA Resins. Anion exchange chromatography often forms the basic part of a combined procedure dedicated for the determination of Pu and Am (and Th) from a single sample aliquot. Pu and Th can be directly separated by the anion exchange resin as mentioned above, while Am together with other sample components is collected in the effluent. After preconcentration, e.g., with Ca oxalate, Am can be separated using a TRU column and further purified from lanthanide impurities either with an anion exchange resin or a TEVA resin. This flow-chart forms the basis of several procedures. Michel et al.29,78 used a sequential separation procedure to determine actinides in 5 g of soil or sediment samples. Pu and Am were separated on anion exchange and TRU resin columns, respectively, and lanthanides were separated from Am on a TEVA column. Recoveries of 89 and 83% were reported for Pu and Am, respectively. Qu et al.98 applied a similar procedure for the determination of Pu and Am in human tissue samples up to 500 g. Actinide resin was used for preconcentration, and there was no need for Am
lanthanide separation. Recoveries were higher than 70 and 80%, respectively. Wang et al.200 determined Pu, Th, Am, Sr, and U in environmental and bioassay samples. Recoveries in the range of 59
85% and 37
68% for Pu and Am were achieved. Spent ion exchange resins were analyzed by Tavcar and Benedik58 using a similar method without 4701

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

Analytical Chemistry Am
lanthanide separation obtaining similar chemical recoveries (83
88% and 54
72%, respectively). In the method of Lee et al.199 for the determination of Pu, U, Th, and Am in sediments and biological materials, the separated Pu, U, and Am fractions were further purified. The Am
lanthanide separation was performed with an anion exchange column. Clean fractions and high recoveries were obtained both for Pu and Am. (See also papers of Moreno et al.135,187 and LaRosa et al.65,91.) After separation of Pu and other actinides with anion exchange resin, Am is sometimes purified by coprecipitation and the Am
lanthanide separation procedure without using TRU resin, but recoveries are often moderate. Lovett et al.76 determined Pu, Am, and Cm in 100
600 L seawater samples by this method and reported recoveries of 64
81% and 42
77% for Pu and Am, respectively. Yamato66 determined Am and Pu in 200 L of seawater and 50 g of environmental samples by a similar method. Recoveries better than 70% were obtained for both elements. Separations Using Anion-Cation Resins. Cation exchange separation of Am has also been combined with the separation of Pu. Agayev et al.79,89 and Lee et al.131 used anion exchange resins to remove Pu followed by the separation of Am on a cation exchange resin column. In the previous procedure, recoveries of 60
80 and 54
68% were reported for Pu and Am, respectively, when 100
200 g soil samples were analyzed. The latter procedure was applied for the analysis of bioassay samples, and acceptable high recoveries were obtained (Pu, 59
100%; Am, 49
100%). Anion and cation exchange chromatography are favored to other methods when ICPMS is the detection technique (Perna20). Separations Using LLC-EC Resin(s). Solvent extraction was often combined with other separation procedures especially in the early times of actinide research when less selective reagents were used, e.g., combinations of MIBK
TTA, TBP
anion resin, etc.,2 but recently they have been replaced by more specific reagents and/or chromatographic techniques resulting in the reduction of the separation steps. Lujanije et al.113 extracted Pu with TOPO and purified it with UTEVA resin. Americium was also extracted with TOPO and separated with TRU resin. In the procedure of Pimpl et al.,32 after extraction of Pu, Th, and U with TOPO, Am was also extracted with TOPO at lower acidity and purified on a TRU column. Americium was separated from lanthanides as a SCN
complex on a TEVA column. Americium recoveries varied between 31 and 75% when 50
100 g soil samples were treated. Ayranov et al.50 extracted Pu from sample solutions (prepared after borate fusion of 1
5 g of soil) with TOPO followed by EC purification of Pu using TEVA and parallel separation of Am using TRU and TEVA columns. Pu recovery was 93%, while that of Am was 87%. Results indicated good accuracy of the method. Extraction of Pu with TOPO has been combined with the separation of Am using HDEHP: Guogang et al.109 reported a procedure for the determination of Pu, Am (and Sr) in soil and sediment samples up to 30 g where recoveries of 85% and 54
99% were achieved. Desideri et al.161 used a similar procedure for the determination of U and transuranium nuclides in depleted U samples, and Am recoveries of 70% were reported. Recently, a great variety of combinations of EC procedures have been reported that seem to be adequate for the simultaneous determination of various actinides. EC has several advantages (see above) but it has one drawback, the low capacity due to the small amount of the extractant that has to be taken into account when separation procedures are developed. A high

REVIEW

concentration of cations and anions in the load solution can result in poor chromatographic separation. To avoid the deleterious effect of the sample matrix, a semiselective preconcentration step is frequently included to remove possible sources of interferences prior to EC (e.g., coprecipitation of actinides with Ca phosphate, Ca oxalate, Ca fluoride, ferric, or ferrous hydroxide). Separations Using Stacked TEVA, UTEVA, TRU, and DGA Resin Columns. It was proposed by Horwitz et al.171 already in 1995 to use different columns together for the separation of the actinides. He recommended the use of the TEVA-UTEVA-TRU column set where the TEVA column separates Th and Np, UTEVA is used for U, and Am and Pu are separated on TRU. In another version, the first TEVA column retains Pu, Np, and Th while U is separated by UTEVA and Am on TRU. Beside the three-stage methods recommended by Horwitz et al. Pilvi€o et al.201 reported about the use of TEVA-TRU-UTEVA columns for the determination of Pu, Am, Th, Np, and U. These methods have not received widespread application probably because of the need for many different EC materials. Smith et al.202 analyzed actinides in soil samples using TEVA and TRU columns, and Moody et al.96 applied a similar method for human tissue samples. Maxwell devised a rapid method for the separation of all actinides using TEVA-UTEVA columns,203 later proposed also TEVA-TRU tandem columns, and most recently used TEVA-TRU-DGA columns for the separation of all actinides from water196 and soil49 samples up to 5
10 g49 or 100
200 g.182 (see details above in paragraph on DGA resin.) In the most recent combined procedure, Maxwell197 applied a TEVA-DGA column set for the determination of Pu, Np, and U in water (see details above in paragraph on DGA). Separations Using UTEVA-TRU Tandem Resin Columns. Several other multistage EC methods were proposed, but the one which became the most frequently used is based on the tandem column set of UTEVA-TRU. Thakkar83 described the detailed procedure in 2001 as the rapid sequential separation of U, Th, Pu, and Am. Actinides were preconcentrated with calcium phosphate (pH = 9), then dissolved in 3 M HNO3/1 M Al(NO3)3. Pu was reduced to Pu(III) with ferrous sulfamate and ascorbic acid. The sample was loaded on the tandem column where UTEVA was the first and TRU was the second column. After loading and washing, the columns were separated. Th and U were removed from the UTEVA column, eluting them subsequently with 5 M HCl/0.05 M oxalic acid, respectively. On the TRU column, Pu was oxidized to Pu(IV) with 2 M HNO3/0.1 M NaNO2, then Am and Pu were stripped after each other with 4 M HCl and 0.1 M ammonium bioxalate, respectively. This procedure resulted in high recoveries for Pu (>80%), Am (>72%), as well as for U (>79%), and also relatively high DFs (in the Pu fraction DF for U > 104). This simple and fast procedure was frequently used after minor changes in different laboratories. Eichrom Technologies, Inc. developed standard procedures for the determination of Am, Pu, and U in urine (ACU02) and in water (ACW03) that were based on the same principle. The latter one was accepted as a standard HASL procedure: SE03. LaRosa et al.91 compared the traditional methods with new EC ones and found that the use of EC in environmental analyses was supplementing or even replacing the old procedures. He confirmed that Pu and Am in large volumes of seawater (50
500 L) could be properly analyzed by the UTEVA-TRU tandem column procedure after preconcentration of actinides by coprecipitation with MnO2 and Fe(OH)3 and optional removal of Si as SiF4. Am and Pu sources had high recoveries (>90%), somewhat higher than in the conventional 4702

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

Analytical Chemistry procedure, and the excellent quality of R spectra had no visible contamination when 7
15 L of seawater was treated. Varga et al.87,88 developed a rapid procedure for the sequential determination of Am and Pu by R spectrometry and ICPMS. Actinides were selectively coprecipitated from 0.5 to 3 g of soil, sediment samples with CaF2, and the UTEVA-TRU procedure was applied. High chemical yields (81
94%) and DF for U (>104) were obtained. Jakopic et al.35 compared the UTEVA-TRU method with the conventional anion exchange one after calcium oxalate coprecipitation and found that the new procedure was superior by obtaining higher Am and Pu recoveries. Pilvi€o95 used the EiChrom procedure for the separation of Pu, Am, and U from bone ash samples after a preconcentration step that was performed with a TRU column. High recoveries (Pu, 81%; Am, 76%; U, 86%; Th, 78%) and contamination free R sources were obtained. The EC procedure was found to be faster and more economic than the conventional anion exchange method. Mellado et al.204 determined Pu, Am, U, and Th in marine sediments by the same method, but chemical recoveries were lower (for Pu 40
60%, for Am 106

U> 100 000

DF

Am

1.7 mBq/kg

0.1 Bq/L or 40 Bq/kg with off-line counting

LD

70%

60
100%

>85%

79%

>70%

not determined

yield

00.13 mBq = 5.10
10 mg

0.5 mBq/L instrument

LD

U: 1400
50 mBq/ 7700, mL (Pu in Np: 32
95%)

>103

DF

Np

85%

100%

86%

yield

>95%

Po > 100 000

U> 100 000

DF

Pu

0.01
2 pg/ mL

1.2 mBq/ kg

0.1 Bq/L or 40 Bq/kg with off-line counting

LD

Analytical Chemistry REVIEW

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

4709

Horwitz, E. P.

Jha, S. K.

82

74

Kalmykov, St. N.

Kenna, T. C.

Kim, C. K.

114

41

150

Joe, K

Holm, E.

67

144

Holm, E.

111

Jia, G.

Holm, E.

146

108

first author

ref

Tc, Np, Pu, Am

Pu, Np

Np, Pu

Np, Pu, Am

Pu, Am

Np

An, gross R

Np (Pu, Am, Tc, Cs)

Am

Am

analyte

Table 2. Continued

seawater

urine

sea waer, sedim.

moss, lichen high bumup PWR spent fuel

R: 10 days

gross R (or Am and Pu)

NAA: 5  1013 n/ cm2, 65 h

R

R (γ)

Q-ICPMS SF-ICPMS comparison

ICP-SFMS (low resol.), TIMS sed.

environm., sedim.

seawater, sed., biol.

R

Np by RNAA: 15
20 h, 1013, Pu: R, LSC

envir.

matrix

R

measureme nt method

5
25 g

5
10 g

0.5 g

2g

100
200 L, 100 g

600 mL

1800 L

200 L, 100 g, 500 g

amount

sample preconcentration

HCl, HNO3, H2O2

TRU (ABBEX)

LLE: TOPO

EC: TOA

anion-HCl; anion-acetic acid

TEVA repeated

UTEVA: Np from Br, Na, Au

anion (Pu,Np); EC: HDEHP repeated

EC: HDEHP, PMBP/TOPO

Fe(OH)3
anion, LLE:TTA Na2SO3

Ca phosphate

aqua La(OH)3 regia HF, in HCl
H2 NH2OH O2, HNO3

1:1 HNO3

HCl

cc. HNO3 digestion, 3h

anion, HDEHP, cation.

anion, cation, anion

separation

anion (20 cm), Ca
Mg hydroxide, LaF3
Np, carbonate hydroxide-Np., (pH LLE: TTA 9
10)

Ca(Mg)CO3, Fe(OH)3

Fe(OH)3 ash: 500 °C, leaching: HNO3 þ HCl þ H2O2

digestion

70%

Cm: 96%

40
80%

30
95%

yield

high DF for Pb

Po: 105, no Pu, Th interference

DF

Am

34 mBq/kg

LD

70%

preirr.: 90%, postirr.: 90%

100%

?

94%, gross R yield >90%

20
50%

yield

DF

Np

Q-ICPMS: 0.01 mBq/ mL; ICPSF-MS:

0.5 fg/g

measured values: 0.1
500 mBq/m3, 1
80 mBq/kg

measured values 1
10 mBq/m3

LD

90%

81
94%

70%

94%

yield

DF

Pu

28 mBq/kg

LD

Analytical Chemistry REVIEW

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

4710

Maxwell, S. L. Pu, Am

Maxwell, S. L. Pu, Np

182

197

Pu, Am, Cm

Np, Pu

Np, (Pu)

lichen

bioassay (urine feces)

sed., biol.

marine samples

forest litter

test: NIST SRM 4341

15 g

1L

7
13 g

amount

soil

R

ICP-Q-MS, water R

seawater

R

200 mL

200 g

100 L (600 L)

SF-ICPMS, marine 20 g R environ.: weed, water

HRICPMS, R

Sr, Th, R Pu, U, Am

Pu, Th, U, Am

Lovett, M. B.

Lindahl, P.

39

ICPMS (R)

U, Am, R Pu, Sr, Cs

Np (Pu)

high burnup fuel solution

matrix

sample

CE coupled groundto ICPMS water, test sample

R

measureme nt method

Np from U, R Th, Pu, Co, Cs, Sr

76

Lindahl, P.

Lee, S. H.

199

147

LaRosa, J. J.

91

Lee, Y. K.

La Rosa, J. J.

42

131

La Rosa, J.

178

Kuczewski, B. Pu, Np

208

Np

Koyoma, S

24

analyte

first author

ref

Table 2. Continued

NdF3/ Np(IV) to separate U

preconcentration

UTEVATRU

yield

anion, TEVA, 67
92% anion, anion, Ca oxalate, TRU, anion

MnO2
Fe(OH)3

anion-8 M HNO3; TEVA

NdF3/Np(IV), Pu(III), TEVA: Np purification

capillary electrophoresis is (CE) using fused silica

anion-8 M HNO3; anion- 12 M HCl

separation

leaching, fusion NaOH

aqua regia

cc. HCl

Ca phosphate (pH 9.5)

TEVA(Pu
Np) DGA(Pu
U)

Ce(OH)3, TEVA, TRU, CeF3 DGA, Am/Ln (H2O2 to separation: TEVA oxidize U)

Fe(OH)2, anion-9 M HCl, Ca oxalate, anion-8 M HNO3, BiPO4 BiPO4, (Am) anion, cation

anion-HCl (Np) anion (Pu), LLE: TTA Pu, U, Po, Ra g 105, Th, Bi, Rn g 104 76
98

1 mBq/kg

25
60%

27
89%

19
54%

30
80%

yield

LaF3 -Np(IV), Fe(OH)3-Pu

1 mBq/sample

high DF

LD

70
90%

42
77%

no cont.

90%

DF

1 M HCl/ anion-HCl, Fe2þ; LaF3 LLE: TTA (Np)

wet ashing carbonate anion-9 M HCl, Fe- 45
100% ppt (pH 8), (OH)3, anion-8 M (HNO3, HCl, HF) Fe(OH)3 HNO3, cation, TRU

acid destruction or leaching

HF, HNO3, HCl

dissolution 8 M HNO3 (HF)

digestion

Am

U > 104

0.1% of U interfered with Np

Pu: 106
7

DF

Np

R: 0.8
8 mBq/L; ICPMS: Np 1 ng/ L, Pu 0.3 ng/L

50 ppb

LD

Pu(R): 95%

61
92%

64
81%

59
100%

74
93%

yield

0.5 mBq/ sample

high DF

LD

U > 106

1 mBq/kg

Am, U, Po g 105

no cont.

90%

DF

Pu

Analytical Chemistry REVIEW

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

4711

Osv
ath, S.

26

Np, Pu, U, Th, 93 Zr

Np, Pu, U, Am

Osv
ath, S.

Pu, Am, Sr

28

Moreno, J.

135

R

water

waste

waste

R, NpICPMS

ICPMS, R

nuclear material

soil

environm.

sediment

ID-NAA: 237 Np, 238 U; ID-R: Pu; γ: 239Np

TIMS

R

Np and Pu, R Th, Am, Sr

Pu, 238U, 239 Np, 237 Np

Moreno, J.

187

Niese, U.

Moody, C. A.

96

Pu, Np, (Am)

142

Michel, H.

29

soil, sed.

Sr, Cs, Th, R (þ U, Pu, Am ICPMS for Pu ratio)

100 mL

100 mL

10 g

10 g ash

5g

2
5 g

20
50 g

amount

sample

synthetic, IAEA 133

Morgenstern, U, Pu, A. Np, Am

Michel, H.

78

matrix

R: Pu þ Np soil

measureme nt method

Th, U, Am, R Pu þ Sr

25

Mellado, J.

204

Maxwell, S. L. Pu, Np

179

analyte

first author

ref

Table 2. Continued

acid digestion

acid digestion

anion-8 M HNO3

UTEVA, TRU for Am

Fe(OH)2
UTEVA repeated Zr(OH)4

Fe(OH)2

50
200 mL

UTEVA

anion, Ca oxalate, TRU

anion- 8 M HNO3

DF

Am

low varying: 89%

Sr Resin, UTEVA >65% (U, Th), anion Pu), anion, cation, TRU, TEVA (Am)

UTEVA, TRU, Sr resin

HNO3
HCl

separation

TRU (2.5 TEVA-TRU, M HNO3) TEVA, TRU

leaching: 8 Fe(OH)3 M HNO3

1. leaching: Ca oxalate HNO3 þ Fe(OH)3 HCl 2. acid destr. HF, HNO3 3. microwave

microwave: HNO3, HF, HClO4

acid leaching:

digestion

preconcentration

anion- 8 M HNO3

LD

66
99% (Zr: 31
99%)

66
93%

93%

varying, reduced at big sample mass

75%

yield

U > 2000

DF

Np

Np losses at high matrix content

LD

65
82%

40
60%

93%

>80%

>89%

89%

75
95

40%

yield

DF

Pu

U > 2000

82%

LD

Analytical Chemistry REVIEW

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

4712

Rameback, H. Np, Pu, Am
Cm

112

Rosskopfova, O.

Schneider, R. A.

Sidhu, R. S.

Sill, C. W.

97

103

93

52

Rosner, G.

Qu, H.

98

143

Qiao, J.

38

Riglet C.

Pimpl, M.

32

105

Pilvi€o, R.

201

aerosol, sed.

environ.

spent fuel solution seawater

environment al

R, γ, X-ray (235 Np, 239 Np)

R

R

R

239

from Ra R through Cf

Pu, Am

Np

Pu, Am (Sr)

239

Np, Np

enriched U solution

bentonite clay

Q-ICPMS, compared with R

ICPMS (Np, Pu), R (Am, Cm), γ

human tissue

50 g

200 L

10 g soil

1g

0.1 g

Diphonix

ActinideCU Resin

Fe(OH)3/ K2S2O5

preconcentration

MnO2
Fe(OH)3

LLE: 10% TOA/ xylene repeated

Pu, Am TRU; Sr: oxalate, Sr resin

anion-9 M HCl anion

LLE: 0.5 M TTA/xylene

LLE_HDEHP repeated

anion-Pu, TRU-Am

anion-8 M HNO3

LLE: TOPO; TRU, TEVA

TEVA, TRU

LLE: 0.5 M HTTA/xylene

separation

fusion: KF, water: LLE: Aliquat-336 pyrosulfate Fe(OH)2; others: BaSO4

leachate

1M HNO3
KBrO3 leach

aqua regis

leaching: 8 M HNO3

up to 500 g HNO3/ H2O2

50
100 g

R

soil

R

U 3 O8 dissolution in 3
4 M HNO3

digestion

0.5
100 g

fuel (Pu, Am) SEM (U,Th)

R

0.2 mL

amount

SI-ICPMS environ. (9 parallel solid samples)

spent fuel dissolver (U, Pu, Am, Fe)

matrix

ICP-OES, γ

measureme nt method

sample

Np

Pu, Am

Pu, Np

(Pu) Am, Cm

Pu, Am

Np

Pant, D. K.

106

analyte

first author

ref

Table 2. Continued

TRU

70
80%

97%

>80%

31
75%

>90%

yield

85%

DF

Am

0 4 mBq

0.0015 Bq/sample, 30 mBq/kg (50 g soil)

LD

>90%

72
83%

82%

>79%

99%

yield

Pu: 2000
10000

U: 103
104

DF

Np

50 g soil: 0.2 fCi/g = 7 mBq/ kg

1 mBq

5 ng Np/g U (ICPMS), 2 ng/g U (R)

237 Np 0.01 mBq

1.5 pg/L

8
10 mg/ L

LD

37
93%

91%

>70%

>79%

>90%

yield

78%

DF

Pu

no cont.

239Pu: 11 mBq

LD

Analytical Chemistry REVIEW

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

4713

Vance, D. E.

Varga, Z.

Wang, J.-J.

Yamamoto, M.

120

87

200

37

Vajda, N.

Thakkar, A. H.

83

72

Tavcar, P.

58

Vajda, N.

Sumiya, Sh.

68

21

Spry, N.

188

Truscott, J. B.

Sill, C. W.

62

54

first author

ref

ICPMS

1g

envir., bioassay

R

R, γ for 239 soil, sed., Np, LSC water

Sr, Th, Pu, U, Am

Np, 241Pu

samples of high U content

0.5
3 g

soil, sed., 5 g, 50 L spent resin, water

soil, sed.

biol. soil

soil, sed.

Q-ICPMS, R

R

30 g

water, urine 2 L

ICP-SFMS, R

Am, Pu

Np

Pu, U, Th, Np

Pu, Am, U, R Th

Th, Np, U, Pu, Am

U, Pu, Am, R Th, Np

Ac, Np, Sr

0.1 L

amount

sample

spent resin

ICPMS (γ)

Np, 99Tc, Pu

R

NPP effluent

R

Np, Ce, Ba, lA

Pu, Am, Sr

matrix air, dust, water, mineral, biol. samples

measureme nt method

γ

analyte

Table 2. Continued

Fe(OH)3

aqua regia

microwave

wet ashing

LiBO2 fusion

microwave/dry and wet ashing

>85%

82%

89%

>72%

54
72%

60%

yield

anion-8 M HNO3, 37
68% Ca oxalate-SR resin, oxalate-TRU, Chelex 100

UTEVA, TRU

LLE: t-amine repeated, Np in aqueous residue

UTEVA, TRU

TRU

TRU: Pu/U

UTEVA, TRU columns and cartridges

anion, TRU, (Sr resin)

anion-HCl, TTA, anion-HNO3, anion-acetic acid

TRU Sr Resin-Sr

Np separation as soluble Np(VI) from BaSO4

separation

Fe removal anion-8 M HNO3 with ether

CaF2 in NH2OH

Fe(OH)2

CaF2/ N2H4

120 mL cc. Ca phosHNO3 phate (pH = 9)

Fenton’s (H2O2 þ Fe2þ)

HNO3 evaporation

digestion

preconcentration

>5800 U, Pu, Th, Np

DF

Am

6
8 mBq/ sample

23 fg g
1

0 6 fg 241Am

0.11 Bq/L

LD

soil: 51
78%, water: 70
82%

91%

97.6%

yield possible La, Zr interfer.

DF

Np

0.1 mBq

ICPMS: 0.037 Bq/ L; R: 0.65 Bq/L

5.10
5 mBq/mL,

LD

59
85%

81
94%

50
70%

75%

>94%

83
88%

56%

yield

DF for U 10000

U 7300

DF

Pu

4
5 mBq/ sample

15 fg g
1

μBq/L in case of 50 L

0.83 fg 239 Pu

Pu: 1.10
2 mBq/mL

0.04 Bq/ L 239,240Pu

LD

Analytical Chemistry REVIEW

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

Analytical Chemistry

U > 104

0.012 mBq/mL

0.1 mBq (10000 s)

no Po interference

>80%

water 72
82%, soil 51
79%

TOA/Teflon

>75% anion(Cm) HNO3, Fe(OH)3, anionHCl, anion dependence)

Ji, Y.-Q. 167

Np

ICPMS

environm.

aqua regia HF

Fe(OH)3, Ca oxalate (pH envir. 50 g (seawater), (200 L) biol. Yamato, A. 66

Am, Pu

R

leaching: HNO3, H2O2

Fe removal anion-8 M with ether HNO3 aqua regia or 10 M HCl or 10 M HNO3
1 M HF 50
200 soil, sediment R Np Yamamoto, M. 36

Abbreviations: sed. = sediment; biol. = biological material; environ. = environmental sample; anion = anion exchange resin; cation = cation exchange resin; interfer. = interference; cont. = contamination; LD = detection limit; IC = ion chromatography; EC = extraction chromatography.

a

>70%

DF DF DF amount analyte first author ref

Table 2. Continued

measureme nt method

matrix

sample

digestion

preconcentration

separation

yield

Am

LD

yield

Np

LD

yield

Pu

LD

REVIEW

’ COMPARISON OF RADIOMETRIC AND MASS SPECTROMETRIC MEASUREMENT TECHNIQUES Many radiometric and mass spectrometric methods are available for the determination of trace quantities of actinides in various sample types. Radiometric Methods. R spectrometry can provide complete isotopic information about R emitting Pu isotopes, 238Pu, 239þ240 Pu, and radioactive Pu tracers, e.g., 242Pu, 236Pu, are also detected straightforward in the same spectra. β-emitting 241Pu cannot be detected by R spectrometry but can be easily measured after a simple procedure to process the R source by LSC.186 By standard R spectrometry, the overlapping peaks of 239Pu and 240 Pu cannot be resolved. R spectrometry as a radiometric method is especially adequate for the measurement of those Pu nuclides that have high specific activities, i.e., this is the most sensitive technique for the measurement of 238Pu. R spectrometry is not regarded as a sensitive technique for the determination of 237Np due to its low specific activity. It can be determined with much higher sensitivity by RNAA (0.01 mBq), but the procedure requires pre- and postirradiation chemical operations, a high flux reactor, and a γ spectrometer and can be regarded as neither simple nor inexpensive. 237Np determination is further encumbered by the lack of an appropriate tracer. R spectrometry is the method of choice for the sensitive determination of 241Am as well as Cm nuclides due to their high specific activities. The 243Am tracer can also be detected straightforward in the same spectra. γ spectrometry is a simple method for the determination of 241Am, but absorption and selfabsorption of the weak γ radiation has to be taken into account, and the sensitivity of γ-spectrometry is relatively low because of the low counting efficiency and the high background of the γ spectrometers. R spectrometry is the most frequently used technique for the routine determination of R emitting transuranium isotopes due to the relatively high sensitivity, selectivity, and the relatively lowcost instrumentation that is well adopted for high-throughput routine analyses using multichamber spectrometers. Detection limits around 0.1 mBq can be achieved by routine measuring techniques. When better resolution or higher sensitivity is required, mass spectrometric systems sometimes offer better possibilities. ICPMS. Recently, ICPMS has become an alternative technique for the determination of Pu isotopes and the 239Pu/240Pu isotope ratio. During the last 10 years, many papers have been published on the comparative evaluation of R spectrometry and ICPMS.50,87,138,161,166,224 The detection limit of a typical quadrupole ICPMS (Q-ICPMS) is 10
100 fg/mL,101 which is in the range of the sensitivity of R spectrometry for 240Pu (t1/2 7000 years), if 0.1
1 mBq Pu is concentrated in a 1 mL sample solution. It means that standard ICPMS should possess higher sensitivities for radionuclides only if the half-life of the nuclide is longer than 1000
10 000 years. In general, the sensitivity of Q-ICPMS for 239Pu and 240Pu is typically not higher than that of R spectrometry, and the basic advantage is that the 239Pu/240Pu ratio can be measured. On the other hand, the short-lived 238Pu can be determined only by R spectrometry, which means that the two techniques are complementing and not replacing each other. The detection limit of the high-performance sector field (SF) and multicollector (MC) ICPMS facilities is about 1 to 2 orders of magnitude lower than that of Q-ICPMS, which makes ICP-SFMS superior also to R spectrometry for those nuclides that have 4714

dx.doi.org/10.1021/ac2008288 |Anal. Chem. 2011, 83, 4688–4719

Analytical Chemistry half-lives longer than about 100 years (if there are no isobaric interferences). Nonetheless, the overall sensitivity of both measurement methods is influenced by chemical processing (sample amount and DFs). The major interference in 239Pu determination is 238U due to tailing and formation of UHþ, and the overall sensitivity of the method depends on chemistry that has to ensure the high DF for sensitive analysis of 239Pu. In general, comparable detection limits were obtained by R spectrometry and ICP-SFMS for 239,240Pu (about 0.1 mBq). ICPMS is the most often used technique to determine low levels of 237Np. The main interference is 238U tailing due to the usual high concentration of natural U in the environment. Several polyatomic interferences are also possible. Detection limits of 1
100 nBq have been reported99 using ICP-SF-MS after radiochemical separation, which is better than the best values reported by RNAA and much better than the lowest detection limit in R spectrometry. Several papers have been published on the comparative evaluation of radiometric and ICPMS techniques.20,27,39,42,73,105,112,120,147,193,197,225 Although mass spectrometric techniques have generally low sensitivity for the “short-lived” 241Am, recently high sensitivity SF-ICPMS has become available for 241Am analysis and a few papers have been published on the comparative evaluation of R spectrometry and ICPMS.50,87,161 The detection limit of the SFICPMS, about 1 fg/mL,100 equals about 0.1 mBq/mL 241Am, which is in the range of the sensitivity of R spectrometry for 241 Am if 0.1 mBq Am is concentrated in 1 mL of sample solution. Regarding the limit of detection, high-performance ICPMS is comparable to R spectrometry if there are no interferences. The main difficulties in ICPMS originate from isobaric and polyatomic interferences such as 241Puþ, 240Pu1Hþ, 206Pb35Clþ, 204 Pb37Clþ, and 205Ti36Arþ. The major isobaric interference of 241 Am is 241Pu that has an order of magnitude shorter half-life, but typically the mass of 241Pu in samples of reactor origin is comparable or higher than that of 241Am. Hence, the determination of 241Am by high-sensitivity ICPMS is possible if high DF for Pu is assured by proper chemical separation that also removes most of the stable elements that form the interfering molecular ions. In general, good agreement was found between R spectrometric and ICPMS results. Other Mass Spectrometric Techniques. Beside ICPMS, there are other highly sophisticated MS techniques, namely, thermal ionization MS (TIMS), accelerator MS (AMS), and resonance ionization MS (RIMS) that are more sensitive for the detection of several transuranium nuclides. TIMS has higher sensitivity for 239Pu and 240Pu determination than ICPMS, and interferences due to U traces are less significant, but it requires a tedious sample preparation to produce the measurement source in the form of a thin filament. RIMS is highly sensitive for 239Pu down to the level of 107 atoms per sample, and the selective excitation of Pu atoms by tuned laser beams does not necessitate extremely selective chemical separation (Roos226). AMS is the most sensitive detection technique for 239Pu (down to about 106 atoms per sample that refers to 0.001 mBq) and has the highest abundance sensitivity, as well due to the lack of molecular isobars (Fifield227). However, the application of these MS techniques is limited by the limited number and availability of very sophisticated facilities. Chartier et al.228 determined Am and Cm in spent nuclear fuel and compared ICPMS with TIMS. He demonstrated that TIMS is the most accurate method for isotope ratio measurement, but

REVIEW

ICPMS results agreed with those of TIMS within 2.5
5.0% depending on the concentration ratios. AMS has also been used for the determination of 237Np, and a detection limit of 0.007 mBq has been reported.100 AMS and RIMS are in principle applicable for the detection of 241Am, but they all suffer from a relatively low detection limit for “shortlived” radionuclides and the high cost of the facilities (Roos,226 Fifield227). Several papers have been published recently about the critical comparison of the radiometric and mass spectrometric techniques, including the comparison of R spectrometry and ICPMS14,100
102,229 as well as AMS34,172,227 and RIMS230 pointing out the advantages and drawbacks of the techniques, but good agreement of the results was confirmed whenever sample preparation method and sensitivity range of the given facility was suited to the analytical demands. Lee et al.224 compared the R spectrometry, LSC, ICPMS, AMS, and TIMS techniques by analyzing Pu isotopes in some environmental samples and concluded that 239,240Pu results by R spectrometry agreed with those obtained by ICPMS and AMS and results of 241Pu determined by LSC agreed with those measured by ICPMS.

’ CONCLUSIONS The radiochemical separation procedures developed and applied for sample decomposition, preconcentration, and selective separation of transuranium nuclides using precipitation, coprecipitation, ion exchange, solvent extraction, and extraction chromatographic techniques have been reviewed in chronological order. The use of anion exchange resins for the selective separation of anionic complexes of Pu and Np forms the basis of many analytical procedures including various standards. Recently, commercially available extraction chromatographic materials started to replace the traditional resins. New procedures have been developed and tested for the determination of Pu and Np, often simultaneously with Am and other actinides. The new organo-phosphorus materials, e.g., TRU and UTEVA resins, and several other extractants offer extremely high selectivity for certain actinides. EC also has the advantages that exchange kinetics are fast, less reagents are used, and less hazardous wastes are produced. The new radiochemical separation procedures provide sources of good quality, high chemical recoveries, high DFs for many nuclides, and thus separation techniques are often equally adequate for R and mass spectrometric analyses. Nuclear measuring techniques applied for the detection of various actinides have been comparatively evaluated. It was pointed out that R spectrometry due to its low-cost instrumentation, acceptable high sensitivity, and selectivity is still a useful tool in the routine determination of the total activity of 239Pu and 240 Pu, and it is the most sensitive technique for the detection of 238 Pu, 241Am, and Cm isotopes. For 237Np determination, R spectrometry proved to be less sensitive technique and RNAA a more sensitive technique. When higher sensitivities are required for 239Pu, 240Pu, 237Np, and 239Pu/240Pu ratio measurements, then high-performance mass spectrometric techniques such as ICPMS and moreover TIMS, RIMS, and AMS are superior. R spectrometry will remain a frequently used technique in routine analysis of many transuranium nuclides. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. 4715

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Analytical Chemistry

’ BIOGRAPHIES N
ora Vajda studied chemical engineering at the Technical University of Budapest, Hungary (BME), where she received her Diploma, Doctor, and Ph.D. degrees in 1976, 1986, and 1995, respectively. She worked at BME for 30 years and finally as an associate professor at the Institute of Nuclear Techniques. Between 1989
1990 and 1992
1993, she worked at the Seibersdorf Laboratory of the International Atomic Energy Agency (IAEA) as a fellow and a scientist. She was a consultant many times at the IAEA and participated in several IAEA organized training courses and missions. From 2008, she is the head of the Radiochemical Laboratory of RadAnal Ltd., Hungary. Her current research activities include the development and application of new analytical techniques for the determination of “difficult-to-determine radionuclides”. Chang-Kyu Kim studied chemistry at the University of Tsukuba (Japan), where he received a Ph.D. degree in 1990. From 1990 to 1998, he was a principal researcher at the Environmental Radiation Assessment Unit, Korea Institute of Nuclear Safety, Korea. He became a project manager in the same institute to develop advanced analytical techniques for the measurement of ultratrace level long-lived radionuclides in environmental samples using high-resolution ICPMS. From 2004 to 2010, he worked as an environment radiochemist in the Terrestrial Environment Laboratory, IAEA Environment Laboratories, Seibersdorf, Austria. During the period, he was in charge of the development of IAEA recommended procedures of radionuclides in environmental samples and the coordination of IAEA ALMERA (Analytical Laboratories for the Measurement of Environmental RAdioactivity). Since February 2011, his current activities include the development of new analytical techniques which can be used for nuclear forensic and emergency cases in the Living & Environment Radioactivity Assessment Laboratory, Korea Institute of Nuclear Safety, Korea. ’ ACKNOWLEDGMENT This work was financially supported under the IAEA subprogram “Provision of reference products for terrestrial environments and laboratory performance support”. ’ REFERENCES (1) B
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