Analytical Methods for the Determination of 90Sr and 239,240Pu in Environmental Samples

Artificial long-lived radionuclides such as 90Sr and 239,240Pu have been long released into the environment by human nuclear activities, which have a profound impact on the ecological environment. It is of great significance to monitor the concentration of these radionuclides for environmental safety. This paper summarizes and critically discusses the separation and measurement methods for ultra-trace determination of 90Sr, 239Pu, and 240Pu in the environment. After selecting the measurement method, it is necessary to consider the decontamination of the interference from matrix elements and the key elements, and this involves the choice of the separation method. Measurement methods include both radiometric methods and non-radiometric methods. Radiometric methods, including alpha spectroscopy, liquid scintillation spectrometry, etc., are commonly used methods for measuring 239+240Pu and 90Sr. Mass spectrometry, as the representative of non-radiometric measurement methods, has been regarded as the most promising analytical method due to its high absolute sensitivity, low detection limit, and relatively short sample-analysis time. Through the comparison of various measurement methods, the future development trend of radionuclide measurement is prospected in this review. The fully automatic and rapid analysis method is a highlight. The new mass spectrometer with ultra-high sensitivity shows strong analytical capabilities for extremely low concentrations of 90Sr, 239Pu, and 240Pu, and it is expected to develop determination methods with higher sensitivity and lower detection limit.


Introduction
With the progress of nuclear energy technology, environmental safety, especially nuclear environmental safety, has become one of the hot issues of common concern for the academic community and the public. Due to human activities, such as nuclear tests, reprocessing of spent nuclear fuel, and nuclear accidents, different artificial radionuclides have been released into the environment. 90 Sr, as a representative of typical artificial radionuclides with high fission yield and long half-life, is easy to be concentrated in the bones and teeth after ingestion and inhalation by the human body, leading to bone cancer, leukemia, etc. [1]. As an extremely toxic transuranic element with a long halflife of thousands of years, Pu is not only harmful to the environment but also will be enriched in organs after entering the human body [2,3]. As shown in Table 1, the 239,240 Pu released by the nuclear weapon tests exceeded 10 16 Bq, and the 90 Sr was close to 10 18 Bq, whose radiation pollution increasingly attracts the public's attention. These radionuclides in the environment can be transferred to the biosphere by dispersion, sedimentation, migration, and accumulation in living organisms through the food chain. It is important for radioactive pollution assessment to monitor the concentration levels of these radionuclides in environmental samples. Table 1. Source and total release of artificial radionuclides (PBq). Adapted with permission from ref. [4].

Radionuclide
Weapon The uptake of radionuclides from soil to plants is generally quantified by the ratio of activity concentrations in plant and soil, namely the transfer factor (TF). In addition to radionuclide concentration, one of the factors influencing the TF is the chemical form of the radionuclides, including oxidation state and formation of complexes [5]. The study of the distribution and migration behavior of 90 Sr and 239,240 Pu in the environment is important work for radioecology. The migration ability of 90 Sr is very strong. The TF of Sr from soil to vegetables varied with species, higher than that of other heavy metals. Among them, the value in cabbage was as high as 0.33 [6,7]. The concentration factor (CF) indicates the ability of a biological organism to concentrate radionuclides and is defined as the ratio of the radionuclide content in the organism to the environmental medium. The CF of 90 Sr by marine organisms was between 0.1 and 10, and it was easy to deposit in shellfish shells, fish scales, and bones [8]. The solubility and mobility of Pu (IV) are relatively low in the pH ranges of 4-10 relevant to the environment ([Pu (IV) aq ] = 10 −10 − 10 −8 mol·L −1 ) [9], while some processes and mechanism, such as the adsorption or complexation of Pu, can increase the mobility. The Pu transport indices for Brassica juncea and Helianthus annuus increased by addition of diethylenetriaminepentaacetic (DTPA) concentration to prepare Pu-DTPA in the nutrient solution [10]. The solution-to-plant TF for the potato tubers was 0.03-0.80, and the Pu uptake from solution greatly increased by addition of the complexing agent EDTA [11]. Generally, each type of nuclear emission has its own unique isotopic composition, that is, "fingerprint" information ( Table 2). Pu in environmental samples is usually derived from global fallout with the value of 240 Pu/ 239 Pu about 0.18. For example, the isotope ratio of 240 Pu/ 239 Pu obtained by Kenna was 0.161684 ± 0.001084 [12]. Understanding the concentration and isotopic composition of 239 Pu and 240 Pu in the environment can provide valuable information about nuclear activities in the affected areas. Therefore, accurate measurement of these radionuclides is very important for environmental risk assessment. The complex matrix of environmental samples with the diverse and unknown existing forms of radionuclides and the trace or ultra-trace concentration level makes accurate measurement quite difficult. Researchers have tried and improved many measurement methods [19][20][21][22][23]. The purpose of this article is to review the separation methods of 90 Sr, 239 Pu, and 240 Pu that have been released in the environment, and discuss commonly used radiometric and non-radiometric measurement methods. By comparing the capabilities of various mass spectrometry technologies, focusing on ICP-MS, the future development of 90 Sr, 239 Pu, and 240 Pu measurements could be deduced.

Sample Preparation
In radiometric technology of samples, the analysis method can be divided into three steps: sample pretreatment, radiochemical separation (concentration, separation, and purification), and source preparation and measurement. The process is shown in Figure 1. Ultra-trace amounts of the target nuclides 90 Sr, 239 Pu, and 240 Pu in environmental samples need to be separated from a large amount of matrix and interfering nuclides, and this is a great challenge for sample preparation and measurement methods.

Sample Pretreatment
The pretreatment is the most critical step when measuring the contents of 90 Sr, 239 Pu, and 240 Pu in environmental samples. The selection of pretreatment method depends on the purpose of analysis, the method of measurement, and the type of sample. The purpose of pretreatment is to destroy organic matter and convert most of the target nuclides into solution phase, so as to facilitate the subsequent separation operation. Interfering substances should be effectively removed without cross-contamination. Choosing the appropriate pretreatment method and shortening the pretreatment time directly affect the precision and accuracy of the analysis results. At present, the commonly used pretreatment methods mainly contain dry-ashing, digestion, melting, etc.
The traditional dry-ashing method usually uses electric heating plates and muffle furnaces [24][25][26] to dry, carbonize, and ash solid samples (soil, plants, etc.). In order to shorten the ashing time, ashing aids such as HNO 3 , H 2 SO 4 , and H 2 O 2 can be added. For animal samples that are more difficult to ash and milk samples that contain more water, the microwave-ashing method has more advantages [27]. The powdery sample has a bulky texture and is not easy to ash. The rising of dense smoke during carbonization and the wind of the fume hood will cause partial loss of the components. Adding a small amount of ultra-pure water to the powder sample can effectively improve this phenomenon and enhance the accuracy and reliability of the results [28]. Considering the volatilization loss of nuclides, GB14883-2016 gives recommendations on the maximum ashing temperature of food (Table 3) [29]. The wet-digestion method has been widely used for the analysis of 90 Sr, 239 Pu, and 240 Pu in various environmental samples (Table 4). In the early days, a single acid reagent was often used. Now, mixed acids or acid reagents with H 2 O 2 are often used to treat samples, such as HNO 3 + HCl (+ HClO 4 , H 2 O 2 ). The conventional wet digestion method has the advantages of rapid oxidation speed, convenient operation, and low loss rate of the nuclides, with a digestion temperature below 300 • C. However, its large amount of acid will increase reagent blanks and is not suitable for processing a large quantity of samples. The microwave-digestion method adopts the pressure principle for digestion, with the advantages of less acid consumption, short time, little loss, low blank value, and easy realization of automatic monitoring. It has been widely used and is suitable for the digestion of biological and geological samples. However, this method has less processing capacity, and it is also not suitable for processing a large number of samples, nor for samples with poor microwave penetration. At present, the microwave digestion-atomic absorption method is widely used to measure trace elements in environmental samples.
In addition, the melting method is sometimes used in the sample pretreatment process [30], such as processing the insoluble residues in the wet-digestion method, accelerating the isotope exchange balance between the tracer and the target nuclide, etc. The melting method can treat kilogram samples, and the treatment effect depends on flux, temperature, and so on.
The same batch of black-tea samples was selected as the research object to investigate the characteristics between different pretreatment methods [31]. The measurement results of different metals by ICP-MS found that the microwave digestion method was significantly better than the dry-ashing method and the wet-digestion method. At present, in order to increase the sample processing capacity, the dry-ashing method is combined with the microwave-digestion method, shortening the processing time and reducing the amount of acid. In order to monitor recovery, a carrier or tracer can be added during digestion.

Chemical Separation
The solution after sample pretreatment contains a large amount of matrix elements, which would interfere with the subsequent accurate measurement. It is necessary to use chemical methods for separation and purification to remove matrix elements and interfering nuclides. There are many chemical separation methods suitable for Sr and Pu, including precipitation, solvent extraction, ion-exchange chromatography, extraction chromatography, or a combination of these methods [36,[39][40][41][42][43][44][45][46]. We mainly focus on the ability of various methods to remove elements that interfere with the analysis of 239 Pu, 240 Pu, and 90 Sr.

Separation Methods of 90 Sr in Environmental Samples
In the sample solution obtained by pretreatment, Sr is usually pre-concentrated by precipitation or co-precipitation via the formation of insoluble nitrates, carbonates, and oxalates. The classical method of strontium precipitation by fuming nitric acid was cumbersome and time-consuming [47]. The precipitate formed by the reaction of alkaline earth metal and carbonate was decomposed with mineral acid in order to separate Sr [48]. Rao et al. [49] used different co-precipitation methods to analyze 90 Sr in environmental and dietary samples byĈherenkov counting of 90 Y.
Alkaline earth metals, such as Sr, can be well separated from other elements via the ion-exchange method. Dowex-50 is one of the commonly used cation exchangers. Calcium, strontium, and barium were separated by cation-exchange resin, using EDTA with appropriate concentration and acidity as eluent, measuring the 90 Sr in biological samples successfully [50]. The solvent extraction method is simple and convenient. The extraction rate of strontium by thenoyltrifluoroacetone (TTA) can be as high as 95%, and Bis (2-ethylhexyl) phosphate (HDEHP) can be used to separate 90 Sr from its daughter 90 Y. By using dicyclohexyl-18-crown-6 in chloroform solution extraction, a small amount of Sr 2+ can be separated from a large quantity of Ca 2+ [51].
A new analytical method was developed for the separation of 90 Sr from 90 Y by using DGA resin, in which the extractant system was N,N,N ,N -tetra-n-octyldiglycolamide (Normal) or N,N,N ,N -tetrakis-2-ethylhexyldiglycolamide (Branched) [52,53]. The working capacity of the two DGA resins is 7.23 mg Sr and 11 mg yttrium per mL of resin. Sr resin is a special resin for separation and enrichment of Sr, with strong selectivity of Sr, and can efficiently separate Sr from Ca and other elements. Sr resin can replace cation exchange resins for low-consumption (significant reduction in resin consumption and elution amount) and high-efficiency chemical separation, with the extractant of crown ether (4,4 (5 )-di-tbutylcyclohexano-18-crown-6) dissolved in octanol. The adsorption capacity of Sr enhances with the increasing of concentration of nitric acid. The study on the adsorption behavior of elements on Sr resin found that the quantitative recovery of Sr can be basically achieved by loading the column with 3 mol·L −1 nitric acid medium and desorbing with 0.05 mol·L −1 nitric acid [54]. Željko et al. [32] compared the separation effect of anionexchange resin, Sr resin, and the combination of the two methods on Sr in environmental samples (soil, vegetation, water, and animal bones) and emphasized their influence on the accuracy of 90 Sr determination, finding that the efficiency of Sr separation depended on the sample type and separation method.
The interfering nuclide 90 Zr needs to be decontaminated when using mass spectrometry to measure 90 Sr. Considering the high concentration of Zr in environmental samples, Sr resin was used for separation, with a 20 v/v% HNO 3 system for sample loading and washing to remove 86% of Zr. Meanwhile, the outflow of Sr was about 1% [23]. With deionized water, more than 98% of Sr and about 0.2% of Zr could be eluted from the resin, which could basically separate Sr from the main interfering Zr. Two-stage extraction separation by using Sr resin could further purify Sr, reducing the Zr concentration in solution to below 5 ng·mL −1 [55].

Separation Methods of Pu in Environmental Samples
It is necessary to eliminate the interference of 214 Am, 210 Po, 224 Ra, 229 Th, 231 Pa, 232 U, and 243 Am when using alpha spectroscopy to determine 238 Pu and 239+240 Pu. Moreover, the 238 UH + and 238 UH 2+ 2 peaks produced by natural uranium interfere with the measurement of 239 Pu and 240 Pu by mass spectrometry. Therefore, after selecting the measurement method, it is advisable to adopt an appropriate separation method to decontaminate the corresponding interfering elements.
Over the past few decades, extraction chromatography has attracted much attention for radiochemical separation and purification of Pu due to its short sample-processing time, small acid reagent dosage, and high selectivity and recovery. Pu in the sample solution presents multiple valence states, namely Pu 3+ , Pu 4+ , PuO + 2 , PuO 2+ 2 , and PuO 3− 5 . Pu (IV) and NO − 3 or Cl − form stable anionic complexes easily, which is the basis for separation of Pu on anion exchange resins or extraction chromatography. Therefore, it is necessary to adjust Pu to Pu (IV) before loading the column. The most common method is to use oxidants (such as NaNO 2 , IO − 3 , etc.) for one-step value adjustment. It is worth noting that, due to the ultra-trace amount of Pu in environmental samples, Pu (VI) may not be completely reduced to Pu (IV) under conventional experimental conditions, and the unconverted Pu (VI) fraction will be lost during the separation process. Therefore, the most appropriate adjustment method is to reduce Pu to Pu (III) with a reducing agent (such as I − and Fe 2+ ), and then use an oxidizing agent to oxidize the generated Pu (III) to Pu (IV) [56].
A variety of extraction resins have been developed at home and abroad, such as TEVA, UTEVA, etc., with large loading capacity and good mass transfer performance, and they are widely used in the sample separation process. TEVA resin [57] is a quaternary ammonium salt, and the stationary phase is trialkylmethylammonium chloride, with a structural formula of CH 3 N(C n H 2n+1 ) 3 Cl (n = 8-10), generally used for the separation of Np and Pu, with the best adsorption system of 2~3 mol·L −1 nitric acid [58]. Using TEVA resin to pre-concentrate Pu in a vacuum box, the decontamination factor (DF) for uranium could reach over 1 × 10 4 , and the DF for thorium and americium was more than 1 × 10 3 [59]. The Pu solution separated from TEVA resin was further purified by DGA resin with the DF for uranium of 1 × 10 6 , eliminating the interference of the uranium hydrogen peak by measuring 239 Pu with ICP-MS, and the short-life isotope 238 Pu was successfully measured by alpha energy spectroscopy [60].
Anion-exchange resins have become attractive methods due to the low purchase cost, and they are widely applied to the separation of Pu for the strong tolerance to matrix elements compared with extraction resins [61]. The commonly used anion-exchange resins are AG-1 × 8, Dowex-1 × 8, etc., with the best adsorption system of 8 mol·L −1 nitric acid. Yamato et al. [39] established a combined separation method of Pu and Am in biological samples. The Pu was adjusted to Pu (IV) to make it adsorb on the anion-exchange column, and Am flowed out with the effluent. Hayashi et al. [62] improved it and established an analysis method for 239/240 Pu in biological samples with a recovery rate of 81.6-91.8%.
The extraction behavior of amine extractants is very similar to that of anion exchangers, with the advantages of rapid extraction speed, high sorption capacity, and high selectivity to Pu (IV) in the nitric acid system. The exchange reaction between tri-n-octylamine (TOA) resin and Pu (IV) is very rapid, and TOA extraction chromatography is a standard method for the determination of 239 Pu and 240 Pu in food by alpha spectroscopy [63]. Ji et al. [64] established a process for the separation of Pu by TOA resin and compared it with the anion-exchange resin (Dowex-1 × 8), which had a slightly poorer decontamination effect.

Measurement Methods of 239 Pu, 240 Pu, and 90 Sr
Radionuclide measurement methods are divided into radioactive measurement methods (such as alpha spectrometry) and non-radioactive measurement methods (such as mass spectrometry). The former is to perform qualitative and quantitative analysis of the target nuclide by measuring the characteristic rays (α, β, or γ rays) emitted by the decay process of radionuclides. Mass spectrometry is the main non-radioactive measurement method, with improved technology, high sensitivity, good precision, and higher equipment popularity than radioactive measurement methods. The various radioactivity measurement methods of 239 Pu, 240 Pu, and 90 Sr are reviewed in this part and compared with mass spectrometry, focusing on mass-spectrometry measurement methods.

Radioactivity Measurement Methods
Different instruments are used to measure different kinds of rays [65]. When a certain nuclide emits different kinds of rays at the same time, the radioactivity obtained by measuring different rays is consistent. The main radiometric methods of nuclides in the environment are shown in Table 5. The radioactive analysis method is the most widely used analysis method for the determination of 90 Sr, 239 Pu, and 240 Pu [21,45]. 90 Sr is a beta emitter, while 238 Pu, 239 Pu, and 240 Pu are alpha emitters (Table 6). Common analytical methods for the determination of 90 Sr in environmental samples are liquid scintillation counting (LSC) and proportional counter [44,67]. Solatie et al. [68] used low-background LSC to measure the concentration of 90 Sr in soil samples near nuclear facilities, with the activity range of 6.2-96.5 Bq·kg −1 . Asgharizadeh et al. [69] measured the activity of 90 Sr in soil and sediment samples from the southern coast of Iran, with the results in the range of 0.40-3.01 Bq·kg −1 . At the 95% confidence level, the minimum detectable activity (MDA) of the LSC method within a 5-h counting period was 0.33 Bq·kg −1 . Lee et al. [69,70] measured the activity concentration of 90 Sr in the volcanic soil of Jeju Island (19.9 ± 2.1 Bq·kg −1 ) and determined that it was higher than that of the Korean Peninsula (8.3 ± 1.7 Bq·kg −1 ) by using low-level LSC. Maxwell [70] reported that the concentration of 90 Sr in the soil samples from the Fukushima Nuclear Power Plant was 1.35 mBq·g −1 by a gas proportional counter. Jabbar et al. [71] used LSC to determine that the average concentration of 90 Sr in soil samples and plant samples from Islamabad was 4.3 ± 0.0005 Bq·kg −1 and 1.05 ± 0.91 Bq·kg −1 , respectively.
There are many methods for measuring Pu in the environment. After chemical separation, Pu is usually measured by using liquid scintillation counting or alpha spectrometry. However, the separation process of these two methods is more complicated and time-consuming. It usually takes about 1 week to complete a measurement period, and sometimes even more than 10 days. At the same time, the alpha-energy values of 239 Pu (5.16, 5.14 MeV) and 240 Pu (5.17, 5.12 MeV) are very close, and this is a great challenge to the energy resolution (at least 25 keV) of the current ion-implanted passivated planar silicon detector [72], leading to the fact that only the total activity of 239+240 Pu can be measured. Beata Varga et al. [73] used alpha spectroscopy to measure the activity of 239+240 Pu of grass samples in the hilly area of Budapest as 0.024 ± 0.01 Bq·kg −1 (fresh samples), with the MDA of 0.02 Bq·kg −1 . Compared with the soil surface, the transfer factor (TF) absorbed by roots was 0.032 ± 0.014 (Bq·kg −1 fresh grass mass)/(Bq·kg −1 dry soil mass), and the TF of forage grass in the literature was in the range of 5 × 10 −5 -0.7. Sherrod et al. [30] determined the activity of 239 Pu in rice samples as 11.8 Bq·kg −1 . Lee et al. [74] measured Pu and Am isotopes in the volcanic soil of Jeju Island and the Korean Peninsula. The concentrations of 239 Pu and 240 Pu were 2.0 ± 0.3 Bq·kg −1 (the error is 1σ) and 0.67 ± 0.1 Bq·kg −1 , respectively. Liu et al. [58] established a method to quickly determine the content of 239+240 Pu in environmental water samples by using low-background LSC, and the detection limit of 5 L water samples was 6 × 10 −3 Bq·L −1 within a 60-min counting time. Dagmara et al. [75] studied the distribution of Pu in the organs and tissues of seabirds and found that the highest Pu concentration was in the digestive organs and feathers, while the smallest was in the skin and muscles. The Pu content was lower in fish-eating seabirds, while it was much higher in herbivorous seabirds. It was analyzed that the main source of Pu in seabirds was global atmospheric sedimentation.
In conclusion, the commonly used methods for measuring ultra-trace 90 Sr and 239,240 Pu in environmental samples are liquid scintillation counting and alpha spectroscopy. The detection limit of these two methods is low, but the measurement time is long, which is not suitable for emergency monitoring. As alpha spectroscopy can only measure the total activity of 239+240 Pu, the ratio of 240 Pu/ 239 Pu is unable to be given to analyze the source of Pu.

Mass Spectrometry Methods
Mass spectrometry technology is widely used in the determination of radionuclides with long life and large mass numbers, such as 235 U, 239 Pu, and 240 Pu. For short-lived radionuclides, radioactivity-measurement methods are more commonly used. In recent years, newly developed mass spectrometry techniques with high resolution, high sensitivity, and advanced interference cancellation functions have also been used to measure radionuclides with a shorter life, such as 90 Sr. This section reviews the mass spectrometry methods, focusing on ICP-MS, to determine 90 Sr, 239 Pu, and 240 Pu in order to better understand the analytical capabilities and potential applications of mass spectrometry.

Measurement Method of 90 Sr
The natural Sr concentration in environmental samples is about nine orders of magnitude higher than that of 90 Sr. 90 Sr is interfered by isobaric 90 Zr interference and the stable peak trailing interference of 88 Sr (82.58% abundance) when determined by mass spectrometry. The separation of 90 Sr and 90 Zr requires a mass resolution greater than 30,000, which exceeds the resolution capability of mass spectrometry. Researchers have tried many methods. Arslan et al. [76] made the first attempt to determine 90 Sr by using accelerator mass spectrometry (AMS) after removing 90 Zr by a chemical procedure. Betti et al. [77] measured the concentration of 90 Sr in soil, sediment, and grass by glow-discharge mass spectrometry (GD-MS). Wendt et al. [78] reported the concentrations of 89 Sr and 90 Sr in soil, grass, milk, and urine after chemical separation by resonance ionization mass spectrometry (RIMS). Bushaw et al. [79] used RIMS to determine 90 Sr in environmental samples with a detection limit as low as 0.8 fg. AMS, especially RIMS, has high element selectivity. However, they are not usually used in the environmental application, due to less accessibility, and complicated operation and maintenance. ICP-MS, with high sensitivity, can perform rapid and relatively cost-effective isotope analysis at ultra-trace levels [80]. High-resolution inductively coupled plasma-mass spectrometry (HR-ICP-MS) has ultra-high sensitivity and strong interference elimination ability. The ionization energy of Sr (5.70 eV) is 1.14 eV lower than that of Zr (6.84 eV). Operating under cold plasma can greatly reduce the ion-ization efficiency of Zr by HR-ICP-MS [33]. The isotope ratio of 90 Sr/ 86 Sr in soil samples near the nuclear facility was 6.02 × 10 −9 , basically reaching the sensitivity of radiochemical method for the determination of 90 Sr [20].
Quadrupole inductively coupled plasma-mass spectrometry (ICP-QMS) [1,23,56] and sector-magnetic-field inductively coupled plasma-mass spectrometry (SF-ICP-MS) [19] are also used for the measurement of 90 Sr. The interference signal is suppressed by improving the sample introduction method and optimizing the operating system conditions, effectively reducing the detection limit. The concentration of 90 Sr was measured by tunable bandpass dynamic reaction cell-ICP-MS (DRC-ICP-MS) with relatively high abundance sensitivity [55]. O 2 was used as the reaction gas to form ZrO + with Zr in the reaction cell, effectively suppressing the interference of Zr. The concentration of 90 Sr in sediments, plants and water samples was consistent with the activity value determined bŷ Cherenkov count and the reference value. The detection limits of the three samples were 0.1 pg·g −1 (0.5 Bq·g −1 ), 0.04 pg·g −1 (0.5 Bq·g −1 ), and 3 pg·L −1 (5 Bq·L −1 ), respectively. Feuerstein et al. [1] used ICP-DRC-MS to quickly determine the concentration of 90 Sr in contaminated soil samples near the Chernobyl nuclear power plant and compared with the results obtained by the previous radiometry ( Table 7). The measurement results were basically consistent, while the accuracy of ICP-DCR-MS was better. Although the MDA of ICP-DRC-MS was inferior to the radiometric methods, it represented a time-saving and economical alternative technology for rapid monitoring of high levels of 90 Sr contamination in environmental samples. The interference signals can also be suppressed effectively by using dual mass spectrometry for mass discrimination. 90 Sr in the reference materials was measured by triple quadrupole collision/reaction cell-CP-MS/MS in single mode and MS/MS mode with relative accuracy and precision of 0.7-3.6% and 3.5-8.1% [34]. The established method was used to measure the artificial radionuclide 90 Sr in soil samples collected from the whole state of Qatar. The average concentration was 0.606 fg·g −1 (3.364 Bq·kg −1 ), with the detection limit of less than 0.04 pg.

Mass Spectrometry Measurement Methods of 239 Pu and 240 Pu
As mentioned above, the use of mass spectrometry to measure 239 Pu and 240 Pu is interfered by the uranium-hydrogen peak. It is necessary to separate 238 U before measurement and absolutely eliminate polyatomic chlorides and oxides. Methods such as improving the sample introduction method, increasing the temperature of the plasma, and changing the flow rate of the atomizer can reduce the formation of hydride ions [83]. Among them, a sample introduction method, such as membrane desolvation, is a very important factor that affects hydride [84]. For hydride generation, Kim et al. [85] compared five injection methods in terms of sensitivity, specific sensitivity, and precision. A high-efficiency sampleintroduction system Apex-Q combined with the sector field ICP-MS could achieve high instrumental sensitivity and reduce the formation of 238 UH + / 238 U [86].
Thermal ionization mass spectrometry (TIMS) is one of the main methods to determine isotope composition and ratio. The accuracy of isotope measurement is less than 5 × 10 −4 , and the abundance sensitivity is better than 5 × 10 −8 . McCarthy et al. [87] measured the isotopic ratios of 240/239 Pu and 241/239 Pu in Oxfordshire soil to be 0.184 (RSD < 1%) and 0.0049 (RSD < 5%). The average concentration of 239+240 Pu was 0.257 Bq·kg −1 , close to the average value of radioactive deposition in Britain [88]. Buesseler et al. [89] found that TIMS was one order of magnitude more sensitive than the traditional alpha-counting technique when used to detect Pu in sediments, seawater, and coral samples.
TIMS has high precision and abundance sensitivity; however, with a complicated sample-preparation process (requiring pure samples), long measurement time, and a large amount of samples, it is not as simple as ICP-MS to operate. By using the sector-field ICP-MS to measure 239 Pu, Ni et al. [85] investigated the transfer of global fallout Pu from paddy soil to rice grain in 12 Japanese prefectures, with the TF ranging from 4.5 × 10 −6 to 1.2 × 10 −4 . Chiappini et al. [90] used high-sensitivity ICP-MS to measure the contribution of Pu deposited in the Mururoa and Fangataufa sediments in the French atmospheric nuclear test for the first time. The contribution was very small and could only be observed in the territorial sea boundary area 22 km away from the atoll. The concentration of Pu in the seawater was only 2 mBq·m −3 higher than that observed in Rangiroa. For standard solutions without interference, the detection limits of ICP-MS/HP/Mistral (high-performance ICP-MS associated with Mistral desolvation atomizer (HP/Mistral)) were compared with alpha spectrometry ( Table 8). The measurement period of alpha spectrometry was 4000 min. It should be noted that alpha spectrometry measures the sum of the activities of 239 Pu and 240 Pu, while the ICP-MS measures the activities of 239 Pu and 240 Pu, respectively. On this basis, ICP-MS/HP and alpha spectroscopy were compared for the measured values of 239+240 Pu in Sellafield sediments, Mururoa lagoon sediments, and grouper samples. These comparisons confirmed that the detection limit of ICP-MS/HP/Mistral was lower than that of alpha spectroscopy when measuring environmental samples. In addition, the introduction of collision/reaction cell (CRC) technology in ICP-MS can effectively reduce the interfering effects of the uranium-hydrogen peaks on measuring 239 Pu and 240 Pu. Recently, a CRC-ICP-MS/MS method reduced 238 UHO + 2 / 238 U 16 O + 2 ratio to 4.82 × 10 −9 , 2-to-3 orders of magnitude lower than previously reported values, and accurately measured the ultra-trace level 239 Pu with the concentration ratio as low as 10 −10 by 239 Pu/ 238 U [91]. HR-ICP-MS is also mostly used when measuring the concentration of 239,240 Pu in the environment. Helal et al. [20] measured the isotope ratio of 240 Pu/ 239 Pu of 0.17 in the soil near nuclear facilities. Stefan et al. [92] used the ultrasonic atomization method to measure the Pu isotope activity and 240 Pu/ 239 Pu isotope ratio in environmental samples, with the measurement accuracy (RSD) of the ratio of 240 Pu/ 239 Pu at about 2%. The total plutonium activity ( 239+240 Pu) measured was very consistent with the data obtained by the alpha spectroscopy, with the detection limits of 239 Pu and 240 Pu to be 5 and 1 fg·mL −1 , respectively. Wang et al. [93] measured the activity of 239+240 Pu in the surface sediments of the Bohai Sea and the North Yellow Sea by the isotope dilution method to be 0.001-0.288 and 0.040-0.269 Bq·kg −1 , respectively. The atomic ratio of 240 Pu/ 239 Pu was 0.173-0.256 and 0.196-0.275, respectively, slightly higher than the global sedimentation value of 0.18.
In addition, Mathew et al. [94] used AMS to distinguish Pu from the FDNPP accident and Nagasaki explosion from global sediments. It was found that the isotope ratio of 241 Pu/ 239 Pu was more sensitive than that of 240 Pu/ 239 Pu when Pu was traced.
Overall, mass spectrometry has been widely used in the measurement of ultra-trace nuclides. In recent years, CRC-ICP-MS/MS is a promising method with ultra-high sensitivity, which can significantly eliminate the 90 Zr and 238 U interferences in the measurement of 90 Sr and 239,240 Pu by using O 2 as the reaction gas.

Conclusions
Concerns have increased regarding the environmental radioactive contamination occurring in large-scale nuclear or radiological accident/incident. Carrying out relevant monitoring is of great significance, because it reflects the affected degree of biological chain to a certain extent. In this work, we reviewed and summarized the pretreatment, separation, and measurement methods for analyzing 90 Sr and 239,240 Pu in environmental samples with the characteristics of ultra-trace concentration and complex matrix. The pretreatment of samples usually costs most of the experimental time. Combining the dry-ashing and microwave-digestion methods greatly increases the processing capacity and shortens the processing time. The classical separation method of Sr is fuming nitric acid precipitation, which is time-consuming and cumbersome. In contrast, the solvent-extraction method is quicker and simpler. In recent years, the commonly used Sr high-efficiency specific resin extraction method has been more selective. As for the separation and purification of Pu in the environment, adjusting the valence state of Pu to Pu (IV) is the primary task. Among the commonly used methods for the separation of trace amounts of Pu, the ion-exchange method is fast and economical, while the exchange capacity of the leaching resin is high. Then we analyzed and compared the nuclear measurement techniques of 90 Sr and 239,240 Pu in the environment. The highly sensitive liquid scintillation spectrometry and alpha spectrometry are the general methods for measuring 90 Sr and 239,240 Pu, as their instrument maintenance is simpler than mass spectrometry. By improving sample introduction methods and optimizing the operating conditions of the instrument, mass spectrometry has the ability to successfully measure the concentrations of 90 Sr and 239,240 Pu in the environment with equal sensitivity and a lower detection limit to radiometric methods.
The pretreatment of a large number of samples and the low detection limit of the instrument are required for the complex matrix of environmental samples and the ultratrace level of radionuclides. Here are some prospects for the radionuclide analysis of environmental samples. First, as sample preprocessing is often cumbersome and timeconsuming, it is critical to establish a rapid, convenient, efficient, and clean pretreatment method in the future. Second, based on the characteristics of radionuclides in environmental samples, research on developing high-capacity, high-efficiency, and specific separation resins for major beta nuclides and actinides is the development trend. Providing rapid and ultra-sensitive analysis technology is of great significance to environmental emergency monitoring. Therefore, automatic separation and measurement technology will become a promising direction due to its fast and convenient advantages. In addition, research on mobility of 90 Sr and 239,240 Pu in the environment is the basis for predicting the degree of ecological environment pollution, as well as the restoration study of soil, vegetation, and water after contamination with these nuclides.