Feasibility study of solid sampling electrothermal vaporization inductively coupled plasma mass spectrometry for the determination of particulate uranium in swipe samples from nuclear facilities
Introduction
Environmental sampling was adopted by the International Atomic Energy Agency (IAEA) as a new technique for the strengthened safeguards system [1]. It aims to assure the absence of undeclared nuclear materials or activities in nuclear plants. One of the analytical methods for environmental sampling is particle analysis, which measures isotope ratios of nuclear material in individual particles. Secondary ion mass spectrometry (SIMS) is known as a powerful analytical method for this purpose. However, it is difficult to analyze the particles with a diameter less than one micrometer (equivalent to about 4 pg for uranium) due to the detection limit of SIMS. A technique based on fission track (FT) has been applied to identification of sub-micrometer particles containing fissile material. This technique combined with mass spectrometry, such as thermal ionization mass spectrometry (TIMS) or inductively coupled plasma mass spectrometry (ICP-MS), enables us to conduct the particle analysis of picogram amounts of nuclear materials.
In the particle analysis using a fission track-TIMS method, a small portion (about μm) of the polycarbonate (PC) film containing fissile materials identified from fission track was cut and transferred onto a Re filament of TIMS by using micromanipulation [2]. Thus, the process of sample preparation is time-consuming and complicated. Also, in the measurement of isotope ratio by TIMS, the number of samples to be analyzed is limited: about six samples a day could be analyzed. If the electrothermal vaporization (ETV)-ICP-MS method can be applied to particle analysis, micromanipulation is omissible because the method of sample introduction to ETV is quite simple. Moreover, analysis of 30 or more samples a day is possible with the aid of the high analytical performance of ETV-ICP-MS as described below. Thus, we have been applying the ETV-ICP-MS method to the particle analysis of environmental swipe samples, to develop an alternative analytical method to FT-TIMS. To analyze a fission track sample, it is necessary to analyze a solid sample because the particles of interest are to be embedded in PC film, which is then irradiated with thermal neutrons to identify a fissile material [2]. In this study, therefore, PC films containing particles were used as samples of preliminary work for analysis of fission track samples.
ICP-MS is a powerful analytical tool for ultra-trace element determination and isotopic analysis. The main advantages of this technique are excellent detection limits, high sensitivity and precision, a wide linear dynamic range and the possibility to perform isotope measurements. Excellent reviews on this technique have been reported in the literatures [3], [4], [5]. One of the most advantages of ICP-MS is the variety of sample introduction techniques according to the purpose.
ETV is one of the promising alternatives to conventional liquid sample introduction system in ICP-MS, and useful method for the introduction of solid samples into an ICP-MS [6], [7]. ETV as a sample introduction technique offers a number of significant advantages over ICP-MS analysis with conventional solution nebulization: (1) analysis of small sample volumes, (2) higher transport efficiencies (20–80%) and therefore higher sensitivity and lower limits of detection, (3) selective volatilization of the analyte from matrix by using a step-wise temperature program. For these reasons ETV-ICP-MS is of great interest when conventional ICP-MS analysis is hindered by metal-oxide or polyatomic interferences [8], [9], or when very small sample volumes or solid samples must be analyzed for many elements. Moreover, solid sampling ETV-ICP-MS offers the avoidance of time-consuming sample preparation. Until now, the applications of solid sampling using ETV-ICP-MS have reported mainly for slurry solid samples [10], [11], [12], [13], [14]. Limited information of solid sampling ETV-ICP-MS is available for dry solid samples [15], [16]. On the other hand, a disadvantage of the ETV sample introduction system compared to the pneumatic nebulization is poor precision in quantitative analysis, presumably due to worse reproducibility of the evaporation efficiency. In contrast, isotope ratios can be measured with better reproducibility. The increasing importance of ETV-ICP-MS has also lead to modifiers being applied to this technique. The modifiers were used to achieve a sensitivity enhancement of the analyte, which was explained by the improved transport of the analyte from the furnace to the plasma [17], [18], [19], [20]. Transport efficiency of analyte in ETV-ICP-MS measurements is one of the most important factors influencing on analytical results, especially for the analysis of ultra-trace amounts of materials like in this study. Therefore, prudent selection and use of appropriate temperature programs and modifiers was the key to the successful application of this technique.
There have been many studies on the effects of several parameters (vaporization temperature, chemical/physical modifiers, matrices) on the determination of U by ETV-ICP-MS [21], [22], [23], [24], [25], [26]. Goltz et al. reported that U carbide form at temperatures above 2273 K and does not vaporize until 2873 K [26]. They showed that the optimal vaporization temperature should be below 2773 K to obtain a single analyte peak. However, a few studies on determination of U have been carried out for a few picogram amounts of analyte. Truscott et al. studied the effect of modifier on U of 3 pg using an ETV-ICP-MS [27]. They reported that signal increased by a 10 times in the presence of CHF3 modifier gas.
The purpose of this work is to investigate the feasibility of solid sampling ETV-ICP-MS for “the particle analysis of safeguards environmental swipe samples”. In particular, the optimization of the PC film sample containing a few picogram amounts of particles, and their analytical results especially isotope ratio measurements that is an important issue in the analysis of safeguards environmental samples, are described in the present paper.
Section snippets
Instrumentation
All measurements were carried out using an HP 4500 ICP-MS in combination with electrothermal vaporization unit (HP 4500 series) of pyrolytically coated graphite furnace of boat type. The ETV unit was interfaced to the ICP-MS with a Teflon tube, 100 cm in length and 0.6 cm in internal diameter. The instrument was optimized for maximum sensitivity for thallium (Tl) solution using conventional pneumatic nebulization, except for the carrier gas flow rate that was optimized after switching to ETV. The
Optimization with solution samples
The measurement of isotope ratios by a quadrupole ICP is very sensitive to the signal measurement parameters used as well as the signal intensity. When the ETV producing a transient signal coupled with the ICP, the accurate and precise measurement of isotope ratios is further complicated. Therefore, to accomplish a higher precision and accuracy of isotope ratio and signal intensity, the use of an ETV system in addition to the ICP parameters, such as temperature program, carrier gas flow rate
Conclusion
Taking advantages of ETV, we have investigated the feasibility of solid sampling ETV-ICP-MS for the particle analysis of safeguards environmental swipe samples. Accurate and precise measurements of isotope ratio in PC film samples containing a few picogram amounts of particles were shown to be possible using ETV-ICP-MS. Results for the PC film samples containing 1 pg of analyte were in good agreement with the reference value. It was also shown that solid sampling ETV-ICP-MS allows a detection
Acknowledgements
A portion of this work was performed under the auspices of Ministry of Education, Cultures, Sports, Science and Technology of Japan. The authors are grateful to Mr. H. Hosomi for assistance with ETV-ICP-MS measurements and for sample preparation.
References (27)
Spectrochim. Acta, Part B
(1999)- et al.
Microchim. J.
(1994) - et al.
Spectrochim. Acta, Part B
(1995) - et al.
Anal. Chim. Acta
(1998) - et al.
Spectrochim. Acta, Part B
(2001) - et al.
Spectrochim. Acta, Part B
(1994) - et al.
Anal. Chim. Acta
(1999) - et al.
Spectrochim. Acta, Part B
(1992) - et al.
Spectrochim. Acta, Part B
(1993) - et al.
Spectrochim. Acta, Part B
(1996)
Spectrochim. Acta, Part B
Spectrochim. Acta
Spectrochim. Acta, Part B
Cited by (5)
Evaluation of electrothermal vaporization for sample introduction aiming at Cu isotopic analysis via multicollector-inductively coupled plasma mass spectrometry
2021, Spectrochimica Acta - Part B Atomic SpectroscopyCitation Excerpt :It has to be remembered that the ETV system provides very short transient signals from small sample volumes, which are not considered as ideal for precise isotopic ratio analysis, especially when coupled to sequential mass analyzers. Still, this level of precision could be enough for isotope dilution calibration [17,18] and, in some particular occasions, for clinical or archaeological applications [19–21], as long as the expected differences between the ratios are high enough. Naturally, in order to deal with transient signals and improve the precision for isotopic ratios, ETV could be coupled to instrumentation with more potential for simultaneous monitoring, such as TOF-MS [22] or, for best precision in isotope analysis, multicollector (MC)-ICP-MS. However, the latter coupling has only been reported twice in the literature.
Electrothermal vaporization-inductively coupled plasma-mass spectrometry: A versatile tool for tackling challenging samples. A critical review
2009, Analytica Chimica ActaCitation Excerpt :Finally, Lee and coworkers performed direct solid sampling analysis of polycarbonate film samples containing a few picogram amounts of Tl, Pb and U particles as a means to assess the feasibility of ETV–ICP-MS for isotopic analysis of swipe samples from nuclear facilities. Sufficiently accurate and precise (<1% RSD) isotope ratio in polycarbonate samples containing 1 pg of the particulate analytes could be obtained [190]. Even if the abovementioned applications might be of considerable interest, it is clear that quadrupole-based ETV–ICP-MS cannot meet the requirements of most isotope ratio applications for which a much better precision is required [191].
ETV-ICPMS for Analysis of Polymers
2012, Mass Spectrometry HandbookElectrothermal vaporization for sample introduction in atomic absorption, atomic emission and plasma mass spectrometry - A critical review with focus on solid sampling and slurry analysis
2008, Journal of Analytical Atomic SpectrometryAtomic spectrometry update. Industrial analysis: Metals, chemicals and advanced materials
2005, Journal of Analytical Atomic Spectrometry