Sergei F. Boulyga

Determination of 236U/238U isotope ratio in contaminated environmental samples using different ICP-MS instruments

This paper considers use of the 236U isotope to monitor the spent uranium from nuclear fallout using inductively coupled plasma mass spectrometry (ICP-MS) in soil samples collected in the vicinity of the Chernobyl Nuclear Power Plant (NPP). Sector field ICP-MS (ICP-SFMS) and quadrupole based ICP-MS without and with hexapole collision cell (ICP-CC-MS) were used for uranium isotope analysis. In addition, a multiple ion collector ICP-MS (MC-ICP-MS) was used for high-precision isotope ratio measurements. A 238U+ ion sensitivity of 18 GHz ppm−1, 12.4 GHz ppm−1 and 16 GHz ppm−1, respectively, was observed in ICP-SFMS, ICP-CC-MS and MC-ICP-MS with ultrasonic nebulizer. An absolute sensitivity of 3600 counts fg−1 was achieved for uranium by using a direct injection high efficiency nebulizer for solution introduction in ICP-SFMS. The detection limit for 236U was in the fg g−1 range and abundance ratio sensitivity for 236U/238U was 5 × 10−6, 3 × 10−7, 6 × 10−7, and less than 3 × 10−7 in ICP-SFMS, MC-ICP-MS, quadrupole ICP-MS and ICP-CC-MS at mass resolution m/Δm = 300, respectively. Interlaboratory comparison yielded a good accuracy (0.4–1.6%) of 236U/238U isotope ratios ranging from 1.5 × 10−3 to 3.2 × 10−4 measured in samples containing 100 ng of uranium. The 236U/238U isotope ratios and spent uranium fraction were determined in Chernobyl soil samples using single ion detector ICP-MS and multiple ion collector ICP-MS (MC-ICP-MS). In comparison to 235U/238U isotope ratio, the 236U/238U isotope ratio provided more sensitive and accurate determination of the portion of spent uranium from Chernobyl NPP in spent/natural uranium mixture in soil samples down to 0.1%. The concentration of Chernobyl spent uranium in upper 0–10 cm soil layers in investigated areas in the vicinity of Chernobyl NPP amounts to 2.4 × 10−9 g g−1 to 8.1 × 10−7 g g−1 depending mainly on the distance to the Chernobyl reactor.

Improvements in routine uranium isotope ratio measurements using the modified total evaporation method for multi-collector thermal ionization mass spectrometry

A new version of the “modified total evaporation” (MTE) method for isotopic analysis of uranium samples by multi-collector thermal ionization mass spectrometry (TIMS), with high analytical performance and designed in a more user-friendly and routinely applicable way, is described in detail. It is mainly being used for nuclear safeguards measurements, but can readily be applied in other scientific areas like geochemistry. The development of the MTE method was organized in collaboration of several “key nuclear mass spectrometry laboratories”, namely the New Brunswick Laboratory (NBL), the Safeguards Analytical Laboratory (SAL, now SGAS—Safeguards Analytical Services) of the International Atomic Energy Agency (IAEA), the Institute for Transuranium Elements (ITU/JRC), and the Institute for Reference Materials and Measurements (IRMM/JRC), with IRMM taking the leading role. Due to the use of the “total evaporation” (TE) principle the measurement of the “major” ratio n(235U)/n(238U) is routinely being performed with an accuracy of 0.02%. In contrast to the TE method, in the MTE method the total evaporation process is interrupted on a regular basis to allow for correction for background from peak tailing, internal calibration of a secondary electron multiplier (SEM) detector versus the Faraday cups, peak-centering, and ion source re-focusing. Therefore, the most significant improvement using the MTE method is in the measurement performance achieved for the “minor” ratios n(234U)/n(238U) and n(236U)/n(238U). The n(234U)/n(238U) ratio is measured using Faraday cups only with the result that the (relative) measurement uncertainty (k = 2) is better than 0.12%, which is an improvement by a factor of about 5–10 compared to TE measurements. Furthermore, the IAEA requirement for the “measurement performance”, defined here as the sum of the (absolute) deviation of the measured from the true (certified) value plus the (absolute) measurement uncertainty (k = 2), for n(236U)/n(238U) ratio measurements is 1 × 10−6, but the MTE method provides a measurement performance which is, depending on the ratio, by several orders of magnitude superior compared to this limit and to the TE method. For routine MTE measurements a detection limit of 3 × 10−9 was achieved using an SEM detector for detecting the isotope 236U. The MTE method is now routinely being used at all collaborating laboratories with the hope that more laboratories will implement this capability in the future as well. Additional applications for the MTE method are presented in this paper, e.g., for absolute Ca isotope measurements.

Ultratrace and isotopic analysis of long-lived radionuclides by double-focusing sector field inductively coupled plasma mass spectrometry using direct liquid sample introduction

Abstract This report is concerned with the investigation of double-focusing sector field inductively coupled plasma mass spectrometry (DF-ICPMS) for ultratrace and isotopic ratio analysis of long-lived radionuclides (226Ra, 230Th, 232Th, 233U, 237Np, 238U, and 241Am) using the direct injection high efficiency nebulizer (DIHEN). A new shielded torch arrangement, known as the guard electrode, improves relative sensitivity by a factor of six when the DIHEN is used. Absolute sensitivity with the DIHEN is on the order of 1300 (226Ra) to 1700 (238U) counts/fg at a solution consumption rate of 5 μL/min. This is a factor of from three to 20 better than the results obtained by a conventional nebulizer-spray chamber arrangement (e.g., ultrasonic and pneumatic nebulizers). The DIHEN-DF-ICPMS is successfully tested for isotope ratio measurements of 235U/238U standards and environmental radioactive waste solutions.

Development of an isotope dilution laser ablation ICP-MS method for multi-element determination in crude and fuel oil samples

An inductively coupled plasma isotope dilution mass spectrometric (ICP-IDMS) method with direct introduction of the isotope-diluted sample into the plasma by laser ablation was developed for accurate, sensitive, fast, and simultaneous determination of trace metals in different oil samples. Metallo-organic solutions of isotope spikes (50V, 53Cr, 65Cu, 57Fe, 62Ni, 68Zn, 113Cd, 117Sn, and 206Pb) were prepared from corresponding aqueous stock solutions by using liquid–liquid extraction of complexed metal ions in isobutyl methyl ketone. The isotope-diluted sample was absorbed by a cellulose material, which was fixed in a special PTFE holder for ablation, using a laser system with high ablation rates. Under these conditions, no time-dependent spike/analyte fractionation was observed for the metallo-organic spike/oil mixtures and the measured isotope ratios of the isotope-diluted samples remained constant over the whole ablation time, which is a necessary precondition for accurate results with the isotope dilution technique. The accuracy of LA-ICP-IDMS determinations was demonstrated by analyzing the fuel oil reference material BCR 1634c, certified for vanadium and nickel, a commercially available oil based standard (SCP-21) for nine metals, and by comparing LA-ICP-IDMS results with those obtained by ICP-IDMS using microwave-assisted sample digestion. Detection limits in the range of 0.02 µg g−1 (V) to 0.2 µg g−1 (Fe) were obtained for LA-ICP-IDMS by total analysis times per sample of only 10 min. Three crude oil samples of different origin with vanadium, nickel, and iron concentrations in the range of 2–60 µg g−1 were also analyzed, indicating iron heterogeneities by the relatively high standard deviation.

Mass spectrometric analysis for nuclear safeguards

Mass spectrometry is currently being implemented in a wide spectrum of research and industrial areas, such as materials science, cosmo- and geochemistry, biology and medicine, to name just a few. Research and development in nuclear safeguards is closely related to the general field of “Peace Research”, representing a specific application area for analytical sciences in general and for mass spectrometry in particular. According to Albert Einstein “peace cannot be kept by force. It only can be achieved by understanding”. Understanding implies a realistic estimation of potential challenges and threats, which is based on the ability to obtain timely, reliable and independent information. A particular task of international nuclear material safeguards is reducing threats that are posed by the proliferation of nuclear weapons. An important part of the International Atomic Energy Agency (IAEA) safeguards system is the “analytical laboratory”, with mass spectrometric techniques, such as thermal ionization mass spectrometry (TIMS), secondary ion mass spectrometry (SIMS), and inductively coupled plasma mass spectrometry (ICP-MS) belonging to the most powerful methods for the analysis of nuclear material and environmental samples collected during inspections. Each of the currently applied techniques provides definite merits (e.g. precision, accuracy, time-effectiveness, high sensitivity, spatial resolution, reduced molecular interference, etc.) for a specific safeguards related application. Thus, taking advantage of each technique helps the analyst to gain a larger quantity of safeguards-relevant information. Along with the analysis of element amounts and isotopic compositions of uranium and plutonium in nuclear material the challenging applications of mass spectrometry include isotopic analysis of micro-samples, age determination of nuclear material as well as identification and quantification of elemental and isotopic signatures of inspection samples in general. Analysis of inspection samples implies strict quality control procedures and it demands the production of suitable certified isotopic reference materials which are used as calibration standards or as quality control samples. This manuscript discusses merits and limitations of presently available mass spectrometric instrumentation for such safeguards applications. It will also highlight the need for further improvements in TIMS, ICP-MS and SIMS performance aimed at obtaining more specific and significant isotopic information.

Reduction of molecular ion interferences with hexapole collision cell in direct injection nebulization–inductively coupled plasma mass spectrometry

A hexapole collision cell was investigated for significant reductions of interferences by molecular ions in inductively coupled plasma mass spectrometry (ICPMS) using a direct injection high efficiency nebulizer (DIHEN). Collision induced reactions with hydrogen reduced isobaric interferences while the addition of helium as a collision gas enhanced analyte ion transmission through collisional focusing. Improved figures of merit were obtained for elements (Ca, Fe, Cr, As, Se) that are typically difficult to analyze with conventional quadrupole instruments. Sensitivities achieved with the DIHEN were higher (by factors ranging from 2 to 9) than those observed with a micronebulizer-spray chamber arrangement. Precision and detection limits were similar to or slightly improved over values obtained using the micronebulizer-spray chamber arrangement. The technique was successfully applied to the determination of Fe, Cr, Co, Cu, Pb, Al, Mn, Zn, Ag, and Sr on silicon wafer surfaces at a concentration range of (0.49–6.5) × 109 atoms cm−2, sampled by a 100 µL drop of H2O–H2O2–HF, as well as for the determination of Cr in DNA.

Determination of 90Sr in soil samples using inductively coupled plasma mass spectrometry equipped with dynamic reaction cell (ICP-DRC-MS)

A rapid method is reported for the determination of (90)Sr in contaminated soil samples in the vicinity of the Chernobyl Nuclear Power Plant by ICP-DRC-MS. Sample preparation and measurement procedures focus on overcoming the isobaric interference of (90)Zr, which is present in soils at concentrations higher by more than six orders of magnitude than (90)Sr. Zirconium was separated from strontium in two steps to reduce the interference by (90)Zr(+) ions by a factor of more than 10(7): (i) by ion exchange using a Sr-specific resin and (ii) by reaction with oxygen as reaction gas in a dynamic reaction cell (DRC) of a quadrupole ICP-MS. The relative abundance sensitivity of the ICP-MS was studied systematically and the peak tailing originating from (88)Sr on mass 90 u was found to be about 3 x 10(-9). Detection limits of 4 fg g(-1) (0.02 Bq g(-1)) were achieved when measuring Sr solutions containing no Zr. In digested uncontaminated soil samples after matrix separation as well as in a solution of 5 microg g(-1) Sr and 50 ng g(-1) Zr a detection limit of 0.2 pg g(-1) soil (1 Bq g(-1) soil) was determined. (90)Sr concentrations in three soil samples collected in the vicinity of the Chernobyl Nuclear Power Plant were 4.66+/-0.27, 13.48+/-0.68 and 12.9+/-1.5 pg g(-1) corresponding to specific activities of 23.7+/-1.3, 68.6+/-3.5 and 65.6+/-7.8 Bq g(-1), respectively. The ICP-DRC-MS results were compared to the activities measured earlier by radiometry. Although the ICP-DRC-MS is inferior to commonly used radiometric methods with respect to the achievable minimum detectable activity it represents a time- and cost-effective alternative technique for fast monitoring of high-level (90)Sr contamination in environmental or nuclear industrial samples down to activities of about 1 Bq g(-1).

Evaluation strategies for isotope ratio measurements of single particles by LA-MC-ICPMS

Data evaluation is a crucial step when it comes to the determination of accurate and precise isotope ratios computed from transient signals measured by multi-collector–inductively coupled plasma mass spectrometry (MC-ICPMS) coupled to, for example, laser ablation (LA). In the present study, the applicability of different data evaluation strategies (i.e. ‘point-by-point’, ‘integration’ and ‘linear regression slope’ method) for the computation of 235U/238U isotope ratios measured in single particles by LA-MC-ICPMS was investigated. The analyzed uranium oxide particles (i.e. 9073-01-B, CRM U010 and NUSIMEP-7 test samples), having sizes down to the sub-micrometre range, are certified with respect to their 235U/238U isotopic signature, which enabled evaluation of the applied strategies with respect to precision and accuracy. The different strategies were also compared with respect to their expanded uncertainties. Even though the ‘point-by-point’ method proved to be superior, the other methods are advantageous, as they take weighted signal intensities into account. For the first time, the use of a ‘finite mixture model’ is presented for the determination of an unknown number of different U isotopic compositions of single particles present on the same planchet. The model uses an algorithm that determines the number of isotopic signatures by attributing individual data points to computed clusters. The 235U/238U isotope ratios are then determined by means of the slopes of linear regressions estimated for each cluster. The model was successfully applied for the accurate determination of different 235U/238U isotope ratios of particles deposited on the NUSIMEP-7 test samples.

Improved abundance sensitivity in MC-ICP-MS for determination of 236U/238U isotope ratios in the 10−7 to 10−8 range

A multicollector inductively coupled plasma mass spectrometer (MC-ICP-MS)—Nu Plasma HR—equipped with an ion deceleration filter (‘high abundance sensitivity channel’) was optimized for the determination of extremely low uranium isotope ratios and applied for the first time for the analysis of 236U/238U isotope ratios in the 10−8 to 10−7 range in isotopic reference material IRMM-184, as well as in two unknown samples obtained in the frame of a round-robin exercise. The application of the ion deceleration lens system in the Nu Plasma ICP-MS allowed a reduction of peak tailing from 238U+ ions at m/z = 236 down to 3 × 10−9, whereas the absolute sensitivity for uranium was reduced by only about 30%. Thus, abundance sensitivity was improved by almost two orders of magnitude and the minimum determinable 236U/238U ratio was improved by more than one order of magnitude compared with conventional sector-field ICP-MS or TIMS. However, interference by 235U1H+ ions deteriorated the accuracy and increased the measurement uncertainty up to 48%, in particular in the case of samples enriched in 235U. Deviation of the measured value from the certified ones varied from −24% to 17.7% but was within the measurement uncertainty.

Direct uranium isotope ratio analysis of single micrometer-sized glass particles

We present the application of nanosecond laser ablation (LA) coupled to a ‘Nu Plasma HR’ multi collector inductively coupled plasma mass spectrometer (MC-ICP-MS) for the direct analysis of U isotope ratios in single, 10–20 μm-sized, U-doped glass particles. Method development included studies with respect to (1) external correction of the measured U isotope ratios in glass particles, (2) the applied laser ablation carrier gas (i.e. Ar versus He) and (3) the accurate determination of lower abundant 236U/238U isotope ratios (i.e. 10−5). In addition, a data processing procedure was developed for evaluation of transient signals, which is of potential use for routine application of the developed method. We demonstrate that the developed method is reliable and well suited for determining U isotope ratios of individual particles. Analyses of twenty-eight S1 glass particles, measured under optimized conditions, yielded average biases of less than 0.6% from the certified values for 234U/238U and 235U/238U ratios. Experimental results obtained for 236U/238U isotope ratios deviated by less than −2.5% from the certified values. Expanded relative total combined standard uncertainties Uc (k = 2) of 2.6%, 1.4% and 5.8% were calculated for 234U/238U, 235U/238U and 236U/238U, respectively.