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The direct injection high efficiency nebulizer (DIHEN) was explored for the ... The analytical methods developed were applied to the determination of ...
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Anal. Chem. 1999, 71, 3077-3084

Ultratrace and Isotope Analysis of Long-Lived Radionuclides by Inductively Coupled Plasma Quadrupole Mass Spectrometry Using a Direct Injection High Efficiency Nebulizer Johanna Sabine Becker* and Hans-Joachim Dietze

Central Department of Analytical Chemistry, Research Centre Juelich, D-52425 Juelich, Germany John A. McLean and Akbar Montaser*

Department of Chemistry, George Washington University, Washington, D.C. 20052

The direct injection high efficiency nebulizer (DIHEN) was explored for the ultrasensitive determination of long-lived radionuclides (226Ra, 230Th, 237Np, 238U, 239Pu, and 241Am) and for precise isotope analysis by inductively coupled plasma mass spectrometry (ICPMS). The DIHEN was used at low solution uptake rates (1-100 µL/min) without a spray chamber. Optimal sensitivity (e.g., 238U, 230 MHz/ppm; 230Th, 190 MHz/ppm; and 239Pu, 184 MHz/ppm) was achieved at low nebulizer gas flow rates (0.16 L/min), high rf power (1450 W), and low solution uptake rates (100 µL/min). The optimum parameters varied slightly for the two DIHENs tested. The detection limits of long-lived radionuclides in aqueous solutions varied from 0.012 to 0.11 ng/L. The sensitivity of the DIHEN was improved by a factor of 3 to 5 compared with that of a microconcentric nebulizer (MicroMist used with a minicyclonic spray chamber at a solution uptake rate of 85 µL/min) and a factor of 1.5 to 4 compared with that of a conventional nebulizer (cross-flow used with a Scott type spray chamber at a solution uptake rate of 1 mL/ min). The precision of the DIHEN ranged from 0.5 to 1.7% RSD (N ) 3) for all measurements at the 10 ng/L concentration level (∼3 pg sample size). The sensitivity decreased to 10 MHz/ppm at a solution uptake rate of 1 µL/min. The precision was about 5% RSD at a sample size of 30 fg for each long-lived radionuclide by the DIHENICPMS method. The oxide to atom ratios were less than 0.05 (except ThO+/Th+ ) and decreased under the optimum conditions in the following sequence: ThO+/Th+ > UO+/U+ > NpO+/Np+ > PuO+/Pu+ > AmO+/Am+ > RaO+/Ra+. Atomic and oxide ions were used as analyte ions for ultratrace and isotope analyses of long-lived radionuclides in environmental and radioactive waste samples. The analytical methods developed were applied to the determination of long-lived radionuclides and isotope ratio measurements in different radioactive waste and environmental samples using the DIHEN in combination with quadrupole ICPMS. For instance, the 240Pu/ 10.1021/ac9900883 CCC: $18.00 Published on Web 06/16/1999

© 1999 American Chemical Society

239Pu

isotope ratio was measured in a radioactive waste sample at a plutonium concentration of 12 ng/L. This demonstrates a main advantage of DIHEN-ICPMS compared with r-spectrometry, which cannot be used to selectively determine 239Pu and 240Pu because of similar r energies (5.244 and 5.255 MeV, respectively). Inductively coupled plasma mass spectrometry (ICPMS) is a popular technique for ultratrace and isotopic ratio analysis of a wide variety of materials.1 For example, measurement of longlived radionuclides in waters, environmental materials, and biological and geological samples is steadily gaining importance2-6 in areas ranging from radiobioassay, decontamination and decommissioning, environmental remediation, nonproliferation and monitoring for clandestine activities, public and occupational health safety, and nuclear waste management. The isotopic ratios of uranium and plutonium7 can reveal the origin of contamination in the environment due to nuclear weapons testing, nuclear accidents, or fallout from nuclear power plants. Very sensitive, accurate, and precise quantification of long-lived radionuclides is also required in the analysis of radioactive waste streams and packages, that is, for characterizing low-level radioactive materials from nuclear reactors for recycling and final storage of the radioactive wastes. Additionally, the variations of isotopic composition in nature, due to radioactive decay of unstable nuclides, is * To whom correspondence should be addressed: (JSB; AM) Telephone: 0049 2461 612698; Telephone: 202-994-6480. Fax: 0049 2461 612560; Fax: 202994-2298. Email: [email protected]; Email: [email protected]. (1) Montaser, A., Ed. Inductively Coupled Plasma Mass Spectrometry; Wiley: New York, 1998. (2) Becker, J. S.; Dietze, H.-J. Adv. Mass Spectrom. 1998, 14, 681-689. (3) Kim, C. K.; Seki, R.; Morita, S.; Yamasaki, S.; Tsumura, A.; Takaku, Y.; Igarashi, Y.; Yamamoto, M. J. Anal. At. Spectrom. 1991, 6, 205-209. (4) Crain, J. S. Spectroscopy 1996, 11(2), 30-39. (5) Becker, J. S.; Dietze, H.-J. J. Anal. At. Spectrom. 1997, 12, 881-889. (6) Becker, J. S.; Dietze, H.-J. 4. Symposium Massenspektrometrische Verfahren der Elementspurenanalyse, 28.9.-1.10.98, Mainz, Book of abstracts, DV 18; Fresenius’ J. Anal. Chem., in press. (7) Passler, G.; Erdmann, N.; Hasse, H.-U.; Herrmann, G.; Huber, G.; Ko¨hler, S.; Kratz, J. V.; Mansel, A.; Nunnemann, M.; Trautmann, N.; Waldek, A. Kerntechnik 1997, 62, 85-90.

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used in geochronology for age determination. In such studies, precise isotopic ratios (%RSD e 0.05) are necessary.2,8,9 The analysis of radioactive samples requires fast analytical methods to measure many samples in a short time with a high degree of accuracy, good precision, and little or no generation of investigative-derived waste. Because of its multielement capability, excellent sensitivity, good precision, ease of sample preparation, and simple measurement procedures, ICPMS is increasingly applied to the determination of long-lived radionuclides at the pg/L concentration level using double-focusing sector field instruments combined with ultrasonic nebulization.2,3 This approach allows simultaneous determination of long-lived radionuclides in aqueous solutions and solid samples. Solid samples are analyzed directly using laser ablation,10 or after digestion and dilution, often with matrix separation to improve detection limits and precision. Recently Becker et al.11 directly measured several long-lived radionuclides in a concrete laboratory standard and reported detection limits of 20 pg/g for 236U using a double-focusing sector field ICPMS. The introduction of aqueous solutions and solid materials in ICPMS is reviewed elsewhere.12-14 To reduce sample consumption and consequently minimize instrumental radioactivity contamination, doses to the operator, and reduce waste generation, high efficiency micronebulizers may be combined with sensitive analytical techniques, such as ICPMS, for the precise and accurate determination of ultratrace and isotope ratios of radionuclides. Various low-consumption nebulizers such as the microconcentric nebulizer (MCN),15-17 the high efficiency nebulizer (HEN),18-23 the direct injection nebulizer (DIN),24-27 and the oscillating (8) Heumann, K. G.; Gallus, S. M.; Ra¨dlinger, G.; Vogl, J. J. Anal. At. Spectrom. 1998, 13, 1001-1008. (9) Platzner, I. T. Modern Isotope Ratio Mass Spectrometry; Wiley: New York, 1997. (10) Gastel, M.; Becker, J. S.; Ku ¨ ppers, G.; Dietze, H.-J. Spectrochim. Acta, Part B 1997, 52B, 2051-2059. (11) Becker, J. S.; Gastel, M.; Tenzler, D.; Dietze, H.-J. In 1998 Advances in Mass Spectrometry, vol. 14, MoPol141, Proceedings of the 14th International Mass Spectrometry Conference, Tampere, Finland, August 25-29, 1997. (12) Montaser, A.; Minnich, M. G.; McLean, J. A.; Lui, H.; Caruso, J. A.; McLeod, C. W. Sample Introduction in ICPMS. In Inductively Coupled Plasma Mass Spectrometry; Montaser, A., Ed.; Wiley: New York, 1998. (13) Becker, J. S.; Dietze, H.-J. Spectrochim. Acta, Part B 1998, 53B, 14751506. (14) McLean, J. A.; Minnich, M. G.; Iacone, L. A.; Liu, H.; Montaser, A. J. Anal. At. Spectrom. 1998, 13, 829-842. (15) Vanhaecke, F.; Van Holderbeke, M.; Moens, L.; Dams, R. J. Anal. At. Spectrom. 1996, 11, 543-548. (16) Augagneur, S.; Medina, B.; Szpunar, J.; Lobinski, R. J. Anal. At. Spectrom. 1996, 11, 713-721. (17) Becker, J. S.; Soman, R. S.; Sutton, K. L.; Caruso, J. A.; Dietze H.-J. J. Anal. At. Spectrom., in press. (18) Nam, S.-H.; Lim, J.-S.; Montaser, A. J. Anal. At. Spectrom. 1994, 9, 13571362. (19) Liu, H.; Montaser, A. Anal. Chem. 1994, 66, 3233-3242. (20) Olesik, J. W.; Kinzer, J. A.; Harkleroad, B. Anal. Chem. 1994, 66, 20222030. (21) Pergantis, S. A.; Heithmar, E. M.; Hinners, T. A. Anal. Chem. 1995, 67, 4530-4535. (22) Liu, H.; Montaser, A.; Dolan, S. P.; Schwartz, R. S. J. Anal. At. Spectrom. 1996, 11, 307-311. (23) Liu, H.; Clifford, R. H.; Dolan, S. P.; Montaser, A. Spectrochim. Acta, Part B 1996, 51B, 27-40. (24) Greenfield, S.; Jones, I. L.; Berry, C. T.; Spash, D. I. Improvements Relating to Spectroscopic Methods and Apparatus. U.K. Patent 1,109,602, 1968. (25) Lawrence, K. E.; Rice, G. W.; Fassel, V. A. Anal. Chem. 1984, 56, 289292. (26) Wiederin, D. R.; Smith, F. G.; Houk, R. S. Anal. Chem. 1991, 63, 219-225.

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capillary nebulizer (OCN),28,29 have been applied in ICP spectrometries to increase analyte transport and nebulizer efficiency. These micronebulizers are important for the analysis of small volume samples and for application in microscale high-performance liquid chromatography (µHPLC), microscale flow injection analysis (µFIA), or capillary electrophoresis (CE).12,14 Recently, a simple and relatively low-cost direct injection high efficiency nebulizer (DIHEN) was introduced by J E Meinhard Associates, Inc., which was based on the work of Montaser and co-workers, for ICPMS.12,14,30,31 As a result of the direct nebulization of sample solution into the plasma, the analyte transport efficiency is 100% and no spray chamber is required, and thus, investigativederived waste is minimized. The investigators observed a 12-fold improvement in absolute detection limits of elements tested and an improvement in precision over conventional pneumatic nebulization.30 The aim of this study is to examine the analytical characteristics of the DIHEN for the ICPMS determination of long-lived radionuclides in the ultratrace concentration range compared with those of the cross-flow nebulizer and the MicroMist microconcentric nebulizer. Furthermore, the DIHEN is explored for isotope ratio measurements and ultrasensitive trace determination of thorium, uranium, and plutonium in radioactive waste and environmental standard reference materials. EXPERIMENTAL SECTION The Direct Injection High Efficiency Nebulizers and the ICPMS Instrumentation. Schematic diagrams of the DIHEN (J E Meinhard Associates, Inc., Santa Ana, CA), the nebulizer tip, and its interface with a typical demountable torch are shown in Figure 1. The ICPMS instrumentation used in this work was an Elan 6000 ICPMS (Perkin-Elmer/Sciex Corp., Norwalk, CT). Details of coupling the DIHEN to a demountable plasma torch are described elsewhere.30 The DIHEN is constructed from borosilicate glass, on the basis of the nebulizer tip dimensions of the HEN.19 The length of the HEN was increased from 75 to 200 mm for the DIHEN to place the nebulizer inside the torch at the base of the plasma.30 Aqueous solutions were introduced to the DIHEN in the continuous flow mode via a syringe pump (Harvard Apparatus, Inc., Holliston, MA). The nebulizer gas flow rate was controlled using the ICPMS mass flow controller (Model 1179 A, MKS Instruments, Andover, MA). To check the accuracy of the nebulizer gas flow, a calibrated external argon flow controller (Model PR 4000, MKS Instruments) was used. For comparison, a cross-flow nebulizer (Perkin-Elmer/Sciex Corp.) and a MicroMist microconcentric nebulizer (Model MicroMist AR30-1F02, Glass Expansion Pty. Ltd., Camberwell, Victoria, Australia) were used with a 97-mL Scott type spray chamber (Perkin-Elmer/Sciex Corp.) and a 20-mL minicyclonic spray chamber (Cinnabar, Glass Expansion Pty. Ltd.), respectively. A schematic diagram of the MicroMist nebulizer is shown in (27) Shum, S. C. K.; Houk R. S. Anal. Chem. 1993, 65, 2972-2976, and references cited therein. (28) Wang, L.; May, S. W.; Browner, R. F.; Pollock, S. H. J. Anal. At. Spectrom. 1996, 11, 1137-1146. (29) Sutton, K. L.; B’Hymer, C.; Caruso, J. A. J. Anal. At. Spectrom. 1998, 13, 885-891, and references cited therein. (30) McLean, J. A.; Zhang, H.; Montaser, A. Anal. Chem. 1998, 70, 1012-1020. (31) Singh, J.; McLean, J. A.; Pritchard, D. E.; Montaser, A.; Patierno, S. R. Toxicol. Sci. 1998, 46, 260-265.

Figure 1. (A) Schematic diagram of the DIHEN and an enlarged view of the nebulizer tip. (B) The DIHEN replaces the injector tube of the ICP torch and is positioned directly below the plasma.

Figure 2. (A) Schematic diagram of the MicroMist microconcentric nebulizer and an enlarged view of the nebulizer tip. (B) Schematic diagram of the 20-mL minicyclonic spray chamber used with the MicroMist nebulizer (courtesy of Glass Expansion Pty. Ltd.).

Figure 2A and the associated minicyclonic spray chamber in Figure 2B. Aqueous solutions were introduced to the cross-flow and MicroMist nebulizers via a peristaltic pump (model Perimax 12, Spetec GmbH, Erding, Germany). The experimental parameters for mass spectrometric measurements are summarized in Table 1.

Measurement Procedures. The DIHEN-ICPMS system was optimized daily by maximizing the ion intensity of 238U+. The total acquisition time per replicate for the measurement of all longlived radionuclides was 60 s (with 3 replicates, i.e., a total measurement time of 3 min for each sample). The sample size was 3 pg for the nebulization of a 10 ng/L solution of each longAnalytical Chemistry, Vol. 71, No. 15, August 1, 1999

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Table 1. Operating Conditions for the Ar ICPMS Instrumenta sample introduction system manufacturer spray chamber solution uptake rate, µL/min nebulizer gas flow rate, L/min measurement sample size,b pg ICPMS system RF power, W lens potential, V For all nebulizers: outer gas flow rate, L/min intermediate gas flow rate, L/min sampling depth, mm scan mode points/mass dwell time/mass, ms sweeps/replicate total acquisition time/replicate, min replicates calibrated detection system dead time, ns a

Cross-flow Perkin-Elmer/Sciex Corp. Scott type, 97-mL, Ryton 1000 0.92 30

MicroMist Glass Expansion Pty. Ltd. Minicyclonic, 20-mL, Cinnabar 85 0.98 2.6

DIHEN J E Meinhard Associates, Inc. 85 0.16, 0.18 2.6

1250 7.0

1250 7.2

1450, 1500 8.2

14 0.8 12 Peak hopping 1 50 30 1 3 53

Unless otherwise noted. b On the basis of a 10 ng/L solution and 3 min analysis time.

lived radionuclide using a solution uptake rate of 100 µL/min. For the cross-flow and MicroMist nebulizers, the sample size (for 10 ng/L concentration) was 30 and 2.55 pg at solution uptake rates of 1 mL/min and 85 µL/min, respectively. Calibration curves and detection limits (3σ) were established by measurements at elemental concentrations of 0.5, 1, 2, 5, and 10 ng/L. For the determination of precision (short-term stability) and sensitivity, the concentration of each radionuclide was 10 ng/L. Materials. A mixture of long-lived radionuclides (226Ra, 230Th,232Th, 233U, 235U, 237Np, 238U, 239Pu, and 241Am) in high-purity water (18 MΩ-cm) was used in the comparison of the three sample introduction systems. Uranium isotope solutions with different 235U/238U isotope ratios (approximately 1, 0.02, and 0.00725) were tested. For isotope ratio measurements, three isotope standards were used: a laboratory standard (CCLU-500, Nuclear Research Center, Prague, Czech Republic),32,33 a certified reference material (CRM U-020, New Brunswick Laboratory, Argonne, IL, (formerly National Bureau of Standards SRM U-020)),34,35 and a spectroscopic uranium standard (solution) with natural isotopic composition (SRM 3164, National Institute of Standards and Technology, Gaithersburg, MD). Furthermore, a soil sample from the vicinity of a nuclear power plant in Eastern Europe (after digestion of solid sample and uranium extraction6) and two aqueous radioactive waste samples were analyzed by ICPMS. RESULTS AND DISCUSSION Optimization of the Mass Spectrometric Method for the Determination of Long-Lived Radionuclides at Ultratrace Concentration Levels by DIHEN-ICPMS. Figure 3 shows the sensitivity for different long-lived radionuclides of Ra, Th, Np, U, Pu, and Am as a function of nebulizer gas flow rate, RF power, and solution uptake rate. These data demonstrate that this DIHEN offers optimal sensitivity for 238U (230 MHz/ppm) at 1450 W, a (32) Source: Dietze, H.-J. Zfl-Mitt. 1979, 27, 101-109. (33) Platzner, I. T.; Becker, J. S.; Dietze, H.-J. Atom. Spectrosc. 1999, 20, 6-12. (34) Russ, G. P. III; Bazan, J. M. Spectrochim. Acta, Part B 1987, 42B, 49-62. (35) Callis, E. L.; Abernathey, R. M. Int. J. Mass. Spectrom. Ion. Processes 1991, 103, 93.

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Figure 3. Plots of the sensitivity of long-lived radionuclides measured with the DIHEN-ICPMS. (A) Effect of nebulizer gas flow rate. (B) Effect of RF power. (C) Effect of solution uptake rate.

solution uptake rate of 100 µL/min, and a nebulizer gas flow rate of 0.16 L/min. The optimal nebulizer gas flow rate (0.18 and 0.16 L/min for the two DIHENs used in this work, respectively) is less than that (0.25 L/min) reported for the DIHEN investigated by Montaser and associates30 and considerably less than those for the cross-flow and MicroMist nebulizers (0.9 and 1 L/min, respectively). These results confirm that the sensitivities of all

Table 2. Sensitivity, Detection Limits, and Precision in the Determination of Long-Lived Radionuclides by ICPMS Using Cross-flow,a MicroMist,a and DIHEN Sample Introduction Systemsb sensitivity (MHz/ppm) radionuclide 226Ra 230Th 237Np 238U

Cross-flow

MicroMist

DIHEN

Cross-flow

MicroMist

DIHEN

Cross-flow

MicroMist

DIHEN

47 57 84 60

22 34 39 45

420 390 430 350

220 570 110 340

4.0 3.8 0.8 5.8

42

560

480

110 28 110 12 63 89

1.3 1.8 1.9 1.2

93

95 190 121 230 184 163

0.9

3.4

0.8 1.4 1.6 0.5 1.6 1.7

239Pu 241Am a

precisionc (%RSD)

detection limit (pg/L)

From ref 17. b Using a 10 ng/L solution. c Measured over 3 min, N ) 3.

long-lived radionuclides are affected by changing the experimental parameters (RF power, nebulizer gas flow rate, or solution uptake rate). At a nebulizer gas flow rate of 0.12 L/min, the sensitivity of long-lived radionuclides is comparable with those observed with a cross-flow nebulizer operated at 0.9 L/min. A further decrease in the nebulizer gas flow rate (below 0.12 L/min) results in an unstable plasma. Measurements with two different DIHENs revealed a slight variation in the optimum parameters. For example, the optimal sensitivity of a second DIHEN varied from 130 MHz/ppm for 226Ra (vs 95) to 180 MHz/ppm for 238U (vs 230) at 1500 W, a sample uptake rate of 85 µL/min, and an injector gas flow rate of 0.16 L/min. Table 2 shows the results for sensitivity, detection limits, and precision under optimal conditions for several longlived radionuclides (226Ra, 230Th, 237Np, 238U, 239Pu, and 241Am) using the DIHEN, cross-flow, and MicroMist nebulizers. Overall, the DIHEN offers improved sensitivity, limits of detection, and precision compared with the other nebulizers tested. The sensitivity of DIHEN-ICPMS ranged from 95 to 230 MHz/ppm for the elements tested. In contrast, the cross-flow and MicroMist nebulizers, both used with a spray chamber, offer sensitivity ranging from 47 to 93 MHz/ppm and 22 to 45 MHz/ppm, respectively. Therefore, the DIHEN (solution uptake rate of 100 µL/min) is 3-5 times more sensitive compared with the MicroMist (85 µL/min) and 1.5-4 times more sensitive than the crossflow (1 mL/min). In terms of absolute sensitivity, the DIHEN offers a factor of 14 to 38 greater sensitivity than the cross-flow nebulizer. However, both in this and the previous study,30 the DIHEN response function exhibits a greater variability in the actinide region than is observed when nebulizer-spray chamber combinations are used. Short-term precision (over 3 min), defined as the percent relative standard deviation (% RSD), for the DIHEN ranged from 0.5 (238U) to 1.7 (241Am) % RSD at very low concentration (10 ng/ L), a general improvement over the other nebulizers tested. These results demonstrate the excellent analytical performance of the DIHEN with ICPMS. The improved precision offered by the DIHEN compared with that of the other nebulizers is attributed to the elimination of noise sources associated with the spray chamber and better counting statistics due to the significantly higher sensitivity. The detection limits were estimated from the calibration curves prepared for long-lived radionuclides in the low ng/L concentration range. A typical calibration curve for 239Pu is presented in Figure 4. The correlation coefficients for the calibration curves

Figure 4. Calibration curve for 239Pu using the DIHEN for determination of the detection limit in high purity water. The DIHEN was operated with an RF power of 1500 W, a solution uptake rate of 85 µL/min, and a nebulizer gas flow rate of 0.18 L/min.

were better than 0.999 for all of the long-lived radionuclides tested. Excellent limits of detection for long-lived radionuclides were measured for DIHEN-ICPMS, ranging from 12 pg/L for 238U to 110 pg/L for 237Np. Table 3 shows the sensitivity and the precision of long-lived radionuclides at different nebulizer gas flow rates at an RF power of 1450 W and a solution uptake rate of 100 µL/min. As shown in Figure 3, the greatest sensitivity and the best precision (0.5-1.7% for 10 ng/L solution) was observed at low nebulizer gas flow rate (0.16 L/min). A change in the nebulizer gas flow rate up to 0.25 L/min6 or down to 0.14 L/min caused a significant decrease in sensitivity and a slight decrease in precision for most radionuclides tested. The dependence of the sensitivities of 226Ra, 233U, 235U, 237Np, 238U, 239Pu, and 241Am on the solution uptake rate is shown in Table 4. With the DIHEN, one can analyze at small sample uptake rates (as low as 1 µL/min) in the continuous flow mode with relatively good sensitivity. Note that the sample size is only 30 fg at a concentration of 10 ng/L for each of the long-lived radionuclides using a solution uptake rate of 1 µL/min. The sensitivity of longlived radionuclides declines with decreasing solution uptake rate (from 100 to 1 µL/min) by a factor of 13 to 17 (Figure 3C). This is accompanied by a decrease in precision. At a solution uptake rate of 1 µL/min, a sensitivity of 7-18 MHz/ppm and a precision of 2.6-5.4% RSD are measured using a 10 ng/L solution over 3 min. Analytical Chemistry, Vol. 71, No. 15, August 1, 1999

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Table 3. Effect of Nebulizer Gas Flow Rate on Sensitivity and Precision with DIHEN-ICPMSa nebulizer gas flow rate (L/min) 0.14 radionuclide

0.18

sensitivity (MHz/ppm)

precision (%RSD)b

sensitivity (MHz/ppm)

precision (%RSD)b

sensitivity (MHz/ppm)

precision (%RSD)b

57 108 87 83 63 136 116 91

2.5 1.9 3.2 1.6 2.7 2.2 0.7 6.5

95 188 167 155 121 230 184 163

0.8 0.6 0.6 0.2 1.6 0.5 1.6 1.7

78 145 124 118 92 189 161 131

2.6 2.0 2.3 2.5 2.3 1.9 1.6 1.7

226Ra 232Th 233U 235U 237Np 238U 239Pu 241Am a

0.16

The DIHEN was operated with an RF power of 1450 W and a solution uptake rate of 100 µL/min. b Measured over 3 min, N ) 3.

Table 4. Effect of Solution Uptake Rate (µL/min) on Sensitivity and Precision with DIHEN-ICPMSa solution uptake rate (µL/min) 1

5

10

50

85

100

sensitivity (MHz/ppm) radionuclide 226Ra 233U 235U 237Np 238U 239Pu 241Am

7 10 9 7 18 13 11

13 23 20 16 35 28 20

20 32 31 25 49 36 29

50 83 81 60 133 93 80

80 136 129 98 206 159 135

95 167 155 121 230 184 163

precision (%RSD)b radionuclide 226Ra 233U 235U 237Np 238U 239Pu 241Am

4.9 4.6 4.7 5.4 2.6 4.2 4.9

3.9 3.1 4.1 3.9 2.3 3.9 2.3

2.1 1.1 2.0 2.6 1.7 3.5 2.5

1.6 0.9 1.7 1.2 1.9 1.5 1.4

0.8 0.4 0.3 2.5 0.6 1.6 1.1

0.8 0.6 0.2 1.6 0.5 1.6 1.7

a The DIHEN was operated with an RF power of 1450 W and a nebulizer gas flow rate of 0.16 L/min. b Measured over 3 min, N ) 3.

Table 5. Relative Intensities of Molecular Ions of Long-Lived Radionuclides Using Cross-flow,a MicroMist,a and DIHEN Sample Introduction Systems with ICPMS relative ion intensity (MX+/M+; X ) O, H) molecular ion

Cross-flow

MicroMist

DIHENb

232Th1H+

2.0 × 10-5 2.1 × 10-5 2.5 × 10-2 2.7 × 10-2

1.6 × 10-5 1.9 × 10-5 2.8 × 10-2 3.1 × 10-2

2.4 × 10-5 2.5 × 10-5 7.5 × 10-2 5.1 × 10-2

238U1H+

232Th16O+ 238U16O+

a From ref 17. b The DIHEN was operated with an RF power of 1450 W, a solution uptake rate of 100 µL/min, and an injector gas flow rate of 0.16 L/min.

Molecular Ion Formation in ICPMS Using the DIHEN. A careful study of molecular ion formation is required to avoid systematic errors in trace, ultratrace, and isotopic analysis. It is well-known that higher oxide ion intensities are observed for direct injection nebulizers because the primary aerosol is injected directly into the ICP without any aerosol conditioning (e.g., spray 3082 Analytical Chemistry, Vol. 71, No. 15, August 1, 1999

Figure 5. Plots of relative oxide ion intensity of long-lived radionuclides using the DIHEN-ICPMS. (A) Effect of nebulizer gas flow rate. (B) Effect of RF power.

chamber) or aerosol desolvation.26,30 The oxide formation with the DIHEN is shown in Figure 5 for long-lived radionuclides as a function of RF power and nebulizer gas flow rate. In general, oxide formation rates increased at lower RF power and for low ( 0.16 L/min) nebulizer gas flow rates. Different isotopes of thorium and uranium (230Th, 232Th, 233U, 235U, and 238U) exhibited an identical oxide-to-atom ratio dependence on the experimental parameters, indicating that these oxide ions are interference-free. The relative oxide ion intensities decrease in the following sequence:

ThO+/Th+ > UO+/U+ > NpO+/Np+ > PuO+/Pu+ > AmO+/Am+ > RaO+/Ra+ Except for ThO+/Th+, oxide to atom ratios are less than 0.05 using the described optimum conditions. The determinations of

Table 6. Precision of the Uranium Isotope Ratio (235U/238U) and the Measured Uranium Isotope Ratio in Different Reference Materials Using the DIHEN, MicroMist, and Cross-flow Nebulizersa reference material CCLU-500 DIHENd MicroMist Cross-flow DIHENd (accepted ratio)

0.06 0.20 0.25 0.9696 ( 0.0014 (0.9995 ( 0.0009)e

SRM 3164b

CRM U-020 precision (% RSD)c 0.17 0.29 0.34*

0.24 0.32 054**

measured 235U/238U isotope ratio 0.02061 ( 0.00003 (0.02081 ( 0.00002)f

0.00727 ( 0.00002 (0.00725)

a The uranium concentration was 1 µg/L, except for cases marked by * (5 µg/L) or ** (10 µg/L). b Natural uranium abundance. c Measured over 3 min. d The DIHEN was operated with an RF power of 1500 W, a solution uptake rate of 85 µL/min, and a nebulizer gas flow rate of 0.18 L/min. e From refs 32 and 33. f From ref 9 (see Table 9.80, p 372).

233U, 239Pu,

and 248Cm could be disturbed by the molecular interferences from 232Th1H+, 238U1H+, and 232Th16O+, respectively. The relative intensities of hydride and oxide ions for the crossflow, MicroMist, and DIHEN are listed in Table 5. Whereas the hydride formation rate ranged from 1.6 × 10-5 to 2.5 × 10-5 across the three nebulizer types, the oxide formation rate is a factor of almost 2-3 higher using the DIHEN. Lower oxide levels are measured in these experiments because the optimum nebulizer gas flow rate is 0.16 L/min, slightly less than the gas flow rate (0.18 L/min) used with the second DIHEN.6 Because of their relatively high intensity, the oxide ions may be used as analyte ions for interference-free analysis or for verifying analytical results. Application of the DIHEN to Isotope Ratio Measurements of Uranium, Thorium, and Plutonium in Isotope Standards and Radioactive Waste. A summary of uranium isotope ratio measurements using DIHEN-ICPMS is presented in Table 6. For a 1 µg/L uranium solution, the 235U/238U isotope ratios of 1, 0.02, and 0.00725 yielded precisions of 0.06%, 0.17%, and 0.24% RSD, respectively. These data are better as compared with the precision obtained for the cross-flow and MicroMist nebulizers. The accuracies of 235U/238U isotope ratios, without mass bias correction, are 2.7, 0.96, and 0.2%, for CCLU-500, CRM U-020, and SRM 3164, respectively. More accurate isotope ratio measurements call for mass bias correction.8,9 The DIHEN and the MicroMist were applied for the isotope ratio measurements of uranium and thorium in a radioactive waste solution (Table 7). The solution contained 236U and 230Th which are not found in nature. The precision of isotope ratio measurements was dependent on the elemental concentration and the magnitude of the isotope ratio. For isotope ratios on the order of 10-4 and lower, the DIHEN generally offers better precision even though it is used at 10 times lower concentration (1 µg/L) compared with the MicroMist (10 µg/L). The determination of the 235U/238U and 230Th/232Th isotope ratios is feasible with DIHEN-ICPMS using both the atomic and oxide species as the analyte ions. The use of oxide ions (see Table 7) for precise isotope analysis is of interest in ICPMS because of the ability to check analytical results. Good agreement between measured isotope ratios was found, but because of the lower intensity of oxide ions, the precision of isotope ratios was decreased compared with analysis using the atomic ions. Table 8 shows a summary of the results for thorium, uranium, and plutonium isotopes in a radioactive waste solution. Note that the

Table 7. Isotope Ratio Determination of Uranium and Thorium in a Radioactive Waste Solution Using a DIHEN and a MicroMist Nebulizer with ICPMS uranium isotope ratios DIHENa (1 µg/L) 234U/238U 235U/238U 235U16O/238U16O 236U/238U

MicroMist (10 µg/L)

ratio

% RSD

ratio

% RSD

0.0000175 0.003421 0.003439 0.000072

1.4 0.27 2.4 2.1

0.0000203 0.003531

1.6 0.24

0.000066

2.5

thorium isotope ratios DIHENa (1 µg/L) ratio 230Th/232Th

0.002265 0.002223

230Th16O/232Th16O

% RSD 2.4 3.4

MicroMist (10 µg/L) ratio

% RSD

0.002218

7.0

a The DIHEN was operated with an RF power of 1500 W, a solution uptake rate of 85 µL/min, and a nebulizer gas flow rate of 0.18 L/min.

Table 8. Isotope Ratio Determination of Thorium, Uranium, and Plutonium in a Radioactive Waste Solution Using DIHEN-ICPMSa element

concn (ng/L)

Th U

0.6 0.5

isotope ratio

measured value

230Th/232Th

0.138 ( 0.019 0.554 ( 0.052 0.352 ( 0.030 0.169 ( 0.009

235U/238U 236U/238U

Pu

12

240Pu/239Pu

a The DIHEN was operated with an RF power of 1450 W, a solution uptake rate of 100 µL/min, and a nebulizer gas flow rate of 0.16 L/min.

isotope ratios 230Th/232Th,235U/238U, and 236U/238U were determined at a concentration of 0.6 ng/L for thorium and 0.5 ng/L for uranium. Furthermore, the concentrations of 241Am and 237Np were determined in the radioactive waste sample with 1 ng/L and 3 ng/L, respectively. The 240Pu/239Pu isotope ratio was measured at a plutonium concentration of 12 ng/L. The latter demonstrates the main advantage of DIHEN-ICPMS compared with R-spectrometry, which cannot be used to determine 239Pu and 240Pu because of their similar R-energies (5.244 and 5.255 MeV, Analytical Chemistry, Vol. 71, No. 15, August 1, 1999

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Table 9. Isotope Ratio Determination of 235U/238U in a Soil Sample Using the Atomic and Oxide Ions as Analyte Ions measured value DIHENa (1 µg/L) concn element (µg/g) U

0.7

isotope ratio 235U/238U 235U16O/238U16O

MicroMist (10 µg/L)

ratio

% RSD

ratio

% RSD

0.02216 0.02218

0.13 0.54

0.02206

0.22

a The DIHEN was operated with an RF power of 1500 W, a solution uptake rate of 85 µL/min, and a nebulizer gas flow rate of 0.18 L/min.

respectively)36 and the time-consuming nature of their sample preparation. Isotope Ratio Measurements of Uranium in a Soil Sample by DIHEN-ICPMS. A careful separation of uranium is required to measure the isotope ratio of uranium in geological or technical samples by ICPMS. This procedure is important to avoid matrix effects, nebulizer clogging, and possible molecular interferences. The uranium-matrix separation procedure involves leaching the geological or technical sample with concentrated nitric acid and uranium extraction with methyl isobutyl ketone (MIBK). After back-extraction, the aqueous solution can be analyzed by ICPMS.6 Table 9 shows the results of uranium isotope ratio measurements in a soil sample using the atomic and oxide species as analyte ions. Uranium was determined in a digested solution of soil samples after leaching of the solid material and extraction of the uranium. A concentration of 1 µg/L uranium was used for the DIHEN compared with 10 µg/L used for the MicroMist. The mass bias correction for uranium isotope ratio measurements was performed using isotope standard reference materials as described elsewhere.8 The results of several measurements are in excellent agreement. CONCLUSIONS It was shown that DIHEN-ICPMS can be applied successfully for the sensitive and precise determination of long-lived radionuclides at very low concentration levels. A significant improvement in sensitivity is obtained with the DIHEN compared with the conventional cross-flow and MicroMist nebulizers. The DIHENICPMS approach allows measurements in aqueous solutions at the ng/L concentration range with solution uptake rates as low as 1 µL/min (very small sample sizes down to the femtogram (36) Lide, D. R., Ed. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, 1995. (37) Kelly, W. R.; Fassett, J. D.; Hotes, S. A. Health Phys. 1987, 52, 331-336. (38) Turner, P. J.; Mills, D. J.; Schro¨der, E.; Lapitajs, G.; Jung, G.; Iacone, L. A.; Haydar, D. A.; Montaser, A. Instrumentation for Low- and High-Resolution ICPMS. In Inductively Coupled Plasma Mass Spectrometry; Montaser, A., Ed.; Wiley: New York, 1998.

3084 Analytical Chemistry, Vol. 71, No. 15, August 1, 1999

range) with very good precision. Application of the DIHEN is of interest especially where sample size is limited, for example, in tracer experiments using enriched stable isotopes in biological or medical research and for the determination of volatile species (e.g., Hg, Se). The significant improvement in absolute sensitivity for the long-lived radionuclides (14-38-fold) and the improvements realized in precision and detection limits, compared with the conventional cross-flow nebulizer, is of special interest for interfacing techniques such as µFIA, µHPLC, or CE with ICPMS detection. Further fundamental studies in molecular species formation, ion formation processes, and matrix effects in DIHENICPMS are required to better understand the greater variability of the response function observed in the actinide region than that observed when nebulizer-spray chamber combinations are used. It is relevant to contrast this study with thermal ionization mass spectrometry (TIMS).9 Of course, TIMS is less prone to elemental memory effects from one sample to the next and has the ability to generate negative ion beams. Compared with TIMS, the DIHEN-ICPMS approach has the benefits of easy sample preparation in terms of analyte separation and purification, is applicable to nearly all elements, offers higher sample throughput, does not suffer from time-dependent mass discrimination effects (e.g., Rayleigh distillation), provides ionization processes that are less affected by elemental ionization potential, and demands less operational skill. In TIMS, the typical sample size ranges from microgram to nanogram quantities, although in some cases a few femtograms can be analyzed.9,37 In contrast, the sample size required in DIHEN-ICPMS is in the low picogram range with quadrupole instruments and can be reduced using double-focusing ICPMS. Clearly, the significance of ICPMS in precise isotope ratio measurements at ultratrace levels is increasing, especially when time-of-flight, multicollector, and/or sector-field instruments are used.38 ACKNOWLEDGMENT The authors are grateful to Mr. M. Girnus (Central Department of Analytical Chemistry, Research Centre Juelich) for his careful operation of the ICPMS instrument. Research by A.M./J.A.M. was sponsored by grants from the U.S. Department of Energy (DEFG02-93ER14320), the National Science Foundation (CHE-9505726 and CHE-9512441), and J E Meinhard Associates, Inc. Scholarship support for J.A.M. was provided by the ARCS Foundation. We would like to thank Michael G. Minnich (George Washington University, Washington, D.C.), Danold W. Golightly (Ross Laboratories, Columbus, OH), and Thomas M. Yoshida (Los Alamos National Laboratory, Los Alamos, NM) for their constructive comments in the preparation of this manuscript. Received for review January 28, 1999. Accepted April 26, 1999. AC9900883