concentration uranium isotope ratios in small volumes of urine. R. Steven Pappas and ... Reagents, base urine pools, NBS U005 depleted uranium standard and ...
TECHNICAL NOTE
www.rsc.org/jaas | Journal of Analytical Atomic Spectrometry
Simple changes improve sample throughput for determination of low concentration uranium isotope ratios in small volumes of urine R. Steven Pappas and Dan C. Paschal Received 5th October 2005, Accepted 15th December 2005 First published as an Advance Article on the web 10th January 2006 DOI: 10.1039/b514040d An increase in the solid phase extraction column wash volume, change of internal standard, magnet mass, and use of an instrumental mass offset has permitted the elimination of digestion and cutting the sample preparation time of the previously published method in half (50 min) without cost to 235U/238U ratio accuracy. A previously reported method for the determination of 235U/ U ratios at low ng L 1 concentrations in 2–4 mL samples of urine addressed issues such as the sensitivity necessary for accurate 235U ion counts, eliminating an organic-based polyatomic interference at mass 235, removing matrix suppression of signal and gas flow issues related to accuracy and precision.1 In that method, 193Ir was chosen as an internal standard due to the license requirement for a more appropriate internal standard such as 233U.2 Microwave digestion was used to eliminate urine organic substances that resulted in interference with counting at mass 235 which would preclude the use of an Aridus desolvation system for enhancing signal intensity. The need for sensitivity to low concentration 235U/238U ratios made eliminating the desolvation system undesirable. The need for increases in throughput compelled additional work to eliminate digestion from sample preparation, as reported here. Reagents, base urine pools, NBS U005 depleted uranium standard and NBS U020 enriched uranium-spiked base urine were obtained from sources previously reported.1 Natural uranium SRM 3164 was obtained from National Institute of Standards and Technology (NIST, Gaithersburg, MD, USA). CRM 115 and CRM 111-A were obtained from New Brunswick Laboratories (Argonne, IL, USA). The 233U internal standard solution was prepared by dilution of CRM 111-A to 100 ng L 1 in 5% v/v nitric acid. Internal quality control (QC) standards were prepared by spiking base urine with dilutions of SRM 3164, NBS U005, and CRM 115 natural and depleted uranium reference materials to the final concentrations reported here. Total uranium for internal urine QC standards was determined as previously described.3 Target ratios for internal QC standards were calculated from natural U in the base urine and the depleted U concentration determined by the difference in total U concentration after spiking with reference material. To 2 mL of urine, urine QC solution or ultrapure water blanks, 100 mL of 233U internal standard solution and 750 mL of nitric acid were added. The acidified solutions were loaded into columns containing 0.22 g of TRU resin (Eichrom 238
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Technologies, Darien, IL, USA) prepared as previously described.1 The solutions were followed with a 2.6 mL container wash and two 6 mL column washes with 4.7 M (30% v/v) nitric acid. This was followed by a 600 mL wash with 1.4 M HF. Uranium was eluted with 800 mL of 1.4 M HF. Eluates were diluted with 200 mL of 5% v/v nitric acid to improve signal stability. Analyses were performed with a ThermoElectron (Bremen, Germany) Element 2, an Aridus desolvation unit (Cetac, Omaha, NE, USA) with Elemental Scientific (Omaha, NE, USA) perfluoroalkoxy (pfa) upgrade spray chamber and a 100 mL min 1 nebulizer with self aspiration. The instrumental conditions were as previously described with the following exceptions.1 The magnet mass was 233.039, with 500 samples per peak, 1% mass and integration windows, 0% search windows and 0.1500 mass offset for 235U and 238U. Gwiazda, et al., have described a polyatomic species associated with urine organic substances at mass 234.8.4 The effects of this substance on the apparent 235U/238U isotope ratio as a function of U concentration have been discussed.1 One approach to improving ratio accuracy is to remove the source of the interference. In the previous report, organic substances were removed by microwave digestion followed by solid phase extraction (SPE) with a small complexing resin column. By increasing the 30% v/v nitric acid SPE wash volume as described above, the U complexed with the resin was washed essentially free of the sources of mass 235 interference prior to elution. Because of the removal of organic matrix components, the Aridus introduction system could be used without prior urine digestion. Thus, sample preparation time was significantly shortened and blank isotopic counts were lower. Addition of a small quantity of nitric acid after elution resulted in a more stable signal than was apparent with the dilute HF eluent alone. However, a source of organic substances that may interfere with the mass 235 signal measurement comes from the TRU chelation compound adsorbed on the resin. If not sufficiently prewashed, the resin contributes excess adsorbent to the eluent, raising the apparent 235U/238U ratio. In order to further minimize the possibility that any small amount of organic content in the eluent might interfere with the analysis, we took advantage of the flat top peaks inherent in magnetic sector instruments and used the mass offset to move the This journal is
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Table 1 Mean and total relative standard deviation of
235
U/238U ratios from urine samples with depleted, natural, and enriched U
Urine U/ng L 1, spike source
Number of runs
Characterized or calculated 235U/238U
Measured 235U/238U total standard deviation
5.1, NIST 20.3, NIST 40.0, NIST 20.9, NBS U005 20.0, NBL 115 500, ORNL U020 (diluted 1/10 : 50.0)
10 15 5 20 20 20
0.007 25 0.007 25 0.007 25 0.004 98 0.002 16 0.020 81
0.007 24 0.007 23 0.007 24 0.005 05 0.002 17 0.020 80
sampling point to the high mass end of the flat top.5,6 This permitted peak sampling at a mass higher than the 235U interference. Isotope ratio analysis of SPE-eluted depleted, enriched and natural U from spiked urine gave accurate ratios with little or no requirement for correction at concentrations as low as 5 ng L 1 of natural U in 2 mL quality control samples (Table 1). The data reported for the NBS U005 502 ng L 1 spike supplied by Los Alamos National Laboratories were obtained from a 1/5 SPE column eluate dilution. The improved method described here is one where high throughput requires minimal sample volume; it issensitive enough to be used with a total urine U concentration of 5 ng L 1 or less, generates excellent within-run precisions (less than 1% ratio RSD) and between-run precisions (0.28–1.8% RSD, Table 1), and is sufficiently accurate to require little or no isotope ratio correction calculations. It has further been
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0.000 07 0.000 04 0.000 02 0.000 05 0.000 04 0.000 08
proven in analyses of clinical samples since the Clinical Laboratory Improvement Act (CLIA) format was updated in 2004.
References 1 R. S. Pappas, B. G. Ting and D. C. Paschal, J. Anal. At. Spectrom., 2003, 18, 1289–1292. 2 C. Chevalier, C. Devillers, R. Hegemann and M. Lucas, Analusis, 1982, 10, 319–322. 3 R. S. Pappas, B. G. Ting, J. M. Jarrett, D. C. Paschal, S. P. Caudill and D. T. Miller, J. Anal. At. Spectrom., 2002, 17, 131–134. 4 R. Gwiazda, K. Squibb, M. McDiarmid and D. Smith, Health Phys., 2004, 86, 12–18. 5 D. Wiederin, Fifth International Conference on Sector Field ICPMS, Omaha, NE, 8/18-20/2004. 6 S. Weyer and J. Schwieters, Int. J. Mass Spectrom., 2003, 226, 355–368.
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