Determination of natural uranium and thorium in environmental samples by ETV–ICP-MS after matrix removal by on-line solid phase extraction Jason B. Truscott,a Lee Bromley,a Phil Jones,a E. Hywel Evans,*a Justine Turnerb and Ben Fairmanb aUniversity of Plymouth, Department of Environmental Sciences, Drake Circus, Plymouth, UK PL4 8AA bLGC, Queens Road, Teddington, Middlesex, UK TW110LY Received 30th September 1998, Accepted 17th February 1999 An on-line solid phase extraction method has been developed for the determination of 238U and 232Th biological certified reference material using inductively coupled plasma mass spectrometry (ICP-MS).Absolute detection limits were 2.7 pg and 3.1 pg for the determination of 238U and 232Th respectively, both being blank limited. The result for the determination of 238U in NASS-4 Open Ocean Sea Water was 2.13±0.28 ng ml-1 compared with a certified value of 2.68±0.12 ng ml-1. The results for the determination of 238U in SLRS-3 River Water was 0.043±0.002 ng ml-1 compared with an indicative value of 0.045 ng ml-1.Results for the determination of 238U and 232Th in NIST 1575 Pine Needles were 14.6±3.4 ng g-1 and 28.3±4.5 ng g-1 respectively compared with certified values of 20±4 ng g-1 and 37±3 ng g-1, using a dry and wet ashing sample preparation method. Results for the determination of 238U and 232Th in NIST 1566a oyster tissue were 121±21 ng g-1 and 29±8 ng g-1 for 238U and 232Th compared to certified and indicative values of 132±12 ng g-1 and 40 ng g-1, using the same method.When a lithium metaborate fusion method was used, results for 238U and 232Th were 23.3±2.0 ng g-1 and 36.2±5.6 ng g-1 respectively in NIST 1575 Pine Needles. The application of electrothermal vaporisation ICP-MS (ETV–ICP-MS) to NASS-4 Open Ocean SeaWater gave 2.81±0.54 ng ml-1 and SLRS-3 River Water 0.045±0.004 ng ml-1 for 238U. When the fused NIST 1575 samples were analysed using ETV–ICP-MS, results for 238U and 232Th were 19.5±1.7 ng g-1 and 38.8±2.2 ng g-1 respectively.Absolute detection limits for ETV– ICP-MS were 30 fg and 9 fg for 238U and 232Th respectively, both being blank limited. cluded that results for the determination of 239Pu in urine were Introduction comparable to those obtained using a-spectrometry. Similarly, Inductively coupled plasma mass spectrometry (ICP-MS) is a 230Th and 234U have been determined in the soil reference technique ideally suited to the determination of the concen- material TRM-4 (ref. 8) using hydrofluoric acid for sample tration and isotopic composition of the actinide elements. The digestion. Chiappini et al.9 have quoted values close to 1.2 fg principal advantages of ICP-MS are speed and sensitivity, detection limits for uranium, using a new high sensitivity ICPwith the capability of determining all the actinide elements MS10 and a high-eYciency desolvating nebulizer. Aldstadt within a minute, at concentrations as low as 1 pg ml-1 in et al.11 have also reported good results for the determination liquid samples.In addition, there is no need to separate the of 238U by FI-ICP-MS using TRU-SpecTM Resin. The use of elements one from another, as there is in a-spectrometry, 209Bi or 205Tl as internal standards has been quoted to be because this is achieved by the mass spectrometer, hence, the applicable for use in thorium and uranium determination in number of sample pre-treatment stages can be greatly reduced.biological samples.12 In this work the application of an actin- However, it is still necessary to separate the radionuclides ide-specific resin for pre-concentration and matrix removal from the matrix, a procedure for which column pre- prior to analysis by ICP-MS, with and without ETV sample concentration methods are ideal. A number of resins have introduction, has been addressed. been used for the pre-concentration and separation of the actinides.Recently a number of very specific chelating resins have become available which are particularly suited to this task. Some extraction procedures and application of these Experimental resins have been addressed by Horwitz and co-workers,1–4 and Pneumatic nebulization ICP-MS detection Crain et al.5 have quoted 20 fg mL-1 detection limits for 239Pu and 235U using TRU-SpecTM resin as a pre-concentration step An inductively coupled plasma mass spectrometer prior to analysis by ICP-MS.Alvardo and Erickson6 obtained (PlasmaQuad 2+, VG Elemental, Winsford, Cheshire, UK) 5 fg and 2 fg detection limits for 238U and 232Th respectively was used. Data was acquired using the time resolved analysis when using electrothermal vaporisation (ETV) coupled with software, which allows time resolved monitoring of multiple ICP-MS and trifluoromethane as a modifier gas, compared to isotopes, and manipulated oV-line using MassLynx software 180 fg and 1600 fg for an unmodified ETV.Wyse and Fisher7 (Micromass Ltd., Manchester, UK). Operating conditions are have reported a potential 3 fg absolute detection limit for shown in Table 1. The flow injection manifold comprising a 500 ml injection loop on a 6 port valve (Model 5020, Rheodyne, plutonium using ICP-MS and TRU-SpecTM resin, and con- J. Anal. At. Spectrom., 1999, 14, 627–631 627Table 1 Operating conditions for ICP-MS VG PQ2+ PE ELAN 5000A ICP— Forward power/W 1350 1080 Plasma gas/l min-1 16.5 15 Auxiliary gas/l min-1 0.7 1.0 Nebulizer gas/l min-1 0.8 0.8 Sampling depth/mm 10 15 Sample flow/ml min-1 0.5 1.0 Torch Fassel (quartz) Fassel (quartz) Nebulizer Concentric (quartz) Cross-flow (Gem-tip) Spray Chamber Scott type (quartz) Interface— Sampler Ni Pt Skimmer Ni Pt Mass spectrometer— Ion masses (m/z) 232Th, 238U, 209Bi 232Th, 238U, 235U Data acquisition Time resolved mode Transient, peak hopping Points per peak 3 1 DAC step 3 n/a Dwell time/ms 20 40 Time-slice duration/s 1 Cotati, CA, USA) was interfaced with the ICP-MS instrument France) in commercially available glass chromatography columns of 3 mm id and 50 mm length (Omnifit microbore as shown in Fig. 1. columns, Omnifit, Cambridge, Cambs., UK). When not in use the columns were filled with 2 M HNO3, and prior to use ETV–ICP-MS detection they were washed with successive portions of 0.1 Mammonium An inductively coupled plasma mass spectrometer (Elan bioxalate and 2M HNO3 at a flow rate of 0.5 ml min-1 for 5000A, Perkin Elmer, Beaconsfield, Bucks., UK) interfaced 6 min, and finally 1 ml of column feed solution.with an electrothermal vaporisation (ETV) sample introduction system (HGA 600MS, Perkin Elmer) was used. Data Reagents were acquired in transient peak hopping mode, which allows time resolved monitoring of multiple isotopes. Operating con- All solutions were prepared using analytical grade reagents ditions for the ICP are shown in Table 1, with the associated and deionised water (Ultra Pure Water, Elgastat Maxima, temperature program for the ETV shown in Table 2.Elga Ltd, High Wycombe, Bucks., UK). Analytical reagents Samples were applied to the column and eluted with 5 ml were: nitric acid, 2 M (Aristar, BDH, Poole, Dorset, UK); of 0.1M ammonium bioxalate into ETV autosampler vials. eluting solution, (0.1M NH4HC2O4 (Fisons Scientific Portions (30 ml ) were pipetted into the ETV furnace tube and Equipment, Loughborough, UK) filtered through a 47 mm the temperature program initiated.diameter 0.45 mm sterile membrane filter paper (Whatman Laboratory Division, Maidstone, Kent, UK); internal stan- Analytical columns dard solution (15 ng ml-1 Bi) to allow correction for instrumental drift; column feed solution, 1 M Al(NO3)3 (Analytical Columns were prepared with a dry powder of resin Grade, Fisher Scientific UK, Loughborough, Leics., UK) (50–100 mm, TRU-SpecTM, EiChrom Europe, 75010 Paris, purified by passing through a 1.2 cm3 bed of Dowex 1-X8 anion exchange resin (BDH, Poole, Dorset, UK) then a 0.6 cm3 bed of Tru-Spec resin; column feed solution, 0.5M Al(NO3)3+2 M HNO3.Standard solution preparation A mixed standard solution of 10 mg ml-1 232Th and 238U, was prepared in 5% HNO3 from 1000 mg ml-1 stock solutions of the individual elements (Johnson Matthey Ltd., Reading, Berks., UK). In order to ensure that the analytes were in the correct oxidation states to be retained on the column [i.e.U (VI) and Th (IV)], 10 ml of the 10 mg ml-1 standard solution was boiled to dryness in two successive 10 ml portions of Fig. 1 Schematic of the flow injection manifold interface with ICP-MS. conc. HNO3. Table 2 Operating conditions and gas flows for the ETV system Internal furnace gas Program step Temp/°C Ramp time/s Hold time/s flow/ml min-1 1 100 10 15 300 (Ar) 2 120 10 60 300 (Ar) 3 800 5 30 10 (CHF3) 4 2500 0.2 2 0 (to ICP) 5 2700 0 1 0 (to ICP) 6 20 15 1 0 (to ICP) 628 J.Anal. At. Spectrom., 1999, 14, 627–631Sample preparation Water samples. Two certified reference materials were studied, namely NASS-4 Open Ocean Sea Water and SLRS-3 River Water (National Research Council, Ottawa, Ontario, Canada). Samples, 10 ml of NASS-4 and 25 ml of SLRS-3 were treated in the same way as the mixed standard solution, except that they were made up to final volumes of 25 ml and 50 ml respectively with column feed solution.Biological samples. Initially the sample preparation procedure was based on a method by Nelson and Fairman.13 Two certified reference materials (CRMs) were studied, namely Fig. 2 Elution profiles for 238U and 232Th. NIST 1566a Oyster Tissue and NIST 1575 Pine Needles (National institute of Science and Technology, Gaithersburg, concentration step was required. The solution was deposited MD, USA). Samples (0.5 g) were weighed into porcelain onto the column by pumping through the manifold using the crucibles, placed in a muZe furnace and dry-ashed at 200 °C tubing normally immersed in the carrier stream.During depos- for 2 hours, 400 °C for 2 h, 600 °C for 2 h, and 800 °C for 2 h. ition the column was diverted to waste. The centrifuge tube This step was omitted for the oyster tissue. Nitric acid (10 ml ) was rinsed with 1.5 ml of 2 M HNO3, to remove any residual was added to each crucible, followed by gentle warming on a sample from the tube, and subsequently with 1 ml of 2 M hot-plate to digest the samples, boiling to dryness and heating HNO3 to flush through any residual column feed solution on a hotplate.This procedure was repeated until a white ash prior to diverting the column to the ICP-MS. The column was was left. On the last iteration the samples were boiled down diverted to the ICP-MS instrument, the analytes eluted with until almost dry, then 10 ml of the column feed solution was 0.1 M ammonium bioxalate, and the analyte masses moni- added to each beaker to dissolve the ash.Samples were made tored. After elution the column was again diverted away from up to final volumes of 50 ml and 25 ml, for oyster tissues and the ICP-MS and flushed with 1 ml of column feed solution to pine needles respectively, with column feed solution. Three remove residual ammonium bioxalate solution prior to further sample blanks were also prepared. deposition. Each injection was repeated at least three times.Fusion of biological samples. Subsequently sample preparations have been performed by lithium metaborate fusion. Results and discussion Similar procedures of lithium metaborate fusions for soil ICP-MS detection samples have recently been used for uranium and plutonium determinations.14 One certified reference material was studied, Elution profiles and detection limits. The elution peaks for namely NIST 1575 Pine Needles. Samples (0.5 g) were weighed 238U and 232Th are shown in Fig. 2, with uranium being eluted into platinum crucibles and 0.8 g of lithium metaborate completely in approximately 50 s and thorium in approxi- (Spectroflux, Johnson Matthey) was added to each, then mately 30 s. The elution times corresponded to volumes of heated over a Meeker burner. A platinum lid for the crucible approximately 0.4 and 0.25 ml for 238U and 232Th respectively. was used to improve heat retention and thus encourage fusion, The standards were deposited from a 500 ml loop so under some flaming was initially observed from the pine needles, these circumstances both 238U and 232Th were eluted in a while the organic matter was burnt oV.The molten fused smaller volume than the sample loop. sample was then quickly poured into a beaker containing Instrumental and method detection limits for 238U and 232Th approximately 30 ml of column feed solution. Any undissolved are shown in Table 3. Instrumental detection limits were fused matter was allowed to dissolve in the solution, mixing determined using solutions prepared in the column-eluting was aided by use of a magnetic stirrer.Samples were made up solution (0.1 M ammonium bioxalate) but had not been eluted to final volumes of 50 ml in column feed solution. Three from the column, thus reflecting the level of the blank in the sample blanks were also prepared. column-eluting solution. Method detection limits were determined by pre-concentrating a 0.5 ml aliquot of column feed Calibration solution onto the column and eluting with 0.1 M ammonium A series of calibration standards containing both 232Th and bioxalate solution. The method detection limits were blank 238U were prepared and deposited onto the column by flow limited and can be improved by a factor of at least 100 if the injection, into a carrier stream of column feed solution at a reagents are purified more eVectively.This will also allow flow rate of approximately 0.5 ml min-1 for 1 min.During greater pre-concentration factors to be realised, thereby deposition the outlet from the column was diverted to waste improving detection limits further. to prevent the column feed solution entering the ICP-MS instrument. After a deposition, the column was rinsed with Analysis of reference materials. The certified reference mate- 1 ml of 2 M HNO3 to remove any residual column feed rials NASS-4 (Open Ocean Sea Water) and SLRS-3 (River solution before the column was diverted back to the ICP-MS, the analytes were eluted with 0.1 M ammonium bioxalate, and Table 3 Instrumental and method detection limits for uranium and the analyte masses were monitored.After elution the column thorium using pneumatic nebulization PN-ICP-MS and ETV-ICP-MS was again diverted away from the ICP-MS and flushed with U Th 1 ml of column feed solution to remove residual ammonium bioxalate solution prior to further deposition. Each injection Absolute/ Relative/ Absolute/ Relative/ was repeated three times.pg pg ml-1 pg pg ml-1 Analysis of samples Instrumental (PN) 2.7 5.4 3.1 6.2 Method (PN) 24 48 60 120 An accurate volume of the prepared sample was either meas- Instrumental (ETV) 0.03 0.9 0.009 0.3 ured into a clean polypropylene centrifuge tube or injected Method (ETV) 0.6 21 0.3 9 into the 500 ml sample loop, depending on whether a pre- J. Anal. At. Spectrom., 1999, 14, 627–631 629Table 4 Results for the determination of uranium in certified reference materials NASS-4, SLRS-3 by PN-ICP-MS and ETV-ICP-MS 238U found/ng ml-1 Analysed without Certified reference Certified value/ column Analysed with Detection material ng ml-1 (10×dilutiona) columna PN NASS-4 2.68±0.12 2.13±0.28 ETV NASS-4 2.68±0.12 1.98±0.11 2.81±0.54b PN SLRS-3 (0.045)c 0.043±0.002 ETV SLRS-3 (0.045)c 0.042±0.002 0.045±0.004d amean±s; b0.5 ml sample; cuncertified indicative value; d2.5 ml sample.Table 5 Results of the determination of uranium and thorium in certified reference materials by PN-ICP-MS after dry/wet ashing U Th Certified reference Certified value/ Founda/ Certified value/ Founda/ Material ng g-1 ng g-1 ng g-1 ng g-1 1566a Oyster Tissue 132±12 121±21b (40)c 29±8d 1575 Pine Needles 20±4 14.6±3.4d 37±3 28.3±4.5d amean±s; bn=11; cindicative value; dn=5.Table 6 Results of the determination of uranium and thorium in pine needles by PN-ICP-MS and ETV-ICP-MS after lithium metaborate fusion, by calibration with and without the column in place U Th Certified value/ Founda/ Certified value/ Founda/ Detection Calibration method ng g-1 ng g-1 ng g-1 ng g-1 PN Calibration with 20±4 23.3±2.0 37±3 36.2±5.6 columnb PN Calibration without 20±4 18.1±1.4 37±3 33.6±6.8 columnb PN Calibration without 20±4 16.6±1.5 37±3 38.1±0.8 column, 5 ml preconc.c ETV Calibration without 20±4 19.5±1.7 37±3 38.8±2.2 columnd amean±s; bn=3; cn=1, 3 injections; dn=6.Water) were analysed by pre-concentrating known volumes of Table 5.For oyster tissue no significant diVerence was found between the found value and the certified mean for uranium the prepared material, eluting and comparing the peaks to the calibration curve after normalising using the Bi internal stan- at the P=0.05 level. For the pine needles, low recoveries for both thorium and uranium were observed in comparison with dard. Results are shown in Table 4, though it was only possible to compare uranium as the reference materials were only the certified mean, though there was no significant diVerence between the found value and the bottom of the certified range certified for this element.One particular problem that was encountered was that the reproducibility for thorium was for both uranium and thorium (i.e. 16 ng g-1 and 34 ng g-1 respectively) at the P=0.05 level. Other workers have reported unpredictable, and this element was prone to carry-over and high blank values.Low recoveries were obtained for uranium losses of uranium through the use of porcelain crucibles,15,16 by adsorption of 238U onto the surface. However, low recover- in NASS-4 samples using pneumatic nebulization (PN)- ICP-MS. However full recoveries were found for uranium in ies could also be the result of analyte losses by volatilisation in the muZe furnace, as by incomplete sample digestion of NASS-4 when using ETV-ICP-MS, with no significant diVerence between the found value and the mean of the certified silicate material.When the lithium metaborate fusion method was used (Table 6) recoveries were within the certified range, value at the P=0.05 level. The analyses were repeated on two separate days and the results were very similar. Good agree- probably due to complete digestion of silicates within the pine needle matrix, with no significant diVerence between the found ment was obtained between the analytical result and the indicative value for SLRS-3, though no firm conclusions can value and the certified mean for both uranium and thorium at the P=0.05 level.In order to try and speed up the analysis, be drawn because this material was not certified. This clearly shows the value of pre-concentration since the indicative value the eVect of calibrating the analysis by simply flow injecting the standards, rather than depositing them on the column, of 0.045 ng ml-1 was close to the detection limit for the ICP-MS instrument used, and was twice the absolute detection was investigated.Results are shown in Table 6 and indicate that full recoveries were obtained for both 238U and 232Th. limit for the method detailed here. However, a preconcentration factor of 5 eVectively raised the level of uran- When the pre-concentration factor was increased by a factor of 10 (i.e. 5 ml were deposited instead of 0.5 ml ) recoveries ium to 10 times the detection limit, making analysis feasible. were still within the certified range, again with no significant diVerence between the found value and the certified mean for Results for the analysis of oyster tissue and pine needles after sample preparation by dry/wet ashing are shown in both uranium and thorium at the P=0.05 level. 630 J. Anal. At. Spectrom., 1999, 14, 627–631Table 6. Results were within the certified range of the reference material. Conclusions The determination of 238U and 232Th in certified reference materials was successfully performed in most instances.Low recoveries were observed for the determination of 238U in NASS-4 Open Ocean Sea Water without matrix removal using the column pre-treatment for ETV-ICP-MS, however, with column pre-treatment full recoveries were obtained. Results Fig. 3 Vaporisation profiles for 238U (3 pg) and 232Th (30 pg) for for the freshwater (SLRS-3) were in good agreement with the ETV-ICP-MS using only argon gas. indicative value. Agreement with certified values was observed for the determination of 238U and 232Th in NIST 1575 Pine Needles after pre-concentration and matrix elimination after lithium metaborate fusion, and detection by ICP-MS and ETV-ICP-MS. However, losses were apparent when using a dry/wet ashing method.The addition of freon gas to the ETV improved sensitivity for 238U and 232Th 10-fold and 50-fold respectively. Acknowledgements The work described in this paper was supported in part by the Department of Trade and Industry, UK, as part of the Government Chemist Programme.References Fig. 4 Vaporisation profiles for 238U (3 pg) and 232Th (3 pg) for ETVICP- MS using CHF3 modifier gas. 1 E. P. Horwitz, New Chromatographic Materials for Determination of Actinides, Strontium, and Technetium in Environmental, ETV–ICP-MS detection Bioassay, and Nuclear Waste Samples, Chemistry Division, Argonne National Laboratory, Argonne, IL, May 1992. EVect of freon gas, elution profiles and detection limits. 2 E. P. Horwitz, M. L. Dietz, R. Chriarizia, H. Diamond, S. L. Vaporisation profiles for 238U and 232Th with and without Maxwel and M. R. Nelson, Anal. Chim. Acta, 1995, 310, 63. 3 E. P. Horwitz, M. L. Dietz, R. Chriarizia, H. Diamond, A. M. freon added during the ashing stage are shown in Figs. 3 and Essling and D. Graczyk, Anal. Chim. Acta, 1992, 266, 25. 4. In the absence of freon (Fig. 3) the peaks were approxi- 4 E. P. Horwitz, R. Chiarizia, M. L. Dietz and H.Diamond, Anal. mately 2.5 s wide, and 232Th vaporised slightly later than U. Chim. Acta, 1993, 281, 361. However, when freon was added (Fig. 4), peak height and 5 J. S. Crain, L. L. Smith, J. S. Yaeger and J. A. Alvarado, peak area signals increased by approximately 10 times and 50 J. Radioanal. Nucl. Chem., 1995, 1, 133. times for 238U and 232Th respectively, resulting in much 6 J. S. Alvardo and M. D. Erickson, J. Anal. At. Spectrom., 1996, 11, 923. improved detection limits.Other workers have also noted the 7 E. J.Wyse and D. R. Fisher, Radiat. Prot. Dosim., 1994, 55, 199. beneficial eVect of freon gas in ETV,6,17,18 which prevents the 8 M. Hollenbach, J. Grohs, S. Mamich and K. Marilyn, J. Anal. At. formation of refractory carbides on the surface of the graphite Spectrom., 1994, 9, 927. tube, however, it is advisable to only introduce the gas during 9 R. Chiappini, J.-M. Taillade and S. Bre�bion, J. Anal. At. the ashing stage. If freon is introduced during the vaporisation Spectrom., 1996, 11, 497. stage, tube lifetimes are reduced substantially. 10 Practical Benefits of an Ultra Sensitive ICP-MS System—Actinide Determination at PPQ Level, Hewlett-Packard Technical Note Instrumental and method detection limits are shown in Pub. No. (43) 5965–5181E, 1996. Table 3 and were determined as before. As for pneumatic 11 J. H. Aldstadt, J. M. Kuo, L. L. Smith and M. D. Erickson, Anal. nebulization, detection limits were blank limited, so improve- Chim. Acta, 1996, 319, 135. ments might be expected if the purity of reagents is improved. 12 Y. Igarahi, H. Kawamura, K. Shiraishi and Y. Takaku, J. Anal. At. Spectrom., 1989, 4, 571. Analysis of reference materials. Results for the analysis of 13 D. M. Nelson and W. D. Fairman, presented at the 36th Annual Conference on Bioassay, Analytical and Environmental water reference materials are shown in Table 4. The samples Radiochemistry, Oak Ridge, Tennessee, USA, 1990. were analysed after straightforward 10-fold dilution, and after 14 I. Croudace, P. Warwick, R. Taylor and S. Dee, Anal. Chim. Acta, pre-treatment on the column. Low recoveries were obtained 1998, 371, 217. for the diluted NASS-4 Open Ocean Sea Water samples 15 W. F. Neumann, R. W. Fleming, A. B. Carlson and N. Glover, without matrix removal on the column, but agreement with J. Biol. Chem., 1948, 173, 41. the certified value was obtained when the matrix was removed 16 A. K. Babko and V. N. Danilova, Zh. Anal. Khim., 1963, 18, 1036. 17 D. Goltz, D. C. Gre�goire, J. P. Bryne and C. L. Chakrabarti, using the column pre-treatment. Similar results were obtained Spectrochim. Acta, Part B, 1995, 50, 803. for the SLRS-3 River Water samples regardless of which 18 B.Wanner, P. Richner and B. Magyar, Spectrochim. Acta, Part B, method was used, reflecting the relative simplicity of this 1996, 51, 817. matrix compared to sea water. Results for the analysis of pine needles reference materials Paper 8/08430K using the lithium metaborate fusion method are shown in J. Anal. At. Spectrom., 1999, 14, 627–631 6