JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 927 Determination of Technetium-99 Thorium-230 and Uranium-234 in Soils by Inductively Coupled Plasma Mass Spectrometry Using Flow Injection Preconcentration" Mark Hollenbach James Grohs Stephen Mamich and Marilyn Kroft RUST Geotech Inc. US. Department of Energy Grand Junction Projects Office PO Box 74000 Grand Junction Colorado 87502 USA Eric R. Denoyer The Perkin-Elmer Corporation 767 Main Avenue Norwalk CT 06859 USA A new method is described for the determination of "Tc 230Th and 234U at ultra-trace levels in soils. The method used flow injection (FI) for on-line preconcentration of 'qc 230Th and prior to detection using inductively coupled plasma mass spectrometry (ICP-MS). The FI-ICP-MS method results in greater sensitivity and freedom from interferences compared with direct aspiration into an ICP mass spectrometer.Detection limits are improved by approximately a factor of 10. The FI-ICP-MS method is also faster less labour intensive and generates less laboratory waste than traditional radiochemical methods. The accuracy of the method was tested for '9Tc by comparison to liquid scintillation counting and for 230Th and *%U b analysis of a US Department of Energy reference soil. Detection limits in the soil for 'Tc "'Th and 'MU were 11 mBq g-' (0.02 ng g-') 3.7 mBq g-' (0.005 ng g-') and 0.74 mBq g-' (0.003 ng g-') respectively. Sample preparation analysis protocol and method validation are described. Keywords Technetium-99 thorium-230 and uranium-234 determination; inductively coupled plasma mass spectrometry; flow injection The United States Department of Energy (US DOE) is con- ducting several large environmental restoration programmes to remediate contamination resulting from decades of nuclear weapons production and testing uranium ore processing and nuclear reactor fuel production and reprocessing.Many thou- sands of soil and water samples will be collected and analysed for radionuclides over the next 30 years at a cost of millions of dollars. Traditional radiochemical methods for alpha- and beta-emitting radionuclides require extensive chemical separ- ations long count times and can produce hazardous and/or radioactive laboratory waste. Developing improved analytical methods for radionuclides can result in a significant cost reduction for the US government.Inductively coupled plasma mass spectrometry (ICP-MS) has been used successfully to measure long-lived radionuclides such as '"I 232Th 237Np and 238U.'-3 However ICP-MS used with conventional sample introduction techniques lacks either the sensitivity or the selectivity to measure shorter lived radionuclides at levels important for environmental monitor- ing. Limits of detection (LODs) required for environmental monitoring of radionuclides are based on the radioactivity of the analyte. As the half-life of the analyte gets shorter the number of analyte atoms required to produce a given level of radioactivity gets smaller. The detection power of ICP-MS is limited by the number of analyte atoms present in a sample. Therefore the shorter the half-life of the analyte the lower the ICP-MS LOD must be to detect the analyte at a given level of radioactivity.Technetium-99 is a beta-emitting radionuclide with a maxi- mum beta energy of 0.29 MeV a half-life of 2.12 x lo5 years and a specific activity of 629mBqng-'. It is produced by nuclear fission and has been released into the environment primarily as a result of nuclear fuel repro~essing.~ It is usually determined by radiochemical methods with detection by liquid scintillation counting Thorium-230 and 234U are alpha-emitting radionuclides with half-lives of 8.0 x lo4 and 2.47 x lo5 years and specific activities of 718 and 229 mBq ng-l respectively. Both are members of the 238U decay series. Thorium-230 is commonly found in * Presented at the 1994 Winter Conference on Plasma Spectro- chemistry San Diego CA USA January 10-15 1994.uranium mill tailings and is of concern because it decays into 226Ra and then 222Rn which is a radioactive gas that can cause cancer if inhaled. The radioactivity of 234U in soils is equal to that of 238U if both isotopes are present at their natural abundances. Soils that are contaminated by synthetically- enriched U may contain 234U at higher radioactivity levels than 238U. Thorium-230 and 234U are usually determined by alp ha-energy spectrometry. In the present work a flow injection (FI) system is used to separate and concentrate radionuclides by solid-phase extrac- tion Methods are presented for measuring 99T~ 230Th and 234U in soils. Samples were fused with sodium peroxide for 99Tc or digested with a mixture of hydrofluoric nitric and perchloric acids for 230Th and 234U. The sample solution is pumped through a column and the analytes are loaded onto the solid-phase adsorber.The analytes are then eluted directly into the nebulizer of the ICP mass spectrometer. The use of FI results in greater sensitivity and freedom from interferences compared with direct aspiration into an ICP-MS instrument. Detection limits are improved by approximately a factor of 10. The FI-ICP-MS methods have LODs comparable to radiochemical methods but are faster less labour intensive and generate less laboratory waste. Experimental Instrumentation and Apparatus ICP-MS The ICP mass spectrometer used in the study was an Elan 5000 (Perkin-Elmer SCIEX Toronto Canada).The system was fitted with a quartz spray chamber (Precision Glassblowing Parker CO USA) and a Type TR-30-3C con- centric glass nebulizer (J. E. Meinhard Associates Santa Ana CA USA) because the Ryton spray chamber and nebulizer supplied with the instrument are not compatible with the 8 mol I-' nitric acid eluent used for "Tc. Two peristaltic pumps were used (Gilson Medical Electronics Middleton W1 USA) one to pump the spray chamber drain and the other to rinse the nebulizer and spray chamber.928 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Flow injection system AS-90 autosampler ( Perkin-Elmer Corporation Norwalk CT USA) were used. used for the eluent and 0.76 mm i.d. tubing was used with the remote pump for rinsing the nebulizer.laboratory with TEVA. Spec extraction resin for "Tc determi- nation and TRU-Spec resin for 230Th and 234U determination Both a FIAS-200 and a F1AS-400MS F1 'Ystem fitted with an The FIAS mini-columns (Perkin-Elmer) were packed in the Fig. is a schematic diagram Of the F1 showing tubing used to construct the (Eichrom Industries Darien IL USA). The volume of the resin contained in each column was approximately 50 and the tubing arrangement* manifold was made of PolY(tetrafluoroethYlene) (PTFE) the of resin was approximate~y 30 mg. (Upchurch Scientific Oak Harbor WA USA). The tubing that connects the valve to the nebulizer was 0.2mm i.d. and the rest of the manifold tubing was 0.76 mm i.d. The tubing Sample preparation apparatus connections were made with Super Flangeless brand or flange- Iess-style fittings (Upchurch Scientific) because they do not use O-rings that are attacked by strong acid and tubing connec- tions can be made quickly and easily without a flanging tool.Super Flangeless fittings made of poly ether ether ketone were used to connect the tubing to the column because the fittings are designed for connections that need to be made and broken often. The rest of the tubing connections were made with Tefzel flangeless-style fittings. The T-connectors were made of Tefzel (Upchurch Scientific). Flow rates through the FIAS manifold are controlled by selection of pump speed and pump tubing i d . Pump tubing (Perkin-Elmer) with 1.52 mm i.d. was used for pumping the sample and column rinse solution 1.14mm i.d.tubing was ( a ) Remote P-pump l3 AS-90 Sample Waste 4,l Sample Load (Valve position 1) Elan Waste 4,1 Rinse El Inject (Valve position 2) solution Model 242-67 pulverizers equipped with ceramic plates (Bico- Braun International Burbank CA USA) were used to grind soil samples. Cross-flow blenders ( Patterson-Kelly Company East Stroudsburg PA USA) were used to blend the samples. Zirconium crucibles with 55 ml capacity (Fisher Scientific Pittsburgh PA USA) and a Model 51894 oven and hearth plate (Lindberg Watertown WI USA) were used for sample fusions. Fused samples were filtered through a 0.45 pm pore size 47mm diameter type HA membrane filter with a glass filter holder (Millipore Corporation Bedford MA USA) and a filtrator (Fisher Scientific). Mixed-acid digestions were car- ried out in 100ml size Type H P heatable plastic beakers (Nalge Company Rochester NY USA) with 65 mm PTFE covers (Berghof/America Concord CA USA) on Type 2200 hotplates (Barnstead/Thermolyne Corporation Dubuque IA USA).Operating conditions and analysis scheme The ICP-MS and the FIAS system were both under computer control. The operating conditions for the ICP-MS are summar- ized in Table 1. A description of the FIAS control programme is given in Table 2. The baseline-to-baseline width of the resulting elution peak was about 17 s and the signal was integrated across the full width of the peak. Typically one measurement was made per sample resulting in a total analysis time of 6.5 min and 10 ml of sample being consumed. A two- point calibration was used for all analyses.Determination of "Tc During the FIAS program the TEVA-Spec column was rinsed with 0.5 mol 1-1 nitric acid after loading the sample. The analyte was then eluted with 8 moll-' nitric acid. Rhenium Table 1 Instrumental operating conditions ICP-MS Forward power/W Plasma gas flow rate/l min-' Auxiliary gas flow rate/l min-' Nebulizer gas flow rate/l min-' Acquisition parameters (all analyses) Dwell time/ms Scan mode Sweeps per reading MCA channels per spectral peak Resolution/amu 10% peak maximum Signal processing Acquisition parameters (99Tc analyses) Readings per replicate Isotopes measured Internal standard Acquisition parameters (230Th and 234U analyses) Readings per replicate Isotopes measured Internal standards 1000 15 0.8 1 .o 50 1 1 0.8 Peak-hop transient Spectral peaks integrated; signal profile counted 75 99T~ loOMo '"Ru lg7Re '"Re for 99Tc 61 9 233u 234u 229~h 230q-h 232~h160 229Th for 230Th and 233U for 234U Fig.1 Schematic of the FIAS manifold showing (a) the tubing arrangement and (b) the two valve positionsJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 929 Table 2 FIAS programme and description of programme steps Speed/rev min - Valve Remote Step Read Time/s Pump 1 Pump 2 position pump Pre-sample - 15 100 0 2 On 1 - 180 25 0 1 On 2 - 30 0 40 1 Off 3 - 3 0 40 2 Off 4 Yes 57 0 40 2 Off 5 - 60 0 40 1 Off 6 - 1 100 0 2 On Programme step Pre-sample 1 2 3 4 5 6 Description of step Sample is pumped at 8 ml min-' through the valve to waste in preparation for loading Sample is pumped through the column to waste at 2.2 ml min-'.Analytes are loaded onto the column Column rinse solution is pumped through the column to waste at 3.5 ml min-' to remove residual sample. This step also improves separ- ation from Ru and Mo for the determination of "Tc Eluent is pumped through the column to the nebulizer at 2 ml min-'. This step is used to delay the start of the read cycle until the analytes approach the nebulizer This is the same as step 3 except that the ICP-MS read cycle is initiated Column rinse solution is pumped through the column to waste at 3.5 mlmin-' to rinse residual eluent and prepare the column for loading of the next sample Pump speed and valve position are set for the autosampler rinse cycle. The autosampler returns to the rinse position for 25 s to rinse the sample inlet line (50 ng 1-') was added to all samples and standards as an internal standard to compensate for any variation in chemical recovery or instrument drift.Rhenium is a very rare element with an average abundance in the earth's crust on the order of 0.01 ng g-1.5 Therefore it was not expected to be in samples at significant concentrations. Determination of 230Th and 234U During the FIAS program the TRU-Spec column was rinsed with 4 moll-' nitric acid after loading the sample. The analyte was then eluted with 0.1 moll-' ammonium oxalate. Thorium-229 and 233U are man-made isotopes with half-lives of 7.34 x lo3 and 1.62 x lo5 years and specific activities of 7880 and 350 mBq ng-' respectively. Thorium-229 and 233U were added to samples and standards at 180 Bq 1-l (22.8 ng 1-') and 18.5 Bq 1-' (52.8 ng 1-I) and used as internal standards for 230Th and 234U respectively.The internal standards are not expected to be in the samples because they are neither naturally occurring nor are they decay products of anything that might be expected to be found in the samples. Reagents De-ionized water (18 MR cm) was used for all dilutions and was prepared using a Milli-Q system (Millipore). Determination of 99Tc The sodium peroxide used for sample fusions was 98% (Aldrich Milwaukee WI USA). The nitric acid used for sample dissolution and preparation of standards was 70% m/m trace metal grade (Fisher Scientific). The nitric acid used to prepare the eluent and column rinse solutions was 70% m/m double sub-boiling quartz distilled (Seastar Chemicals Seattle WA USA).Standard solutions of "Tc used for instrument calibration sample spiking and quality control standards were prepared by dilution of SRM 4288 Technetium-99 (National Institute of Standards and Technology Gaithersburg MD USA). The independent-source 99Tc standard used to verify calibration was prepared by dilution of a 38.5 kBq (61.2 pg) standard (Isotopic Products Laboratories Burbank CA USA). All "Tc standards were prepared in 0.5 mol I-' nitric acid. Standard solutions of Re Mo and Ru were prepared by dilution of 1000 pg ml-' stock solutions (Inorganic Ventures Lakewood NJ USA). (Note Laboratories must have procedures for safe handling of radioactive material and management of radioac- tive waste.Laboratory personnel must be trained to handle radioactive materials safely.) Determination of 230Th and 234U The nitric and perchloric acids were 70% m/m trace metal grade the hydrofluoric acid was 49% m/m analytical-reagent grade and the ammonium oxalate was certified ACS grade (Fisher Scientific). Standard solutions of 230Th used for calibration spiking and quality control standards were prepared by dilution of a 12.4 kBq (17.3 pg) standard (Isotopic Products Laboratories). The 229Th standard used as an internal standard for 230Th was prepared by dilution of a 72.2 kBq (9.16 pg) standard (Isotopic Products Laboratories). Standard solutions of 234U used for calibration spiking and quality control standards were prepared from a 2.36 kBq ml-' (10.3 pg ml-I) standard that was made by dissolving a portion of certified reference material (CRM) U500 Uranium Isotopic Standard (New Brunswick Laboratory Argonne IL USA) in nitric acid.The 233U standard used as an internal standard for 234U was prepared by dilution of a 37.0 kBq (106 pg) standard (Isotopic Products Laboratories). The independent-source standard used to verify calibration of 230Th and 234U was prepared from a 925mBqml-' (1.29 ng ml-' 230Th and 4.04 ng ml-' 234U) standard that was prepared by dissolving a portion of Reference Material No. 101-A Pitchblende Ore-Silica Mixture (New Brunswick Laboratory). Standard solutions of 232Th were prepared by dilution of a 1000 pg ml-1 Th standard (Inorganic Ventures). Preparation of Soil Samples Soil samples were dried in an oven at 103°C for 12-24 h ground to pass through a 325 mesh screen (maximum particle size 45 pm) blended and transferred to polyethylene bottles.Sample dissolution for determination of "Tc A 0.25 g sub-sample was placed in a zirconium crucible. For the spiked sample 50 pl of 7.40 Bq ml-' (11.8 ng m1-l) 99Tc were added to the sample and dried on a hot-plate at 100°C. A 2.25 g portion of sodium peroxide was added to the crucible and mixed with the sample using a small metal spatula. The crucible was placed on a hearth-plate that was then placed in an oven pre-heated to 470 "C for 30 min removed from the oven and allowed to cool to room temperature. The specified oven can hold 24 crucibles. Approximately 45 ml of water were added to the crucible and the fusion mixture was allowed to dissolve for 1 h.A 4ml volume of 70% m/m nitric acid was added to the crucible. The sample solution was diluted to 100 ml with water and filtered through a 0.45 pm membrane filter to remove the small amount of turbidity present in the sample solutions. For the radiochemical 99Tc method 5 g of sample were leached with a mixture of sulfuric acid and potassium persulf- ate. Technetium was extracted from the leachate with tri-n- butyl phosphate (TBP) that was previously equilibrated with sulfuric acid. The TBP was mixed with a scintillation cocktail and the "Tc activity was measured by liquid scintillation counting.6930 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Sample dissolution for determination 0j230Th and 234U A 0.5 g sub-sample was placed in a heat resistant plastic beaker. For the spiked sample 1.54 Bq (2.14 ng) of 230Th and 0.590 Bq (2.58 ng) of 234U were added to the sample.A 10 ml volume of 70% m/m nitric acid 5 ml of 70% m/m perchloric acid and approximately 3 ml of 49% m/m hydrofluoric acid were added. The beaker was covered with a PTFE cover and heated on a hot-plate at approximately 100 "C for 15 min. The covers were removed and the temperature of the hot-plate was raised to approximately 300 "C. Heating was continued until the sample formed a moist bead and the beaker was removed from the hot-plate to cool. (Note Samples containing high concentrations of organic material should not be heated to dryness with perchloric acid because an explosion could result.) A 10 ml volume of water and 1 ml of 70% m/m nitric acid were then added to the beaker and heated on a hot-plate at approximately 150 "C just to boiling.A 12.5 ml portion of 70% m/m nitric acid was added the sample solution was diluted to 50ml with water and any insoluble material was allowed to settle overnight before analysis. Quality Control The following performance check was done daily. A standard containing 100 pg l-' each of Ba Ce Co In Li Mg Pb Rh and T1 was aspirated conventionally in order to verify mass calibration resolution background count rate sensitivity oxide level and the level of doubly-charged species. The instrument was calibrated with a blank and one stan- dard. Calibration was verified by analysing a standard obtained from an independent source.A standard was analysed to verify calibration accuracy at low concentration. Two standards were analysed to verify control of the interferences described below. The first contained the interfering species listed below and the second contained analyte plus the interfering species. Analysis of the interference check standards verified that the interfering species did not give false-positive results and that the analyte could be accurately measured in the presence of the interfering species at the levels tested. The calibration was verified by analysing a standard at half the concentration of the calibration standard and a calibration blank after every ten samples and at the end of the analysis run. One preparation blank and at least one spiked sample were analysed for every 20 samples.Determination of 99 Tc The instrument was calibrated with a 7.40 Bq 1-' (1 1.8 ng 1-') 99Tc standard and a blank. The low concentration verification standard contained 185 mBq I-' (0.294 ng 1-') 99Tc. The first interference check standard contained 100 pg I-' Mo and 25 ngl-' Ru and the second contained lOOpgl-' Mo 25 ng 1-' Ru and 3.70 Bq I-' (5.90 ng 1-') 99Tc. Determination of 230Th and 234U The instrument was calibrated with a 30.9 Bq 1-' (43.0 ng 1-') 230Th and 11.8 Bq 1-' (51.5 ng 1-') 234U standard and a blank. The low concentration verification standard was 309 mBq I-' (0.430 ng 1-I) 230Th and 236 mBq 1-' (0.103 ng 1-') 234U. The first interference check standard contained 0.1 mg 1-l Th and the second contained O.lmgl-' Th and 5.90Bql-' (25.8 ng 1-') 234U.Results and Discussion Sample Preparation The grinding and blending process produced a homogeneous sample with a particle size that was readily attacked by the sodium peroxide fusion or the mixed acid digestion. Determination of 99Tc A sodium peroxide fusion was selected because it is effective for dissolving soils Tc is stable in an alkaline medium and peroxide oxidizes Tc to pertechnetate (Tc04-). The pro- portions of sample flux and nitric acid were selected to optimize sample dissolution yet yield a relatively dilute nitric acid solution. Having Tc in solution as pertechnetate is required for optimum extraction and dilute nitric acid is a suitable matrix for extraction of T c . ~ The small amount of turbidity present in the sample solutions was removed by filtration to prevent it from plugging the extraction column.Determination of 230Th and 234U The mixed-acid digestion is adequate for the determination of Th and U in most soils. If the presence of Th or U in refractory material is a concern a more rigorous digestion such as fusion with lithium metaborate may be desirable. The final sample solutions were prepared to contain 4 moll-' nitric acid because that acid concentration has been reported' to be suitable for the extraction process. Properties of the Extraction Resin The extraction resins used were found to be very durable. The resins are specific for certain analytes and most of the sample matrix constituents pass through the column without being retained. Columns are frequently used for 200 or more analyses. The TEVASpec resin usually has a longer life-time than the TRUSpec resin. Technetium is retained strongly on TEVASpec resin in dilute nitric acid solutions and is retained very weakly in strong nitric acid.7 Rhenium behaves similarly to Tc with TEVA-Spec resin and can act as a surrogate indicating recovery of Tc through the separation.Rhenium is added to samples and standards and 187Re is designated in the analysis pro- gramme as the internal standard for 99Tc. In this way compen- sation for varying recovery through the separation and for instrument drift is achieved in one step. Uranium and Th are retained strongly on-TRUSpec resin in strong nitric acid solutions and are retained very weakly in dilute nitric acid.8 Eluting with 0.1 moll-' ammonium oxalate produces a sharper elution peak than eluting with either dilute nitric acid or water. Use of 229Th and 233U as internal standards for 230Th and 234U is ideal because the chemistry of the internal standards in the separation is the same as that of the analytes.Elution Profiles Fig. 2 shows elution profiles obtained for 99Tc and Re stan- dards. Fig. 3 shows an elution profile obtained for a soil sample that contains 430 mBq g-' (0.684 ng g-') 99Tc. The concen- tration of 99Tc in the sample solution was 1.07Bq1-' (1.70 ng 1-I). Fig. 3 also shows the graph for '"Ru which was monitored to check for the potential interference by 99Ru. A 3min load time was used because the LODs obtained were adequate for this study. Lower LODs could be obtained using longer load times.This was determined by varying the load time of a 7.40 Bq 1-l (11.8 ng 1-') standard. The peak area of the standard increased linearly with respect to load time up to 8 min loading at a rate of 2.2 ml min-'. Fig. 4 shows an elution profile obtained for 229Th 230Th 233U and 234U standards. Fig. 5 shows an elution profile obtained for a soil sample that contains 1.63Bqg-' (2.27 ng g-') 230Th and 1.65 Bq g-' (7.20 ng g-') 234U. The concentrations of 230Th and 234U in the sample solution are 16.3 Bq 1-' (22.7 ng 1-') and 16.5 Bq 1-' (72.0 ng l-') respectively. In all cases the peaks rise sharply and return to the baseline rapidly with minimal tailing.JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 931 22 20 r lm 15 m C 3 0 Y m z 3 10 Y .- m 8 Q) C w - E ' I I 1 0 5 10 15 20 Time/s Fig.2 Elution profile for a standard containing 50 ng 1-' of Re and 7.40Bql-'(11.8ngl-') o ~ ~ ~ T c 900 800 7 600 CJY v) c 3 0 > .C 400 C 4- .. Y - 200 0 5 10 15 20 Timels Fig.3 Elution profile for a soil sample containing 429 mBqg-' (0.682 ng g-') of 99Tc Interference Management Determination of99Tc The measurement method could be subject to interferences from 99Ru because 99Tc cannot be distinguished from 99Ru. Ruthenium is a very rare element with an average abundance in the earth's crust of the order of 1 ng g-' of which 99Ru makes up 12.7%. Naturally occurring Ru is not expected to present a problem because it is so scarce and it is separated by the extraction column. Ruthenium-99 is also a fission product produced from the decay of 99Tc by beta particle emission.However 99Ru resulting from 99Tc decay is also expected to be scarce because of the 212000 year half-life of 99Tc and the fact that 99Tc has only been produced from fission for approximately 50 years. High concentrations of Mo could cause an interference if the "'Mo peak is large enough to overlap with mass 99. The magnitude of the problem depends on the abundance sensitivity 0 5 10 15 20 Time/s Fig. 4 Elution profile for a standard containing 180 Bq 1-' (22.8 ng 1-') of 229Th 30.9 Bq 1-' (43.0 ng 1-') of 230Th 18.5 Bq 1-' (52.8 ng 1-I) of 233U and 11.8 Bq I-' (51.5 ng 1-') of 234U I ' I \ \ 0 5 10 15 20 Ti me/s Fig. 5 Elution profile for a soil sample containing 1.63 Bq g-' (2.27 ng g-') of 230Th and 1.65 Bq g-' (7.20 ng g-') of 234U of the ICP mass spectrometer that is used.Newer instruments generally have better abundance sensitivities than older systems (typically greater than 1 x lo6). The efficiency of separating Tc from Ru and Mo was determined with the following experiment. A standard contain- ing lOOpgl-' each of Mo Ru and Re was analysed by conventional aspiration to determine response factors for Mo and Ru relative to Re. A standard containing lOOpgl-' Mo 1 pg 1-l Ru and 50 ng 1-l Re was analysed by the FI-ICP-MS method. Using the peak areas obtained for Mo Ru and Re the previously determined response factors and the known concentration of Re in the standard the concentrations of Mo and Ru were estimated in the eluent. Based on the estimated concentrations in the eluent and the known concentrations in the original standard the separation efficiency was calculated.The separation efficiency varied slightly between columns from 97% to greater than 99.5%. Similar experiments were per-932 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 Table 3 Comparison of 99Tc results obtained by FI-ICP-MS and radiochemical methods for the PORT11 soil sample I FI-ICP-MS Radiochemical method6 mBq g-' ng g-' mBq g-' ng 8-l 33 30 40 38 32 36 42 33 43 35 Mean result +95% confidence interval 37-13 35 35 58 31 35 36 34 39 39 0.05 2 0.048 0.060 0.05 1 0.057 0.067 0.053 0.068 0.0515 0.064 0.056 0.056 0.092 0.049 0.056 0.057 0.054 0.062 0.062 52 41 44 56 41 48 41 44 41 44 0.083 0.065 0.070 0.089 0.065 0.076 0.065 0.070 0.065 0.070 0.059 k 0.005 45*4 0.072 +_ 0.006 Table 4 Comparison of 99Tc results obtained by FI-ICP-MS and radiochemical methods for the PORT13 soil sample FI-ICP-MS Radiochemical method 455 407 440 448 418 422 433 414 396 Mean result k 95% confidence interval mBq g-' 427 9 426 426 459 396 422 422 429 426 440 0.723 0.647 0.700 0.71.2 0.665 0.6?1 0.688 0.658 0.630 0.678 k 0.014 0.677 0.677 0.730 0.630 0.67 1 0.671 0.682 0.677 0.700 mBq g-' 426 437 433 429 455 437 41 1 433 440 433 433 * 8 ng g-' 0.677 0.695 0.688 0.682 0.723 0.695 0.653 0.688 0.700 0.688 0.689 k0.013 Table 5 Results obtained for 230Th and 234U in the soil reference material TRM-4 Mean result i- 95% confidence interval Reference values L 95% confidence interval 230Th mBq g-' 1680 1610 1700 1700 1710 1670 1640 1670 1620 1710 1671 L27 1643 i- 10 ng g-' 2.34 2.24 2.37 2.37 2.38 2.33 2.28 2.33 2.26 2.38 2.33 k 0.04 2.29 f 0.01 234u mBq g-' 1690 1700 1620 1640 1640 1620 1650 1670 1690 1650 1657+ 21 1650f9 ng 8-' 7.38 7.42 7.07 7.16 7.16 7.07 7.21 7.29 7.21 7.38 7.24 -t 0.09 7.20 -t 0.04 formed using soil samples spiked with Ru and Mo prior to fusion and comparable results were obtained.The interference check standards described above under Quality Control were analysed at the beginning and end of each analytical run to verify that separations from Mo and Ru are effective. Molybdenum-100 and "'Ru were monitored in each analysis in order to verify the absence of interferences. Determination of 230Th and 234U No significant interferences were observed for the determi- nation of 230Th.Thorium-232 present in samples could interfere with the determination of 234U by formation of ThH which overlaps with 233U the internal standard for 234U. The ratio of ThH to 232Th is approximately 0.01%. Natural Th is essentially 100% 232Th. Any 232Th present in the samples is concentrated by the FI process. Thorium-232 oxide was monitored at mlz248 in each analysis to give an indication of the amount of 232Th present in the samples without exposing the detector to the much higher count rates that would be observed at mlz232. These data were used to flag any potential problems. The 232Th concentrations in the samples analysed for this study were not high enough to cause problems. The concen- tration of 232Th in the interference check standards was selected to demonstrate that the method could tolerate 232Th at the approximate levels found in the samples studied.Several options exist if 232Th is present in samples at high enoughJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1994 VOL. 9 933 concentration to interfere with 233U. The samples could be diluted or a shorter load time could be used however the LOD would be raised. An inter-element correction factor could be used to correct the 233U signal based on the observed 232Th'60 signal. This approach will be investigated in this laboratory in the future. Method Performance Detection limits Detection limits were determined by analysing a blank ten times and multiplying the standard deviation obtained for the ten readings by 3.The LOD for "Tc was 26 mBq 1-l (0.04 ng 1-') in solution or 11 mBq g-' (0.02 ng g-') in the soil. The LOD for "Tc by conventional aspiration ICP-MS was 200 mBq 1-I. The LODs for 230Th and 234U were 37 mBq 1-'(0.05 ng 1-l) and 7.4 mBq 1-l (0.03 ng 1-') respectively. This corresponds to 3.7 mBq g- (0.005 ng g- ') and 0.74 mBq g- ' (0.003 ng g-') in the soil for 230Th and 234U respectively. The LODs for 230Th and 234U by conventional aspiration were 960 mBq 1-' (1.3 ng 1-') and 160 mBq 1-l (0.7 ng 1-') respectively. The LOD for 99Tc by the radiochemical method was 11 mBq g- ' (0.02 ng g-').6 Accuracy and precision of the method for " Tc There are no soil reference materials with known amounts of "Tc available to verify the accuracy of the method. Therefore samples of two soils contaminated with "Tc were obtained and prepared for use as reference materials.Approximately 10 kg of a soil designated PORT1 1 and 3 kg of a soil designated PORT13 were obtained from the Portsmouth Gaseous Diffusion Plant site in Piketon OH USA. The samples were dried and ground as described above under Preparation of Soil Samples blended for 1 d and packaged in 100 g bottles. Ten bottles of each sample were shipped to the Portsmouth Plant laboratory for determination of "Tc by the radiochemi- cal method described under Experimental. The samples were also analysed by the FI-ICP-MS method. The results are given in Tables 3 and 4. The 95% confidence intervals were calculated as described by Natrella' based on the t-distribution by multiplying the appropriate t value by the estimated sample standard deviation and dividing by the square root of the number of measurements.A statistical test for comparing results obtained by different techniques described by Natrella" was used to determine if the mean results obtained by the two methods agreed. The test uses the estimated variances of the data sets and the t statistic to calculate a critical value for the difference in the means. If the absolute value of the difference in the means is not greater than the critical value there is no reason to believe that the means differ. For the PORT11 data the test concluded that the means of the FI-ICP-MS and radiochemical results do differ statistically at the 95% confidence level indicating that a bias exists in at least one of the data sets.This is not of great concern considering the proximity of the results to the LOD of the method and that the agreement of the two means is good enough to satisfy the objectives of the projects that the methods are used to support. The statistical test of the two PORT13 data sets concluded that there is no reason to believe that the mean values differ at the 95% confidence level. Recoveries of pre-digestion spikes were 93.3 and 99.7% for PORT11 and PORT13 respectively by the FI-ICP-MS method. Accuracy and precision of the method for 230Th and 234U Accuracy and precision of the 230Th and 234U method were assessed by analysing a soil reference material designated TRM-4 that was prepared and characterized by the US DOE Grand Junction Projects Office laboratory.'' The results obtained by FI-ICP-MS are given in Table 5.The 95% confi- dence intervals were calculated as described for "Tc. The recommended values were established for the reference soil by alpha-energy spectrometry. The data provided in ref. 11 were used to calculate the 95% confidence intervals for the rec- ommended values and to statistically compare the means of the FI-ICP-MS and alpha-energy spectrometry data sets as described for "Tc. The test concluded that there is no reason to believe that the mean values for either 230Th or 234U obtained by the two techniques differ at the 95% confidence level. Recoveries of pre-digestion spikes were 93.0 and 99.0% for 230Th and 234U respectively for TRM-4 by the ICP-MS method. Conclusions The FI-ICP-MS method described has been used in this laboratory regularly over the past year and has proven to be accurate cost-effective and reliable.The FI approach using solid-phase adsorption on a mini-column is effective only not in improving sensitivity and LODs but also in reducing physical and spectroscopic interferences. The lower cost and higher sample throughput compared with radiochemical methods make the method especially attractive for environmen- tal applications such as those involved in many US DOE environmental restoration and waste management projects. Work by RUST Geotech personnel was performed under DOE contract No. DE-AC04-861D12584 for the U.S. Department of Energy. Radiochemical "Tc analyses were conducted by Billy Short Martin Marietta Utility Services Radiochemistry Division Piketon OH USA. 1 2 3 4 5 6 7 8 9 10 11 References Cox R. J. Pickford C. J. and Thompson M. J. Anal. At. Spectrom. 1992 7 635. Grohs J. F. and Hollenbach M. H. paper presented at the 30th Rocky Mountain Conference Denver CO USA July 31-August Riglet C. Provitina O. Dautheribes J. and Revy D. J. Anal. At. Spectrom. 1992 7 923. Nichols S. Sanders T. W. and Blaine L. M. Sci. Total Environ. Cotton F. A. and Wilkinson G. Advanced Inorganic Chemistry 3rd edn. Interscience New York 1972 p. 990. Short B. W. Piketon OH USA 1993 private communication. Horwitz E. P. and Chiarizia R. unpublished work from the Chemistry Division Argonne National Laboratory Argonne IL USA 1991. Horwitz E. P. Chiarizia R. Dietz M. L. and Diamond H. Anal. Chim. Acta 1993 281 361. Natrella M. G. Experimental Statistics National Bureau of Standards Handbook 91 US Government Printing Office Washington D.C. 1966 pp. 2-1-2-4. Natrella M. G. Experimental Statistics National Bureau of Standards Handbook 91 US Government Printing Office Washington D.C. 1966 pp. 3-26-3-28. Donivan S. and Chessmore R. Uranium Reference Materials publication number UNC/GJ-36(TMC) UNC Geotech Technical Measurements Center US Department of Energy Grand Junction Projects Office Grand Junction CO 1987. 5 1988. 1993 130-131,275. Paper 4/01 1720 Received February 2 1994 Accepted May 18 1994