Anal. Chem. 1996, 68, 3783-3788
Determination of 234Th in Marine Samples by Liquid Scintillation Spectrometry Jacqueline M. Pates,*,† Gordon T. Cook, Angus B. MacKenzie, Robert Anderson, and Sarah J. Bury‡
Scottish Universities Research and Reactor Centre, Scottish Enterprise Technology Park, Rankine Avenue, East Kilbride, Glasgow G75 0QF, U.K.
A liquid scintillation spectrometry method for the determination of 234Th in seawater with 230Th as the yield tracer has been developed and validated. 234Th is separated from the dissolved phase by an Fe(OH)3 precipitation and is then purified using ion exchange chromatography. The counting source is prepared by taking the sample to dryness in a vial, redissolving in acid, and mixing with a scintillation cocktail. The instrument employed has a relatively low background (11 cpm) and the ability to separate r from β activity on the basis of pulse shapes. The 234Th + 234mPa counting efficiency is 50% over the counting window employed. The limit of detection, using the above parameters, a 20 L sample, and a 400 min count is found to be 0.04 dpm L-1. It was also demonstrated that less advanced instruments, without r/β separation, can also be used effectively. In recent years, there has been an increasing level of interest in the use of 234Th/238U disequilibrium in the marine environment to study geochemical processes with short time scales (up to 100 days), particularly those associated with carbon cycling in the oceans1-3 and the partitioning of pollutants between the dissolved and particulate phases.4,5 However, the analysis of 234Th is constrained by its short half-life and its low concentration in seawater, so appropriate analytical techniques must be rapid and sensitive and preferably should allow shipboard analysis. Traditionally, 234Th has been analyzed by gas proportional counting (GPC) of β particles emitted by 234mPa, using sample volumes ranging between 20 and 100 L, depending on detector efficiency and background.5-7 Since the analysis requires preconcentration and purification of the sample and electrodeposition onto a planchette, a yield monitor is requiredstypically 228Th, 229Th, or 230Th.8,9 The samples then require a minimum of two † Present address: Department of Geology and Geophysics, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, U.K. ‡ Present address: Department of Biological Sciences, Napier University, 10 Colinton Rd., Edinburgh EH10 5DT, U.K. (1) Buesseler, K. O.; Bacon, M. P.; Cochran, J. K.; Livingston, H. D. Deep-Sea Res. 1992, 39, 1115-1137. (2) Coale, K. H.; Bruland, K. W. Limnol. Oceanogr. 1985, 30, 22-33. (3) Schmidt, S.; Nival, P.; Reyss, J.-L.; Baker, M.; Buat-Menard, P. Oceanol. Acta 1992, 15, 227-231. (4) Huh, C.-A.; Ku, T.-L.; Luo, S.; Landry, M. R.; Williams, P. M. Earth Planet. Sci. Lett. 1993, 116, 155-164. (5) Kershaw, P.; Young, A. J. Environ. Radioact. 1988, 6, 1-23. (6) Coale, K. H.; Bruland, K. W. Limnol. Oceanogr. 1987, 32, 189-200. (7) Buesseler, K. O.; Michaels, A. F.; Siegel, D. A.; Knap, A. H. Global Biogeochem. Cycles 1994, 8, 179-193. (8) Tanaka, N.; Takeda, Y.; Tsunogai, S. Geochim. Cosmochim. Acta 1983, 47, 1783-1790. (9) Martin W. R.; Sayles, F. L. Geochim. Cosmochim. Acta 1987, 51, 927-943.
S0003-2700(96)00503-3 CCC: $12.00
© 1996 American Chemical Society
countssone to determine the β activity and another to determine the R activitysand each count requires independent calibration of detection efficiency.10 Modern GPC instruments are capable of providing low backgrounds (∼0.3 cpm) and extremely good accuracy and precision (∼2%) and have been used at sea.7 An alternative approach is γ spectrometry using HpGe γ photon detectors. This technique meets most of the requirements for 234Th analysis since no chemical manipulation of the sample is required and the detectors are sufficiently rugged to be used at sea. However, the low absolute intensity of the 63 keV γ photon emissions (3.8%), combined with the relatively low detection efficiency of γ spectroscopy systems, results in large sample volumes (300-600 L) being required for the analysis.11,12 These sample sizes can be achieved through the use of in situ pumps and manganese cartridges,11 which scavenge thorium from seawater.13 These systems avoid the problems of bottle-associated sampling artifacts, such as thorium losses to the vessel walls and particles sinking below spigots, and enable sampling of rare large particles. However, the pumping system is relatively expensive and time-consuming to use, restricting the number of depths that can be sampled simultaneously. In addition, only a simple split of particulate and dissolved fractions can be achieved, with more detailed size fractionation, such as that required for the determination of colloidal 234Th, requiring an alternative method. This second point is of particular relevance with the growing realization that (i) colloids play a critical role in both carbon cycling and trace metal scavenging and (ii) 234Th/238U disequilibrium is a useful technique for elucidating this role.14-16 Liquid scintillation spectrometry (LSS) is a technique suitable for the analysis of both R and β emitters, with much higher detection efficiencies than either R or γ spectrometry using semiconductor detectors or GPC. For R emitters, the LSS detection efficiency is ∼100%, while for β emitters with Emax > 156 keV (14C), detection efficiency is >95%. LSS has been applied to the determination of 234Th in seawater with 230Th as a yield tracer,17 however, the 234mPa spectrum overlaps that of 230Th (Figure 1), and mathematical techniques are required to resolve the individual spectra. Modern R/β LSS instruments can separate (10) Aller, R. C.; DeMaster, D. J. Earth Planet. Sci. Lett. 1984, 67, 308-318. (11) Buesseler, K. O.; Cochran, J. K.; Bacon, M. P.; Livingston, H. D.; Casso, S. A.; Hirschberg, D.; Hartman, M. C.; Fleer, A. P. Deep-Sea Res. 1992, 39, 1103-1114. (12) Buesseler, K. O.; Andrews, J. A.; Hartman, M. C.; Belastock, R.; Chai, F. Deep-Sea Res. II 1995, 42, 777-804. (13) Livingston, H. D.; Cochran, J. K. J. Radioanal. Nucl. Chem. Articles 1987, 115, 299-308. (14) Moran, S. B.; Buesseler, K. O. J. Mar. Res. 1993, 51, 893-922. (15) Niven, S. E. H.; Kepkay, P. E.; Boraie, A. Deep-Sea Res. II 1995, 42, 257273. (16) Huh, C.-A.; Prahl, F. G. Limnol. Oceanogr. 1995, 40, 528-532.
Analytical Chemistry, Vol. 68, No. 21, November 1, 1996 3783
Figure 1. Liquid scintillation spectrum of 230Th and 234Th/234mPa from a seawater sample counted on the Packard Tri-Carb 2550TR/ AB LSS.
R from β events on the basis of their pulse shapes and allocate them to separate multichannel analysers (MCAs). They are also rugged enough to be taken to sea,18 although this has not been done in this study due to the short duration of the cruises. R/β LSS has been employed for the analysis of freshwater samples, where a 228Th yield tracer was used.19 However, 228Th has a series of rapidly ingrowing R and β emitting daughters, necessitating the use of counts by both LSS and R spectrometry and negating the benefits of simultaneous R/β detection. Here, we present an R/β LSS method for the determination of 234Th in seawater using 230Th as the yield tracer, requiring a single count. METHODS Instrumentation. A Packard Tri-Carb 2550TR/AB liquid scintillation spectrometer (Packard Instrument Co.) was used throughout this study. This instrument employs electronic pulse shape discrimination (PSD) both to separate R from β events and to reduce background in the β MCA. R/β Separation. Scintillation events result in electronic pulses being generated at the photomultiplier tube (PMT) anodes which have different timing characteristics, depending on the origin of the scintillation event (R or β decay). The length of each pulse is determined and compared to an operator-set time discriminator, called a pulse decay discriminator (PDD). Those events of longer duration than the PDD are classified as R events and those of shorter duration as β events. Each event is then allocated to either the R or β MCA. As pulse lengths vary not only with particle energy but also with the vial type, cocktail, and quenching conditions,20 it is necessary to optimize the PDD for individual applications. Background Reduction. In addition to R/β separation by pulse shape determination (PSD), the low-level (LL) mode of the counter (17) Anderson, R.; Cook, G. T.; MacKenzie, A. B.; Harkness, D. D. In Liquid Scintillation Counting and Organic Scintillators; Ross, H. H., Noakes, J. E., Spaulding, J. D., Eds.; Lewis Publishers: Chelsea, MI, 1991; pp 461-470. (18) Queirazza, G.; Roveri, M.; Delfanti, R.; Papucci, C. In Radionuclides in the Study of Marine Processes; Kershaw, P. J., Woodhead, D. S., Eds.; Elsevier Applied Science: London, 1991; pp 94-104. (19) Morris, H. W.; Livens, F. R.; Nolan, L.; Hilton, J. Analyst 1994, 119, 24032406. (20) Pates, J. M. Ph.D. Thesis, University of Glasgow, 1995.
3784 Analytical Chemistry, Vol. 68, No. 21, November 1, 1996
also examines each pulse being allocated to the β MCA using a technique termed burst counting circuitry (BCC). The β MCA receives the β events and the vast majority of background events. The latter can be subdivided into quenchable and nonquenchable (principally Cerenkov) events. A nonquenchable event comprises a prompt pulse and a number of afterpulses of much smaller amplitude. In contrast, β events, particularly those of low energy, have fewer afterpulses. The BCC determines the number of afterpulses occurring a specified time after the onset of the prompt pulse (delay before burst, DBB), and prompt pulses with sufficient afterpulses to exceed the preset threshold values are rejected.21 The delay between the onset of the prompt pulse and commencement of afterpulse measurement has a user preset range of between 75 and 800 ns. Reagents and Tracers. Bio-Rad AG 1-X8 100-200 mesh chloride-form anion exchange resin (Bio-Rad Laboratories) was used throughout. The LSS cocktail used was Ultima Gold AB (Packard Instrument Co.), a cocktail formulated specifically for R/β separation.22 All other reagents were of analytical grade. Preparation of High-Activity 234Th and 230Th Sources. A small quantity of uraninite (∼0.5 g) was dissolved in HNO3/HCl (aqua regia), and the γ spectrum of the resulting solution was recorded using a HpGe γ photon detector, to ascertain that the U series nuclides were in equilibrium to at least 226Ra and that an insignificant activity of 232Th was present. The solution was separated into fractions containing either U (238U, 234U, and a small quantity of 235U) or Th (230Th and other short-lived isotopes, including 234Th) by ion exchange. The solutions were then aged for ∼1 year to allow 234Th to grow toward equilibrium in the uranium stock solution (after 1 year, 99.997% of equilibrium is reached) and to allow short-lived thorium isotopes, including 234Th, to decay completely in the thorium stock solution. When required, a high-activity 234Th solution was prepared by passing the uranium stock solution through a chloride-form column and collecting the eluent. 230Th was prepared by taking a small aliquot of the thorium stock solution and passing it through a nitrate-form column to remove decay products. The 230Th was then eluted with 9 M HCl. The resulting solutions, each containing ∼25 000 dpm total activity, were taken to dryness in 7 mL glass scintillation vials and then redissolved in 0.5 mL of 0.1 M HCl. Finally, 4.91 g of scintillation cocktail was added. The uranium stock solution was recovered by eluting the chloride column with 1 M HCl, taking the eluent to dryness, and redissolving in 9 M HCl. Dilutions of the stock solutions (called working solutions) were made to produce ∼50 dpm mL-1 238U for efficiency determinations and ∼50 dpm mL-1 230Th for use as a yield tracer. The exact concentration of 238U in the working solution was found to be 53.8 dpm mL-1 by ICPMS analysis. The exact concentration of 230Th was not determined, as only relative yields are required for the analysis. Instrument Optimization. The optimum PDD was determined by counting high activities of 234Th and 230Th in separate vials over a range of PDD settings and calculating the degree of misclassification at each point (Figure 2). At the optimum PDD, total misclassification of R events into the β MCA and vice versa is 2.8%; however, misclassification of β events into the R counting (21) Passo, C. J.; Kessler, M. J. In Liquid Scintillation Spectrometry 1992; Noakes, J. E., Schonhofer, F., Polach, H. A., Eds.; Radiocarbon: Tucson, AZ, 1993; pp 51-57. (22) Pates, J. M.; Cook, G. T.; MacKenzie, A. B.; Thomson, J. J. Radioanal. Nucl. Chem. Articles 1993, 172, 341-348.
Figure 2. Instrumentally determined cross-over plot for 234Th and 230Th, showing the optimum PDD: O, R events in the β MCA; b, β events in the R MCA; - - -, optimum PDD.
window (110-280 keV)was 0.13%, while misclassification of R events into the β window (0-120 keV) was 0.08% of the total count rate. These values are negligible compared with the count rates from 10-20 L seawater samples and are disregarded in subsequent calculations. When high-energy β particles are being counted, the DBB must be optimized to obtain the best background reduction without losing significant counting efficiency. The background (B) and 234Th counting efficiency (E) were determined for a range of DBB settings using the high-activity 234Th vial and a similarly prepared background, and figures of merit (E2/B, Figure 3) were calculated for each setting. These indicated that DBBs above 500 ns were necessary for optimization. A value of 800 ns was chosen for counting samples, as this minimized misclassification of β events (and therefore minimized efficiency losses) and maintained a low background. 234Th Separation Procedure. The method has been adapted from an established procedure for thorium analysis of sediments by R spectrometry23 and employs sample sizes of 10-20 L. Method Description: (1) Immediately after sampling, the water is filtered and then acidified with 2.5 mL L-1 of 11 M HCl. Next, 10 mg of Fe (as Fe(NO3)3) per liter of sample and 50 dpm 230Th yield tracer are added. The sample is stirred and allowed to equilibrate for 24 h. (2) A 5 mL L-1 portion of 18 M NH4OH is added to precipitate Fe(OH)3, which scavenges the thorium (and uranium).The sample is stirred for 20 min, allowed to settle, and then passed through a 0.45 µm cellulose nitrate filter, where the precipitate is retained. (3) Fe(OH)3 is dissolved in
