Anal. Chem. 2004, 76, 5510-5517
Direct Determination of Lead Isotopes (206Pb, 207 Pb, 208Pb) in Arctic Ice Samples at Picogram per Gram Levels Using Inductively Coupled Plasma-Sector Field MS Coupled with a High-Efficiency Sample Introduction System Michael Krachler,*,† James Zheng,†,‡ David Fisher,‡ and William Shotyk†
Institute of Environmental Geochemistry, University of Heidelberg, Heidelberg, Germany, and Terrain Sciences Division, Geological Survey of Canada, Ottawa, Canada
Adopting strict cleanroom procedures, ice samples from the Canadian High Arctic have been analyzed for Pb concentrations and Pb isotopes (206Pb, 207Pb, 208Pb) using ICP-SMS. The detection limit for Pb (0.06 pg g-1) was ∼2 orders of magnitude lower than the lowest concentration of Pb in the ice samples (range, 4.3-1660 pg g-1; median, 45 pg g-1). Acidification of ice samples with high-purity HNO3 for stabilization purposes contributed only 0.004 pg of Pb g-1, which is an insignificant source of Pb. Using a new sample introduction system consisting of a heated (140 °C) minicyclonic spray chamber and a Peltier cooled condenser (2 °C) and by replacing the conventional sample cone with a high-performance cone, signal intensities for Pb were increased by ∼1 order of magnitude. Thus, it was possible not only to measure Pb isotope ratios directly using ICP-SMS but also to achieve reasonable precision (∼0.2%) at low picogram per gram concentrations of total Pb. This precision is comparable to that achievable by thermal ionization mass spectrometry at such low Pb concentrations, but the ICPSMS requires much less sample volume (∼2 mL), needs no sample pretreatment, and therefore is considerably faster and less expensive than the conventional approach. Even though absolute Pb concentrations in two ice samples dating from 1974 and 1852 were very similar (9 and 6 pg g-1) their fundamentally different isotopic signature (206Pb/207Pb: 1.169 ( 0.002 vs 1.147 ( 0.003) clearly indicates different sources of Pb. The analytical procedures described here, therefore, offer great promise for fingerprinting the predominant sources of atmospheric Pb in polar snow and ice. During the past decade, ice cores have been extensively exploited to assess global atmospheric lead contamination. Several * To whom all correspondence should be addressed. Tel.: +49 62 21 54 48 48. Fax: +49 62 21 54 52 28. e-mail:
[email protected]. http:// www.uni-heidelberg.de/institute/fak12/ugc/mkrachler/krachler.htm. † University of Heidelberg. ‡ Geological Survey of Canada.
5510 Analytical Chemistry, Vol. 76, No. 18, September 15, 2004
historical records of atmospheric Pb deposition are available from ice cores in Greenland, Antarctica, and Alpine regions of Europe, for example.1-5 Important improvements in sampling, decontamination, and cleanroom techniques allow for the reliable determination of Pb at picogram per gram and even femtogram per pram concentration levels in snow and ice.6-9 Practical detection limits obtained using inductively coupled plasma-sector field mass spectrometry (ICP-SMS), now the most frequently employed instrument for quantification in this concentration range, are typically limited by blank levels and not by instrumental sensitivity. Information on the origin of Pb in such samples, however, can only be derived from the isotopic composition. For environmental studies, the three stable, radiogenic 206Pb, 207Pb, and 208Pb isotopes are frequently exploited to trace the origin of Pb.10 Multicollector (MC)-ICPMS potentially providing highly precise isotopic information has not been considered for the determination of Pb isotope ratios in ice samples so far. Even though thermal ionization mass spectrometry (TIMS) provides the highest accuracy and precision for Pb isotope ratio measurements for all sample matrixes, ICPSMS is gaining popularity for that purpose.1,2,4-7,11-13 One reason for the increasing use of ICP-SMS is the fact that the ultimate (1) Hong, S.; Candelone, J.-P.; Turetta, C.; Boutron, C. F. Earth Planet. Sci. Lett. 1996, 143, 233-244. (2) Vallelonga, P.; van de Velde, K.; Candelone, J.-P.; Morgan, V. I.; Boutron, C. F.; Rosman, K. J. R. Earth Plant. Sci. Lett. 2002, 204, 291-306. (3) Planchon, F. A. M.; van de Velde, K.; Rosman, K. J. R.; Wolff, E. W.;Ferrari, C. P.; Boutron, C. F. Geochim. Cosmochim. Acta 2003, 67, 693-708. (4) Schwikowski, M.; Barbante, C.; Doering, T.; Gaeggeler, H. W.; Boutron, C.; Schotterer, U.; Tobler, L.; van de Velde, K.; Ferrari, C.; Cozzi, G.; Rosman, K.; Cescon, P. Environ. Sci. Technol. 2004, 38, 957-964. (5) Veysseyre, A. M.; Bollho¨fer, A. F.; Rosman, K. J. R.; Ferrari, C. P.; Boutron, C. F. Environ. Sci. Technol. 2001, 35, 4463-4469. (6) Vallelonga, P.; van de Velde, K.; Candelone, J.-P.; Ly, C.; Rosman, K. J. R.; Boutron, C. F.; Morgan, V. I.; Mackey, D. J. Anal. Chim. Acta 2002, 453, 1-12. (7) Krachler, M.; Le Roux, G.; Kober, B.; Shotyk, W. J. Anal. At. Spectrom. 2004, 19, 354-361. (8) Barbante, C.; Cozzi, G.; Capodaglio, G.; van de Velde, K.; Ferrari, C.; Boutron, C.; Cescon, P. J. Anal. At. Spectrom. 1999, 14, 1433-1438. (9) Krachler, M.; Zheng, J.; Fisher, D.; Shotyk, W. J. Anal. At. Spectrom., in press. (10) Sangster, D. F.; Outridge, P. M.; Davis, W. J. Environ. Rev. 2000, 8, 115147. 10.1021/ac0496190 CCC: $27.50
© 2004 American Chemical Society Published on Web 07/30/2004
precision for Pb isotope ratios typically obtained by TIMS is not necessary for many environmental studies.7,11 Reported precisions for 206Pb/207Pb and 206Pb/208Pb ratios using ICP-SMS amount to 0.14% for ice/snow samples with Pb concentrations of >1000 pg g-1.4,13 At lower Pb concentrations, precision worsens rapidly. We recently developed an analytical protocol using ICP-SMS to achieve similar precision at Pb concentrations that are ∼1 order of magnitude lower.7 As a result, it became possible to determine Pb isotope ratios in snow samples from Germany with Pb concentrations as low as 88 pg g-1.7 Unfortunately, Pb concentrations in Arctic ice are distinctly lower, with “background” concentrations approaching the low-picogram per gram level.14 In Antarctic ice dating from preanthropogenic times, Pb concentrations can even fall to femtogram per pram levels.1 At present, therefore, the limiting factor for Pb isotope ratio measurements using ICP-SMS is the lack of signal intensity in these types of samples. In other words, poor counting statistics are obtained at such low Pb concentrations. Consequently, the precision of Pb isotope measurements is poor and these data do not allow a detailed, reliable interpretation of the results. Therefore, there is an urgent need to improve the precision of Pb isotope ratio measurements, and this could be achieved if the sensitivity of the ICP-SMS could be increased. The main goal of the present study was to improve the accuracy and precision of Pb isotope ratios at very low (pg g-1) concentrations of total Pb by increasing sensitivity. There are presently two possible ways to do this directly: (1) using a new sample introduction system (APEX) and (2) replacing the conventional ICP-SMS sample cone with a high-performance “X” cone. Here, we have evaluated the sensitivity of Pb determinations using the APEX sample introduction system with and without the X cone. In addition to absolute sensitivity, we have also evaluated the effects of these modifications on the mass discrimination behavior for the Pb isotopes of interest (206Pb, 207Pb, 208Pb). Once the procedures described here were optimized, they were then used to determine the isotopic composition of Pb in 120 samples of ice from the Canadian Arctic (Devon Island) representing the past ∼150 years of snow accumulation. The accuracy and precision of the Pb isotope ratios in the samples containing the lowest Pb concentrations (a few pg g-1) were found to be comparable with TIMS analyses of Antarctic ice containing comparable Pb concentrations. EXPERIMENTAL SECTION Laboratories and Instrumentation. All sample handling and the preparation of all standards were performed in cleanrooms under laminar flow clean air benches of at least class 100 to minimize the potential risk of contamination. Ice core decontamination was carried out in a clean air bench of class 100 and further processing (melting, acidification, bottling) of the ice samples was (11) Chillrud, S. N.; Hemming, S.; Shuster, E. L.; Simpson, H. J.; Bopp, R. F.; Ross, J. M.; Pederson, D. C.; Chaky, D. A.; Tolley, L.-R.; Estabrooks, F. Chem. Geol. 2003, 199, 53-70. (12) Shotyk, W.; Weiss, D.; Appleby, P. G.; Cheburkin, A. K.; Frei, R.; Gloor, M.; Kramers, J. D.; Reese, S.; van der Knapp, W. O. Science 1998, 281, 1635-1640. (13) Do ¨ring, T.; Schwikowski, M.; Ga¨ggeler, H. W. Fresenius J. Anal. Chem. 1997, 359, 382-384. (14) Boutron, C.; Go¨rlach, U.; Candelone, J.-P.; Bolshov, M. A.; Delmas, R. J. Nature 1991, 353, 153-156.
Table 1. Operating Conditions of the ICP-SMS forward power coolant gas flow rate auxiliary gas flow rate sample gas flow rate sample cone skimmer cone resolution sample uptake rate (self-aspirating) ion sampling depth ion lens settings take up time washing time between samples scan type mass window magnet settling time dwell time per isotope scan duration per scan number of scans per replicate total time per sample (5 replicates including uptake)
1250 W 16 L min-1 ∼ 0.6-1.0 L min-1, optimized dailya ∼ 1.0 L min-1, optimized dailya Ni, 1.1-mm aperture i.d. or X-cone Ni, 0.8-mm aperture i.d. m/∆m 300 ∼0.22 mL min-1 adjusted dailya adjusted when appropriate 100 s 1 min magnet fixed at 206Pb, electric scanning over other masses (E-scan) 5% 1 ms (default minimum) 5 ms 78 ms 1200 9 min 30 s
a Optimized in order to obtain a stable and high 208Pb signal and the lowest possible oxide formation rate (see text for details).
carried out in clean air benches of class 10 at the Geological Survey of Canada (GSC). All HDPE bottles and PFA vials in contact with the samples were thoroughly cleaned using HNO3 baths of increasing purity.8 All ICPMS measurements were carried out using an Element 2 ICP-SMS (Thermo Finnigan, Bremen, Germany) equipped with a guard electrode to eliminate secondary discharge in the plasma and to enhance overall sensitivity. A microvolume autosampler (ASX 100, Cetac Technologies, Omaha, NE) and a HF-resistant sample introduction kit consisting of a microflow PFA nebulizer, a PEEK microcyclone spray chamber, and a sapphire injector tube were employed for the quantification of total lead in the ice samples. For isotopic measurements (206Pb, 207Pb, 208Pb), the conventional spray chamber was replaced by a high-efficiency sample introduction system (APEX Q, made from quartz, without optional membrane desolvation unit, Elemental Scientific Inc, Omaha, NE) consisting of heated spray chamber and a Peltier cooled condenser. Additionally, the normal sample cone was replaced by a high-efficiency cone (X-cone, Thermo Finnigan) further increasing sensitivity of the ICP-SMS. The low-flow PFA nebulizers were operated in the self-aspirating mode to reduce the risk of contamination by the peristaltic pump tubing. The detector dead time was determined according to the manufacturer’s recommendations and amounted to 19 ns. Details of the entire analytical protocol have been discussed at length elsewhere.7 ICPSMS operating conditions, data acquisition, and reduction parameters are summarized in Table 1. Reagents and Standards. For the preparation of all solutions, high-purity water (18.2 MΩ cm) from a MilliQ-Element system designed for ultratrace analysis (Millipore, Milford, MA) was used. Nitric acid (65%, analytical-reagent grade, Merck, Darmstadt, Germany) was further purified by doubly sub-boiling distillation (MLS GmbH, Leutkirch, Germany). Both the water purification system and the sub-boiling distillation unit were operated in cleanrooms. Analytical Chemistry, Vol. 76, No. 18, September 15, 2004
5511
Lead calibration solutions were prepared daily by appropriate dilution of a 10 mg L-1 stock standard solution (Merck) to concentrations of 0.5, 1, 5, 10, 100, and 500 ng L-1 with 0.14 mol L-1 high-purity nitric acid. All reported Pb concentrations have been converted to a weight basis considering the density of the corresponding solutions. For isotopic analysis, the National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 981 Common Lead Isotopic Standard was diluted to a Pb concentration of ∼250 µg g-1 with 1% (v/v) high-purity HNO3. To correct for mass discrimination effects, this standard solution was further diluted to ∼100 pg Pb g-1 for daily analysis. Collection of Ice Samples and Sample Treatment. A 63.72m-long firn core was drilled in May 2000 from Devon ice cap (Devon Island; 75 °N, 82 °W; 1860 m asl) using a “clean drill”.15 The cores were shipped back frozen to the GSC laboratories in Ottawa and stored at -20 °C before processing and analysis. The cores were mechanically decontaminated using a method comparable to that developed for Greenland cores.16 Samples were acidified with high-purity nitric acid while they melted and stored at -20 °C in precleaned HDPE bottles until analyses by ICP-SMS. Quality Control. A certified reference material for the determination of Pb concentrations or Pb isotope ratios in ice is currently not available. Therefore, the riverine water reference material SLRS-4 from National Research Council Canada, Ottawa, Canada, was used to assess the accuracy of the determination of total Pb. The experimentally established Pb concentrations (0.079 ( 0.004 µg L-1, N ) 9) were in good agreement with the certified concentration of 0.086 ( 0.007 µg L-1. Additionally, Pb concentrations in selected ice samples have been also determined using USN-ICP-QMS at the GSC yielding a correlation coefficient between the two data sets of R2 ) 0.95. The accuracy of Pb isotope ratio measurements was assessed previously by comparative measurements of identical samples using TIMS. The 207Pb/206Pb and 208Pb/206Pb ratios obtained using ICP-SMS were found to agree within 0.1% of the TIMS values. Experimentally established 207Pb/206Pb and 208Pb/206Pb ratios in SRM 981 differed from the certified values by not more than 0.04%.7 Using the high-efficiency sample introduction system described in this study for the analysis of SRM 981, comparable accuracies (207Pb/206Pb ) 0.9154 ( 0.002 and 208Pb/206Pb ) 2.1677 ( 0.002) were obtained, underpinning the reliability of the analytical protocol adopted. Procedural Lead Blank. Acidification of the melted ice samples with high-purity nitric acid to 0.5% contributed only 0.004 pg of Pb g-1 and was negligible: the actual Pb concentrations determined in the ice samples were at least 3 orders of magnitude higher than this blank contribution. Potential contributions of the high-purity sample introduction system of the ICP-SMS made from PFA and PEEK are extremely small and are already included above (0.004 pg g-1). In that context more importantly, however, is the assessment of potential leaching of Pb from the precleaned storage bottles. Following extensive multistep cleaning of the storage bottles, leaching experiments revealed that concentrations of Pb in the 0.5% high-purity nitric acid solutions stored in the processed HDPE (15) http://thrust_2.tripod.com/glaciology/icedrill.html. (16) Candelone, J. P.; Hong, S.; Boutron C. F. Anal. Chim. Acta 1994, 299, 9-16.
5512
Analytical Chemistry, Vol. 76, No. 18, September 15, 2004
storage bottles were always below the detection limit (
