Continuous Ice-Core Chemical Analyses Using Inductively Coupled

Environ. Sci. Technol. 2002, 36, 7-11

Continuous Ice-Core Chemical Analyses Using Inductively Coupled Plasma Mass Spectrometry JOSEPH R. MCCONNELL,* GREGG W. LAMOREY, STEVEN W. LAMBERT, AND KENDRICK C. TAYLOR Desert Research Institute, University and Community College System of Nevada, Reno, Nevada 89512

Impurities trapped in ice sheets and glaciers have the potential to provide detailed, high temporal resolution proxy information on paleo-environments, atmospheric circulation, and environmental pollution through the use of chemical, isotopic, and elemental tracers. We present a novel approach to ice-core chemical analyses in which an ice-core melter is coupled directly with both an inductively coupled plasma mass spectrometer and a traditional continuous flow analysis system. We demonstrate this new approach using replicated measurements of ice-core samples from Summit, Greenland. With this method, it is possible to readily obtain continuous, exactly coregistered concentration records for a large number of elements and chemical species at ppb and ppt levels and at unprecedented depth resolution. Such very-high depth resolution, multiparameter measurements will significantly expand the use of ice-core records for environmental proxies.

Introduction A number of chemical species and physical properties are routinely measured in ice-cores, yielding high-resolution proxies of paleo-environmental information at subannual to millennial time scales. Among other applications, such records have been used to infer past atmospheric chemical concentrations (1), reconstruct past regional and hemispheric atmospheric circulation (2), estimate changes in cosmic-ray flux (3), and evaluate environmental pollution (4, 5). However, the relatively coarse measurement resolution of a number of parameters, many of which require discrete sampling methods, has limited investigation of high-frequency phenomena observed in ice-core records. These include rapid climate change events such as those during the HoloceneYounger Dryas transition approximately 12 000 y B.P., when ice-core records suggest major changes in climate occurred over a few decades or less (6). Moreover, relatively coarse depth resolution and ambiguity between the limited number of routinely measured chemical parameters add uncertainty to ice-core depth-age relationships, especially for cores collected in lower-accumulation regimes. Accurate depthage relationships are central to the use of ice-cores for detailed study of modern phenomena. For example, when ice-core accumulation measurements are used to develop and validate precipitation models over ice sheets (7, 8) or to study * Corresponding author phone: (775)673-7348; fax: (775)673-7363; e-mail: [email protected]. 10.1021/es011088z CCC: $22.00 Published on Web 11/30/2001

 2002 American Chemical Society

circulation phenomena such as El Nin ˜ o/La Nin ˜ a and the North Atlantic Oscillation (9), even a single-year error in the depth-age scale can preclude meaningful comparisons between ice-core and instrumental records. Previous discrete sampling of snow and ice-cores and chemical analyses using ion chromatography (10), Inductively Coupled Plasma Mass Spectrometry (ICP-MS) (11-14), Graphite Furnace Atomic Absorption Spectroscopy (15, 16), and other analytical methods have yielded concentration measurements of a large number of chemical species and elements but at substantial cost in both time and resources and only at relatively coarse depth resolution. Conversely, continuous measurements using an ice-core melter and Continuous Flow Analyses (CFA) have yielded high depth resolution, but measurements are limited to only a small suite of species such as hydrogen peroxide (H2O2), ammonium (NH4+), calcium ion (Ca2+), and nitrate (NO3-) (17, 18). Moreover, interpretations of core data that depend on analyses of different samples are limited because exact coregistration of samples is difficult. Here, we present a method in which an ice-core melter is coupled with both an ICP-MS and a traditional CFA system to rapidly obtain exactly coregistered, continuous concentration measurements of 11 elements (Na, Mg, Al, K, Mn, Rb, Sr, Zr, Ba, Nd, Pb), four chemical compounds and ions (hydrogen peroxide (H2O2), ammonium (NH4+), calcium (Ca2+), nitrate (NO3-)), and electrical conductivity. We demonstrate this new method, referred to as Continuous Flow Analysis with Trace Elements (CFA-TE), through replicated analyses of recently collected ice-core samples from Summit, Greenland. These elements were selected both because of their environmental and biogeochemical significance and because expected concentrations in Greenland snow and ice are above detection limits of the ICP-MS.

Methods The snow and ice chemistry facilities at the Desert Research Institute consist of a class-100 clean room, a cold laboratory, and a wet chemistry laboratory. Pass-through ports connect all three laboratories. The ice-core melter is situated in the cold room (maintained at -16 °C), the ICP-MS in the clean room, and the CFA system in the wet chemistry laboratory (Figure 1). The ice-core melter consists of a plastic (LDPE) stand that holds a longitudinal slice from an ice or firn core with a square cross section ∼3 cm × ∼3 cm, vertically above a melter head. Note that firn is compacted, granular snow with a density of ∼0.4 to ∼0.8 g cm-3. A plexiglass box surrounds the entire melter assembly to limit air movement around the core while in the melter stand (and so the potential for contamination, particularly when analyzing permeable firn core). The melter head is in thermal contact with a heated aluminum block and temperature just below the melter head is held constant by a temperature controller. Ice in contact with the custom-machined, nickel-plated brass melter head melts, and the meltwater is split into three regions by square, ∼0.5-cm-deep ridges engraved into the melter head (Figure 1). Contamination of ice core samples can occur during drilling operations, shipping, and sample handling. Except at core breaks, contamination is generally confined to the outer regions of the core with the potential for contamination decreasing toward the center. Therefore, melt from the innermost 1.4 cm × 1.4 cm region (zone 1 with an area of 2 cm2) is pumped through a heated Teflon tube to a debubbler located in the clean room. While the use of the nickel-plated VOL. 36, NO. 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1. Typical ICP-MS Instrument Settings for the Summit Study RF forward power argon gas flow plasma auxiliary nebulizer nebulizer type peak scan parameters scans replicate-1 replicates sample-1 scan time replicate time ion lens voltages extraction lens first lens second lens photon stop third lens fourth lens sample depth

FIGURE 1. Schematic of the coupled ice-core melter, CFA and ICPMS systems. The ice-core melter is located in a cold room and the CFA and ICP-MS systems in adjacent wet chemistry and class-100 clean laboratories, respectively. brass melter head in this application precludes meaningful measurements of Ni, Cu, and Zn, tests with a titanium melter head indicate that measurements of these elements are likely possible with a melter head of a different composition. Melt from the 2.3 cm × 2.3 cm middle region (zone 2 with a net area of 3 cm2 after subtracting the area of zone 1) is pumped through a heated Teflon tube to the wet chemistry laboratory where the stream is split, with a small portion pumped directly through a low-volume electrical conductivity cell and the remaining water pumped to a debubbler. Melt from the potentially contaminated outer region (zone 3 with a net area of 4 cm2 after subtracting the areas of the zones 1 and 2) is discarded. Because the sample cross section is approximately square, we developed a melter head with square regions rather than the circular regions found on most other ice-core melter systems. To minimize contamination of the sample streams by melt from the outside of the core, pump rates for the three regions are designed to ensure that lateral flow on the melter head is strongly outward. Because heat flow across the melter head is approximately constant, conversion of ice to water is at a constant rate. Therefore, melt rates vary inversely with firn and ice density, although for core samples deeper than ∼5 m, density variations are relatively small, so melt rates (with respect to depth) are nearly constant. Melter depth resolution for firn is ∼2 cm and is limited primarily by capillary forces that pull water up into the firn during melting. For analysis of compacted firn and ice, dispersion within the tubing, debubblers, and flow cells limit depth resolution to ∼1 cm. Meltwater from zone 1 is pumped from the melter head through a debubbler located in the clean room and then to a Cetac (model U-6000AT+) Ultra-Sonic Nebulizer (USN) and Varian Ultramass-700 ICP-MS equipped with an active-film multiplier detector and nickel cones. The ICP-MS is set to collect measurements continuously with an effective sample rate of ∼9 s. The instrument settings used in this study are listed in Table 1. While cold plasma settings provide higher sensitivity for lighter elements and hot plasma settings give higher sensitivity for heavier elements, the settings in Table 1 were designed to provide sensitivity over a large mass range. Prior to continuous injection into the USN, the unacidified, bubble-free sample stream (∼1.2 mL minute-1 flow rate) is 8

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1.27 kW 20.0 L min-1 1.15 L min-1 0.95 L min-1 Cetac U-6000AT+ 5 25 1784 ms 8.92 s -450 V -200 V -12.8 V -12.2 V 0.8 V -52 V 6.0 mm

combined with a second continuous aqueous stream containing acidified internal standards (∼1.0 mL minute-1 flow rate). Calibration of the ICP-MS over a range of concentration levels that bracket the expected sample concentrations is critical. This is especially difficult at the very low (ppb to ppt) concentrations of analytes found in most polar ice-cores. The ICP-MS calibration standards used for the analysis of the Summit ice-core are listed in Table 2 along with typical linear correlation coefficients between ICP-MS instrument counts (corrected for any temporal variations in the responses using internal standards) and standard concentration. For most analytes, correlation coefficients were nearly always greater than 0.99 over the range of calibration, demonstrating the linearity of the ICP-MS response over large concentration ranges. Typical instrument counts for blanks and standards are also given in Table 1. Deionized (18.2-megaohm) water, with additional purification provided by a Labconco WaterPro PC polishing station, was used to prepare all blanks and standards. Internal standardization is commonly used to compensate for temporal variations in ICP-MS response caused by partial blockage of the sampler and skimmer cone orifices, signal suppression or enhancement, or other matrix effects. Elements used for internal standards, which obviously must exist only at low concentrations in the samples, are selected to be both close in mass to the analytes under study and to avoid introducing isobaric interferences with the analytes. The internal standards used in this method are 6Li, 45Sc, 71Ga, 89Y, 103Rh, 115In, 159Tb, 165Ho, and 209Bi. To compensate for any temporal changes in instrument response, the measured counts per second at each sample time for those internal standards with atomic masses that most closely bracket an analyte are linearly interpolated to that analyte’s mass. The raw counts per second for the analyte are divided by the interpolated internal standard value, and this ratio or normalized response is then used for all calibrations and analyte determinations. The CFA system used in this application is similar to that described by Anklin et al. (18) and Rothlisberger et al. (19). The system includes analyses for electrical conductivity and aqueous concentrations of H2O2, NH4+, Ca2+, and NO3-. Although additional continuous flow analytical methods are available for sodium (Na+) (19), chloride (Cl-) (20), formaldehyde (HCHO) (21), and sulfate (SO42-) (22), these have not yet been implemented in our laboratory. The sample stream from the debubbler that is located in the wet chemistry laboratory is split into four parts and pumped at a rate of

TABLE 2. Typical ICPMS Standards, Instrument Counts, and Correlation Coefficients element (isotopes) Na (23) Mg (24) Al (27) K (39) Mn (55) Rb (85) Sr (86, 87, 88) Zr (90) Ba (137) Nd (143, 144) Pb (206, 207, 208)

units

blank

1

ppb

0 (92) 0 (1.2) 0 (3.8) 0 (75) 0 (1.1) 0 (0.04) 0 (0.30) 0 (0.02) 0 (0.01) 0 (0.01) 0 (0.23)

0.2 (116) 0.04 (2.9) 4.0 (3.5) 0.04 (79) 1.0 (1.3) 1.0 (0.18) 1.0 (0.15) 1.0 (0.05) 1.0 (0.02) 0.2 (0.02) 1.0 (0.25)

ppb ppt ppb ppt ppt ppt ppt ppt ppt ppt

standard (instrument counts × 103) 2 3 4 1.0 (148) 0.2 (7.4) 20.0 (4.4) 0.2 (86) 5.0 (1.6) 5.0 (0.29) 5.0 (0.31) 5.0 (0.18) 5.0 (0.04) 1.0 (0.03) 5.0 (0.29)

∼0.5 mL minute-1 into four separate analytical systems. Reagents are added as necessary. H2O2, NH4+, and Ca2+ are measured using established fluorimetric methods and NO3using adsorption spectroscopy described in detail by Anklin et al. (18). The detection limits for similar systems and details for the reagents used in the CFA system are given by Rothlisberger et al. (19). Exact coregistration of sample analyses is critical to quantitative interpretation of multiparameter, high-resolution glaciochemical records. For example, errors in coregistration can lead to misinterpretation of the relative timing of deposition for different species and thus to incorrect associations such as common source region and atmospheric transport processes. To ensure exact coregistration between the CFA and the continuous ICP-MS measurements, a timing standard that consists of a mixture of selected elements measured by the ICP-MS and all species measured by the CFA system is used. To keep the melter head clean, it is continuously irrigated with ultraclean water before and between analyses of groups of contiguous ice cores. To determine the different residence times and associated delays in response, the timing standard is temporarily pumped to the melter head instead. The delays associated with each analysis system are determined by the relative timing of the response to the timing standard, and these delays are removed during data processing.

Results and Discussion Replicated measurements from the CFA-TE system (Figure 2) for a ∼3-m section of ice-core collected in 1999 at Summit, Greenland (72.3°N, 38°W) show that the measurements are highly reproducible and that distinct annual cycles in chemical concentrations are readily observed. Note that the Summit core was collected with an electromechanical drill and without the use of drilling fluids. The core samples, from a depth of 70.5 to 73.5 m, are dated at ∼1790 A.D. Measurements on a ∼1-m section were replicated by melting and analyzing adjacent parallel longitudinal sections of the Summit core, each with a cross-sectional area of ∼3 cm × ∼3 cm. Because the longitudinal sections were adjacent in the core, the overlapping chemical profiles with depth are likely to be similar, although the profiles will not be identical because of small-scale spatial variability in snow chemistry.

10.0 (583) 2.0 (65) 200.0 (12) 2.0 (154) 50.0 (3.9) 50.0 (2.0) 50.0 (2.7) 50.0 (1.6) 50.0 (0.25) 10.0 (0.25) 50.0 (1.7)

20.0 (1088) 4.0 (133) 400.0 (21) 4.0 (235) 100.0 (6.5) 100.0 (4.2) 100.0 (5.9) 100.0 (3.3) 100.0 (0.53) 20.0 (0.48) 100.0 (3.4)

5 100.0 (4553) 20.0 (668) 2000.0 (94) 20.0 (862) 500.0 (26) 500.0 (20) 500.0 (27) 500.0 (16) 500.0 (2.5) 100 (2.4) 500.0 (16)

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correlation coefficient 0.998 0.998

10000 (430)

0.997 0.986 0.996 0.995 0.987 0.996 0.997 0.996 0.997

FIGURE 2. Concentration profiles (ppb, *ppt) with depth for a 3-m section of ice-core from Summit, Greenland. Core breaks are shown as vertical dashed lines. Replicated analyses were made on different days (day 1 dashed (red), day 2 solid (black)) using adjacent ∼3-cm × ∼3-cm longitudinal sections of the core. Note the excellent agreement in the measured concentrations for the ∼1-m overlapping section of core. The ice is dated at ∼1790 A. D. All breaks in the ice-cores are shown as vertical dashed lines. Breaks develop during drilling, shipping, cutting, and handling and are the primary source of contamination of the inner core. While both intra-annual and interannual variability in chemical concentrations are high in Summit snow and ice (Figure 2), favorable comparisons between published mean concentrations from ice cores for ice of similar age and those measured in this study suggest that the CFA-TE system is producing accurate results. For example, mean concentrations of Na, Mg, and K ions for the GISP2 B core for ice dated VOL. 36, NO. 1, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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between 1797 and 1803 A.D. are 6.2, 1.3, and 2.8 ppb (2), while 11-year mean elemental concentrations measured with the CFA-TE system are 5.2, 1.6, and 1.4 ppb, respectively, for ice dated at ∼1787-1797 A.D. These differences are well within the observed interannual variability. Candelone et al. (15) reported Al concentrations of 4.3 and 5.6 ppb for 1773 and 1817 A.D. ice collected near Summit, while we measured an 11-year mean Al concentration of 1.2 ppb, with a range of 0.2-7.6 ppb (Figure 2). The reliability of CFA measurements for Ca2+, NH4+, NO3-, and H2O2 are well established (21). One challenge in determining trace element concentrations with the ICP-MS in the ppt range is isobaric interference between two different elements or molecular species with the same mass-to-charge ratio. An example of isobaric interference is neodymium-144 (Nd144) and samarium-144 (Sm144). Such isobaric interferences are relatively few and are estimated and removed through correction equations in the instrumental software (23). For example, using the relative natural abundance of the samarium isotopes Sm144/Sm147 ) 0.2053, the interference-corrected Nd144 is

Nd144(corrected)dNd144(measured)0.2053*Sm147(measured) (1) Additional interferences come from various oxides that have the same mass-to-charge ratio as the elements under study. However, this problem is largely eliminated by the use of the USN in the sample introduction process. The USN not only gives an increase in sensitivity because of its added efficiency but also produces an aerosol with greatly reduced water content, thereby largely eliminating the major source of oxygen (water) for oxide formation. Other interferences due to doubly charged ions and argon molecular ions are minimized by careful tuning of plasma conditions. Contamination of the sample stream during melting and analysis is a primary concern. Clean-suits and plastic gloves were worn by personnel while cutting the longitudinal sections of core at the National Ice Core Laboratory (NICL) and the sections were immediately bagged and sealed in plastic lay-flat tubing. During melting, the cores were handled only with gloved hands and, except for scraping with a precleaned plastic knife as the core pieces were loaded into the melter stand, care was taken to eliminate all contact with the ends of the longitudinal sections. To ensure that the melter head is kept clean, ultraclean water is continuously applied to the melter head before and between sample analyses. Frequent measurements are made to compare elemental concentrations in ultraclean water both before and after contact with the melter head to ensure that the melter head is not a source of contamination. Contamination of the inner core sample stream for the analytes measured by the ICP-MS in this study appears to be minimal. No correlation between the core breaks and the chemical measurements was found, and the measured concentration profiles with depth were essentially the same in the analyses of adjacent cores sections (Figure 2). Both would be unlikely if contamination was significant. In addition, most of the measured concentrations in this study were similar to or below those reported in the literature for earlier ice-core measurements in Central Greenland. For example, after using extensive mechanical decontamination procedures, Candelone et al. (15) reported Pb concentrations of 12, 45, and 18 ppt for ice collected near Summit dated at 1805, 1928, and 1933 A.D., respectively. This compares to average concentrations of 13, 45, and 22 ppt in this study for ∼0.25-m sections of ice of similar age. Given the significant intra-annual variability in Pb concentration (Figure 2), the agreement with the previously published concentrations is excellent. A major benefit of the CFA-TE is that far less sample 10

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FIGURE 3. The very close agreement in the variation with depth for two continental dust indicators, Al and Ca2+, demonstrates the exact coregistration of the sample analyses. Note that Al was measured using the ICP-MS and Ca2+ using the CFA system. preparation and decontamination is required compared with previously published techniques (15). A major benefit of the ice-core analysis method presented here is exact coregistration of the measurements. While dissimilar tubing lengths, flow-cell volumes, and other instrument characteristics result in different residence times within the melter assembly, the CFA analytical systems, and the ICP-MS, the associated delays in instrument responses are measured through the use of a timing standard and then removed in data processing. Note that within the ICP-MS itself, measurements of the various analytes are exactly coincident in time. Validation of this coregistration of ICPMS and CFA-measured analytes is shown by the close correspondence in depth variations of Ca2+ (measured with CFA) and Al (measured with the ICP-MS) concentrations, both of which are strongly controlled by deposition of dust from continental sources at Summit (Figure 3), although a fraction of calcium found in ice cores also comes from ocean water. While Ca concentration can also be determined by ICP-MS, the necessary settings would significantly reduce the instrument sensitivity for many of the other elements that were measured. Impurities in ice cores consist of both insoluble and soluble fractions. The former is in the form of small dust or volcanic tephra particles and there is concern that some of these particles may stick to the walls of the sample flow tubing. While we have found no evidence of this, preliminary analyses of CFA-TE measurements of ice associated with known volcanic events suggest that the particles pass through the flow system and are vaporized in the plasma. Thus, the elemental concentrations measured with the CFA-TE system appear to be the combined insoluble and soluble fractions.

Acknowledgments This work was supported primarily by NSF grant 99-77252 to the Desert Research Institute, with additional support from the Vice-President of Research fund at the Desert Research Institute. We are grateful to R. Stone for the role that he played in developing the continuous measurement method for the ICP-MS and R. Edwards for discussions on trace elements analyses of polar snow and ice. University of Nevada, Reno graduate student C. Kirick and undergraduate student C. Harrison diligently helped with the core analysis. M. Hutterli, D. Belle-Oudry, and others collected the Summit core in Greenland in 1999 under NSF grant 98-1331 to the University of Arizona and the Desert Research Institute. We also thank M. Hutterli and D. Belle-Oudry for helpful discussions on CFA operation and ice-core melter design and R. Jacobson for critical review of the manuscript.

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(14) Townsend, A. T.; Edwards, R. J. Anal. At. Spectrom. 1998, 13, 463-468. (15) Candelone, J.-P.; Hong, S.; Pellone, C.; Boutron, C. F. J. Geophys. Res. 1995, 100(D8), 16605-16616. (16) Wolff, E. W.; Suttie, E. D.; Peel, D. A. Atmos. Environ. 1999, 33, 1535-1541. (17) Fuhrer, K.; Neftel, A.; Anklin, M.; Stafflebach, T.; Legrand, M. J. Geophys. Res. 1996, 101, 4147-4164. (18) Anklin, M.; Bales, R. C.; Mosley-Thompson, E.; Steffen, K. J. Geophys. Res. 1998, 103, 28775-28783. (19) Rothlisberger, R.; Bigler, M.; Hutterli, M. A.; Sommer, S.; Junghans, H. G.; Wagenbach, D. Environ. Sci. Technol. 2000, 34, 338-342. (20) Standard Methods for the Examination of Water and Wastewater, 19th ed.; American Public Health Association: Washington, DC, 1995. (21) Sigg, A.; Fuhrer, K.; Anklin, M.; Staffelbach, T.; Zurmuhle, D. Environ. Sci. Technol. 1994, 28, 204-209. (22) Madsen, B. C.; Murphy, R. J. Anal. Chem. 1981, 52, 1924-1926. (23) Test Methods for Evaluating Solid Wastes, Physical/Chemical Methods; EPA-955-001-00000-1, SW846 Method 6020, Revision 0; U.S. Environmental Protection Agency, Office of Solid Wastes, U.S. Government Printing Office: Washington, DC, 1994.

Received for review June 25, 2001. Revised manuscript received October 3, 2001. Accepted October 17, 2001. ES011088Z

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