Accuracy of Continuous Ice-Core Trace-Element Analysis by Inductively Coupled Plasma Sector Field Mass Spectrometry STEFANIE KNU ¨ SEL,* ,†,‡ DAVE E. PIGUET, ‡ MARGIT SCHWIKOWSKI,* ,‡ AND HEINZ W. GA ¨ GGELER †,‡ Paul Scherrer Institute, CH-5232 Villigen PSI, CH-5232 Switzerland, and Department of Chemistry and Biochem istry, University of Berne, Freiestrasse 3, CH-3012 Berne, Switzerland Trace elements trapped in glaciers are important indicators for the characterization of past biogeochemical cycles, the identification of numerous sources and their varying strength, and thus indirectly provide insight into past climate variations. However, this necessitates highly resolved and continuous records of trace elements in ice. To obtain records corresponding to these requirements, a continuous ice-core melting (CIM) device was coupled to an inductively coupled plasma sector field mass spectrometer (ICP-SFMS). Accuracy of this newly developed method was tested by replicate analysis of longitudinally cut ice- core sections (reproducibility) and by comparing results of the continuous method with the conventional decontami- nation and analysis procedure. The new, fast method is suited to accurately determine concentrations of a number of elements, such as Li, Na, Mg, Ca, Mn, Co, Br, Sr, Mo, and Tl. However, for 18elements (including Al and lanthanides) observed concentrations were underestimated when analyzed using the continuous method.Possible explanations of these low concentrations are (i) incomplete dissolution of mineral dust particles contained in the ice resulting from a delayed acidification step and/or (ii) adsorption of dissolved trace elements or mineral dust particles on the surface of the ice melting device. Introduction Impurities trapped in ice cores give precious information on the paleo atmosphere and consequently deliver insight into past climate variations,knowledge ofwhich is a precondition for the prediction and interpretation offuture climate.Trace elements (herein referred to elements with a concentration range in ice of pg g -1 to μgg -1 , although some may be major or minor constituents of the earth crust) are of increasing interest because they provide the ability to identify various sources (1, 2) and to understand biogeochemical cycles (3). In particular the study of trace elements in dust contributes to the understanding of past circulation systems and the estimation of source strengths (4-7). Additionally trace elements analysis is suited for the determination of natural background concentrations as well as anthropogenic pol- lution in ice cores from around the globe. Recent investiga- tionsofAntarcticice coresprovided evidence that quiescently degassing volcanoes are the major source of trace metals, whereas continental dust is of minor importance (8). An- thropogenic pollution was observed in both Antarctic (9) and Arctic ice cores, where for instance it has been demonstrated that lead pollution in the atmosphere in the second halfofthe 20th centurywas caused bylead additives in gasoline (10).Increased concentrationsofplatinum group metals in Greenland snow dated to the mid-1990s (11) and decreasing lead concentrations in Greenland snow over the past 20 years appear to be related to the use of automobile catalytic converters and consequently the diminishing use of lead additives in gasoline (12). Ice-core records of trace elements from mid- and low-latitudes have also provided insight into pollution on a more regional scale, e.g. in the Alps (13-15) or Andes (16). Conventional methods to analyze trace elements in ice include a decontamination procedure performed by chisel- ling away outer, possibly contaminated layers under ex- tremely clean conditions (17, 18). The chiselling procedure also includes the risk of carrying contamination from the outer layers to the inner, “clean” portion of the core. Furthermore, this procedure is labor-intensive and time- consuming, because the very low natural concentrations of trace elements in snow and ice can easily be influenced by contamination.Therefore the number ofsamples processed as well as the spatial resolution is limited. Thus most of the trace element records available in the literature are discon- tinuous (e.g., refs 8, 11, 15,and 16) or recorded with a coarse depth resolution of about 10-20 cm (8, 12, 19). There is a need for continuous, highly resolved records oftrace elementsparticularlyin areaswith lowaccumulation or in glaciers with strong layer thinning. To obtain higher spatialresolution,newdecontamination methodshave been developed includingthe use ofan ice holder,which provides a depth resolution of 2 -3 cm (20). Another approach was presented byReinhardt et al.(21)who applied laser ablation inductively coupled plasma mass spectrometry (ICP-MS) to frozen ice cores and who achieved an extremely high resolution of 0.3-1 mm. However, this method might be disturbed by either the inhomogeneity of dust-containing ice layers or the inclination of layers. McConnell et al. (22) reported a promising method which included the coupling ofa meltinghead to a quadrupole ICP-MS.The meltinghead decontaminated the ice by separating the meltwater from the inner part of the core from the surface meltwater, thus greatly simplifying the decontamination procedure. In ad- dition to the benefits ofa shorter procedure and the reduced riskofcontamination ofthis modified process,a high spatial resolution of about 1 cm was obtained. In the study of McConnell et al. (22) only the reproducibility of the continu- ous method was tested, whereas the results were not compared with those from the conventional method for analyzing trace elements in ice cores. Additionally, neither procedural blank concentrations nor detection limits were reported. Here we present a detailed study of the analysis of trace elements in ice usingcontinuous ice melting(CIM)ICP sector field mass spectrometry (SFMS), including a determination of procedural blanks and reproducibility as well as a comparison with the conventionalanalysismethod.Ourmain findings are that this new method appears to be suitable for the analysis of one group of trace elements, while serious difficulties were observed for a number of trace elements that often occur in nature as silicates. *Corresponding authors phone: +41 56 310 4397; fax: +41 56 310 4435; e-mail: stefanie.knuesel@iac.unibe.ch (S.K.) and margit. schwikowski@psi.ch (M.S.). † University of Berne. ‡ Paul Scherrer Institute. Environ. Sci. Technol. 2003, 37, 2267-2273 10.1021/es026452o CCC: $25.00  2003 American Chemical Society VOL. 37, NO. 10, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2267 Published on Web 04/17/2003