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Collaborative Research: Molecular Level Characterization of Organic Matter in Ice Cores using High-resolution FTICR mass spectrometry

This award supports a project to fully develop the analytical protocols needed to exploit a relatively new technique for the analysis of soluble organic matter in ice core samples. The technique couples Electrospray ionization to high resolution Fourier transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS). We demonstrate that one can analyze a small volume (100 mL) of ice core melt-water with sufficient mass spectral sensitivity to describe the molecular composition of the DOM. Following method optimization a suite of ice core samples from several Antarctic, Greenland, and North American locations were examined to address several hypothesis driven research questions. DOM is thought to represent an assemblage of complex molecules that, among them, could contain proxies for climatological conditions of the past. Studies of the DOM in ice cores have recently taken on great importance, because they generally record past histories of atmospheric conditions at time of deposition. Several important questions related to developing a better understanding of the nature and paleo record of organic matter in ice cores are being addressed. The mass spectra of the ice core DOM consist of multitudes of peaks spanning the range of 200-700 m/z, and complexity is apparent from the detection of up to 20 peaks per nominal mass at nearly every mass throughout that range. A molecular formula calculator generates formula matches, from which van Krevelen plots are constructed. Analysis was conducted using ice core samples from 5 sites in Antarctica (Vostok, WAIS Divide, Taylor Dome, Siple Dome, RIDS), 1 site in North America (Fremont Glacier) and 1 site in Greenland (GISP2). All cores were obtained from the National Ice Core Laboratory and a total of 34 different samples/depths were analyzed. Changes in the types and amounts of CHO-, CHOS-, CHON-, and CHONS-containing molecular formulae are observed. Samples at lower depth (i.e., younger in age) have higher O/C ratios than those at higher depth (i.e., older). There is little variation in the S/C ratios amongst samples, there was a notable difference in the N/C ratios between the Greenland and Antarctic samples. The nitrogen content was lower for Greenland samples relative to the Antarctic samples. The large datasets generated from the FTICR-MS analysis of DOM can make data mining difficult, and multivariate statistical analysis (such as principal component analysis, PCA) assists in the data interpretation. In general, the DOM in all of the ice cores is aliphatic in nature, as indicated by the vast majority of molecular formulas having H/C values greater than 1.5. However, there are many formulas with a low H/C (and thus high double bond equivalents), indicating the presence of some aromatic and condensed aromatic species. Based on the multivariate principal component analysis, the 2 Greenland samples are remarkably similar and dominated by CHO-only formulas, with less contribution from nitrogen- and sulfur-containing species. The Antarctic ice cores contain a much higher number of formulas containing nitrogen, but fewer containing sulfur. The deepest/oldest samples (from Siple Dome and Vostok) separate from the other younger ice core samples. These samples contains formulas with lower O/C ratios than the other samples’ molecular formulas.

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