Trace Element Analysis of Canadian Surface Soil and Associated Quartz by LA-ICP-MS: Preliminary Development of a Geo-location Database for Forensic Investigations.

The purpose of this project is to produce a regional map of element concentrations in surface soil (i.e., 0-20 cm depth) across different provinces in Canada, which will be used to identify, or eliminate, geographical areas of interest as applied to a forensic investigation based on characteristics of soil from an unknown source.  Over this 4 year project, we estimate a total of ~1000 soil samples will be collected and analyzed within Canada.  The developed method consists of analyzing a pressed pellet of soil by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) for major, minor and trace element concentrations.  The pressed pellet is spiked with indium metal as the internal calibrating mass and standardized externally to NIST glasses (SRM-612 and -610) and BKA glass (FGS 2).  Two quality control pellets made of NIST SRM-2711a Montana / GBW 07401 (GSS-1) China soils are used to test the system for day to day reproducibility.  Using the optimized grinding procedure and LA-ICP-MS conditions, the measured in-run reproducibility (n=20) is better than 15% for over 40 elements (Li up to Bi) with some elements within 2 standard deviations of the certified values : Li, Be, B, Na, Mg, Al, Si, P, K, Ca, Sc, V, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Nb, Mo, Ag, Cd, Sn, Sb, Cs, Ta, Pb and Bi (ref. NIST SRM-2711a & GBW 07401).  Other elements such as Ti, Co, Ga, Sr, Y, Zr, Ba, Hf, W and most REE’s (i.e., La-Lu) indicate important bias (lower or higher than certified values) or uncommonly high Relative Standard Deviation (RSD%), which can mean difficulty in reproducibility and/or characterization by LA-ICP-MS for future forensic applications.  The calculated Minimum Detection Limit values range from ~120 to 0.002 µg/g for Si and Bi (MAX and MIN, respectively).  Within the same soil samples, a representative aliquot of handpicked quartz crystals (n=10 per soil sample) will be analyzed by LA-ICP-MS to support the soil characterization and to widen its forensic application.  Preliminary results will be shown for a few selected samples collected in the eastern provinces of Canada.