Quantification of potentially toxic elements in food material by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) via pressed pellets.

Here, we validated a protocol for the elemental analysis of plant and food materials using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and pressed pellets in compliance with the Green Chemistry principles, i.e. avoiding the use of hazardous reagents. The protocol included sample pre-treatment, preparation of pellets, and LA-ICP-MS optimisation for the food sample matrix using leaf certified reference materials. Sample homogenisation and particle size reduction (∼10 µm) for samples with larger initial particle sizes proved crucial in improving signal stability and analytical results, whereas the raster mode was preferred over spot mode for greater accuracy. The validation of the technique was successful for determination of Mn, Ni, Cu, Zn, As, Se, Cd and Pb (z-score values  ≤2). We propose LA-ICP-MS using pressed pellets as an alternative to ICP-MS in food analysis.

A crystallographic study of crystalline casts and pseudomorphs from the 3.5†Ga Dresser Formation, Pilbara Craton (Australia).

Crystallography has a long history of providing knowledge and methods for applications in other disciplines. The identification of minerals using X-ray diffraction is one of the most important contributions of crystallography to earth sciences. However, when the crystal itself has been dissolved, replaced or deeply modified during the geological history of the rocks, diffraction information is not available. Instead, the morphology of the crystal cast provides the only crystallographic information on the original mineral phase and the environment of crystal growth. This article reports an investigation of crystal pseudomorphs and crystal casts found in a carbonate-chert facies from the 3.48 Ga-old Dresser Formation (Pilbara Craton, Australia), considered to host some of the oldest remnants of life. A combination of X-ray microtomography, energy-dispersive X-ray spectroscopy and crystallographic methods has been used to reveal the original phases of these Archean pseudomorphs. It is found with a high degree of confidence that the original crystals forming in Archean times were hollow aragonite, the high-temperature polymorphs of calcium carbonate, rather than other possible alternatives such as gypsum (CaSO4·2H20) and nahcolite (NaHCO3). The methodology used is described in detail.

3-D microstructure of olivine in complex geological materials reconstructed by correlative X-ray µ-CT and EBSD analyses.

We reconstruct the 3-D microstructure of centimetre-sized olivine crystals in rocks from the Almirez ultramafic massif (SE Spain) using combined X-ray micro computed tomography (µ-CT) and electron backscatter diffraction (EBSD). The semidestructive sample treatment involves geographically oriented drill pressing of rocks and preparation of oriented thin sections for EBSD from the µ-CT scanned cores. The µ-CT results show that the mean intercept length (MIL) analyses provide reliable information on the shape preferred orientation (SPO) of texturally different olivine groups. We show that statistical interpretation of crystal preferred orientation (CPO) and SPO of olivine becomes feasible because the highest densities of the distribution of main olivine crystal axes from EBSD are aligned with the three axes of the 3-D ellipsoid calculated from the MIL analyses from µ-CT. From EBSD data we distinguish multiple CPO groups and by locating the thin sections within the µ-CT volume, we assign SPO to the corresponding olivine crystal aggregates, which confirm the results of statistical comparison. We demonstrate that the limitations of both methods (i.e. no crystal orientation data in µ-CT and no spatial information in EBSD) can be overcome, and the 3-D orientation of the crystallographic axes of olivines from different orientation groups can be successfully correlated with the crystal shapes of representative olivine grains. Through this approach one can establish the link among geological structures, macrostructure, fabric and 3-D SPO-CPO relationship at the hand specimen scale even in complex, coarse-grained geomaterials.