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Ph.D. Candidate, Graduate Student, Rensselaer Polytechnic Institute


Development of multimodal approach to get maximum chemical information from precious samples

Recent advances have led to the ability to generate comprehensive chemical maps of solid samples with high spatial resolution. Unambiguous analyte identification from these samples necessarily requires different analyses performed on the same sample, often termed ‘multimodal chemical imaging’. While tandem imaging methods provide a wealth of information, they often suffer from weak sensitivity, poor selectivity, or compromised spatial resolution as dictated by the spectroscopic method employed. Greater success in comprehensive chemical imaging has been achieved with mass-spectrometry-based instruments, due to the excellent sensitivity and selectivity. Here, we present our recent work towards the development of a multimodal chemical-imaging apparatus capable of providing simultaneous molecular and elemental information from the exact same spatial location at high spatial resolution (30 microns). This dual imaging approach is achieved through laser sampling of samples, at atmospheric pressure, followed by simultaneous mass-spectrometric and optical-emission measurements. Aerosolized particles from the resulting ablation process were swept with a gas stream to a flowing atmospheric-pressure afterglow (FAPA) molecular ionization source and, subsequently, mass-analyzed with an Orbitrap mass analyzer. At the same time, atomic emission from the laser-induced plasma (LIP) formed during the ablation event was recorded to provide elemental information on the laser-sampled area. By Raster scanning the laser across the sample, followed by appropriate data processing, dual atomic and molecular chemical images were generated. The effect of various laser-ablation parameters, such as laser fluence and ablation-gas composition, in conjunction with FAPA operating conditions were explored in detail for a variety of sample types including pharmaceutical tablets, biological tissues, and organic minerals. Different approaches to obtain multimodal atomic and molecular images, as well as the data processing needed to generate and compare chemical images will be presented. Finally, approaches to enable quantitative information for both molecular and atomic analyses will be presented.


Toward Three-Dimensional Chemical Imaging of Ternary Cu–Sn–Pb Alloys Using Femtosecond Laser Ablation/Ionization Mass Spectrometry

Abstract: Femtosecond laser ablation/ionization mass spectrometry (LIMS) has been applied to probe the spatial element composition of three ternary Cu–Sn–Pb model bronze alloys (lead bronzes: CuSn10Pb10, CuSn7Pb15, and CuSn5Pb20), which were recently identified as high-performance cathode materials in the context of electro-organic synthesis (dehalogenation, deoxygenation) of pharmaceutically relevant building blocks. The quantitative and spatially resolved element analysis of such cathode materials will help in understanding the observed profound differences in their electrochemical reactivity and stability. For that purpose, we developed a measurement procedure using the LIMS technique which allows analyzing the element composition of these ternary alloys in all three spatial dimensions. Their chemical composition was determined spotwise, by ablating material from various surface locations on a 4 × 4 raster array (50 μm pitch distance, ablation crater diameter of ∼20 μm). The element analyses show significant chemical inhomogeneities in all three ternary bronze alloys with profound local deviations from their nominal bulk compositions and indicate further differences in the nature and origin of these compositional inhomogeneities. In addition, the element analyses showed specific compositional correlations among the major elements (Cu, Sn, and Pb) in these alloys. On selected sample positions minor (Ni, Zn, Ag, and Sb) and trace elements (C, P, Fe, and As) were quantified. These results are in agreement with inductively coupled plasma collision/reaction interface mass spectrometry (ICP–CRI-MS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) reference measurements, thus proving the LIMS depth profiling technique as a powerful alternative methodology to conventional quantification techniques with the advantage, however, of a highly localized measurement capability.

Pub.: 04 Jan '17, Pinned: 28 Jun '17