Imaging of Al/Fe ratios in synthetic Al‐goethite revealed by nanoscale secondary ion mass spectrometry. (26th March 2018)

• Record Type: Journal Article
• Authors: Pohl, Lydia; Kölbl, Angelika; Werner, Florian; Mueller, Carsten W.; Höschen, Carmen; Häusler, Werner; Kögel‐Knabner, Ingrid
• Is Part Of: Rapid communications in mass spectrometry. Volume 32:Number 8(2018)
• Page Start: 619
• Page End: 628
• Publication Date: 2018-03-26
• Subjects: Mass spectrometry -- Periodicals; 543.65
• DOI: 10.1002/rcm.8088
• Languages: English
• ISSNs: 0951-4198

Abstract

Abstract : Rationale: Aluminium (Al)‐substituted goethite is ubiquitous in soils and sediments. The extent of Al‐substitution affects the physicochemical properties of the mineral and influences its macroscale properties. Bulk analysis only provides total Al/Fe ratios without providing information with respect to the Al‐substitution of single minerals. Here, we demonstrate that nanoscale secondary ion mass spectrometry (NanoSIMS) enables the precise determination of Al‐content in single minerals, while simultaneously visualising the variation of the Al/Fe ratio. Methods: Al‐substituted goethite samples were synthesized with increasing Al concentrations of 0.1, 3, and 7 % and analysed by NanoSIMS in combination with established bulk spectroscopic methods (XRD, FTIR, Mössbauer spectroscopy). The high spatial resolution (50–150 nm) of NanoSIMS is accompanied by a high number of single‐point measurements. We statistically evaluated the Al/Fe ratios derived from NanoSIMS, while maintaining the spatial information and reassigning it to its original localization. Results: XRD analyses confirmed increasing concentration of incorporated Al within the goethite structure. Mössbauer spectroscopy revealed 11 % of the goethite samples generated at high Al concentrations consisted of hematite. The NanoSIMS data show that the Al/Fe ratios are in agreement with bulk data derived from total digestion and demonstrated small spatial variability between single‐point measurements. More advantageously, statistical analysis and reassignment of single‐point measurements allowed us to identify distinct spots with significantly higher or lower Al/Fe ratios. Conclusions: NanoSIMS measurements confirmed the capacity to produce images, which indicated the uniform increase in Al‐concentrations in goethite. Using a combination of statistical analysis with information from complementary spectroscopic techniques (XRD, FTIR and Mössbauer spectroscopy) we were further able to reveal spots with lower Al/Fe ratios as hematite.