TOWARDS THE CALIBRATION OF AN SEM-EDS SYSTEM FOR THE ANALYSIS OF TEPHRA SAMPLES AT THE UNIVERSITY OF ST. THOMAS, MN

The goal of this project was to improve our quantitative capabilities with a new Scanning Electron Microscope Energy Dispersive Spectroscopy (SEM-EDS) in the geology laboratory at the University of St. Thomas. This is a crucial step towards being able to chemically fingerprint tephra samples and perform tephrochronology in areas of research interest. Most investigations of tephrochronology use an electron microprobe, but it is also possible to do precise and accurate work with an SEM-EDS given significant attention to instrument calibration. In order to ensure reliable quantitative measurements for future projects, we calibrated the SEM-EDS. First, we prepared the tephra samples, and created a custom holder for the samples and standard blocks. Next, we established known values for reference standards using an elemental standard block (Oxford Standard) and a tephra standard block with published and widely used tephra standards. We used a combination of single elements and mineral standards to calibrate the EDS and then analyzed a fixed list of elements commonly found in tephra (Si, Fe, Al₂O₃, CaSiO₃, GaP, Mn, KAlSi₃O₈, MgO, Ti, and NaAlSi₃O₈). When selecting the elements and minerals, our goal was to correct for matrix effects and avoid overlapping wavelengths which impact measurements. Using an absorbed current terminal (ACT) and picoammeter, we monitored standard conditions between measurements. Initially, our error levels were high, with an average error of 10.1%. After the addition of the picoammeter and ACT, we saw some level of improvement with an average error decreasing to 7.07% for a sample of interest. Ultimately, we strive to achieve accuracy within 5% for major elements in these analyses. In this presentation, we share our improved results in the calibration process and detail our progress developing the requisite technical skills. We encountered obstacles, collaborated to find solutions, and ultimately improved the calibration of the instrument. In this project, we worked with tephra from an Icelandic soil profile and lake cores, but with the workflow we have developed for calibration, St. Thomas students and faculty can work with a broader range of tephra from other areas of interest, including ongoing project work in the Great Basin.