UW Madison Geoscience 777 Electron probe microanalysis Scanning

UW- Madison Geoscience 777 Electron probe microanalysis Scanning Electron Microscopy EPMA - SEM Preface: What’s EPMA and SEM all about? How does Geology 777 work? What can you learn? Updated 1/21/19

UW- Madison Geology 777 Why? You are taking this class because you need to be able to intelligently use an electron beam instrument – the Hitachi S 3400 SEM, the CAMECA SX 51 or SXFive FE electron microprobe. These instruments work on the same physical principles, but have differences.

UW- Madison Geology 777 EPMA - what is it? EPMA is a tool to get precise and accurate quantitative chemical analyses of micronsize domains of our samples. A focused beam (“spot”) of high energy electrons interact with the atoms in the sample, yielding X-rays (and other signals), which we quantify and compare with counts from standards. It is nominally nondestructive.

UW- Madison Geology 777 SEM - what is it? SEM is a tool to produce images -- pictures -- of our samples. A rastered (scanned) beam of high energy electrons sweeps across the surface, interacting with the atoms in the sample, yielding backscattered electrons, secondary electrons, auger electrons, and in some cases photons in the visible light range (CL). It is nominally non-destructive.

UW- Madison Geology 777 SEM - is it for me? 1) This technique is rather simple and one can learn the essentials in a short time. 2) It provides images easily, though one needs to understand the various parameters (e. g. working distance, resolution, etc) to not make mistakes compromising image quality. 3) Samples may be imaged with little or no preparation (coating, mounting+polishing), though this may complicate detailed examination. 4) It is very easy to make mistakes using the easy EDS software, especially for attempts to get chemistry of small particles.

UW- Madison Geology 777 EPMA - is it for me? Traditionally “EPMA” has meant using a dedicated electron microprobe (also known as electron probe microanalyzer. . EPMA) to do quantitative chemical analysis at the micron level. EPMA traditionally has meant an instrument with wavelength dispersive spectrometers (WDS) and a rigorous calibration procedure, using standards. However, there are many people who attempt to also do quantitative chemical analysis at the micron level in SEMs using a different type of detector (EDS). One key theme of the class is to understand the similarities and the differences between WDS and EDS.

UW- Madison Geology 777 EPMA - is it for me? 1) It uses standards (reference materials). EDS and WDS may be using very different kinds of standards: be aware of what is going on! 2) It is a micro-technique, and for multiphase samples provides discrete compositions, not the bulk composition. 3) Under “normal operating conditions”, it samples volumes (widthsdepths) on the order of ~1 -3 um, limiting its usefulness for smaller inclusions or thin films. 4) It provides major and minor element quantification, and has limited capacity for trace element analysis. (What do you mean by “trace”? ) 5) Despite being non-destructive, samples need to be mounted and polished; they can be reanalyzed many times. 6) It is relatively inexpensive and accessible 7) Some degree of complexity; there can be a sharp learning curve

UW- Madison Geology 777 Goal of this course The goal is to provide useful background information to make SEM and EPMA less a ‘black box’ for you and to help you make better decisions about how to analyze your samples, and to understand when data is good and when it is not. This class will provide the basic instructions for the use of our Hitachi SEM and Oxford AZTec EDS. It will point out errors that can occur with EDS spectral interpretation. This course provides some directed exercises with our CAMECA SX 51 probe. The electron probe is more complicated than the SEM and experience has shown that individualized training is the best way to go. This happens when the student has his/her samples ready to analyze do we set up a 4 -8 hour appointment.

UW- Madison Geology 777 How this course is structured Weekly class meetings: ~1. 5 hours, discussion of assigned materials (PPT presentations, assigned readings) Instructional materials will be provided as PPT slides and reprints, all on class web page: www. geology. wisc. edu/~johnf/g 777/ Weekly quiz: at start of each class, on the assigned material (emphasis on previous week’s lecture) Weekly labs: ~2 hours. Complete lab report and turn in following week Weekly assignments: Calculations and computer exercises. There will be a class project assigned.