Electron probe microanalysis - Electron microprobe analysis EPMA (EMPA)

1. UW- Madison Geology 777 Electron probe microanalysis - Electron microprobe analysisEPMA (EMPA) Preface: What’s EPMA all about? How does Geology 777 work? What can you learn? Updated 9/4/07

2. UW- Madison Geology 777 EPMA - what is it? EPMA is a tool to get precise and accurate quantitative chemical analyses of micron-size domains of our samples.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 non-destructive.

3. UW- Madison Geology 777 EPMA - is it for me? This technique has its own characteristics, strengths, weaknesses. It pays to consider whether it is the best technique to get the information you need. It is a micro-technique, and for multiphase samples provides discrete compositions, not the bulk composition. It samples volumes (depths) on the order of ~1 um, limiting its usefulness for small inclusions or films. It provides major and minor element quantification, and has limited capacity for trace element analysis. (What do you mean by “trace”?) Despite being non-destructive, samples need to be mounted and polished; they can be reanalyzed many times. It is relatively inexpensive and accessible

4. UW- Madison Geology 777 Goal of this course The goal is to provide useful background information to make EPMA less a ‘black box’ for you and to help you make better decisions about how to analyze your samples, and to understand when probe data is good and when it is not. This course will not, by itself, teach you how to use our Cameca SX51, though you will have some opportunity to get hands on experience in weekly lab sessions. SX51 training requires more than 20 minutes a week (per person in weekly lab), and experience shows that it best occurs in 4-8 hour blocks once/twice per week when you have your samples ready. It is relatively software intensive.

5. UW- Madison Geology 777 How this course is structured Weekly class meetings: ~1.5 hours, discussion of assigned materials (PP presentations, readings); students will be responsible for many of them Weekly quiz: at start of each class, on the assigned material Weekly labs: ~2 hours. Complete lab report and turn in following week Weekly assignments: Calculations and computer exercises. Lab practical in late Oct-early Nov Projects (hands on with SX51)

6. UW- Madison Geology 777 Use for Reference--In Library on Reserve Goldstein et al, 3rd Edition. 2003 New:$75

7. UW- Madison Geology 777 Also On Reserve in Geo Library Reed (1996) 201 pages Paper: New:$36 Hard:New: $95 Used $80 Reed (1993) Paper: New:$55 Used:$35? Hard:New ~$95

8. UW- Madison Geology 777 EPMA - “ideal” case Simple assumptions: We have stuck our sample in epoxy, cut and polished it (or made a thin section epoxied to a glass slide and polished it). There are standards, either user-supplied, or in the probe lab. We sign up on the schedule, get some “on the job” training, and analyze our samples. We return to our lab with the data we need.

9. UW- Madison Geology 777 The devil in the details Optimal case for “easy” EPMA: The samples are flat, well polished, conductive, non-porous, infinitely thick (to e- beam), homogeneous, clean. Standards exist and have the same 5 features. Materials are oriented at 90° to the electron beam (not tilted). Background positions well chosen with no peak or background interferences (in unknown and standard). Detector pulse distribution well centered. Constants for matrix correction (e.g. mass absorption coefficients) well known. Sample is not able to be altered by beam.

10. UW- Madison Geology 777 EPMA - worst case details Possible complications: The actual materials being probed are scratched or etched, insulators, porous, multiphase (eutectic) assemblages, with polishing oil in pores. 1” round has surface that is not normal to walls, resulting in tilted surface to electron beam. Background and peak positions have interferences. Detector pulse distribution on standard depressed (cut off) on low end. Mass absorption coefficients poorly known. Specimen is a thin film. Specimen is hydrous or sensitive to alteration or damage by the beam electrons. A main goal of this course is for you to understand when the optimal conditions are met -- and when they are not -- and if there is a way to make this thing work!

11. UW- Madison Geology 777 EPMA - so what to do? How to trust the results?: Evaluate “secondary” standards. “Should” get 100 wt% totals. Evaluate stoichometry if able to. How to get the best results: Get the sample preparation right. Have multiple standards for difficult samples. Take some time at the start: Don’t be in a hurry with a new sample/suite of elements. Do wavescans first to consider peak and background interferences.

12. UW- Madison Geology 777 Which is it? EMPA or EPMA? Good question. The instrument is an electron microprobe, but traditionally the practitioners in the field refer to the technique as electron probe microanalysis (like Goldstein)-- and some call it electron microprobe analysis (like Reed). It’s a toss up ... I did a search in one or two reference databases for keywords, and there were a lot more using EPMA vs EMPA, so I tend to use EPMA.