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Microscopy Today, May 2012 has an article by Ian Holton asking whether EDS is a substitute for EPMA. He does a decent job touching on the many factors.
Personally, I have been performing EDS for over 30 years now. I took on the operation of a probe lab a little more than two years ago. I'd like to think I can comment on both sides of the issue. By the way, most of the applications are in materials science rather than geology.
The biggest strength of the probe has been the precision of the analyses. The same factors that allow low detection limits also result in good precision. I have a pair of charts showing the difference between EDS and WDS analyses of a concentration profile. The WDS results showed much less noise than did the EDS results, and that is for someone who tries to do EDS well. I find many users idling by at 600 cps and 7% DT and then getting large fluctuations in their results. They aren't counting like they could. I do wonder how much the new SDD detectors will change the game. They should narrow the gap.
Energy resolution is often touted as a reason for EPMA analyses. I agree that it helps, but decent EDS software ought to deconvolve most (not all) overlaps if used correctly. However, I do not count on out-of-the-box EDS with factory standards to properly sort out the overlaps. In-house profiles are probably necessary and they should be prepared by a trained operator. (One adventurous user went through the steps to do his own standardization but he neglected an essential point. His standards were about 20% off from all of the other standards.)
Good peak-to-background ratios lead to lower detection limits in the probe. Long counting in the EDS will help some, but will not eliminate the gap.
All told, I think EPMA's biggest strength is its sensitivity to low levels and to small changes/gradients. If there is a diffusion problem which requires determining the magnitude of a gradient, then the precision of a probe is required. That can occur in materials science as well as geology. I also encounter applications where other properties are very sensitive to the concentration of an element. Some users need to know whether the substitution of Co for Fe is 1.3% or 1.4%. I cannot yet think of tackling that by EDS.
Most of our EDS users simply do not require the precision of probe analyses. They may quote results to two or three decimal places, but all they are usually looking for are qualitative results. I doubt that you will pull them away from the SEM.
I wish you well trying to find enough applications to justify the new probe. It will probably involve educating users to help them realize what applications do require the use of a probe. Also make sure they understand the need to keep analytical expertise on campus. Whether WDS or EDS, you want someone around who can tell what the results mean - or don't mean.
Warren
-----Original Message-----
From: JEOL-Focused Probe Users List [mailto:[log in to unmask]] On Behalf Of Ritchie Sims
Sent: Thursday, June 14, 2012 2:12 AM
To: [log in to unmask]
Subject: [PROBEUSERS] Case for a new EPMA
JEOL Probe Users Listserver
Moderator: Ellery Frahm, [log in to unmask],
Electron Microprobe Lab, University of Minnesota
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Hi
For the past ten years we have been using a JXA-840A with EDS for quantitative geological
work, it's been OK, for analytes > about 0.2%, but there is a chance now to make a case for
a brand-new 'proper' EPMA.
The complication is that we have to find as many potential users, from within and without the
University, to support us.
In the School of Engineering there are a couple of SemS, with close ties to local industry, and
because their standardless analytical packages give element concentrations to two decimal
places, they and their users accept that their results are quantitative (!).
They are likely to oppose our getting a new EPMA as it would probably reduce their revenue
stream.
I know virtually nothing of probe applications outside Geology.
I would really appreciate hearing of EPMA projects and applications in non-geological fields
to help me prepare a case. It would be great to list those for which an EPMA beats an SEM
hands-down.
cheers
Ritchie
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Ritchie Sims Ph D Phone : 64 9 3737599 ext 87713
Microanalyst Fax : 64 9 3737435
Department of Geology email : [log in to unmask]
The University of Auckland
Private Bag 92019
Auckland
New Zealand
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