NetNotes
path or even HT board as others pointed out in the comments. I’ll check these later as needed. Guosheng Liu
gul417@mail.usask.ca Fri Aug 28
SEM: preparing microbeam standards
Does anyone have any tips or a good method for mounting standards in a standard block for SEM/Probe analysis? We got the Smithsonian microbeam standards recently and are trying to fi gure out the most eff ective way to create our own in-house standards block. Any help or advice would be much appreciated. Erin Summerlin
es.smrln@
gmail.com Mon Jul 13
We mount all of our standards in acrylics pucks with 35 pre-drilled holes. T e advantage is that when the epoxy shrinks as it cures, the whole mount shrinks without introducing any cracks that catch abrasives, oil, etc. Details are here: http://probesoſt
ware.com/ smf/
index.php?topic=
172.msg1436#msg1436 John Donovan
donovan@uoregon.edu Mon Jul 13
SEM: imaging of starch grains
A food scientist here is interested in gluten and gluten-free baked goods and dough, and is looking at starch grains from things like breadfruit fl our. T ese are all “gushy” preps. What we’ve been doing is freezing pieces in liquid nitrogen (decided it wasn’t worth trying to use a better cryogen for this), then throwing them down on the benchtop to “cryofracture” them, then freeze-drying them, since food scientists tend to have good freeze-dryers. T en mount on stubs, coat, and they’ve been pretty good. Of course, in this case, trying to decide what’s a starch granule vs. a fat glob has been fun, but these guys think they know. (I reserve judgment.) If the material would not fracture by dropping it on the bench, we used the razor-blade-hammer-pop it open technique. Tina (Weatherby) Carvalho
tina@pbrc.hawaii.edu Sat Aug 15 Starch grains are fun. I did some from barley in the past - the USDA people in the barley lab (because of the brewing industry in Wisconsin). What are you trying to image? Specimen prep: If dry, like corn kernels, just break open the kernel. Cryofracture is fun, but not needed. Poke out the starchy endosperm and spread it on the stub. Sputter coat with Au/Pd as per usual. If wet - dissected from fresh, moist grains, then: Either dissect and allow to air dry - you won’t affect the structure of the starch grains themselves and treat as above or fix with a normal formaldehyde/glutaraldehyde fix, use an extended—like an hour or more—dehydration series and critical point dry. Dissect some more and spread on a stub. If they’re looking at how the grain is digested once the seed germinates—as the barley people were, then you must fix and dehydrate. But! It’s also worth going the simple “do as little as possible” route. Starch grains are very tough and are very hard to break open—hitting with a hammer just produces individual grains. I’ve even tried cryofracture and not broken open a grain. But, the seeds do use enzymes and “open” the starch grains, producing pits. The walls of the pits have really neat light-dark layering. Starch grains left behind by baking, I don’t know. I’ve looked at bread dough and not seen starch grains, nor did I see any in the (fully baked) pretzels I did. Donuts, though... there’s a hole in my studies, there. Phil Oshel
oshel1pe@cmich.edu Sat Aug 15
Is there a reason they haven’t tried the old fashioned way and looked at these samples with crossed polars in a light microscope?
58
Starch grains give a distinctive Maltese cross. Barbara Foster bfoster@
the-mip.com Sat Aug 15
I wondered this myself! But for imaging starch breakdown - seeing the holes develop, and also for getting a quicker idea of the relative size, shape and abundance of small and large granules, SEM is quick, and you can keep the samples and look at them again if necessary. Here, people from the starch lab extract the starch, wash to remove protein and other contaminants, dry it, spread on a sticky carbon tab on a stub, image with BSE at 10 Pa (no coating necessary). For higher magnifi cation or resolution, gold-coat then image at 20 kV under HV. Generally no fi xation or other processing because in the various rinse steps you tend to lose the smaller B-granules which people here are very interested in. It’s also easy to do and this way once trained, the starch folk can do this without needing my input. Just have to make sure not to dwell on the grains too much when focusing or they tend to crack, especially if uncoated. Rosemary White
rosemary.white@csiro.au Sat Aug 15
SEM and EDS: elemental Hg analysis
Anyone have experience looking for trace amount of elemental mercury in samples? Fern Stones
stones.fern@dol.gov Tue Jul 7 I examined some river sediment with EDS, and while I wasn’t
specifi cally looking for Hg, I did fi nd many elements. I also had enough overvoltage to see even the K - lines. T ere are two problems that you face: T e fi rst is that EDS is not sensitive to amounts much smaller than 1%, and the second is that Hg is volatile, so the amount under the beam will be continually decreasing. If you have access to WDS, it will be easier to fi nd small amounts of Hg before it goes away. Bill Tivol
wtivol@sbcglobal.net Sat Jul 11
My acquaintance with mercury in the SEM is with a tooth fi lling, a mercury amalgam! Acquired when a student broke a tooth, the “specimen” proved to be very interesting as an EDX investigation. It was made up of mercury, silver, tin and copper. We picked up mercury if we jumped to a new area but, aſt er dwelling too long on an area, the mercury may not show. For those who would like a nice EDX test specimen, ask a dentist, they oſt en have a pot full of potential specimens. Without any doubt, specimens like this make great teaching material. Steve Chapman
protrain@emcourses.com Sun Jul 12 SEM/EDS sensitivity can be much better. I’ve gotten better than
500 PPM detection limits, using an SDD at high currents (5 nA or so, but still less than a probe) for about 1 minute acquisition times (~15–20 million counts in the spectrum) with very careful sum-peak stripping. Can’t speak to volatility, but I suppose a cold stage plus area scanning might work if a yes/no answer is all that’s needed and that meets your defi nition of “trace”. T e Hg Lα line is in a nice place WRT potential overlaps, with the exception of Ge K , which is still more than 100 eV away. Depending on the sample and the spatial resolution requirement, XRF might be a better tool for this if the sample is something like Bill’s river sediment and you need one or two more orders of magnitude in sensitivity. No beam heating issues. Rick Mott
rmott@pulsetor.com Sun Jul 12
I would suggest inductively coupled plasma- optical emission spectroscopy (ICP-OES) or inductively coupled plasma mass spectrometry (ICP-MS) if you have access to them. ICP-MS is capable of ppq in the right conditions, and frequently in the ppt. Samples do not need to be liquid, they are digested or ashed, then digested, for introduction into the instruments. While SEM-EDS or XRF might be capable of detecting the element, the conversion from excitation volume, ZAF correction, and peak area to a ppm or similar number is non-trivial. Allen J. Hall
ajhall@prairienanotech.com Sun Jul 12
www.microscopy-today.com • 2015 November
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