NetNotes
quite quickly. I had a similar incident with a client who also worried about changing from their 200 kV JEOL. I had the client take test pictures of the gold lattice from 200 kV down to 80 kV in 20 kV steps; she was 100% successful! Steve Chapman
protrain@emcourses.com Tu Jul 28 Just to add what may be an obvious note about operating at a lower
accelerating voltage. If the 2100 has never been aligned at anything other than 200 kV, you may need to have a service engineer in to do initial alignments at lower voltages. When our JEOL JEM-3010 was installed, the engineers did alignments at 50 kV intervals from 50 to 300 kV and stored those values for future use. On the rare occasions when I use a lower accelerating voltage, I just have to do some minor touch-up to the alignment. Also, you’ll want to check your magnification calibration at the lower accelerating voltage. Elaine F. Schumacher
eschumacher@mccrone.com Tu Jul 28 I also needed to adapt biological sections for use in a 200 kV
TEM. I routinely purchase carbon/Formvar-coated copper grids (200 mesh) and collect the sections on the ‘light’ non-carbon side of the grid (less phobic and easier to collect sections from the boat water). Te stability of the carbon plus support film permits the use of thinner sections (less than 70 nm) and there is less driſt of the material under the electron beam. Te electron beam can be focused to a fine spot on these grids for making height adjustment and for x-ray analysis. I did not like carbon-only coated grids because of the increased background texture. I stopped trying to make these grids myself— the time involved and inconsistent quality of my homemade grids was not worth it. Te increased grid cost is offset by your reduced photography cost. Changing kV to 120 in the instrument required additional magnification calibrations and although this may improve contrast in the biological material on the screen, the digital camera we use provides excellent contrast in the final image. So depending on the camera and your final images, a lower kV may improve contrast but I would definitely recommend carbon/Formvar-coated grids. Susan Belfry
belfry@unb.ca Tu Jul 28 Just one additional thought: did you insert the objective
aperture? Perhaps on the 200 kV (EDX?) you mainly work with the aperture out. It needs to be inserted otherwise there is a big chance that you blow up your section. Stephane Nizets
nizets2@yahoo.com Wed Aug 3
SEM:
ion pump problem We are currently experiencing a problem with our Carl-Zeiss
EVO 40 SEM. We have fitted this SEM with a LaB6 filament, and the gun area is pumped with an ion pump. We occasionally (every 3–4 months) bake the gun area to reach ~2–3 × 10-7 mbar vacuum levels as a requirement for the LaB6 filament operation. During the last bake, we found that the ion pump stopped working during the cool down stage while the final vacuum level in the system was good around 10-7 mbar levels. However, since then we were not able to keep the ion-pump working. When we turn the ion pump on, it slowly dies away. Tat is, the reading of the vacuum level in the gun area is thru the ion current (there is not an extra ion gauge in the system), and the vacuum level starts at 10-8 mbars and within 5 min goes down to 10-11 mbars. Finally, the ion pump shuts down. Tis pattern repeated aſter several restarts of the system. I tried gently tapping the ion pump with a plastic handle of a screwdriver, which kept vacuum level around 10-8 and the pump running, but I think that is not a viable option for running any SEM with decent resolution. Erman Bengu bengu@fen.
bilkent.edu.tr Wed Jul 20 Ion pumps have a long, but limited lifetime. Most likely internals of your pump became coated with dielectric deposits, and you will
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need to replace it. If new pump is beyond the budget, then you can send your original pump to be rebuilt.
vray@partbeamsystech.com Wed Jul 20 Gentle taps with a screwdriver? I can remember the Phillips
engineers hitting the magnet of the ion pump on the Bristol EM400 with a 2″ by 4″ block of wood. Tis was to knock the whiskers off the inside of the pump which tended to cause it to short out and not operate. If you are going to have to replace the pump anyway, a couple of fairly brutal knocks might be in order, just to see if you can eke out a couple more months. A rebuild is probably in the offing, but you might be able to stretch the life of the current one. John Mansfield
jfmjfm@umich.edu Wed Jul 20 Use a hammer and pound it. If you are doing it right, you should
also use hearing protection and eye protection. John Mardinly John.
Mardinly@asu.edu Wed Jul 20 Dr. Mardinly’s comments about using a hammer to dislodge
whiskers inside an ion pump leads me to point out that if the whiskers are not too firmly established it is oſten possible to get rid of them by turning the high voltage supply on for a few seconds (not too long or you may damage the power supply) three or four times with the pressure in the pump above 1 Pa (10-2 Torr), If this approach works it is gentler, and quieter, than the hammer method. Incidentally, this method (but not the hammer method) and other characteristics of ion pumps, are described on page 295 of my book, Vacuum Methods in Electron Microscopy. Wilbur C. Bigelow
bigelow@umich.edu Wed Jul 20
Tis is the correct procedure how to remove whiskers, however
I do not think that Erman’s problem is caused by whiskers in the ion pump. Presence of whiskers causes additional field emission current and as a result it appears like additional (leakage) current. Such ion pump shows higher ion current and therefore it measures higher pressure than expected. Erman’s ion pump shows unexpectedly low pressure. Terefore, it could be a problem of the ion pump controller (e.g., lower voltage than usual); ion pump is not necessarily bad. I would not use a hammer in this case. Tomas Hrncir
tomas.hrncir@
tescan.cz Tu Jul 21
SEM:
Epon blocks Has anyone examined Epon blocks in a SEM? I tried to image
my muscle samples embedded in Epon with a SEM. (the same block was previously used for ultra-thin sections, so its surface was pretty flat). Te only thing I could see were brighter than background spots representing the muscle fibers. I’ll try to remove some resin from the surface with sodium ethoxide, so that only the sample “sticks out” a little bit over the resin, unembedded. Has anyone else done something similar (seeing Epon blocks in SEM)? Could you please share your experience? Josif Mircheski
jmircheski@us.es Tu Jul 21 I haven’t seen any replies to your question on the listserver.
Yes, I have done SEM on Epon thick sections without etching them, which work better than using the block. Use a razor blade and hand-trim a “thick” section (actually as thin as you can trim manually) from an Epon block aſter you have cut some thin sections from the block face to smooth the face off, and mount this section with the smooth side facing up on some double-sided carbon tape. Use a piece of plastic wrap (Saran Wrap, sandwich wrap, plastic bag) to press the section onto the tape. If you have access to a carbon coater, give the sample a light carbon coating to reduce charging. Put a couple of marks on the tape so you can find the section easily in the SEM, either by pin pricks or dots of silver paint. Now observe your section using your backscattered electron detector.
www.microscopy-today.com • 2011 November
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