[3dem] volta phase plate recovery time

[Matthias Wolf]

Hi Anchi,

We see those, too. I call them scars. Compared to a real PP potential, however, they have a minimal effect, so we simply don't worry about them, even when they are close to the new potential (which is then visible as a small twin satellite peak at low res in the FFT and one can use a high pass filter on these images to get rid of it). As Craig said, moving the objective aperture control mechanism can shift the position, but we found that that the FEI tracking algorithm seems to follow an absolute pattern and such offset is only temporary - maybe leginon could control the obj aperture position directly for adding a small offset? Alternatively, FEI should add an offset control to their VPP OCX for heavily used phase plates.

Thanks for pointing out the faster contrast development on used spots. We looked at them by EELS and it turns out that they are not due to mass loss or gain (no erosion or addition of contaminants by the beam - the thickness of the VPP carbon film remains the same), but apparently due to a different modification of carbon, which is visible in a changed spectrum and which seems to be not amorphous. Because of this apparent change in structure of the VPP carbon film, I believe that the residual scars won't go away, even after their charge has dissipated. The amount of the structural change may depend on the deposited electron dose, which would explain why some scars go away more readily.

As for a number on wait time: it also takes us more than 2 months to get through all 700+ preset positions. But many scars are still there after one year, which is consistent with a non-charge-based effect and a permanent change in structure.

   Matthias

On Mar 24, 2017, at 19:20, Anchi Cheng <acheng at nysbc.org<mailto:acheng at nysbc.org>> wrote:

Hi, Wim,

Thanks for replying.  No, these are regular-spaced spots that show up after
using the patch position.  Similar to Figure 2 in Rado’s 2015 PNAS paper.
The spacing corresponds also the distance between the defined distance
between patches.  Incidentally, this phase plate is the cleanest I have ever
seen.  It was hard to find a feature on it to align.

We do have some burn spots disappeared after 1.5 month.  There were the
ones that was irradiated less.

I am hoping I get a number from you or others with phase plates longer than
we have how long you wait before using the same phase plate patch position
again.

Best,

Anchi

On Mar 24, 2017, at 4:15 AM, Wim Hagen <wim.hagen at embl.de<mailto:wim.hagen at embl.de>> wrote:

Hi Anchi,

Maybe it is contamination? I see some spots in some areas on our brand new phase plate, the contrast of the spots is reversed w.r.g. to a Volta patch. Here are some images with many bad spots and one 'quick and dirty fresh' Volta patch, 'above and below' plane:

https://oc.embl.de/index.php/s/j3SlNyNwsspH00U

Best,

Wim

On Thu, Mar 23, 2017 at 18:33, Anchi Cheng <acheng at nysbc.org<mailto:acheng at nysbc.org>> wrote:
Hi, all,

I am wondering what people’s experience in the length of time
they wait before they reuse a patch position on FEI volta phase plate.

We can still see rows of the different contrast (burn spot, we call it) at low mag
and very high defocus (-100 mm) after a month and a half.

If I return to a patch in 2 weeks, the phase shift development is faster.  In addition,
there is no guarantee that I am back at the exact position.

Anchi

NRAMM
Simons Microscopy Microscopy Center
New York Structural Biology Center
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