Re: paleonet Acid Etching for SEM

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On Wed, 10 Sep 2003, Peg Yacobucci wrote:

> Date: Wed, 10 Sep 2003 15:52:45 -0400
> From: Peg Yacobucci <mmyacob@bgnet.bgsu.edu>
> Reply-To: paleonet@nhm.ac.uk
> To: PaleoNet@nhm.ac.uk
> Subject: paleonet Acid Etching for SEM
>
> Howdy all,
>
> Here's a question I'm forwarding from a student.
>
> I am looking for microfossils (bacteria, algae, etc.) in travertines.  I
> have multiple samples that I want to run under SEM and it has been suggested
> that I should acid etch the samples to try to bring out detail in the
> fossils.  My questions are:
> - What acid should I use?
> - What concentration of acid will work without destroying microfossils?
> - How (dunking, fumes. etc.) and how long should I expose the samples to the
> acid?
>

Dear Peg,
Dear Justin,

at a first glance it might be possible that some readers smile
after reading the following lines. We had however a related case
with acids and resulting equipment deterioration.

So:

The answer splits between:

(a) you use an Ommicron ESEM vs.

(b) you use a "conventional" SEM.

First (b), conventional SEM) is answered:

Under (b) vacuum is generated. If you treated your samples with
acid and you are not sure that either the acid reacted completely
with your specimen or disappeared with its reactive component
you might initiate, due to the low pressure,  evaporation
of acid remnants and subsequent corrosion (not everywhere).

Checking of the SEMs materials in the vacuum chamber and their
(lacking/not lacking) ability to react with the acid, is thus useful.

In principle (getting details more pronounced) an acid treatment and
stopping the reaction after different times might (not always)
improve the picture.

Before embarking on such fairly brutal methods I would do the
following (some SEMs permit this, an older SEM I know does not
permit this):

(1) Find out the spotsize in world coordinates (such as nanometers)
at different turntable settings, e.g. which spotsize number
equivalates which diameter in nanometer)

(2) Find out how much scans in numbers (from y=0 to y=top of picture)
your SEM makes under which setting.

(3) Calculate your spotsize such that each scan (as band of a certain
width) lies next to each other.

For some SEMs this is trivial, as the turntables are numbered accordingly;
for some SEMs this is in principle possible though sometimes even
the service personnel does not always know how to recalculate the
spotsize numbers to real widths.

(4) Experimenting with various qualities of sputtering might in
some cases also help i.e.:

(4a) either intentionally getting edge effects low or
(4b) getting intentionally edge effects slightly higher.

In principle all thoughts from light microscopy, e.g. applying
sometimes light from the side (here: orientation of your specimen)
apply with particularls of SEMicroscopy
kept in mind (such as electrostatic charging of edges) also here.

Above spot-size thoughts result from experiments with a Phillips SEM.

(a) If you have an ESEM:

There you don t have vacuum near the sample. Thus acelerated evaporation
of acid-remnants is unlikely to occur.

Further above other conditions apply.

Therefore, also with an ESEM, care is neccessary when applying
acid in the sample chamber.

In any case, particularly as an ESEM is very good but expensive,
I am not liable for any damages that might result.

If an ESEM can now tolerate acids (for example if they have a component
to exchange atmospheres above the sample) above thought does not
apply (= then they are much better than I thought, I do not want
to critisize them).

Above thoughts do not apply to "immediate corrosion" (above experienced
example was an extreme case). They might however apply to slow
corrosion with nobody knowing the reason for problems some years later.

> Thanks,
>
> Justin Richardson
> Dept. of Geology
> Bowling Green State Univ.
>

Best regards, Peter

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Dr. Peter P. Smolka
University Muenster
Geological Institute
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