SEM-EDS
Analysis
Correlative
microscopy,
Filter-Handling,
PSE
operating modes,
Spectra
and material databases
JOMESA PSE: Material analysis with SEM-EDS |
Requirements for Filter Preparation
To be able to
investigate filter membranes in the optical microscope (JOMESA HFD)
as well as in the scanning electron microscope (JOMESA PSE), several
requirements have to be met.
1.
The filter must be –
due to the limited depth of focus - sufficiently flat.
Methods
like covering with glass plates must be excluded because glass is
intransparent for electrons and so an investigation in the SEM-EDX
is impossible.
2.
The particles must be affixed onto the
filter membranes because charged particles may be kicked out of
place in the SEM-EDX. Furthermore during the transport of the filter
membrane from the optical microscope to the SEM-EDX it must be
ensured that particles will not change their position or
orientation. This is very critical for large (>500 mm)
particles.
A fixation may not affect the light optical nor the
electron optical image; otherwise a correct judgement of the
particle origin may be hindered. The fixation must not cover the
particle; the EDX analysis may be limited or even impossible.
Glue-based solutions which cover particles – even when optical
transparent - must be excluded.
A manual picking of particles
from the filter onto a sticky carbon tape may not be considered just
because of time and cost reasons in serial analysis.
3.
Due
to the non-conductivity of the filter membranes we often observe
charging effects in the SEM-EDX which make it nearly impossible to
obtain good SEM images and so it´s hardly possible to face the
correct area of the EDX analysis – even if the particle is
properly fixed.
The use of so called „low-vacuum“ or
„variable pressure“ conditions in the SEM is a
compromise only: the charging effects are reduced, but the quality
of the EDX spectra is severely degraded. The coating of the sample
(with e.g. gold or carbon) is not an option: the metallic coating
affects the analysis of filters in the optical microscope. There is
a risk that all particles are detected as metallic. Also the
EDX-spectra are overlayed by the signal of the coating layer.
Additionally the process and the equipment necessary for coating is
time consuming and expensive.
The use of conductive, ionic
liquids in the SEM-EDX is established, the application and the
conditions for particle analysis on filter membranes is not state of
art yet.
4.
The filter should have markers to synchronize
the motion of optical microscope and SEM-EDX. The marker should be
permanently fixed and the filter should be asembled in a low-priced
mount. The marker must be visible in the optical microscope as well
as in the SEM; so simple printed markers can be excluded.
5.
The
optical analysis is much faster and cheaper then the SEM-EDX
analysis, so the filter should be covered and protected from
contamination.
6.
It should be possible to archive the
analyzed filter to allow a repetition or cross-check of the
analysis.
As
a consequence of these issues JOMESA developed a handling procedure
for PET and Polyamide (Nylon) filter membranes (patent pending).
Step 1: JOMESA SEM filter mount SM60x60
SEM60x60
filtermount consists of a bottom part (left pricture), onto the
outer side information about the filter can be written. On the
inner side, where the filter is placed there is a small copper
platelet on the right top corner. This will be used for a precise
correlative alignment of the filter in the HFD and the PSE.
Furthermore it can be used to calibrate the EDX detector of the
PSE. There is space on the SM60x60 to attach a QR code for sample
identification. |
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Step 2: The JOMESA Fixation Solution: Secure fixation of the particles on the filter
The JOMESA Fixation Solution is a monomer solved in acetone. The filter membrane will be placed onto the the filtermount, wetted with the solution. The acetone will evaporate. The polymerisation will take place between the particle and the filter membrane. The particles will be fixed by the polymerisation. The upper side of the particles will not be covered; the results of the analysis are unaltered.
JOMESA Fixation Solution:
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Conventional fixation glues
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How will the JOMESA Fixation Solution applied?
0.2 ml of the JOMESA Fixation Solution FS-ACR1will be drawn into a syringe. |
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The JOMESA Fixation Solution FS-ACR1 will be distributed equally inside the circular mark. |
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The filter will be placed into the solution. Let it dry. |
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To
protect the filter from dust, it may be covered with the glass.
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Step 3: Measuring the filter under the optical microscope (JOMESA HFD)
The filter under the optical microscope (JOMESA HFD). Before starting the scan the system will move to the alignment marker top right automatically. |
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The alignment mark in the image of the HFD: the Cu disc has a small hole in the center. This hole will be centered in the image automatically. It is the reference for the coordinate system of this filter. |
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Step
4: JOMESA Conductivity Solution CS-IL1
The
JOMESA Conductivity Solution is an organic liquid to make particles
and filter conductive. This ionic liquid contains only the elements
C, O, N and H.
Using
the Conductivity-Solution CS-IL1 is optional. It is recommended if
electrostatic charging in the high-vacuum worsens the image quality
and perfect SEM images are required. It is not necessary if only EDX
spectra should be acquired. It is highly recommended when the exact
place of the EDX spectrum must be determined.
Without JOMESA Conductivity Solution |
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With JOMESA Conductivity Solution |
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JOMESA
PSE works in high vacuum. The arguments are:
"... X-ray
microanalysis in the VPSEM and ESEM can be a useful tool to
complement SEM imaging, but the analyst must recognize the
inevitable limitations that result from gas scattering compared to
the level of analytical performance achieved in a conventional high
vacuum SEM. … While it is usually possible to achieve useful
results for major constituents, minor and trace constituents are
likely to be severely compromised. … Quantitative analysis of
areas with micrometer dimensions is severely compromised.... ."
Dale
E. Newbury, National Institute of Standards and Technology:in:
Journal of research of the National Institute of Standards and
Technology 2002 Nov-Dec; 107(6): 567–603.
How will the JOMESA Conductivity Solution be applied?
0,2 ml JOMESA Conductivity Solution CS-IL1 will be drawn into a syringe. |
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The JOMESA Conductivity Solution CS-IL1 will be equally dropped over the filter with the particles affixed. Let it dry. |
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Step 5: Measuring the filter with the scanning electron microscope (JOMESA PSE)
The SEM-filter mount can be analyzed directly in the PSE electron microscope. A special holder will be used. |
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As in the HFD light microscope the Cu alignment marker will be used for the correlation of the filter position. |
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Components Overview:
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