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4.2.3 Polymers
Polymer samples include plastics and glass and are typically mounted flat or on a
c
ross section.
Because of their organic and / or siliceous composition, polymers also tend to charge
even at low magnifications and accelerating voltages.
A light gold coating can eliminate most charging effects and optimize resolution.
Polymers should be fixed down to the sample stub with colloidal graphite, silver paint,
double sided carbon pad, or a clamping mechanism.
4.2.4 Metals
Samples containing metal may have a certain affinity for the electromagnets within
the SEM. This affinity may be strong enough to pull the sample from the stub and up
into the detector region. This will cause degradation of imaging capability and will
require a service technician to remove this material from the detector.
To avoid this scenario, all metal samples should be firmly fastened to the SEM stub.
Metalpowders can be fastened to the SEM stub in the same fashion as
non-metal powders. Please reference the 4.2.2 ‘Particle
samples’ section for instructions.
M
etalfilings can be attached to the stub in a similar fashion to metal
powders. However, larger filings should be attached using
colloidal graphite or even a clamping stub.
Ferrous, or iron-containing samples should be mounted onto an SEM stub with great
care. There are several powerful electromagnets inside the SEM that will attract iron
containing materials within the sample chamber. Loose ferrous material can be pulled
off the sample and onto the detector. This will cause a considerable, if not total, loss
of imaging capability and can seriously damage the detector. Make sure the sample is
firmly attached to the sample stub with colloidal graphite, silver paint, adhesive pad,
or a clamp before imaging in the SEM.
4.2.5 Biological samples
Biological samples may be more difficult for SEM imaging.
High vacuum, low conductivity, and regular outgassing are all factors that reduce the
clarity of biological samples in the SEM.
To best image biological samples make sure the sample is dry.
Here are a few methods of sample preparation for the Phenom.
AirDrying will remove a fair amount of moisture from a biological
sample allowing for better imaging. The drawback to air
drying is that surface morphology of the sample is sacrificed.
CriticalPointDrying(CPD) is a complex way of drying biological samples without
seriously disrupting surface morphology. Although
sample preparation time will greatly increase with the
use of CPD, samples will look much truer to their
original state than with air drying.
HeavyMetalStaining using heavy metals such as osmium can be used for
samples to be imaged in the Phenom. Heavy metal staining
is also a complicated and potentially hazardous form of
sample preparation. This type of preparation should only be
attempted by a trained individual in an appropriate lab
environment.
Biologicalsamples are also fairly non-conductive and may charge under the
SEM. A gold coating can reduce charging in these types of
samples. Refer to 4.2.6 ‘Heavy Metal Coating’.
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