Filtration and Physical Properties of SPI-Pore™ Polycarbonate
Membrane Filters
The general procedure is to critical point dry the wet sample, without ever
giving it a chance to dry out.
When the sample is dehydrated, always use alcohol and never acetone, since
acetone will dissolve away the polycarbonate substrate! So we recommend using
whatever dehydration protocol one would normally use, going either from the
alcohol directly or, as we would prefer, via n-butyl acetate, into the critical
point dryer.
In the case of this kind of sample being critical point dried, it is
critical that it be done in an environment of zero turbulence of the flowing
liquid CO2.
The SPI Critical Point Dryer unit has a large viewing glass just for this
kind of reason. Without the viewing glass, it is not possible to validate the
absence of turbulence.
One should ask, just what kind of container should the membrane filter be
contained in while in the CPD unit during the drying process. And we would
suggest cutting out small pieces of the wet membrane and placing each one in a
different SPI Microporous Specimen Capsule.
There is a choice of three different pore sizes and the selection then will
depend on the features being collected on the membrane filters and the
possibility of for sample cross-contamination. Note however that as the pore
size gets smaller, the exchange time of the starting liquid with the liquid
carbon dioxide will increase substantially.
Once the sample is dry, we recommend mounting the small squares of now dried cut
out membranes on an SEM mount using only dry adhesives, such as the double sided
adhesive conductive carbon tapes,
sheets, or discs.
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Thursday March 11, 2010
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