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Fritted glass is manufactured
from particles of glass which are fused, or sintered into a solid,
but porous glass body, which can be sealed into glass tubing as a filter made entirely
of glass. In our discussions with
customers and vendors regarding fritted glass for filtration, column
support for packings and resin beads, and solid phase synthesis, we
have found a plethora of standards for a variety of uses. For our
own edification we have concatenated some of this information; we'd
like to pass this along to you, both to help you make intelligent
choices about porous glass filters, and to make it easier for us to serve you once
you do. Frit porosities are defined
in three different systems, as shown here.
The size of filtered particles, the size of packing in chromatography
columns, or resin beads for peptide and small molecule synthesis will
determine the choice of porosity for your application. See our notes
about mesh designations.
| Fritted
discs are made by heating glass particles or fibers at a high
enough temperature that they fuse together sufficiently that
they become a relatively strong object. In our case, the glass
is borosilicate glass, and is the same as the standard material
for laboratory apparatus, i.e. Pyrex 7740, Kimax KG-33, or
Schott Duran. The Kimble and Corning, and Robu frits are made
from particulate glass, while the Ace frits are made from short
pieces of fiber, and are alleged to be stronger. Once formed,
the fritted disc can be sealed into tubing as part of a variety
of apparatus, for filtering, supporting column packings or
resin beads, etc. |
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Porosities of sintered glass filters
are determined dynamically: air is forced through a frit just immersed in water
until a bubble goes through. The pressure required correlates with the pore
size, according to theory: pore diameter (in microns) = 30 * surface
tension (in dynes/cm) / pressure (in mm HG). This should correlate
with the average pore size of the whole frit.
Pore
volumes vary with the porosity. A graph
of flow rates for air and water
for various porosities (in the European ISO 4793 system) is available as
a pdf.
The
realities of fusing a frit into a tube provide endless possibilities of
variation. Small diameter frits and the finer
porosities in particular can be problematic. Your
mileage may vary.
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