Vacuum Degasser User manual
Vacuum Degasser
Page 15
Principles of degassing using Teflon AF®
membranes
This relatively recent addition Teflon AF® membranes to the field of degassing is
due to their having properties not found in other fluoropolymers. The fully amor-
phous nature of this fluoropolymer and its molecular structure creates a molecular
level porosity unlike the mechanically induced porosity in PTFE extruded tubing.
In addition, unlike the process used in extruding PTFE, no extrusion agents are
needed (like kerosene, etc.) which contaminate titrants until they are extracted by
the titrant over time. Likewise, this molecular structure, combined with the very
small surface areas required to degas the titrant, reduces the possibility of carryover
from one titrant to another to virtually zero.
Teflon AF® is so non-polar that it is both solvophobic and hydrophobic. This fea-
ture of Teflon AF® reduces the possibility of cross-channel contamination from
one channel to another, and when combined with the ultra-low internal volumes
of Teflon AF® channels needed for low flow rates, all but eliminates any cross con-
tamination concern. Teflon AF® has been used in certain optical systems associ-
ated with HPLC for a few years without concern for normal HPLC solvents.
However, Teflon AF® is soluble in certain solvents (Refer to the
Chemical Com-
patibility
chapter) and
must
not be used to degas these solvents when used for
titration.
Teflon AF® is permeable to some degree to water vapour whereas PTFE is not.
While the vacuum pump in the Vacuum Degasser contains internal provisions for
sweeping water or titrant vapour from the pump continuously, it is possible that
over time, high concentration buffers may form crystals within the channel due to
the loss of water within the channel. The same precautions should be taken to pre-
vent crystallization within these channels as are taken for the MPT-2 Autotitrator.
Refer to the
Short-term Shutdown
procedures.
Extending the degassing flow rate range
Certain organic titrants outgas upon mixing with water, if not properly degassed.
These titrants are generally alcohols (e.g. methanol), acetonitrile and tetrahydro-
furan. Passing water and methanol through a single channel is generally sufficient
to degas these titrants so outgassing does not occur upon mixing when a 75% meth-
anol: 25% water mixture is generated by the Vacuum Degasser or pump at a flow
rate of 2 mL/min. If outgassing does occur, or if a flow rate higher than 2 mL/min.
is required, it is a general rule that only the organic portion of the titrant needs to be
passed through a second degassing channel to ensure outgassing does not occur.
This is due to the ability of all organic titrants (e.g. methanol) to hold at least 10
times more dissolved atmosphere than water can.
MAN0376-2.0 - Vacuum Degasser.book Page 15 Friday, January 12, 2007 2:49 PM
Summary of Contents for DEG0003
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