Chapter:
Topic:
INJEcrOR
&
GASVALYES
SRI SeptaOverview
Side
view
FroatView
Side
view
Froalview
Septum
with
added
coating
Cut
septum
o
Molded
septum
.-I
by
SRI
EO@
In
order toplace a sample into thecolumnof a gas
chromatograph without de-pressurizing the injection port and column
or
interrupting
the carrier
gas
flow, some
type
of penetrable,
resealable membranemust be
used.
The membranemust be penetrable
to permit
the
introduction of the syringe needle into the injection port,
but must
also
havethe ability to re-seal itself.
If
it couldnot re-seal
itself, each injection would leave a leak
that
would permit carrier gas
to escape from
the
system. Each subsequent injection wouldworsen
the condition, adversely affecting retention times and sensitivity.
Silicone rubber
is
commonly
used
toproduce injection port septa.
Silicone, due to its formulation, is soft yet maintains the ability to seal
puncture wounds created by syringe needles. Although septa differ in
formulation, proper care
will
prolong the lifeof
any
septum. A
silicone septa (CRS 800-327-3800,
part
number
23n40)
is
installed in
all
SRIinjection
ports
whenshipped. This septum
is
very softand
resealable. It demonstrates lowsilicone bleed anddoes not affect
sample component elution times. Additionally,
this
septumexhibits
negligible "coring" for betterdurability andperformance. This septum
seala well in the tapered interior of the
liS"
modified Swagelok
injection
port.
The exampleat right illustrates thedifference in
physical appearance between
this
septum
and
the staodardcut septa
machine-stamped fromsilicone sheets. Coated
septa
are manufactured
this
way. The coating
is
intended to reduce septum bleed and
increase
resealability.
All septaare
made
of silicone rubber that contains polymerizedsilicone gums. Acatalyst
is
used
toobtain
the
polymerization that produces
the
elasticity sought fromtheseptummaterial.
Unfortunately, some silicone oils remain unpolymerized in
the
septum
and
freely diffuse out of the
silicone septum. These oils vaporize into the injection port and are
known
as septableed. Some
manufacturers
insert
additional
oils,
making
the septa softer and easier
to
remove from their molds.
This inereases theamount of septa bleed in thosepieces. Most lowbleed septaare manufacturedby
extending the duration ofpolymerization, resulting ina harder siliconewith lessbleed.
The
septa
used
bySRI exhibitextremely lowbleedwhile remaining soft
and
highly resealable.
When silicone oils bleed into the
COIUnul
over time, the efficiency and performance of the
column
is
degraded. Columns with a silicone liquid phase, suchasQV-l or SE-30 types,
will
not
display
the
effectsof septableedas readily as would a phase suchas
Carbowax
20M, whichwould
be adversely affected by theeffectsof siliconebleed.
In
other columns,
the
condition maygo
unnoticed
initially, especially during isothermal operation until the development of a highunsteady
baseline occurs, acoompanied in some instancesby increasednoise.When thecolumn temperature
is
ramped
asoccurs during temperature-programmed operations, the silicone oilsbegin to elute as they
are heated to their respective boilingpoints. These siliconecomponents
will
elute
through
several
runs, producing spurious
peaks
withoften reproducible retention times.
This
can influence
component identification
and
measurement negatively.
In
some
work,
where sensitivity
is
not
great,
septa
bleed
is
nOl a concern.To identify septa
bleed, especially where temperature programming
is
employed, cool theunit toambient temperature
and hold for ten to fifteen minutes. Then ramp the temperature
up
to
the
maximum running
temperature normally
used,
with the sensitivity set tohigh. Any
peaks
or baseline
drift
can be
attributed to septableed. Onemethod to
minimize
bleed is that of baking septainan oven prior to
insertion into the injection port in order to volalize the siliconeoils. The septamay
alsobe
baked
in
the injection
port
overnight, as long as the column oven
is
maintained
atthe same temperature as the
injection
port
to
avoid the accumulation of bleed products. Regardless of septum
type,
septa should
never
be
handled except
with
tools. Finger oilsmay appear onchromatograms asadditional peaks.
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