stationary phases are more resistant to oxidation (methyl and
phenyl/methyl polysiloxanes), all stationary phaseswill
eventually degradewhen exposed to oxygen in the carrier gas
at high temperatures.
Oxygen can be removed using getters, ormaterials that
adsorb or chemically reactwithoxygen. Some gettersmust
be heated to effectively remove oxygen, while others canbe
used at room temperature. Getters also differ in their capacity
and mechanism to remove oxygen. Heated getters may
release hydrogen or other impurities into the carrier gas
stream, whereas most room temperature getters simply bind
or reactwith oxygen. Some room temperature getters are
extremely reactivewhen broken, therefore caremust be taken
not to break the trap or expose the trap material. Getters can
also remove tracemoisture but this diminishestheir capacity
to remove oxygen. Removingmoisturewithmolecular sieve
traps is more effective and will extend the lifetime for most
getters. Molecular sieve traps exhibit excellent capacity for
removing trace levels ofmoisture from carrier gas. Indicating
molecular sieve traps are available, however, the indicating
media is only sensitive to high levels ofwater and are not
usually recommended. The l/8” “S” typemolecular sieve
traps (cat& 20686) are usually the best choice for
chromatographers. They are packed, activated at oven
temperatures of 300°C sealed, and are ready to-use. Because
of their small size, they can be reconditioned in a GC oven
when contaminated.
Awide assortment of traps are available for
carrier, makeup, and
detector
fuel
gases.
Hydrocarbon impurities in the carrier gas lines will result in
detector instability, ghost peaks, and in extreme cases will
result in column contamination. High levels of hydrocarbon
impurities are not usually present in commercially available
carrier gas sources, therefore most chromatographers do not
find hydrocarbon traps necessary.
Hydrocarbon and solvent contamination is frequently
removed using activated coconut charcoal. Since indicating
hydrocarbon traps are not available for carrier gas lines, the
analyst must note the date of installation and change the trap
after approximately six months of use. Indicating traps,
which are available from compressed air lines (1/8'”:cat.#
20637), should be usedwhen oil lubricated air compressors
are used as the FID air source.
What are the differences between indicating and
non-indicating traps?
Some traps can indicate oxygen,moisture, or hydrocarbon
removal by changing color. Indicating traps aremadewith
glass housings to allow visual inspection of the color change.
Although glass housings are fragile, they prevent oxygen
from diffusing into the carrier gas and allow visual indication
of the purifier activity level. Plasticmaterials are permeable
to oxygen and are not recommended for any trap installed on
a carrier gas line. Non-indicating traps are generally con-
tained in a metal housing for strength and ruggedness. It is
important that indicating oxygen traps are made with either
glass or metal housings.
Indicating traps change
colors as they .are depleted,
telling you when it is time to
change them.
Indicating traps have an advantage over non-indicating traps
since you can visually determinewhen to install a new trap.
With non-indicating traps, it is impossible to accurately
determinewhen the trap needs to be replaced. Non-indicat-
ing, high capacity traps should be installed prior to an
indicating trap.When the indicating trap shows a color
change, the non-indicating trap has been depleted and should
be changed.
In what order should the traps be installed?
The order inwhich the traps are placed inthe carrier gas flow
path and their proximity to the GC is very important. Figure 9
on page 10 shows the recommended order for installing
carrier gas traps. The hydrocarbon trap should be placed first
in line from the carrier gas tank. This is to prevent trace
hydrocarbons from contaminating themolecular sieve trap.
Themolecular sieve trap should be placed after the hydrocar-
bon trap to removewater. The oxygen trap should be placed
closest to the GC bulkhead fitting. In general, traps should be
installed on eachGC as close to the bulkhead fitting as
possible. Traps installed near the gas cylinderwill not remove
oxygen thatmay enter the carrier gas from leaky fittings
downstream.
’
Traps should be installed vertically to avoid channeling.
Channeling results from the packingmaterial settlingwhich,
when a trap is positioned horizontally,may allow carrier gas
to pass throughwithout sufficient interactionwith the
packing.
9
9
1998
