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12
tom of the liner (Figure 2). The benefits of using this type of injection are that
upwards of 3µL to 4µL of sample canbe injected, and injectionport discrimi-
nation is reduced.
A split/splitless injectionport alsomay be adapted for direct injections using
Uniliner
®
liners. In this mode of operation, the split/splitless injection port
split valve (purge valve) must be turned off. Additionally, it is beneficial to
make a leak-free connection (press-tight seal) between the liner and the col-
umn. Refer to the steps presented in the supplied product information sheet,
or found inour catalog, to install aPress-Tight
®
connector. It is alsohelpful to
use graphite ferrules for this connection because the Vespel
®
ferrules may
cause the column connection to fracture if overtightened. Finally, before in-
serting the column into the connector, dip the end of the column into a vial of
methanol for 30 seconds. This causes the polyimide coating on the column to
swell, resulting in a better seal.
A third direct injection technique used for pesticide analysis involves large-
volume injections into a cold injection port. The injected solvents and com-
pounds are cold-trapped on the injectionport linerwalls. The injectionport is
heated to about the boiling point of the solvent, and the solvent is vented out
of the system. The vent is turned off and the injection port is heated rapidly,
allowing the trapped analytes to transfer from the liner to the inlet end of the
analytical column.
Pesticide methods generally require a second column analysis for confirma-
tion, to give a higher degree of confidence in reported analytes. For dual-
column analyses, we recommend that these injections bemade into a single
injection port and split onto two columns using a glass “Y” fitting (Figure 3).
Although there are alternative ways to set-up a dual-column system, this
method provides the best reproducibility, while achieving the required detec-
tion limits andminimizing instrument maintenance.
Splitless Injections
Splitless injection involves operating a split injection port with the split valve
(purge valve) closed during the injection, allowing all the carrier gas to be
directed into the column. The split valve remains closed for a short time (30
sec. to 2 min.) after the injection in an attempt to transfer as much of the
sample extract as possible from the injection port onto the column. After this
splitless hold, the purge vent is opened, and the remaining solvent and non-
transferred sample are vented out of
the injection port. The purge vent
shouldhavea carbon trapattached to
remove any pesticides and other or-
ganic compounds before being vented
into the laboratory.
Splitless injection requires optimiza-
tion of the purge time to ensure that
the maximum amount of analyte is
transferred to the column, and mini-
mizes the amount of solvent. Gener-
ally, the purge time is determined by
maximizing the area count of the last
eluting analyte. For additional infor-
mation, please contact Restek’s tech-
nical service and ask for the technical
guide
OperatingHints forSplit/Splitless
Injectors
(lit. cat.#59880).
Splitless injection isprone to inertness
problemsbecauseof theresidence time
Figure 2: A press-tight seal
connects the liner to
the column.
Figure 3: “Y” fitting provides best dual-column system connection
analytical
columns
Press-Tight
®
“Y” or
Vu-Union
™
“Y” connector
guard
column
split/splitless
or direct
injection inlet
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Press-tight
connection
Uniliner
®
Metal sleeve
adaptor
