3
In contrast, a direct injection is
made into a 2mm or 4mm ID
injection port liner, and the large
buffer volume of the liner ensures
adequate space for sample vapor-
ization. The vaporized sample is
transferred to a 0.32mm or
0.53mm ID column. Larger sam-
ple volumes can be usedwith
direct injections, withminimal
sample backflash and solvent
peak tailing.
Direct injection also reduces the
need for columnmaintenance rel-
ative to on-column injection. In
on-column injections, the tip of
the syringe needle enters the col-
umn bore and there is a potential
for chipping the edge of the col-
umn or damaging its inner sur-
face. Fragments of fused silica
and scratches in the stationary
phase coating in the interior of
the column inlet can produce
adsorptive sites for active sample
components, thereby requiring
portions of the inlet side of the
column to be removed periodical-
ly. Direct injection, however, reduces the frequency ofmaintenance cycles because there is
less physical damage to the column inlet. On the other hand, direct injection dictates that the
injection port liner be changed or cleaned during routinemaintenance.
When samples are dirty or contain non-volatile residue, direct injection should be used
instead of on-column injection. The injector liner will trap the non-volatile sample residue
and keep it from entering the column.Maintenance isminimal—simply clean or replace the
injector liner. In contrast, columnmaintenance as a consequence of on-column injections of
dirty samples requires removing the inlet end of the column from the injection port, discard-
ing several loops of the column, then reinstalling the column. The only potential advantage
to on-column injection over direct injection is that delivery of the sample into the bore of an
inert fused silica columnmight reduce the possibility of analyte adsorption, compared to
depositing the sample in an injector liner. However, using deactivated injector liners (all
Restek injector liners are fully deactivated) helpsminimize the likelihood of analyte adsorp-
tion during the sample vaporization and transfer process. Because direct injection results in
significantly less columnmaintenance and allows a larger injection volume than on-column
injection, direct injection is becomingmuchmore popular as a sample introduction tech-
nique in capillaryGC.
1
1
Most factory-installedGC injectors are designed for split or splitless injections, but these injectors can
be adapted for direct injections simply by purchasing liners designed for direct injection. For informa-
tion about the availability of direct injection liners for yourGCmodel, refer to pages 14-16, or call
Restek’s Technical ServiceGroup at 800-356-1688 or 814-353-1300, ext. 4.
What are the advantages of direct injection over splitless injection?
Because both direct and splitless injections attempt to deliver the entire sample vapor cloud
onto the column in the narrowest band possible, both techniques are used primarily for trace-
level sample analysis. Only in a direct injection system, however, does the injector liner
make a positive seal with the column inlet. This leak-tight connection between the liner and
the column ensures that the entire sample enters the column, thereby enhancing the overall
sensitivity of the analysis.Adsorption of active species isminimized and responses are
greater for highermolecular weight compounds.
Figure 1.
Direct injection and on-column injection differ
inwhere the syringe needle terminates during
sample vaporization.
Press-Tight
®
connection
direct
injection
on-column
injection
Press-Tight
®
connection
capillary
column inlet
needle termination
capillary
column inlet
