retention gap. When the oven temperature is increased the sample components will start to move (there
Is very little retentlon...that's why It's called a retention "gap"). When reaching the analytical column, the
components will focus In the stationary phase resulting In a narrowing of Injection band width (Figure 1).
As
these retention gaps are mainly used for on-column Injection, the Inside diameter Is usually 0.32mm up
to 0.S3mm since the needle of an on-column syringe must be able to enter the retention gap. For coupling
the retention gaps to the analytical column, we need generally coupling devices that can deal with
different diameter capillary tubing.
Retention gaps and splltless injection
While on-column injection minimizes discrimination and provides the best quantitative data, especially for
thermolabile components, it can be challenging to perform. Many laboratories will choose a splitless
method for ease of use. For splitless injection we generally do not require a retention gap. The sample is
Injected In a hot Injection port, evaporated, and transported with a carrier gas flow of approximately
lmL/mln Into the capillary. The amount of solvent vapor that enters the column per unit time Is much
smaller than with on-column injection. Although with splitless injection the oven temperature is also 10–
15°C below the boiling point of the solvent, there is little chance of the solvent condensing. The high
concentration of solvent entering the capillary column will cause a strong focusing
effect
for the
components, generating a narrow injection band. If, in splitless injection, a method is used where the
initial (injection) oven temperature is much lower than the boiling point of the solvent, the risk of solvent
condensation (forming a llquld plug) will increase. This can cause unwanted broadening of the injection
band. Coupling a retention gap will also fix this problem.
Wettability of the retention gap
An important factor for good performance is the wettability of the retention gap surface.
It
is critical that
the solvent spread evenly over the surface. This means that nonpolar solvents (hexane, methylene
chloride, Isooctane, benzene) require non/Intermediate deactivated retention gaps and more polar
solvents (metha noll will require polar deactivated retention gaps. If the polarity of the retention gap and
solvent do not match, the solvent will form droplets inside the capillary. The carrier gas will "push" this
droplet along the retention gap into the analytical column. The result is a broadened injection and possibly
even peak splitting.
Retention gaps for large volume injection
Instead of Injection of
1~1
on a 1-2m retention gap, one can also Inject much larger amounts on much
longer retention gaps. Here we talk about large volume Injection technique where retention gaps of 8–
10m are used. Such retention gaps can be loaded with
100-200~1
of sample. Injection must be slow to
allow the solvent to evaporate while passing through the retention gap. With large volume injection,
detection limits can be reduced a factor of 100. The technique requires some skill to optimize all the
injection parameters. Additionally, the large volume retention gaps do pollute relatively quickly due to the
large amounts of sample Introduced.
Guard columns and retention gaps are useful tools to the practicing chemist and it is important to
understand the difference between them. In Part 2 of this article we will review guard columns and
discuss a new segment coating technology that allows retention gaps and guard columns to be built
directly into the analytical column tubing. This new technology eliminates column coupling, substantially
reducing analytical problems related to leaks and dead volume.
Read Part 2:
Using Guard Columns and Retention Gaps in
GC
REFERENCES
1. Grob, K., Joumal of Chromatography 237:15 (1982).
2. Hinshaw J.,
LC/GC
Europe 17(9):
46~6
(2004).
2J2
2012
2
