31
GC COLUMNS |
CAPILLARY COLUMNS
Guard/Retent ion Gap Columns
Guard Columns and Retention Gaps
Guard columns and retention gaps are widely used in gas chromatography. The concept
of the guard column is to trap nonvolatile material at the head of the column, not
allowing the material to reach the analytical column. The concept of the retention gap
is to help focus the compounds transferred from the inlet to a small band at the head
of the analytical column in order to reduce chromatographic peak broadening. Both
concepts (trapping nonvolatile material and refocusing the target analytes) may take
place when a piece of deactivated tubing is connected to an analytical column as in
Figure 1.
Analyte Focusing
There are two injection techniques where the retention gap is used to help focus target
analytes at the beginning of the analytical column, cool on-column injection and split-
less injection.
For cool on-column injection, the purpose of a retention gap is to help focus the
sample components when introducing a liquid sample directly into the retention gap.
The cool on-column injection is performed by inserting the syringe needle into the
retention gap (this can be accomplished with a 0.53mm ID retention gap and a 26s
gauge syringe) and transferring the liquid sample directly into the retention gap. The
injection is made with the injector and column oven set below the boiling point of the
solvent. As the solvent is evaporated, the volatile target analytes migrate in the solvent
towards the analytical column, and the heavier analytes will be distributed over the
retention gap. As the oven temperature increases, the target analytes vaporize and move
unretained down the retention gap column until the compounds reach the liquid
stationary phase of the analytical column. At this juncture, the target analytes are
trapped/focused by the liquid phase forming a narrow injection band.
The retention gap may also be useful in hot vaporization injections when the transfer
of the compounds from the inlet to the column does not form a focused band. Typical
applications include water injections or injections using small ID columns, where split
or tailing peaks would indicate an unfocused band. In these applications, the target
analytes are trapped in a nonuniform or longitudinally diffuse band at the head of the
retention gap (Figure 2a). As the oven temperature is increased, the solvent and target
compounds are vaporized and move unretained through the retention gap (Figure 2b).
When the target compounds come in contact with the stationary phase, they are refo-
cused in a narrow band (Figure 2c), improving the chromatography.
Figure 1
A guard/retention gap column connected to an analytical column
Injector
Guard/Retention Gap
Column
Detector
Analytical Column
Vu2 Union
®
Connector
did you
know
?
We test our guard columns/ transfer lines
with a comprehensive test mix to ensure
high inertness.
please
note
For superior inertness, try our Siltek®
guard columns!
See page 33 for details.
Having trouble making a leak-free
connection? Try our “built in”
Integra-Guard® columns!
See page 35 for details.
Figure 2
Retention gaps are
used to focus components in a
tight band at the beginning of
the analytical column.
a)
Sample introduction: a liquid film of
solvent and sample is deposited in the
first length of capillary.
b)
As oven temperature increases, the
solvent evaporates and the target
compounds elute unretained through
the retention gap until they contact the
analytical column.
c)
When target compounds come in
contact with the stationary phase, they
are refocused on the analytical column,
resulting in a narrow initial band width.
a
b
c
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Mar 2011
2012
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