31
GCCOLUMNS |
CAPILLARYCOLUMNS
Guard/Retention Gap Columns
GuardColumnsandRetentionGaps
Guardcolumnsand retentiongapsarewidelyused ingaschromatography.Theconcept
of the guard column is to trap nonvolatile material at the head of the column, not
allowing thematerial 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.
AnalyteFocusing
There are two injection techniqueswhere the retention gap is used tohelp focus target
analytes at thebeginningof 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 ismadewith the injector and columnoven set below the boilingpoint of the
solvent.As the solvent is evaporated, the volatile target analytesmigrate in the solvent
towards the analytical column, and the heavier analytes will be distributed over the
retentiongap.As theoven temperature increases, the target analytes vaporizeandmove
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/focusedby the liquidphase forming anarrow injectionband.
The retention gapmay also be useful in hot vaporization injections when the transfer
of the compounds from the inlet to the columndoes not form a focusedband.Typical
applications includewater 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 anonuniformor longitudinally diffuse band at the headof the
retention gap (Figure 2a).As the oven temperature is increased, the solvent and target
compounds arevaporizedandmoveunretained through the retentiongap (Figure2b).
When the target compounds come in contact with the stationary phase, they are refo-
cused in anarrowband (Figure 2c), improving the chromatography.
Figure1
Aguard/retentiongapcolumnconnected toananalytical column
Injector
Guard/RetentionGap
Column
Detector
Analytical Column
Vu2Union
®
Connector
didyou
know
?
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withacomprehensive testmix toensure
high inertness.
please
note
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guardcolumns!
Seepage33 fordetails.
Having troublemakinga leak-free
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Seepage35 fordetails.
Figure2
Retentiongapsare
used to focus components ina
tightbandat thebeginningof
theanalytical column.
a)
Sample introduction: a liquid filmof
solvent and sample isdeposited in the
first lengthof capillary.
b)
Asoven temperature increases, the
solvent evaporates and the target
compounds eluteunretained through
the retentiongapuntil they contact the
analytical column.
c)
When target compounds come in
contactwith the stationaryphase, they
are refocusedon theanalytical column,
resulting inanarrow initial bandwidth.
a
b
c
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TelNo : 03 9762 2034 . . . inAUSTRALIA
Mar 2011
