32
GCCOLUMNS |
CAPILLARYCOLUMNS
Guard/Retention Gap Columns
Protecting theAnalyticalColumn
The concept of a guard column is to protect the analytical column from becoming
contaminatedwith nonvolatile compounds. The guard column is used to retain non-
volatilematerial, usually in the first 10-20 cm, not allowing thismaterial to elute onto
the liquidphase of the analytical column.As the oven temperature increases, themore
volatile target compounds vaporize, elute down the guard column, and refocus at the
head of the analytical column without interference from the nonvolatilematerial left
behind.
Using guard columns is advantageous, because they prevent contamination that can
cause active sites aswell as change the conditions of the focusing zone of the analytical
column.Another advantage is that the resolutionof closelyeluting compoundswill not
be affectedwhen the column is trimmed duringmaintenance, because the guard col-
umndoes not contribute to the resolvingpower of the analytical column.Using guard
columns is a simple, cost-effectiveway to extend analytical column lifetime.
In summary, the retentiongapandguard columnare essentially the sameproducts,but
areused fordifferentpurposes.Thedeactivated tubinghelps focus target analytes at the
headof the analytical column for on-column and splitless injections, and alsoprevents
nonvolatilematerial from contaminating theheadof the analytical column.
What typeofguardcolumnshouldbeused?
Whenusingaguardcolumn, it is important tomatch thepolarityof the solvent and the
polarity of the surface deactivation. Rxi® Guard tubing is good for a wide variety of
applications andallowsmost common solvents (methylenechloride,hexane, isooctane,
toluene) to easilywet and create auniform filmon the tubing surface.
If more polar solvents such as methanol or water are used, a polar-deactivated guard
column is recommended toallow the solvent towet the tubing surface.However,polar-
deactivatedguardcolumns arenot resistant toharsh“watervaporization”,whichoccurs
whenwater in the liquid state is injected into the tubing and rapidly vaporizes (such as
in steam cleaning).Hydroguard®deactivation is analternative fordirect aqueous injec-
tions.However, aHydroguard®-deactivatedguard columnwill not allowpolar solvents
towet the tubing surface,andmaycause solventbeading if theoven temperature is20°C
below the solvent boiling point. Siltek® deactivation creates a highly inert surface for
very active compounds such as chlorinated and organophosphorus pesticides. Base-
deactivated guard columns reduce adsorption and tailing for amines and other basic
compounds.
How isaguardcolumnconnected to theanalytical column?
To connect the guard column to the analytical column,Vu2-Union®,Press-Tight®, and
other connectors are available. MXT® unions, typically used for connecting metal
columns together, are now available for fused silica columns. See pages 287 to 292 for
information about these connectors.
it’s a
fact
Toeliminateconnections thatmay leakand
toensure longer column lifetime, useour
unique Integra-Guard®Column. See
page35
.
Vu2Union®Connector
(Seepage289.)
Press-Tight®Connectors
(Seepages 287-288.)
MXT®UnionConnector Kit
for FusedSilica
(Seepage292.)
Connectors forFusedSilicaColumns
Website :
E-mail :
TelNo : 03 9762 2034 . . . inAUSTRALIA
Mar 2011
