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GC COLUMNS |
CAPILLARY COLUMNS
Guard/Retent ion Gap Columns
Protecting the Analytical Column
The concept of a guard column is to protect the analytical column from becoming
contaminated with nonvolatile compounds. The guard column is used to retain non-
volatile material, usually in the first 10-20 cm, not allowing this material to elute onto
the liquid phase of the analytical column. As the oven temperature increases, the more
volatile target compounds vaporize, elute down the guard column, and refocus at the
head of the analytical column without interference from the nonvolatile material left
behind.
Using guard columns is advantageous, because they prevent contamination that can
cause active sites as well as change the conditions of the focusing zone of the analytical
column. Another advantage is that the resolution of closely eluting compounds will not
be affected when the column is trimmed during maintenance, because the guard col-
umn does not contribute to the resolving power of the analytical column. Using guard
columns is a simple, cost-effective way to extend analytical column lifetime.
In summary, the retention gap and guard column are essentially the same products, but
are used for different purposes. The deactivated tubing helps focus target analytes at the
head of the analytical column for on-column and splitless injections, and also prevents
nonvolatile material from contaminating the head of the analytical column.
What type of guard column should be used?
When using a guard column, it is important to match the polarity of the solvent and the
polarity of the surface deactivation. Rxi® Guard tubing is good for a wide variety of
applications and allows most common solvents (methylene chloride, hexane, isooctane,
toluene) to easily wet and create a uniform film on the tubing surface.
If more polar solvents such as methanol or water are used, a polar-deactivated guard
column is recommended to allow the solvent to wet the tubing surface. However, polar-
deactivated guard columns are not resistant to harsh “water vaporization”, which occurs
when water in the liquid state is injected into the tubing and rapidly vaporizes (such as
in steam cleaning). Hydroguard® deactivation is an alternative for direct aqueous injec-
tions. However, a Hydroguard®-deactivated guard column will not allow polar solvents
to wet the tubing surface, and may cause solvent beading if the oven temperature is 20°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 is a guard column connected to the analytical 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
To eliminate connections that may leak and
to ensure longer column lifetime, use our
unique Integra-Guard® Column. See
page 35
.
Vu2 Union® Connector
(See page 289.)
Press-Tight® Connectors
(See pages 287-288.)
MXT® Union Connector Kit
for Fused Silica
(See page 292.)
Connectors for Fused Silica Columns
Website :
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TelNo : 03 9762 2034 . . . in AUSTRALIA
Mar 2011
