Optimizing the Analysis of Volatile Organic Compounds

20
Universal Press-Tight ®
Connectors
Description cat.#
Universal Press-Tight ® Connectors, 5-pk. 20400
Universal Press-Tight ® Connectors, 25-pk. 20401
Universal Press-Tight ® Connectors, 100-pk.
Universal Angled “Y” Press-Tight
20402
®
Connector, ea.
Universal Angled “Y” Press-Tight
20403
®
Connectors, 3-pk. 20404
For additional connectors, see page 64.
www.restekcorp.com
Direct Connection
A union between the purge and trap transfer line and the capillary column bypasses the
injection port, eliminating the problems associated with the injection port: loss of sample
through the septum purge, adsorption of active compounds, bleed from Viton ® O-rings or
septum, and — most important — dead volume. Two direct connections are described
below. The disadvantage of a direct connection is it eliminates the ability to make manual
injections when attempting to isolate a chromatographic problem. Therefore, this connection
technique works best for experienced analysts and for instruments that undergo regular
maintenance.
Metal Transfer Line: This is the easier of the two direct connection methods. Using the
transfer line provided by the instrument manufacturer and an MXT ® low dead volume connector
(cat.# 20394, see our catalog), connect the trap to the capillary column. This configuration
significantly improves peak shape, compared to injection port connections, especially
with an electrolytic conductivity detector (ELCD).
Fused Silica Transfer Line: A fused silica transfer line further reduces dead volume, relative
to the original equipment line. We recommend using Siltek ® fused silica tubing for VOC
or other sensitive analyses because it is not affected by moisture and is inert to active compounds.
To configure the line, disconnect the metal transfer line from the Valco ® six-port
valve, then remove the metal ferrule and 1/16" nut by cutting the end of the tubing. While
wearing insulated gloves, heat the line to 200°C to melt the glue that holds the line in place,
then use pliers to pull the line out of the heater jacket. Cool the line, install a piece of metal
tubing (cat. #21503, see our catalog) inside the line, then install the Siltek ® -treated fused silica
transfer line within the metal tubing (cat. #10027, page 63). The metal tubing will prevent
the transfer line from being scratched or broken. Base the ID of the metal tubing on the OD
of the transfer line: 0.02" ID for a 0.25 or 0.32mm ID fused silica line, 0.30" ID for a 0.45
or 0.53mm ID line. In turn, base the ID of the transfer line on the ID of the analytical column;
we recommend using a transfer line with an ID equal to or slightly smaller than that of
the column. A transfer line with an ID slightly smaller than that of the column will increase
backpressure, enhancing the resolution of early-eluting compounds. Use a Press-Tight ® connector
(cat. #20400 or 20403, page 20) to connect the fused silica transfer line to the analytical
column (Figure 17). Use the correct ferrule for connecting the column to the 6-port valve
(Figure 18); we recommend a one-piece fused silica adaptor (cat. #20137, page 64).
Figure 17.
A dual-column configuration splits the sample equally between separate detector systems.
Angled “Y”
Press-Tight ®
Connector
Figure 18.
Connect the fused silica line directly to the 6-port valve. Notice a small (5mm)
septum helps determine how far the column is inserted into the valve, preventing breakage
at the column end that could allow shards of fused silica to enter the valve.
MXT ® Metal Tubing
(used to protect fused
silica column)
Fused Silica
Septum