10
✶
TransferLine
After the headspace sample iswithdrawn from the vial, it ready to be transferred to
theGC. In balanced-pressure and pressure-loop systems a short piece of tubing
called a transfer line is used to transfer the sample from the autosampler to theGC.
Transfer linematerial must be chosen that suits the sample analytes.Many different
materials can be used as transfer line tubing, including stainless steel, nickel, fused
silica, and Silcosteel
®
- or Siltek
™
-coated tubing. Stainless steel provides a strong,
flexible tubingmaterial, but can be adsorptive towardsmore active analytes such as
alcohols, diols, and amines. Nickel and Silcosteel
®
tubing are highly inert towards
active compounds and provide ruggedness similar to stainless steel. Fused silica and
Siltek
™
tubing are extremely inert
towards active compounds, however
they are not as rugged as nickel or
Silcosteel
®
tubing.
The internal diameter of the
transfer line should be chosen
depending on the internal
diameter of the analytical
column, the column flow rate,
and the flow rate delivered
from the autosampler. To elimi-
nate tubing dead volume, use the smallest diameter tubing possible. For
example, compound residence time in a 1.0mm ID transfer line is 3.6 times greater
than in the same length of 0.53mm ID tubing. Reducing the residence time of the
headspace sample in the transfer line helps tominimize band broadening. Therefore,
the flow rate should be set as high as possible to quicklymove the sample cloud
through the tubing andminimize any dead volume effects.
Transfer line temperature should be set depending on the analytes of interest and the
samplematrix. Typical transfer line temperatures range from 80°C to 125
°
C. To
minimizematrix problems and prevent water condensation from aqueous samples,
use a higher transfer line temperature (~125°C to 150
°
C).
InjectionPort Interface
The quality of the connection of the transfer line to the analytical column greatly
affects sample bandwidth. Inmost cases, the transfer line has a smaller internal
diameter than the injection port liner, and the vaporized headspace sample carrying
the compounds of interest will be diluted into a larger volume of carrier gaswhen
the sample elutes from the transfer line into the inlet liner. This can lead to broader
peaks, tailing peaks, lower sensitivity, and loss of resolution. Because headspace
samples are already in a gaseous state (vapor cloud)when they enter the injection
port, there is no need to use a large buffer volume in the liner to allow for sample
expansion aswhen analyzing liquid samples. Using injection port liners that have
smaller internal diameters and lower buffer volumeswill helpmaintain a narrow
bandwidth as samplesmove from the end of the transfer line to the head of the
analytical column. 1.0mm ID deactivated injection port liners are recommended for
most headspace applications to achieve the lowest injection port dead volume.
If band-broadening due to excess dead volume in the system is still a problem, peak
shapemay be improved by refocusing sample analytes at the analytical column head.
Highly volatile compounds can be trapped at the column head and refocused into a
narrow bandwidth by reducing the initial oven temperature below the boiling point of
compounds of interest.After the sample is completely transferred to the column, the
oven temperature can be increased tomove the compounds through the column.
Use an inert transfer
line when optimizing
pressure-loop systems.
Restek’s technical service is here
to help. If you still have questions
after reviewing this guide,
please call us at 800-356-1688
or 814 353-1300, ext. 4, or call
your local Restek representative.
