User`s guide

Troubleshooting
373
or trap into the gas lines of the system. For sample tubes, this
involves routinely ensuring that the retaining gauze disks for the
packing are in place, and not using a mesh adsorbent that is too fine.
Use 60-80 mesh minimum size. 80-100 mesh is too fine.
In the case of solid, resinous or liquid samples, using a small
adsorbent bed inside the tube is highly desirable to prevent high-
boiling components of the sample (e.g., resins) from getting into the
ATD/TD plumbing. Contamination of the ATD/TD at high
concentration levels can be very time-consuming to remove. This
will ensure that none of the matrix material or other high-boiling
compounds migrate into the gas lines of the system. Select an
adsorbent suitable for your application. A PTFE sample tube insert is
also recommended. See Direct Sampling for Thermal Desorption on
page 336.
If system contamination is a problem, you must first eliminate the
carrier gas, sample tube, trap and filter disks as the source of
contamination. If conditioning doesn't help, the trap, connecting
tubing/connectors and the transfer line may have to be dismantled
and cleaned or replaced. Contact your PerkinElmer service engineer
for help.
Poor Recovery or Loss of Sample
Cause: Excessive desorption temperature degrades the
sample.
Action: Recovery of thermally unstable components
may be improved by increasing the desorption
time and gas flow while reducing the desorption
and flow-path temperatures.
Action: During the desorption of extremely labile
components, glass wool alone should be used as
trap packing where possible. A lower trap
heating rate (~5 deg C/sec) can also be used
during the desorption of labile components.
Cause: Incomplete primary or secondary desorption.
Action: Ensure that the gas flow rate recommendations
are being adhered to and that there is sufficient
flow through the sample tube, trap and
analytical column at all stages of the analysis.