User`s guide

Troubleshooting
370
matrix is not escaping from the tube or trap and
migrating into the rest of the system. Use glass
wool plugs in the tubes and trap, ensure that the
filter disks at both ends of the trap are installed
correctly. The trap packing material must be
packed sufficiently well so that it does not shift
back and forth when the flow direction changes.
Cause: Contaminated filters
Action: This is probably the most frequent cause of
“carryover”. During desorption, some of the
sample is split into the exit filters on the inlet,
outlet and desorb vents. Even if these vents are
closed, the volume of the filter is sufficient to
take up some volatiles. Try replacing and or
cleaning the filters. Maintain a regular
replacement schedule. Periodically open the
splitters to ensure some positive flow through
them, especially when high level analyses are
being performed.
System Contamination
System contamination can be avoided with proper care and
maintenance of you instrument. Contamination can also be avoided
by ensuring that your method is suitable for he compounds that you
are analyzing.
When you are troubleshooting the ATD/TD/GC system, you should
first eliminate the GC as the source of the contamination. Disconnect
the ATD or TD from the GC. If the baseline signal drops to the
normal detector background, the ATD or TD is most likely the
source of contamination.
NOTE: Some polymeric adsorbants such as Tenax will always
produce some peaks.
Cause: The carrier gas supply is a very common source of
contamination in thermal desorption.
Action: Contaminants can be introduced through the
gas itself, cylinder regulators, gas lines or
carrier gas filters. To establish whether or not