User`s guide

Method Development
206
The process of developing thermal desorption methods can be
greatly simplified by keeping the principles given below in mind
when parameters are initially selected.
Optimizing Primary (Tube) Desorption
To optimize the primary (tube) desorption, the analysis parameters
should be set following these general guidelines:
• The sample tube temperature needs to be set as high as possible.
The desorption temperature is limited by the adsorbent/sample
matrix stability, the lability of the components of interest and the
temperature limitations of the system.
• The carrier gas flow through the sample tube (desorb flow +
inlet split flow) must also be as large a total volume as possible.
NOTE: If desorb flows in excess of 120 mL/min are used with
helium or hydrogen as the carrier gas, the cold trap may
not be able to maintain its low temperature during primary
desorption.
• The desorb time should be kept to a minimum in order to
maximize sample throughput.
• The packing material in the sample tube, must be weak enough
to allow 100% desorption of the components of interest within
the temperature and flow range of the TD. The sample tube is
backflushed during desorption, and therefore the mass of
packing is not normally critical.
• The cold trap temperature must be set as low as possible during
primary desorption.
• The carrier gas flow through the cold trap (desorb flow) must be
as low as possible.
• The cold trap packing (adsorbent) must be strong enough to
retain all of the components of interest during tube desorption
with the selected temperature and flow conditions. The mass of
packing must be sufficient to retain the components of interest
and should be at least 10 mg. The upper limit is not as critical
provided that the packing stays within the 2 markers.