User`s guide

Method Development
221
Table 13 Guide to Adsorbent Selection for Thermal Desorption
NOTE: Always store the adsorbents in a clean environment, away
from organic solvents, and always condition them before
use. Adsorbents will trap and preconcentrate organic
volatiles from the atmosphere. Even trace level compounds
escaping from closed solvent bottles will yield interference.
Microporous co-polymers such as Tenax TA and GR, Chromosorb
102 and 106 and carbon molecular sieves are used as adsorbents for
sampling volatile organic compounds. The co-polymers have
differing adsorption properties and breakthrough volumes and
therefore are used to sample compounds of different molecular
weight ranges.
Tenax is a weaker adsorbent than Chromosorb 102 and 106 while the
molecular sieves have the highest adsorbent properties. The co-
polymers are hydrophobic, so they are suitable for air sampling
without retention of water vapor. As a general guide Tenax is most
suitable for substances with a boiling point range of 100-250°C,
Chromosorb 106 for 30-150°C and carbon molecular sieves for -30
to 80°C
Tenax TA and Tenax GR are a recently introduced porous polymeric
material originally developed as a GCS column packing. It is
hydrophobic and is excellent for adsorbing volatiles from the
atmosphere at room temperature yet is stable enough at high
Molecular
Sieve**
-60°C to + 80°C 350 Used specifically for 1, 3-
butadiene and nitrous
oxide.
Charcoal* -80°C to + 50°C 400 >1000 Rarely used for thermal
desorption because metal
content may catalyze to
analyte degradation. Use,
with care, for ultra-
volatile C
2
, C
3
, C
4
hydrocarbons.
* These sorbents exhibit some water retention. Safe sampling volumes should be reduced by a factor of
10 if sampling at high (> 90%) relative humidity.
**
Significantly hydrophilic. Do not use in high humidity unless silicone membrane caps can be fitted
for diffusive monitoring purposes.