User`s guide

Method Development
227
The separate tube technique is also useful if sorbents of totally
different substrates are used, since each tube can be conditioned
individually under optimum conditions.
Selecting Packing Material for
the Cold Trap
Since the cold trap is maintained at sub-ambient temperatures during
the primary (tube) desorption process and because the trap packing
is only required to retain the components for a few minutes, it is
usually possible to select a weaker adsorbent for the trap than that
required in the sampling tube.
You want to minimize the mass of packing selected, typically to
20-25 mg, in order to reduce band spreading during the secondary
(trap) desorption process.
The ATD and TD are shipped from the factory with the trap
configured in the backflush configuration. Analyte vapor is
transferred into and out of the cold trap through the same opening at
the back end of the cold trap area. The direction of carrier gas flow
during secondary desorption is the opposite of the direction of flow
during primary desorption, when analytes are refocused on the cold
trap. This allows the use of multiple adsorbents in the trap if desired.
The standard trap packing is Tenax (TA).
Tenax TA is a microporous polymer based on 2,6-diphenyl-p-
phenylene oxide and has largely replaced Tenax GC for adsorption
tubes and GC columns. It has excellent thermal stability up to 350°C
and a surface area of 35-40 m
2
g
-1
and pore size of approximately
200 nm. Tenax TA has lower retentive properties than Porapak Q or
Chromasorb 101, 102 and 106 with a low affinity for water and so is
suitable for packing adsorbent tubes. Tenax GR is a graphitized form
of Tenax containing up to 30% graphitized carbon incorporated in
the co-polymer during the polymerization process. GR has less
affinity for water than TA. It also has higher breakthrough volumes
and produces highly symmetrical GC peaks and high column
efficiencies making it more suitable than TA for adsorption tubes.
1
1. A. Braithwaite and F.J. Smith, Chromatographic Methods, Fifth
Edition, (Glasgow, 1996), p 205