C3
Short Path
TM
Thermal Desorption TD-5
Scientific Instrument Services, Inc.
TM
FAX: 908-806-6631
www.sisweb.com/catalog/08/C3
Theory of Operation
T
he Thermal Desorption Unit sits directly on top of the
injection port of most GC's (Fig.2), where it provides
direct desorption of both volatile and semi-volatile samples
into the GC injection port and column. The system delivers samples
into the GC along an optimal "short path," thereby eliminating trans-
fer lines and cross-sample contamination issues found in previous
systems.
Samples to be analyzed are collected into stainless steel
desorption tubes (either glass lined GLT or silco-treated)(Fig.1). A
solid sample of interest may be packed directly into the tube and
subjected to direct thermal desorption. Alternately, the tube may be
packed with adsorbent resin such as Tenax
TM
TA or activated carbon
for indirectly trapping analytes from liquid or air samples.
To analyze a prepared desorption tube (Fig.4), a syringe nee-
dle is first attached to the desorption tube, which is then attached to
the connector tube on the Desorption Unit (#1). The desorption
blocks heat to initial temperature (#2). Carrier gas is sent through
the desorption tube for an initial purge time (#3). The system then
injects the desorption tube into the GC injection port (#4) where flows
are readjusted as required by the method of analysis, i.e. split/split-
less, etc. The hinged heating blocks close around the desorption
tube (#5) to ballistically heat the sample tube, optionally with a tem-
perature ramp. The combination of the heat applied and the carrier
gas flow through the desorption tube will purge the desired compo-
nents into the GC injection port and onto the front of the GC column.
To obtain sharp chromatographic peaks when desorption typically
lasts 5 to 15 minutes, it can be desirable to focus desorbed compo-
nents at the head of the GC column. This focusing can be improved
by installing a Cryo-Trap, which cryogenically cools the head of the
GC column (with liquid CO
2
or LN
2
) during desorption to trap des-
orbed analytes. After desorption, the Cryo-Trap rapidly heats to
volatilize the trapped analytes and quickly release them through the
GC column for separation (#6).
Purge
Gas Off
Desorb
Blocks Off
Cryo Off
0
Purge
Gas Off
Desorb Blocks
Heating
Desorb Blocks
Heating
Cryo Cooling
Cryo Cooling
Cryo Cooling
Purge
Gas On
Purge
Gas On
Desorb Blocks
Heating
Cryo Cooling
Purge
Gas On
Desorb Blocks
Heating
Cryo Heating
Purge
Gas Off
Desorb
Blocks Off
1
2
Detachable
Needle
Quartz Wool
Quartz Wool
Screw Threads
Solid Sample or
Adsorbent Resin
Figure 1 - Desorption (sample) tube
Figure 2 - Desorption Unit mounted on an Agilent 6890 GC
Figure 3 - Schematic of TD-5 System Connected to GC
Sample Loaded
Temperatures ON
Purge Gas ON
Injecting
Desorbing Sample
GC Analysis
Figure 4 - TD-5 Run Cycle
X
X
3
4
5
6
X
tip...
