System Description
Short Path Thermal Desorption Model TD-5
A new PC-controlled Short Path Thermal
Desorption System for the analysis of volatiles and semi-volatiles
by GC and GC/MS systems.
Figure: Diagram of TD-5 components and how they connect
to the GC. The Thermal Desorption Unit sits on top of the GC injection
port and is powered by the Electronics Controller, which communicates
with Control Software on the PC computer. An optional Cryo-Trap
device cryo-focuses analytes at the top of the GC column.
Main Features:
- High sensitivity Thermal Desorption and Direct Thermal
Extraction
- No memory effects - individual flow path for each sample
preventing contamination of transfer lines.
- Short Path from sample to GC
- Eliminate tedious sample cleanup by other techniques such
as solvent extraction.
- Mounts overtop GC injection port, easily removable and
transferable
- Controlled by Windows-based software on the PC
- Automatic injection of sample into GC via pneumatic
driven injector
- Desorption samples at temperatures from 20° to 400°C
either isothermal or temperature program at up to
100 C/min.
- Program desorption time from 1 second to 100 minutes
- Automatic programming and control of GC Cryo-Trap
Accessory
- Glass lined stainless steel (GLT) sample tubes are both
inert to samples and strong for sample handling and
transportation.
Description of the Model TD-5 Short Path Thermal Desorption System
A Complete Description of the Theory of Operation is presented
in Technical Bulletin
No. 1.
The new TD-5 Short Path Thermal Desorption System
consists of a Thermal Desorption Unit, Electronics Control Unit,
Control Software, and optional Cryo-Trap (see figure above). The
Thermal Desorption Unit sits directly on top of the GC injection port
of most GC's, where it is utilized for the direct desorption of both
volatile and semi-volatile samples into the GC injection port and
column. Due to its "short path" of sample flow, the TD-5
overcomes the shortcomings of previous desorption systems by
eliminating transfer lines (which can be easily contaminated by samples)
and by providing for the optimum delivery (and therefore maximum
sensitivity) of samples to the GC injector via the shortest path
possible, i.e. direct injection into the GC, like using a GC injection
syringe.
When ready to be analyzed, a syringe needle is attached to the
desorption tube which is then attached to the connector tube on
the Autoinjector Assembly of the Desorption Unit.
Purging: The carrier gas
through the Desorption Unit is turned on via the electronics
console microprocessor and the flow through the desorption tube
is adjusted via the flow controller, rotameter and/or the
pressure transducer between 1.0 ml/min and 120 ml/min.
Injecting: The desorption tube is injected with the needle
attached. The desorption tube will pass through the opening in the
middle plate of the Desorption Unit base to position the desorption
tube in proper alignment with the GC injection port and the normally
open desorption block assembly.
When injection is complete, the flows are
readjusted as required by the method of analysis, i.e.
split/splitless, etc. In this position the sample is not being
desorbed into the GC since the heating blocks have not yet closed
around the desorption tube. The temperature of the tube remains
close to room temperature due to the action of the cooling fan,
which pulls in room air from the front of the Desorption Unit,
thru the view port and across the desorption tube. Carrier gas
flows, desorption temperatures, and GC parameters can be adjusted
as required.
Blocks close and desorption: The Microprocessor Control
actuates an air valve which delivers air to the air powered solenoid
and moves the hinged heating blocks from the open to the closed
position around the desorption tube. The desorption tube will
ballistically heat up to the set temperature or the temperature
program program ramp for the heater blocked will begin. The
combination of the heat applied and the carrier gas flow through the
Desorption Tube will purge the desired components into the GC
injection port and onto the front of the GC column.
The various parameters are set and utilized according to the
application requirements. Normally desorption temperatures
between 70 degrees C and 250 degrees C are suitable for most
applications. The maximum desorption temperature permissible with
the system is 400 degrees C.
The heater blocks can be temperature programmed at ramp rates
up to 100 degrees/min. Normal desorption times vary from 3 minutes
to 15 minutes, however longer desorption times up to 100 minutes
are permitted. Since the column is normally maintained at
subambient temperatures, the desorbed compounds of interest are
trapped on the front of the GC column in a narrow band utilizing
GC oven cooling or the GC Cryo-Trap Accessory. Despite the long
desorption times, the peaks eluted from the column are extremely
sharp and well resolved.
Desorption complete, cryo-heat, GC start:
After the desorption is complete, the desorption heater blocks are
opened, the normal carrier gas flow to the GC injection port is turned
on, and the GC, Mass Spec, GC Cryo-Trap Accessory is switched to the
heating mode and recorders are started via accessory ports on the back
of the Electronics Console cabinet which act as remote out
switches.
The temperature and time parameters are configured
in the software.
Samples to be analyzed are collected on
Glass Lined Stainless Steel (GLT) Desorption Tubes containing an
adsorbent resin such as Tenax® TA or activated carbon.
Alternatively, samples of small size (mg) can be packed into the
tube and subjected to direct thermal extraction.
When ready for analysis the GLT desorption
tubes are fitted with a syringe needle and attached to the
Desorption Unit. This permits the trapped samples to be heated by
the Desorption Tube Heater Blocks, desorbed from the adsorbent
resin and injected directly into the injection port of a Gas
Chromatograph, GC/MS or GC/FTIR via the shortest path possible,
i.e direct injection into the GC much like a syringe. The GC
column, either capillary or packed, is normally maintained at
subambient temperatures (or at a low enough temperature to retain
any samples at the front of the GC column) during the initial
desorbing of the sample into the GC. Alternately the TD-5 permits
the control (both heating and cooling) of the GC - Cryo-Trap for
the trapping of volatiles at the front of the GC column inside
the GC cover. This enables the desired components to be collected
in a narrow band on the front of the GC column over a long period
of time (5 to 15 minutes). As an alternative to cryofocusing, the
same effect can be achieved by using a thick film capillary
column or a packed column with a high loading capacity. These are
available at numerous chromatography houses. When the sample has
been fully desorbed into the GC column, temperature programming
is commenced to volatilize the organics and to elute and separate
them into the desired components.
The Short Path Thermal Desorption System
provides several unique advantages over other desorption systems:
- It enables the sample, which is
trapped on an adsorbent media contained in a glass lined
stainless steel tube (GLT desorption tube), to be subjected to
rapid heating.
- The desorbed component can be
easily and efficiently transferred into the injection port of the
gas chromatograph from a glass lined stainless steel sample tube
and its associated injection needle. This provides for a short
transfer path for the sample in an inert environment to minimize
the degradation of labile sample components which often decompose
upon contact with the hot catalytic metal wall surfaces of the
transfer path of other systems.
- Third, each sample has its own individual
adsorbent trap tube and needle to eliminate the possibility of
cross-contamination from sample to sample, thus preventing any
"memory effect" due to overloading of the sample in the
GLT desorption tube or due to residues from previous samples.
The technique of Short Path Thermal
Desorption has been developed to permit the analysis of organic
compounds present in air or compounds which can be easily purged
from solid and liquid samples. Samples such as volatile and
semi-volatile organics in air, flavors and fragrances in foods
and cosmetics, manufacturing chemical residues in
pharmaceuticals, volatiles in packaging materials and building
products, and aromatic residues in forensic arson samples are
just a few of the applications for which this technique has been
applied. A few specific applications are the detection of benzene
and chlorinated hydrocarbons in food and other manufactured
products, the identification of natural occurring insect
repellents in plants, the identification of flavors in black
pepper and other spices, and the identification of volatile
contaminants in commercial shipping containers. The technique
eliminates the need for solvent extractions in many analysis. See
applications pages.
An alternate method of analysis using the
Short Path Thermal Desorption System is called Direct Thermal
Extraction. This technique permits the analysis of low moisture
content samples which have been placed directly in the GLT
desorption tubes. Samples such as spices, paint chips, packaging
films, pharmaceuticals, plastics, pine needles, and fibers can be
analyzed directly using this technique. Water vapor must be
minimized since it will condense and plug the GC column. See
application pages for examples of applications. Alternatively,
adsorbent materials such as Tenaxª, Carbowax¨, Porapak¨ or
Carbotrap¨ can be used for high moisture containing samples
using the Thermal Desorption Technique.
 The Direct Thermal
Extraction technique permits the direct thermal extraction of
volatiles from solid matrix samples into the GC injection port.
Control Software
The TD-5 is operated by control software residing on a PC computer.
This software enables the user to configure run parameters
(e.g. set-point times and temperatures), initiate runs, and monitor
run status (e.g. actual temperatures and pressures).
The PC software permits
automated operation of the desorption cycle including
purging, injecting, desorbing, and remote starting
of the GC, as well as control of the GC cryo-trap (cold-trap)
accessory.
Figure: Thermal desorption control software showing the
configuration of parameters for a thermal desorption method.
Here, times and temperature set-points are specified.
These are displayed graphically on the right side.
Figure: Thermal desorption control software showing a
run in progress. Shown are the time sequence of the desorption process and
the temperature program (Method parameters), the history of previous
samples run (Sample Log Book), and the status of the current run (times,
temperatures, pressures, and animated graphic of the desorption system).
The Electronics Console consists of a user
adjustable microprocessor electronics control which is interfaced
via a 20 pushbutton keypad with a yellow fluorescent display to
control the operation of the desorption system. The Main Power
Switch controls the power to the entire Desorption System. The
Heater Switch turns on the power to the heater cartridges in the
desorption tube heater blocks and begins their heating cycle. The
Cryo-Trap heater switch controls the power to the circuit for the
GC Cryo-Trap Accessory Unit. A Platinum Resistance Thermometer
(PRT) in the heater blocks provides for the accurate (+/- 0.1%)
temperature readout and also provides for feedback to the
temperature controller to maintain the heater block temperatures.
A single cable assembly provides the
connection between the Electronics Console and the Desorption
Unit. The cable has connectors on each end which screw into the
electrical feedthru fittings located on each of the two
assemblies. The cable can be used reversibly since the connectors
on both ends are identical.
The TD-5 Controller
automatically controls both the cooling and heating of the GC
Cryo-Trap accessoty.
See Ordering Information
for the Model TD-5 Short Path Thermal Desorption System
Tenax® is a registered trademark of Buchem BV.
© 1996-2008 Scientific Instrument Services, Inc.
[Trademarks]
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