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.

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.

Theory of Operation

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.

Purge & Trap Technique

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.

Direct Thermal Extraction Technique

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 Tenax�, 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).

Thermal Desorption Electronics Console

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

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