The design of the sorbent tubes used for sample collection and thermal desorption in Dynatherm instrumentation is a major factor contributing to the overall performance of the equipment.

Trapping Requirements           

• retain desired compounds during sample collection while passing carrier gas, air, and water.
  
  
• desorb quickly and completely during the thermal desorption cycle.
  
  
• exhibit low background within a reasonable temperature limit, 250º to 350ºC.

Sorbent Layers
The sorbent tubes sold by CDS are generally packed with layers of various sorbent materials, so that a broad range of volatile and semi-volatile compounds, polar and non-polar, may be trapped on an appropriate sorbent. The more volatile compounds break-through the initial layers of sorbents, but are trapped by succeeding layers. Each sorbent layer protects the next increasingly active layer, preventing a compound from being held so tenaciously that it cannot be desorbed quickly and completely during the heat cycle without degradation.

Direction of Flow During Sampling
An example of a good combination of adsorbent materials is: Tenax-TA and Carboxen 1000. These are packed in the sorbent tube in the order described above, with the Tenax layer adjacent to the glass frit fused to the tube. It is important that the flow during sample collection enters the sorbent tube at the least active layer of sorbent material (next to the glass frit) and exit through the most tenacious layer. The higher molecular weight compounds will be adsorbed on the Tenax layer, while the lower boiling components will pass through the Tenax layer and stop on the Carboxen layer. Each layer of adsorbent material protects succeeding, more active layers.

Direction of Flow During Desorption
During thermal desorption, carrier gas passes through the tube in the reverse direction of sample collection flow. Since the collected sample is then backed off of  the adsorbent material, the higher boiling compounds do not come in contact with the more tenacious sorbents as they elute. Consequently, when the tube is heated to desorb the sample, the heat energy required for volatilization of the components is kept to a minimum. Each molecular weight range is trapped on an appropriate sorbent material from which it is easily released. Nothing is held so tenaciously that it decomposes during desorption.

Sequential Trapping
Trapping material selections are based on both collection efficiency, the ability to retain a particular class of analytes, and on desorption efficiency, the ability to quickly release trapped compounds. In Dynatherm thermal desorption instruments, two sorbent traps operate in series: the first is the sorbent (sample) tube, 4 or 8mm I.D. high capacity tubes designed to retain a desired range of compounds from large sampling volumes. The second is the focusing trap, 0.9 to 1.5mm I.D. devised to inject the collected sample onto a capillary column in a narrow-band plug. Transferring the analytes to the trap improves the efficiency of the injection, since liters of flow may have been required for sample collection. The low internal volume of the focusing trap permits efficient desorption at typical capillary column flow rates, 1 to 3cc/minute.

Material Selection
Materials chosen for sorbent tubes may differ slightly from those used in a trap. In general, materials with optimum retention characteristics for the analytes to be collected are chosen for the sorbent tube, while materials in the focusing trap are selected based on their desorption efficiency. For example, silica gel would not be a good choice for sampling in a saturated atmosphere due to its water retention characteristic, but it would be an excellent material in a focusing trap, providing good desorption efficiency for low-boiling polar compounds, since the water will be purged off the collection tube before the sample is transferred to the focusing trap.

The Keys to Performance
Sequential trapping and the dimensional relationship of the sorbent tubes, the special sorbent materials used, and the rapid heating rate integral to all Dynatherm thermal desorbers combine to provide high efficiency trapping and capillary inletting, eliminating the need for external cooling or cryogenic focusing devices.