Preparation and Conditioning of Desorption Tubes and Resin Beds

Packing Desorption Tubes

Before packing desorption tubes, be sure they are clean and free of any residue or obstructions inside the tubes. If necessary clean with a small metal brush (S.I.S. part # SMS04016003 or SMS04016004) to remove any stubborn residues inside the desorption tubes. Wash thoroughly and rinse in water and then bake dry in an oven.

Normally silation of the tubes is not required. However if required they can be silated using Pierce silation reagent (Part No. 83410 - Pierce Chemical Company - Phone: 815-968-0747). After silating wash thoroughly with solvents and dry in an oven at 100 degrees for at least 1 hour.

Figure 1 - Preparation of Thermal Desorption Tube

Insert a small silanized glass wool plug in one end of the desorption tube. Silanized Glass Wool is available from Supelco (Part # 2-0411). Weigh into the desorption the required amount of adsorbent resin. A small funnel will aid in inserting the adsorbent resin (S.I.S. Part No. 781122). Then insert another glass wool plug into the other end of the desorption tube to hold the resin in place. Normally resin beds between 50 and 200 milligram are used. These is no need to use larger beds since these will just trap more water and contribute more background to the chromatogram. If larger resin beds are required due the low breakthrough volumes for a particular analyte, then either select another resin with higher breakthrough volumes or consider the use of a mixed bed adsorbent resin tube.

Conditioning of Resin Packed Thermal Desorption Tubes

Before they can be used the desorption tubes must be conditioned. Conditioning involves the continuous flow of a high purity carrier gas (either nitrogen or helium) while the desorption tubes are heated at elevated temperatures. Only high purity gases should be used containing less than 1 ppm of oxygen. Oxygen can be detrimental to the adsorbent resin especially the Tenax® Resins.

Attach the carrier gas line to the desorption tube containing the adsorbent resin packed inside. Turn on the high purity gas to a flow rate of 10 to 50 ml/min and allow to purge for at least 10 minutes to remove all traces of oxygen from the adsorbent resin inside the desorption tube.

Figure 2 - Conditioning a Thermal Desorption Tube

Begin to heat the desorption tube while continuing to purge gas through the adsorbent resin inside. We recommend a temperature program of 4 to 10 degrees per minute from room temperature up to the maximum temperature required for conditioning. After the desorption tube reaches the maximum temperature, continue the conditioning process for 2 to 4 hours. Temperature should be maintained at the upper limit during this time phase and continuous flow must be maintained through the desorption tube and resin at all times.

The selection of the upper temperature limit is a function of both the adsorbent resin and the temperatures at which it will be thermally desorbed when the samples are analyzed. In no case should the conditioning temperature exceed the maximum rated temperature for the adsorbent resin. If possible the desorption tube should be conditioned 25 to 50 degrees higher than the temperature at which it will be desorbed. The higher the temperatures at which you condition, the lower the life of the resin. For example Tenax TA has a maximum temperature of 350 degrees C. However in our applications we typically desorb our samples at temperatures between 200 and 250 degrees C. Therefore we never condition our desorption tubes over 300 degrees C. This gives a good safety margin for the tubes. We use the same desorption tubes over possibly a hundred times within a 6 month time period. After 6 months, we remove the Tenax resin bed and replace it with a new resin bed. We routinely replace the adsorbent resin every 6 months, not because the resin has gone bad, but just as a precaution and to assure good clean resins with minimal background.

After the heating cycle time is complete, the desorption tubes should be removed from the oven and cooled to room temperature. Gas flow must be maintained through the adsorbent resin during this cooling cycle. The cooling should normally take 5 to 10 minutes. Do not purge with carrier gas any longer than necessary, since any volatiles in the carrier gas (even though present in less than ppb levels) will be trapped on the resin which is now at room temperature After the cooling is complete, turn off the carrier gas and seal or cap the desorption tubes until ready for use.

The contribution of adsorbent resins to the GC chromatogram background should be minimal. Many users have incorrectly blamed adsorbent resins for the noise GC chromatogram backgrounds. Our studies have proven that most of the noisy GC backgrounds are due to contaminated GC injection ports. Technical Bulletin No. 3 describes this background problem and demonstrates steps that can be taken to improve it.

Theoretically, the conditioned desorption tubes can be stored several weeks before use. But whenever possible it is always best to use fresh tubes. In our facilities we always use desorption tubes that have been conditioned within the past two days.