Determination of Back Pressure Data for Adsorbent Resins

A desorption tube filled with adsorbent resin can produce significant back pressures for the sampling pump when collecting a sample. If the back pressure is too high the air sampling pump will fail. For efficient operation the Back Pressure should not exceed 25 inches of water. The smaller the diameter of the desorption tube, the longer its length or the more adsorbent resin contained inside the higher the backpressure that will be created at the end of the desorption tube near the pump. Backpressure is not normally a problem in positive pressure systems in which a gas is forced through the adsorbent resin bed from a tank of compressed gas. In applications utilizing the small personal air pumps, a high backpressure in excess of 25 inches of water will cause these pumps to fail.

In order to determine the Back Pressure at the pump end of the thermal desorption tube the setup shown above was constructed. A small Gillian Air Sampling Pump (Model LFS-113) was used. This pump will maintain a constant and accurate air velocity through the desorption tube regardless of the backpressure up to about 40 inches of water. A pressure gauge was attached in line between the air pump and the desorption tube. The Gillian pump was set up and the flows calibrated utilizing a Gilibrator Air Flow Calibration System. This device is a primary air flow calibration system which accurately measures flows with 3 digit accuracy. Flows were calibrated through the desorption tube packed with the measured amount of resin and the pressures recorded as a function of flow for both 3.0 and 4.0 mm I.D. desorption tubes. The backpressures were recorded for the various tube sizes and resin beds.

A Typical result are shown below for Tenax® TA.

Tenax TA Back Pressures for 3.0 mm I.D. Desorption Tubes

Back Pressure expressed in inches of water

Tenax TA Back Pressures for 4.0 mm I.D. Desorption Tubes

Back Pressure expressed in inches of water

From this data it can be seen that for a 3.0 mm I. D. tube packed with 100 milligram of Tenax TA, the sample pump should not exceed 50 ml/minute. For a 4.0 mm I.D. tube packed with 100 milligram of Tenax TA, one can safely collect samples at flows up to 175 ml/minute.

Carbosieve SIII Back Pressures for 3.0 mm I.D. Desorption Tubes

Back Pressure expressed in inches of water

Carbosieve SIII Back Pressures for 4.0 mm I.D. Desorption Tubes

Back Pressure expressed in inches of water

Backpressure varies significantly with adsorbent resin. The Tenax resins are probably the most restrictive whereas many of the charcoal resins will permit much larger bed volumes in the desorption tubes. An example of the backpressure data for Carbosieve SIII is shown above.

From this data it can be seen that for a 3.0 mm I. D. tube packed with 200 milligram of Carbosieve SIII, the sample pump should not exceed 100 ml/minute. For a 4.0 mm I.D. tube packed with 200 milligram of Carbosieve SIII, one can safely collect samples at flows in excess of 200 ml/minute. This is a significant improvement over the Tenax resins.

In summary, the backpressure data must be evaluated when selecting an adsorbent resin for a particular application. Normally an adsorbent resin is selected based on is Breakthrough Volume data, however the amount of resin that can be placed into a desorption tube as well as the sampling flow rate that can be used for the resin are restricted by the backpressure data. Backpressures should always be less than 25 inches of water when using personal pumps for sample collection.

A more detailed description of Breakthrough Volumes is in the article entitled: "Calculation and Use of Breakthrough Volume Data." and in Application Note 32.

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