Use of Mixed Resin Beds

The selection of an adsorbent resin for an application is based on a large number of variables. These variables originate from three sources and each area and parameter must be evaluated before a resin is selected. The Breakthrough Volume tables are a very useful aid in the selection of a resin or group of resins for a particular application. In many cases one resin will not enable the analysis of all the analytes of interest. This is especially the case when a wide range of boiling point analytes are present in the sample. For example the analysis of a sample that contains very volatile organics such as acetone or butane but also contains very large hydrocarbons. For these applications, mixed bed or multiple bed resins are frequently utilized.

The more appropriate term for these type tubes is probably multiple layer resin beds. A large number of resins are available which can be selected for their ability to trap and release analytes of interest. These resins can be evaluated and selected by using the Breakthrough Volume Tables. Each adsorbent resin has its own unique properties and the ability to trap and release a certain range of organic compounds. Resins such as glass beads are very good for trapping and releasing the very large hydrocarbons (C20 and larger). But volatile analytes and most moderate boilers will pass right through this resin. Tenax® is the most popular resin for the analysis of a wide array of analytes including the aromatics and terpenes. If large hydrocarbons were trapped on this resin it would be near impossible to desorb them off the resin. By the same token the very volatile organics such as propane will pass right through this resin. Carobsieve SIII is the finest of the charcoals for the trapping of the most volatile organics. Even compounds down to ethane can be trapped with this resin. However if any of the larger compounds were trapped on this resin, it would be impossible to release these analytes off the adsorbent resin. The use of multiple layer resin beds permits the trapping and analysis of a wide boiling point range of organics using moderate desorption temperatures.

Due to these unique trapping properties of each resin, it is important that the resins be packed in the correct order as shown above. The resins, such as the charcoals, which trap the smallest analytes should always be the top layer of the multiple layer resin beds. Likewise the resins which trap only the largest molecules and do not retain any of the lighter volatiles (such as the glass beads) must be the bottom layer. When packing these mixed bed resins, it is important that the resins be layered, one on top of the other with minimal mixing of the resins. If the resins were mixed together, the packed desorption tubes would not perform as expected because the most adsorbing of the resin (i.e. the charcoals) would be exposed to the larger molecules during sample loading and would not release these analytes during the thermal desorption process.

Once the multiple layered desorption tubes are packed, they must always be used in the correct orientation. It would be helpful to always mark the charcoal end of the trap. When collecting a sample for analysis it is important that the gas volume being sampled enters the end of the desorption with the least retaining resin (the glass beads end). This will enable the glass beads to adsorb the heavy hydrocarbons and pass all the other analytes. The next layer will adsorb the aromatics and medium boilers and the final charcoal layer will trap the lowest molecular weight compounds. This sample collection direction shown via the blue arrow above must always be used during the sample collection process.

During the thermal desorption process, the direction of gas flow is reversed as shown by the red arrow in the above diagram. By reversing the flow through the desorption tube the large molecular weight compounds are never exposed to the other resins which would trap these compounds and retain them in the resin. This assures complete delivery of all the analytes to the GC injection port for analysis. This reversal of gas flow between sample collection and desorption is good practice not only for multiple layer desorption tubes but even for single layer adsorption tubes. This permits the easiest release of analytes off the adsorbent resin in the trap.

Packing of Multiple Layered Resin Beds

Multiple resin beds are packed much like the single layer resin bed described previously. Glass wool plugs at the ends of the desorption tubes hold the resin bed in place. The resins are added to the desorption tube one layer at a time and care should be taken to minimize the mixing of the resins at the interface of the two resin beds. Another technique is to put a small glass wool plug between each resin bed. This will eliminate the mixing of the resin beds, but it will also take up space within the desorption tube which is needed to hold the resin beds. The packed desorption tubes must be flow conditioned as described previously using care not to exceed the temperature limit of any of the adsorbent resins.