Back in the 1970s, many multi-point systems were being deployed, for monitoring carbon monoxide in parking garages and vehicular tunnels. Since long and otherwise exposed sample tubing runs needed to be installed, copper tubing was used. Because CO is not very reactive, and copper tubing is both flexible and robust, this was a preferred choice.
Bear in mind, though, that the internal tubing in these instruments was either type 316 stainless steel, Teflon, polyethylene or polypropylene—all known for their chemical resistance, and to a lesser extent, for their lack of a tendency to absorb (or adsorb) the fluid going through it (chemisorption).
For many years, a variety of mostly proprietary tubing formulations sold under the brand name Tygon® has been available, and has dominated the laboratory and food and beverage markets. A very popular embodiment has been the clear, flexible PVC formulation. Notably, there are dozens of tubing formulations carrying the name “Tygon®,” so further details beyond the brand must be provided in any formal methods and materials write-up.
As laboratory-trained technicians moved into toxic gas detection, many of them started using Tygon® PVC tubing. However, when handling certain reactive gases such as chlorine and hydrogen sulfide, they would often observe a staining of the clear tubing. While this staining was never formally characterized, good analytical technique would shy away from such a development.
Thus, there was an immediate shift to materials known to be chemically impervious, such as Teflon, polyethylene, and polypropylene. Another popular formulation, known as Bev-A-Line IV, is a co-extrusion of a polyethylene liner with ethyl vinyl acetate shell.
As far as we are aware, no extensive testing of a variety of chemicals with a variety of tubing materials with regard to gas sampling has ever been published. We were able to locate this paper, entitled “The loss patterns of reduced sulfur compounds in contact with different tubing materials.” Not surprisingly, Teflon showed the best performance. As this research paper indicates [refer to chapter 4], when the setting requires analytical precision, Teflon is the most frequent choice.
For most toxic gas detection applications, we recommend Bev-A-Line, or even better—the more expensive Teflon.