When sampling for volatile organic compounds (VOCs), why are sorbent tubes commonly used, and what dictates the choice of sorbent?

• A. They concentrate VOCs for laboratory analysis, increasing sensitivity.
• B. They remove moisture from the sample.
• C. Sorbent tubes concentrate VOCs for laboratory analysis, increasing sensitivity. Choice depends on VOC volatility and polarity (e.g., activated carbon for general VOCs, Tenax for semi-volatile organics).
• D. They provide immediate results in the field.

Answer: (C)

Sorbent tubes are used because they trap and concentrate VOCs from the air, turning trace-level ambient concentrations into a sample that can be reliably analyzed in the laboratory. As air is drawn through the tube, VOC molecules adhere to the sorbent material. After sampling, the tube is sealed and sent to the lab, where the VOCs are thermally desorbed and introduced into a GC or GC–MS system for quantification. This concentration step is what gives the analysis the needed sensitivity for accurate measurements at low concentrations.

The choice of sorbent depends on the characteristics of the target VOCs, especially how volatile they are and their polarity. For a broad range of VOCs, activated carbon is a common general-purpose sorbent because of its high surface area and strong adsorption across many compounds. For semi-volatile organics with different desorption and breakthrough behavior, a polymerlike sorbent such as Tenax is often preferred, since it provides good retention for those compounds and works well with thermal desorption systems. In practice, the selection is guided by the volatility and polarity of the target compounds, with moisture control and compatibility with the analytical method also influencing the choice.

Moisture management can be a concern in sampling, but it’s not the primary reason sorbent tubes are used. The key idea is concentration and compatibility with the VOCs’ properties to maximize recovery and sensitivity in the lab.