The collection of volatile organic compounds (VOCs) from ambient air is either achieved actively by sampling onto a sorbent tube using a sampling pump, or passively using the tube as a diffusive sampler without pumping air through it. Active sampling is preferable because of easier quantification, shorter sampling time, and wider compound range that can be determined using a single tube.
There are various national and international standards dealing with the analysis of VOCs via thermal desorption coupled with gas chromatography and a mass spectrometry (TD-GCMS). Following thermal desorption from the tube, analytes are focused in a cold trap using cryogenic cooling, (commonly liquid nitrogen). For labs that cannot use liquid nitrogen, a novel cryogen free trapping technique for VOCs was developed by scientists at GERSTEL: Dynamic Focusing.
The dynamic focusing technique has been thoroughly tested for use with methods such as U.S. EPA TO-17 and ISO 16000-6. While ISO 16000-6 focuses on the determination of VOCs ranging in volatility from n-C6 to n-C16, or in boiling point from 50 °C to 260 °C, EPA TO-17 includes very volatile organic compounds (VVOCs) such as propene. The use of Dynamic Focusing produces method compliant results over the full range of volatility covered by these methods, without the need for either cryogens or problematic inline valves.