Tubes used in purge and trap analyses generally are packed with multiple beds of adsorbent materials, so that a broad range of polar and nonpolar, high and low molecular weight compounds can be trapped in a single tube. Each bed protects the next, increasingly active bed, by preventing compounds from being held so strongly that they cannot be desorbed quickly without decomposition. During the purge phase of sampling, lower molecular weight compounds pass through the initial adsorbent beds but are trapped by succeeding beds. During desorption, the carrier gas passes through the trap in the reverse direction of purge flow, so that higher molecular weight compounds never come in contact with the stronger (innermost) adsorbents.
