EVALUATION OF SOLID SORBENT SAMPLING METHODS BY BREAKTHROUGH VOLUME STUDIES

Organic vapours in air can be sampled by concentration on a solid sorbent. A known quantity of air is passed through a tube containing the sorbent, and the adsorbed vapours can subsequently be desorbed and analysed. To match sorbent and sorbate correctly, the maximum volume of air that can be sampled without significant breakthrough of the sorbate into the effluent stream must be known. Many factors might influence the course of breakthrough, including vapour concentration, temperature, humidity, interferences, flow-rate and sorbent bed geometry. A thorough investigation of the effects of these factors on the breakthrough Of organic vapours on two porous polymers is presented. The dependence of breakthrough volume on concentration that is predicted by existing theories of frontal (gas-solid) chromatography is confirmed, and the relationship is shown to be closely linked to the shape of the adsorption isotherm. The results were checked using sample tubes with back-up sections to detect breakthrough. Sample rates as low as 20 ml min-1 confirmed predicted breakthrough volumes. Breakthrough volumes determined using the technique of continuous vapour assault under carefully controlled 'worst-case' conditions (the 'direct' method) can be used to predict safe sample volumes, according to NIOSH and OSHA protocols. Safe sample volumes derived from breakthrough volumes determined using a technique based on chromatographic retention volumes (the 'indirect' method) are not comparable. To avoid confusion when referring to values obtained by the indirect technique the term 'safe sample volume' should not be used. Where sample tubes without back-up sections are employed, so that breakthrough cannot be detected, there must be confidence that sample loss has not occurred.