Production of test gases in the ppb range for round-robin tests and quality assurance measures during the measurement of VOCs

Abstract

Workers exposed to volatile organic compounds (VOCs) very often suffer damage to health, and VOC odours may also represent a serious nuisance. The compounds concerned may be numerous and highly diverse; the concentrations of the individual substances however are generally relatively low. Quality assurance measures such as round-robin tests for the measurement of VOCs in indoor areas or at workplaces have not so far been available. A particular reason for this is the challenge presented by the production of test gases in the necessary concentrations. The BGIA test gas facility has therefore been modified for the production of test gases in the µg/m³ range. A two-stage primary gas purifier, a continuous test gas generator with multi-stage dilution, a capillary evaporator for low-volatility compounds and an online thermodesorber have been installed specifically for this purpose.

Purification of the primary gas proved to be particularly difficult: the available pre-purified compressed air contained organic trace impurities which fluctuated over time. The essential requirement, i.e. the injection into the test gas stream of a primary gas which was constant over time and exhibited the lowest possible contamination, could be satisfied only by catalytic decomposition of the organic residual impurities and with a supplementary adsorber unit. Even with these measures in place, small quantities of organic substances remain present, and the blank readings must still be monitored continuously. Following completion of all modifications, it proved possible to produce test gases with a high degree of reproducibility with substance concentrations in the range of 1 to 500 µg/m³.

Since 2007, the BGIA has been offering round-robin tests for VOCs with local sampling in the range from 5 to 50 µg/m³. The first round-robin tests show that good results are obtained at concentrations from 10 to 50 µg/m³. At the lower concentration of < 15 µg/m³, however, the number of participants experiencing deviations from the reference value was substantially higher. The round-robin tests also involved the taking of blank readings prior to the start of test gas dosing. Some two-thirds of participants were unable to detect any interfering components; the remaining participants produced blank readings the levels of which could not always be explained. The problem may be caused in this case by the carrier material Tenax TA which is usually employed.