Monitoring volatile organic compounds in ambient air inside and outside buildings with the use of a radio-frequency-based ion-mobility analyzer with a micromachined drift tube

A radio-frequency-based ion-mobility analyzer with a micromachined drift tube was operated continuously to monitor volatile organic compounds (VOCs) in ambient air inside a building and in an open space near the union of I-10 and I-25 at Las Cruces, New Mexico. Air was drawn directly, without enrichment or preseparation, through the analyzer, which was regulated to 35 °C. The ion source was a photo-discharge lamp at 10.6 eV, providing a preliminary level of selectivity in response to chemicals with low ionization potentials. The compensation voltage was scanned continuously from −40 to +20 V at rates of 60 V/s, providing profiles of ions obtained from VOCs in air. Solvents were detected at 1-ppm levels as fugitive emissions from other experiments under way in the laboratory from 8:00 a.m. to 6:00 p.m. However, patterns in VOC levels from 1 to 5 ppb between 6:00 p.m. and 7:00 a.m. and on weekends was attributed to air exchange between ambient air and the ventilation system of the building. The mobility analyzer results were consistent with VOCs from traffic on major city thoroughfare adjacent to the building. In-field studies near two interstate highways demonstrated that analyzer response could be correlated to traffic patterns and exhibited diurnal trends. These findings demonstrate the concept and practice of micromachined mobility analyzers as continuous monitors for VOCs as airborne vapors in buildings and on site. © 2000 John Wiley & Sons, Inc. Field Analyt Chem Technol 4: 297–308, 2000