Roving GC/MS: Mapping VOC gradients and trends in space and time

Abstract

Obtaining representative VOC (volatile organic compound) measurements in ambient environments which exhibit complex concentration gradients and/or trends is difficult when relying upon limited numbers of analyses obtained by simple pooling or averaging techniques. A more effective approach is to perform large numbers of analyses over a period of time to permit detailed mapping or profiling of local gradients and trends. Until recently, use of GC/MS (gas chromatography/mass spectrometry) techniques for rapid profiling or mapping operations was not feasible because of sample speed limitations. This article describes a roving GC/MS system based on the combination of a Hewlett-Packard model 5972 MSD (mass selective detector), a FemtoScan Enviroprobe repetitive vapor sampling inlet with short capillary GC column, an Alcatel Micro HV olliess vacuum pump stack, and a Pentium notebook PC running under Windows 95. The roving system is further equipped with differential GPS (global positioning system) and radio transceiver capabilities thereby permitting remote tracking of vehicle location and local VOC concentrations. Laboratory tests demonstrate lower detection limits of approx 4 ppb for BTX (benzene, toluene, and xylene), corresponding to minimum detectable quantities of approximately 25 femtograms, and maximum analysis speeds of up to 5 GC peaks per second on a mixture of volatile ketones. Demonstrated outdoor performance, using a zero-emission electric vehicle, includes measurement of low ppb BTX levels along a 6 km urban route at 15 s (∼150 m) intervals while moving at an estimated average speed of 35 km h−1. Indoor measurements of toluene concentrations in the low to mid ppm range at 6 s (∼5 cm) intervals along a 6 m long path reveal a high degree of spatial and temporal variability in VOC concentrations. Mobility, specificity, sensitivity, and speed of the roving GC/MS method make this a promising candidate method for rapid outdoor and indoor screening, monitoring, and mapping of VOCs. © 1996 John Wiley & Sons, Inc. Field Analyt Chem Technol 1: 109–116, 1996.