Unlike Electro-chemical, PID, FID and GC/MS based detection methods, Cerex UltraViolet Differential Optical Absorption, UVDOAS, delivers simultaneous monitoring of ppb levels of individual VOC species, such as Benzene, Toluene, Ethylbenzene, and Xylene, in real time. The method is unaffected by ambient humidity, requires no sample conditioning filter change prior to monitoring, requires no carrier gas, requires no wet chemistry, and incurs no analytic costs or sample handling costs. Results are immediate. The raw data containing all the information necessary for gas identification and quantification is permanently saved.