Comparison of open path and extractive long-path FTIR techniques in detection of air pollutants

A comprehensive comparison of long-path extractive and open-path FTIR techniques on the bases of the available literature and on our own experience at the wastewater treatment plant of a chemical factory has been made. Two equalization basins were investigated and it was supposed that all the surface of the wastewater emits polluting compounds smoothly, since the atmospheric dispersion was found to be ideal at both sites, there were no significant chemical reactions, and the basin dimensions were rather small (7 x 16 m2 and 37 x 79 m(2) , respectively). Since the possibilities were different, rather different optimal spectral parameters (resolution, scan numbers, path length) were chosen for field and laboratory measurements. The S/N ratio of laboratory spectra was about 50-100 times higher than that of field spectra, which resulted in higher precision and lower detection limits of the measurements, and comparing to field measurements an additional compound (chloroform) was detected. On the other hand, the extremely polar ammonia was not detectable by the extractive technique. With the open-path method, time-dependent concentration changes of the pollutants were monitored. The presence of the theoretically banned organic phase in wastewater was demonstrated by detection of xylenes, isopropanol, and methanol in the air above the basin. Comparing the results measured by the two different techniques the discrepancies in the concentrations were dependent on the compound under determination. In some cases, the concentrations agreed well; in other cases, not. Consequently, to application of both methods is desirable in some special cases; for example, when the extractive method is used for preliminary investigations.