OCR Text |
Show ) is also worth noting that sample concentration can be greatly increased by intentionally condensing the toxic species on a cold finger. After this condensation the more volatile stack gases could be pumped off and the toxic species could then be volatilized, diluted in helium , and finally expanded through the nozzle. The use of volatilization techniques to examine species condensed on particulates or concentrated on cold fingers requires a slight increase in analysis time due to the necessary sample handling. Nonetheless significant increases in sensitivity might be made by sacrificing some speed. In summary we have shown how free-jet expansion cooling can be used effectively in conjunction with LIF and REMPI to provide ppb level detection of the mono- and dichlorinated benzenes and naphthalenes. These techniques are also species and isomer selective and can be implemented in quasi-real time. When used in conjunction with reflectron TOF MS, REMPI provides a mass spectrum that can be used for additional selectivity, either in sorting out parent ions at low laser intensities or in identifying species based on their ion fragmentation patterns at high intensities. For highly chlorinated aromatics in which intersystem crossing to the triplet manifold is very fast, the quantum yield of fluorescence is too small to make LIP a viable detection scheme. The DCN's represent such a case, and it is highly likely that more complicated toxic species will fall into this category as well. REMPI can be used effectively in such instances as our results on the DCB's and DCN's illustrate. In extreme cases it may be necessary to use schemes that detect the triplet state by using two-color REMPI to ionize it or by using sensitized phosphorescence to observe iL When it is applicable REMPI is the technique of choice since it provides higher sensitivity than LIP and additional selectivity via the mass spectrum. Much basic research into the jet-cooled spectroscopy and photophysics of the dioxins, PCB's, furans, etc. must be done before we can ascertain whether the techniques discussed here can be applied successfully to trace detection of these molecules. Parts-per-billion concentrations should be sufficient for detection of such toxic species in the stacks of incinerators since their concentration is greatly reduced when released to the environment. For example, emission data from three typical municipal waste incinerators2(a) indicates that a 1 ppb concentration of TeDD in the exhaust stack corresponds to a maximum ground level concentration of -2xlO- 12g/m3 . Finally it seems feasible to implement jet-cooled LIF or REMPI with TOF MS in an incinerator effluent stream, although it may be necessary to include additional preliminary separation techniques such as gas chromatography to help reduce matrix effects. 17 |