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Show with two fixed wavelength lasers in which the first laser detects the molecule of interest while the second is tuned away from the absorplion line and provides a baseline calibration. In this case detection could take as few as ten laser shots, or 1 s at 10 Hz. All of the times mentioned above satisfy the quasi-real-time requirement in that they could provide feedback to the process in a time short enough to prevent significant release of toxics to the environment. Finally let us consider just how jet-cooled LIF or REMPI with TOF MS might be implemented in the "real world" environment of an incinerator exhaust stream. We assume that such an exhaust stream is at roughly atmospheric pressure and elevated temperature. Gas phase samples might be removed from the stream via a heated transfer line, pressurized in the carrier gas (He), and then expanded through a nozzle into a vacuum chamber. LIF could be performed in this source chamber or REMPI could be performed on a skimmed molecular beam in a second, differentially pumped chamber that contains the TOF MS. If the stream temperature is not too high, pulsed sampling and pulsed valves can be used. For extremely high temperatures a continuous nozzle may be necessary. A continuous expansion has the drawback of requiring substantially more vacuum pumping speed in the source chamber. The LIP or REMPI laser(s) can be located some distance from the vacuum apparatus (for instance in a clean room) and the beam(s) propagated to the appropriate location. This makes it feasible to use standard, commercially available laser systems rather than to develop miniature, rugged lasers for use in harsh environments. The disadvantage is that such remote laser placement may require extensively stabilized alignment. The LIF or REMPI signals would be acquired by electronics similar to those described here and processed by computer. The computer would also need to execute the necessary software for raw data analysis to provide species concentration and to provide a warning or feedback information to the incineration process. A critical question concerning the applicability of jet-cooled LIF or REMPI in such an environment is the nature of the "matrix" in which the sample resides. Are the species of interest present in trace quantities among a large number of more abundant species? Are there particulates present and, if so, are the molecules to be detected partially condensed on them? If the problem is simply that it is impossible to sort out the large number of species based on jet-cooled REMPlrrOF MS alone, it may be feasible to use a preliminary separation technique prior to expansion to help simplify the matrix in the molecular beam. For example jet-cooled LIF has been used to detect methylnaphthalenes eluting from a gas chromatography column and this method has been used successfully on a crude oil sample.IS It is easy to imagine using a GC column to provide initial separation just prior to free-jet expansion in our "device". Particulates pose another problem. If none of the species of interest are condensed upon them, then simple filtration will prevent the nozzle from clogging. This approach of ignoring the particulates can also be used when the minimum detectable concentration of toxic species in the gas phase is sufficient to provide warning of an unaccepatble release even though most of the toxics are condensed on particulates. Finally, if this criterion is not met and essentially all the toxic species are condensed on particulates, it should be possible to re-volatilize them simply by heating or by using the laser desorption technique mentioned. It 16 |