Laser-Based Techniques for the Detection of Cholorinated Aromatic Hydrocarbons

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I. Introduction The detection of chlorinated aromatics, such as polychlorinated biphenyls (PCB's) and polychlorinated dibcnzo-p-dioxins (PCOO's), is currently a problem of intense concern because of the toxic and carcinogenic nature of these species and their widespread presence in the environment l Combustion has been implicated as one of the primary sources of such species. both via the incomplete destruction of toxic wastes upon incineration and via their formation during the burning of chlorinated feedstock in municipal waste incinerators.2 There is a clear need for techniques that provide trace detection of chlorinated aromatics in the exhaust streams of incinerators. The requirements of such gas-phase detection techniques are: (l) ultrahigh sensitivity (at least part-per-billion in molecular concentration). (2) isomeric selectivity, and (3) real-time or quasi-real-time applicability. Useful techniques must be able to differentiate not only among species with different chemical formulas but also among positional isomers of a specific chlorinated aromatic, for example the 22 isomers of tetrachlorodibenzo-p-dioxin (TCOO). This requirement arises because different isomers often have dramatically different toxicities.3 Real-time capability together with ultrahigh sensitivity ensures that the release of toxies to the environment can be minimized. Finally a practical detection device must operate reliably in the real-world (harsh) surroundings of an incinerator and should also be as inexpensive as possible. Present analytical techniques for chlorinated' aromatics satisfy some but not all of the requirements enumerated above. For example high performance gas chromatography-mass spectrometry (GC-MS) is capable of isomerically selective detection of dioxins down to very low detection limits.4 however GC-MS analyses are usually very costly and take weeks to perform. This is obviously an undesirable situation since the incinerator operators incur a large expense to find out if they have released toxics to the environment after the fact This example demonstrates the need for at least quasi-real-time techniques, i.e. techniques that yield results within seconds or minutes rather than weeks. In this paper we describe our investigations into several laser-based spectroscopic techniques that may satisfy the requirements listed above. All of these techniques depend on the electronic excitation of the molecule of interest, usually making use of the aromatic ring as a chromophore that absorbs laser light The pertinent energy levels of a representative chlorinated aromatic (ArCl) and the schemes we will consider are shown in Fig. 1. The ground electronic state of the molecule is a singlet state in which all electron spins are paired; this state is generically referred to as SO. All of the processes we shall consider begin with absorption of a laser photon at wavelength A.l that excites the molecule to the first excited singlet state, S}. In molecular orbital language this transition is most likely to be of the 1t-1t * type; a transition from a bonding orbital on the aromatic ring to an anti-bonding orbital on the ring (This simple picture is disturbed to a large degree by the presence of substituent groups on the rings.). Once excited to S 1 the molecule may fluoresce back to SO, this transition is strongly allowed and typically occurs within nanoseconds to microseconds. This is the first technique that we shall consider. and in laser spectroscopy 2 ) J