Benzene photoionization

Reactions (19)-(21) represent the dissociation of benzene and reactions (22)-(26) represent the detection of fragments by VUV laser photoionization. The line-shape images resulted from these reactions. 

One method (EPA 8020) that is suitable for volatile aromatic compounds is often referred to as benzene-toluene-ethylbenzene-xylene analysis, although the method includes other volatile aromatics. The method is similar to most volatile organic gas chromatographic methods. Sample preparation and introduction is typically by purge-and-trap analysis (EPA 5030). Some oxygenates, such as methyl-f-butyl ether (MTBE), are also detected by a photoionization detector, as well as olefins, branched alkanes, and cycloalkanes. 

Certain false positives are common (EPA 8020). For example, trimethylben-zenes and gasoline constituents are freqnently identified as chlorobenzenes (EPA 602, EPA 8020) becanse these componnds elnte with nearly the same retention times from nonpolar columns. Cyclohexane is often mistaken for benzene (EPA 8015/8020) becanse both compounds are detected by a 10.2-eV photoionization detector and have nearly the same elntion time from a nonpolar colnmn (EPA 8015). The two compounds have very different retention times on a more polar column (EPA 8020), but a more polar column skews the carbon ranges (EPA 8015). False positives for oxygenates in gasoline are common, especially in highly contaminated samples. 

Tubaro, M., Marotta, E., Seraglia, R., and Traldi, P. (2003). Atmospheric pressure photoionization mechanisms. 2. The case of benzene and toluene. Rapid Commun. Mass Spectrom. 17, 2423—2429. 

In the U.S.S.R. photo-mass spectrometry has been used in a study of the photoionization and photodissociation of methylated benzene derivatives and the aromatic amines,16 and recently the hydrazine derivatives17 (Sec. III). Moreover, for many of the aromatic compounds the kinetic energy distribution of the photoelectrons has been measured in both the gaseous and the solid state (Secs. IV and V). 

TABLE II. Photoionization Thresholds in Gaseous Benzene and Its Derivatives Obtained by Mass Spectrometry... 

In our original survey of the literature concerned with photoionization of benzene we inadvertently missed important contributions by J. C. Person220 and by Dibeler, Reese, and Krauss.230 Both of these studies provide evidence for an isotope effect in the photoionization of benzene, hence implying competition among the several decay channels available to the resonant state formed by photon absorption. 

The first approximation to the description of Rydberg levels treats the benzene ion-core as a monopole. This description is known not to be quantitatively accurate. Calculations which include the symmetry of the molecular ion, and the charge delocalization, lead to an energy level spectrum in much better agreement with experiment. Thus, it seems unlikely that the geometric structure of the molecular ion can be completely neglected in the study of photoionization. 

C,HJ (Benzene) sf6- >0 Unspecified Photoionization -mass spectrometry Single-source electron-impact ionization-mass spectrometry CeH (QHj —)C,2H SF6 (XFVSFJXFj- X-Se.Te,U SF6-(TeF6 SF6,F)TeF5- The ion SF - produced by thermal electron capture of SF in a vibrational degree of excitation equal to electron affinity of the neutral 116a... 

In this manner, a nearly universal and very nonselective detector is created that is a compromise between widespread response and high selectivity. For example, the photoionization detector (PID) can detect part-per-billion levels of benzene but cannot detect methane. Conversely, the flame ionization detector (FID) can detect part-per-billion levels of methane but does not detect chlorinated compounds like CCl very effectively. By combining the filament and electrochemical sensor, all of these chemicals can be detected but only at part-per-million levels and above. Because most chemical vapors have toxic exposure limits above 1 ppm (a few such as hydrazines have limits below 1 ppm), this sensitivity is adequate for the initial applications. Several cases of electrochemical sensors being used at the sub-part-per-million level have been reported (3, 16). The filament and electrochemical sensor form the basic gas sensor required for detecting a wide variety of chemicals in air, but with little or no selectivity. The next step is to use an array of such sensors in a variety of ways (modes) to obtain the information required to perform the qualitative analysis of an unknown airborne chemical. 

As discussed in Chapter 6, the primary method of analyzing benzene in body fluids and tissues is gas chromatography (GC) in conjunction with either mass spectrometry (MS), photoionization detection (PID), or flame ionization detection (FID). For detection of benzene metabolites, both GC/FID and high-performance liquid chromatography (FIPLC) with ultraviolet detection (UV) have been used. A recent article describes the development of simple and sensitive methods for determination of blood and urinary benzene levels using gas chromatography headspace method (Kok and Ong 1994). 

Screening methods are available for analysis of benzene in feces and urine (Ghoos et al. 1994) and body fluids (Schuberth 1994). Both employ analysis by capillary GC with an ion trap detector (ITD). Benzene in urine has been determined by trapping benzene stripped from the urine on a Carbotrap tube, followed by thermal desorption GC/flame ionization detection (FID). The detection limit is 50 ng/L and the average recovery is approximately 82% (Ghittori et al. 1993). Benzene in urine has also been determined using headspace analysis with capillary GC/photoionization detection (PID). The detection limit is 40 ng/L (Kok and Ong 1994). 

Kok PW, Ong CN. 1994. Blood and urinary benzene determined by headspace gas chromatography with photoionization detection application in biological monitoring of low-level nonoccupational exposure. Int Arch Occup Environ Health 66(3) 195-201. 

For example, Vincow and coworkers (205) have prepared radical cations of benzene, hexamethylbenzene, perylene, naphthalene, etc., by photoionization of the molecules in a rigid glass, while Hulme and Symons (206) have... 

A brief review and reassessment of data on the photophysics of benzene has been presented by Pereira. Evidence for the l E2g valence state has been obtained by u.v. two-photon spectroscopy.Slow electron impact excites fluorescence in thin films of benzene at 77 K as well as emission from isomers." The fluorescence yields and quenching by chloroform of alkyl-benzenes and 1-methylnaphthalene after excitation into Si, Sz, and S3 states and after photoionization have been measured. The channel-three process has been reconsidered in terms of the effects of local modes and Morse oscillator potentials. Excited-state dipole moments of some monosubstituted benzenes have been estimated from solvent effects on electronic absorption spectra, Structural imperfections influence the photochemistry of durene in crystals at low temperatures. Relaxation time studies on excited oxido-substituted p-oligophenylenes have been made by fluorescence depolarization... 

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