Oxygen photoionization

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. 

A further argument was advanced by Veal et al.. A linear relationship was found to exist between the XPS intensity of the main oxygen valence band and the oxygen-to-uranium ratio of the different uranium oxides investigated (Fig. 23). This was interpreted as indicating that this band consists entirely of 2p states (perhaps 6 d hybridized) with no 5f contribution, as expected in a localized 5f picture. (The 5f contribution, if present, would have caused deviations from this hnear relationship, especially because of the very large photoionization f cross-section.)... 

Radical cations can be formed by irradiation of unsubstttuted aromatic hydrocarbons such as naphthalene, and this makes possible the photochemical displacement of hydride ion by a nucleophile such ascyanide f3.10). Oxygen is not necessary for the success of this type of reaction if a good electron-acceptor is present, such as p-dicyanobenzene (3.11), which enhances the initial photoionization and also provides for reaction with the displaced hydrogen. 

More recent studies have made possible direct determination of the rate constants k7a and k7b. McNeal and Cook47 have followed the concentration of 02(1A9) in a discharge-flow system by the photoionization technique (Sect. III-E). In the presence of ozone, 02(1A,) decays by a predominantly first-order mechanism, so that, presumably, the second-order pooling process (11) does not contribute significantly to the loss of 02(1A9). If it is assumed that the only loss process for Oa(1A9) is reaction with ozone in reaction (7a), then k7a lies between 1 x 106 and 2 x 106 liter mole-1 sec-1. This value is considerably lower than the rate constant measured by Mathias and Schiff87 if Oa in reaction (7) is largely O Aj). McNeal and Cook consider the possibility that their photoionization current decreases by less than that expected on the basis of [02(1A9)J decay, as a result of the formation of vibrationally excited oxygen in reaction (15d)... 

In order to study the photochemical action of solar radiation on tropospheric, stratospheric, and mesospheric constituents, the solar spectrum must be divided in various ranges.1 The radiation at wavelengths less than 100 nm, which is absorbed by nitrogen and oxygen in the thermosphere above 100 km, leads essentially to ionization processes and is, therefore, not considered there. Only X-rays of wavelengths less than 1 nm can penetrate into the atmosphere below 100 km, and lead indirectly to the dissociation of molecular constituents. Nevertheless, their principal role is the photoionization in the D region of the ionosphere below 100 km where the solar line Lyman-a at 121.6 nm ionizes the nitric oxide molecule, NO. 

Photoionization at 193 nm in oxygenated solution of poly(C) causes strand breakage with high efficiency, half of which occurs at times < 4 ms, the other half with a half-life of 7 ms (Melvin et al. 1996 Table 11.8). This kinetic behavior is very different from what is seen after -OH-attack and points to the direct involvement of the Cyt radical cation. In poly(U), the (biphotonic) photoionization shows similar results (Table 11.8). With poly(A), the formation of strand breaks is 20-times less efficient as compared to poly(C) (Table 11.8), and this is in agreement with the above conclusion that the A-+ or A- do not cause strand breakage to any major extent. 

Volatile aromatic and chlorinated compounds are usually analyzed with the photoionization detector/electrolytic conductivity detector (PID/ELCD) combination in EPA Method 8021. In this method, the PID detects aromatic compounds, typically the volatile constituents of petroleum fuels (BTEX) and oxygenated additives, and the ELCD detects chlorinated solvents. Both detectors are considered to be selective for the target analytes of EPA Method 8021. But are they sufficiently selective for making unambiguous decisions on the presence and the concentrations of these analytes ... 

Above the mesopause, Tg increases rapidly. In this region, termed the thermosphere (Fig. 2), absorption of short wavelength solar radiation is occurring (Fig. 3) which results in the efficient photodissociation of molecular oxygen, and the photoionization of the O atoms so produced and of the 02 and N2 molecules. Thus, Tg increases beyond 1000 K, approaching 2000 K at times. Whereas below 100 km the neutral gas particles, the ions and the electrons in the plasma all possess the same kinetic temperature, above 100 km, due to the lower pressure and the subsequent reduced electron/heavy particle collision frequency and the large amount of energy imparted to the photoelectrons, the electron temperature, Te increases above Tg (and Tj the ion temperature, which is Tg, see Fig. 2). 

Photoionization indicated an ionization potential of 10.565 eV, which is in excellent agreement with the value of 10.5 eV calculated from the UV spectrum. The oxiranes have higher ionization potentials than that observed for dimethyl ether (lO.OeV) this demonstrates the less effective nature of the delocalization of the 2p 7T-electron pair in the case of oxirane than in simple ethers. Support for this is provided by the experimental fact that the oxiranes exhibit a charge shift from the oxygen towards the carbon atoms, which may cause an increase in the ionization potential of the 2p tt-electron pair. 

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