Photochemical degradation characteristics

Some X-ray photoelectron spectrometers are equipped with monochromators that can be used to remove unwanted radiation, such as the continuous radiation and even some of the weaker characteristic X-rays such as K<,3, K 4, Kas, and Ko,6, from the emission spectrum of the anode. A monochromator can also be used to resolve the K i,2 line into its two components K i and Ka2- Using a monochromator has at least two beneficial effects. It enables the narrow, intense K<, line to be used to excite spectra at very high resolution. A monochromator also prevents unnecessary radiation (continuous, K<,2, Ka3, K<,4, Kas, and Ka6) that might contribute to thermal or photochemical degradation from impinging on the sample. 

The Bureau of ATF Ink Library of standard ink samples is essentially complete with respect to all domestic and most European-produced inks for the past few decades. However, the library does not contain all of the inks produced in the world nor will this ever be possible. This does not detract from the practicability of the ink identification technique despite the rare occurrence of a non-identification. However, since in actual practice it is not possible to obtain all of the inks in the world, the comparison is based on the probability or degree of certainty that a questioned ink matches an existing standard ink. The non-identification of questioned inks can occur when the corresponding standard ink was not supplied, or when the questioned ink characteristics changed sufficiently due to deteriorating conditions such as photochemical degradation caused by extreme exposure to light. 

Another explanation was the following. The organomontmorillonite used was a natural montmorillonite that contained iron. Chemical analysis of the clay confirmed the presence of a low amount of iron. It was recalled that iron and, in more general terms, metals are likely to induce the photochemical degradation of polymers. Iron at low concentration had a prooxidant effect that was due to the metal ion of iron that can initiate the oxidation of the polymer by the well-known redox reactions with hydroperoxides [93]. It was concluded that the transition metal ions, such as Fe, displayed a strong catalytic effect by redox catalysis of hydroperoxide decomposition, which was probably the most usual mechanism of filler accelerating effect on polymer oxidation. A characteristic of such catalytic effect was that it did not influence the steady-state oxidation rate, but it shortened the induction time. 

The rate at which a pollutant photochemically degrades depends on numerous chemical and environmental factors, such as the light absorption properties and reactivity of a compound, the light transmission characteristics, and the intensity of solar radiation. 

The characteristics of plastics can change depending on the exposure to thermal, mechanical (shear), oxidative and photochemical degradation processes. The characteristics of the recyclate may be quite different from those of the original virgin plastic. 

Kalnins, M.A. Surface Characteristics of Wood As They Affect Durability of Finishes. Part II. Photochemical Degradation of Wood, pp. 23-60. U.S. Eorest Service Research Paper no. 57 Forest Products Laboratory, Madison, Wisconsin (1966)... 

In this chapter we briefly discuss the potential of the microbial food web as a source and sink for FDOM and examine how the fluorescence characteristics change as a result of microbial processing. In addition, the combined effects of microbial and photochemical degradation on FDOM characteristics are summarized. The literature on the subject is vastly expanding and it is not our aim to provide an exhaustive review but rather to highlight specific studies as examples of how fluorescence spectroscopy is being applied to studying the microbial turnover of DOM in natural aquatic enviromnents. 

Using l,8-diphenyloctatetra-l,3,5,7-ene, (DOT), as a model compound either in dilute, ( 10-5m), hexane or ethanol solutions or incorporated into a film of undegraded PVC confirmed that in the presence of HC1 it underwent a photochemical reaction which resembled that of the polyenes in thermally degraded PVC. The results indicated that the initial rates of reactions proceeding in either solvent showed a second order dependence on HC1 pressure and that the reaction was considerably slower in ethanol than in hexane. Further, when cast in PVC films, the characteristic absorption maxima of DOT were shifted about 16nm to longer wavelengths compared with their absorption in hexane and there... 

One attractive possibility is the use of the a-keto-oxime chromophore. It has a strong absorption at 220 nm whose tail, which extends to 240-250 nm, would improve the absorption characteristics of PMMA. Also, the esters possess a N-O bond which is photochemically labile yet sufficiently thermally stable so as to be compatible with the various processing steps. The solution degradation of a-keto oximino methacrylate esters upon irradiation with light of X365 nm has been reported by Delz-enne (2), and we proceeded to investigate the solid state photodegradation of similar copolymers and their possible utility as deep UV photoresists. 

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