Surface Contamination experiments

Carrier tests with dried cells on surfaces are not suitable for the evaluation of biocide efficacy towards biofilms. In general, microbial cells dried on carriers are less susceptible to biocides compared to planktonic cells however, established biofilms grown on surfaces usually display enhanced resistance compared to organisms simply dried on carriers (e.g., Samrakandi et al., 1994 Ntsama.Essomba et al., 1997). A possible reason may be physiological changes which are associated with biofilm formation, and can result in enhanced resistance to biocides. These processes are not involved in carrier tests. 

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CO oxidation catalysis is understood in depth because potential surface contaminants such as carbon or sulfur are burned off under reaction conditions and because the rate of CO oxidation is almost independent of pressure over a wide range. Thus ultrahigh vacuum surface science experiments could be done in conjunction with measurements of reaction kinetics (71). The results show that at very low surface coverages, both reactants are adsorbed randomly on the surface CO is adsorbed intact and O2 is dissociated and adsorbed atomically. When the coverage by CO is more than 1/3 of a monolayer, chemisorption of oxygen is blocked. When CO is adsorbed at somewhat less than a monolayer, oxygen is adsorbed, and the two are present in separate domains. The reaction that forms CO2 on the surface then takes place at the domain boundaries. 

On the other hand, the objection of some surface physicists is that the 0 of Hg under the conditions of experiments carried out with a stream would be different from 4.50 eV because of surface contamination. If this is the case, the actual work function would be higher. However, contamination normally leads to a decrease in work function, especially if the contaminating species is water or an inert gas. An increase in work function would be possible if oxygen were chemisorbed, which has been ruled out experimentally. If an oxide layer is formed, a decrease in 0 is also expected. 

In conclusion, acceptance of 4.8 V as the potential of the SHE in the UHV scale leads to apparently contrasting arguments on one hand, the experiments with the streaming electrode leading to 4.44 V are vitiated by surface contamination of Hg, whose actual 0 would be about 4.8 V during the experiments. On the other hand, a decrease in 0 upon contact with... 

Due to the low volatility of cyromazine and the use of water-soluble bags for packaging the Trigard formulation, the main routes of exposure were expected to be from direct contact with the product or spray mixture on contaminated surfaces. Previous experience with pesticides worker exposure studies indicated that exposure from vapors or spray mist would be a minor factor. This can easily be confirmed by the PHED or similar published sources however, the extent of exposure from inhaling the product as dust is less well known. This route of exposure was also assumed to be minor, particularly with the use of water-soluble bag packaging. Given the low mammalian toxicity of cyromazine, the operators did not wear respiratory protection. 

Metallic adherends were cut from. 032" (.08 cm) thick sheets into lx4-inch (2.54 cm x 10.16 cm) specimens. Cleanliness of the surface, is required in order to facilitate good adhesion. Steel, as bonded within the automotive industry, often experiences a variety of surface contaminants which are not removed prior to bonding. 

This experiment requires skill and careful technique in order to obtain accurate results. As with most surface chemistry experiments cleanliness is of paramount importance. High-energy liquids such as water easily pick up surface-active contaminants from the air in a laboratory and great care should be taken to reduce exposure. Contaminants generally do not adsorb at the surface of low-energy liquids such as hexane and hence are less of a problem. 

Surface diffraction experiments have to be done in UHV. Otherwise the surfaces are covered with a monolayer of adsorbed molecules. At this point the reader might ask why do we not have to use UHV in scanning tunneling or the atomic force microscope In both techniques the tip penetrates through the surface contamination layer. In the scanning tunneling microscope it is often just invisible because contamination layers are usually not good conductors. In... 

Jones, I.S. Pond, S.F. (1966) Some experiments to determine the re-suspension factor of plutonium from various surfaces. In Surface Contamination, ed. B.R. Fish, pp. 83-92. Oxford Pergamon. 

Another effect, "surface contamination", must also be taken into account This may arise from residual gas and small leaks in the vacuum system, but a further possible source should not be overlooked, namely the following Polymers are capable of entrapping appreciable amounts of gas in their free volume, and these molecules are released under the effect of vacuum and of particle bombardment. In the present experiments the polymers were deliberately not degassed before plasma treatment, as such a pretreatment would not likely be economical in an industrial plasma process. The released molecules, primarily air and water vapor, evidently can participate chemically during plasma treatment by intermixing with the feed gas molecules. 

The chlorine present on the surface m be a result of surface contamination or possibly a chemically bound form in the surface structure. Note that the SiC phase in these materials is a pyrolysis product of methyltrichlorosilane, which conceivably could introduce some chlorine in the final structure. Our previous experience with a variety of surfaces has shown that ultrasonically cleaning the surface in absolute methanol usually eliminates most surface contaminants caused by handling. Unfortunately, no means were available for depth-profiling the alloyed carbons to determine the presence of chlorine below the surface. 

Although a surface in an ambient environment may appear clean, in reality the surface is covered with a layer or layers of adsorbed species. These species may be either physically or chemically adsorbed to the surface, but in both instances prevent the study of a truly clean surface. Under a vacuum environment, such surface contaminants can be removed into the gas phase by sputtering the surface with energetic ions. Once the surface contaminants have been removed, ultrahigh vacuum conditions are required to keep the sample clean. For example, at a pressure of 1 X 10 Torr, a sample receives a flux of approximately 5 X 10 " molecules s cm . Assuming a surface density of 10 atoms cm and a sticking probability of 1 for the adsorption of the gas phase species, the sample would be covered by one monolayer of adsorbed species in seconds. Conversely, by working at pressures below 1 x 10 Torr, a sample can be easily maintained free of contaminants for times on the order of hours, the time frame required for most surface science experiments. 

The susceptibility of solid surfaces to contamination often results in a requirement for an ultrahigh vacuum (UHV) chamber for preparation and observation of particular samples. For many materials, including metals such as platinum and nickel, adsorption of hydrocarbons and chemisorption of oxygen are quite fast at atmospheric pressure, and the surface must be isolated in UHV to prevent rapid degradation. In addition, a sample in UHV may be subjected to surface analytical techniques such as X-ray photoelectron and Auger spectroscopy to verify or corroborate Raman results. As a result, much of the early and well-characterized surface Raman experiments were carried out in UHV chambers operating below 10 torr (12). 

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