Contamination mechanisms

Use handling technique that avoids airborne contamination Mechanical handling... 

Turchi CS, Ollis DF. Photocatalytic degradation of organic water contaminants mechanisms involving hydroxyl radical attack. J Catal 1990 122 178-192. 

When introduced into the lungs of debilitated patients through contaminated mechanical ventilators. 

Chu FS Mycotoxins Food contamination, mechanism, carcinogenic potential and preventive measures. Mutat Res 1991 259 291-306. 

Regardless of placement of an aerobiological sampler in a laminar airflow work zone, it can at best measure the effect of the process at some point downstream from the product. For example, the mouth of a flask may be situated in first air issuing from the HEPA filter, while air impinging on the surface of a plate adjacent to it will be affected by disruptions of the airstream caused by the flask. Contamination found on the plate then results from a different set of conditions than those to which the product is subjected and does not exactly parallel the product contamination mechanism. Only a media-fill process simulation can fulfill this function. Aerobiologic sampling immediately downstream of the critical orifice can, however, detect downward shifts in the overall cleanliness of the critical process air, which in turn may indicate increased contamination potential near the product. 

V. Bertagna, F. Rouelle, G. Revel, and M. Chemla, Electrochemical and radiochemical study of copper contamination mechanism from HE solutions onto silicon substrates, J. Electrochem. Soc. 144, 4175,... 

Contamination Studies. Schmidt and Pearce (22,). in a classic study, using NAA evaluated sources of contamination of silicon during device processing. Their work demonstrated clearly that high-temperature oxidation steps are particularly likely to add transition-metal contaminants. Mechanical polishing compounds, boats and handling tools, and components of ion-beam equipment were also identified as being potentially troublesome. 

Bound residues of tetracyclines may occur in bones of slaughtered animals for months after treatment. Theoretically, these could reach the food chain via contaminated (mechanically deboned) meat or meat and bonemeal. The accumulation of tetracyclines in tissues is illustrated by the findings of Toutain and Raynaud for oxytetracycline in calves (Table 2.8). Concentrations of oxytetracycline were relatively high in liver and kidney compared to the extrapolated zero-time concentration for serum (4.2 mg/1). The time required for residues to deplete to 0.1 mg/1 in serum was 143 hr, considerably shorter than the time required for residues to deplete to 0.1 mg/kg in liver and kidney, but similar to the depletion time for muscle. The data nicely illustrate the importance of tissue elimination half-life in determining decrease to the 0.1 mg/kg concentration despite an almost three-fold higher initial concentration... 

The mobile phase may be contaminated with something that is present at very low levels and slowly accumulates on the column. Check the quality of the solvents The use of HPLC-grade solvents is highly recommended. Water can easily be contaminated. Water that is deionized using an ion-exdiange process should be treated subsequently with activated charcoal to remove small amounts of the ion-exchange resin, which otherwise would accumulate on a reversed-phase column and cause retention-time drift for compounds that can interact with the resin. Other column contamination mechanisms are discussed in more detail below. 

Turchi, C. S., D. F. Ollis (1990) Photocatalytic degradation of organic water contaminants Mechanisms involving hydroxyl radical attack. Journal of Catalysis 122, 178-192 Turco, R. R, R. C. Whitten, O. B. Toon, X B. Pollack and P. Hamill (1980) DCS, stratospheric aerosols and climate. Nature 283, 283-285... 

Harmful effects due to environment and products, e.g. vibration, impact, static strain forces, thermal loads, corrosion, contamination, mechanical wear, and lack of electromagnetic compatibility must be accounted for. 

Another important parameter in controlling the buildup of radiation fields is employment of an optimum primary coolant chemistry throughout plant operation, even when all the facts mentioned above indicate that deposition and activation of corrosion products in the reactor core do not represent the most important contamination mechanism. As was discussed in detail in Section 1.3., the primary aim of coolant chemistry is to keep the corrosion of the materials and, consequently, the release of metal atoms to the coolant as low as possible. With the main source of radioactive atoms being the in-core materials, this requirement particularly concerns the conditions prevailing inside the reactor pressure vessel. A second requirement which is of equal importance is to keep the concentration of... 

Contamination modeling is an important aspect of fuel cell development. It is required to interpolate and extrapolate experimental results to expected conditions in real-world operation, as it is impractical to test all combinations of reactant concentrations and fuel cell operating conditions. Modeling also assists in the development and validation of hypothesized contamination mechanisms. Model development for the anode is more extensive than that for the cathode contamination. The majority of the modeling deals with the kinetic effects associated with adsorption of contaminant species on the cathode and anode catalysts. 

In order to establish a fundamental understanding of contamination mechanisms, develop effective contamination control strategies, and then improve the reliability/durability of PEM fuel cells, it is important to recognize the sources and their possible effects on fuel cell operation. In this chapter, we review the origins of contaminants and their chemistries, with some description of their effects on PEM fuel cell operation. 

The effect of CO contamination on fuel cell performance is shown in Figure 2.1. Evidently, even a small amount of CO (and only a trace amount of CO) in the fuel cell feed stream causes a dramatic decrease in performance [9,25], and a higher CO concentration can cause an even larger drop. In order to mitigate the effect of CO contamination on fuel cell performance, it is important to understand the contamination mechanism. 

At the fuel cell cathode side, the prominent contaminants are NO (NO and NO2) and SO (SO2 and SO3). These mainly arise from electricity generation and fossil fuel combustions. NO, adsorb weakly on the Pt catalyst surface, and the contamination mechanism is mainly via the formation of NHJ, which... 

Since only a few groups have studied NOx contamination mechanisms, there is as yet no consensus about these processes. Thus far, two primary mechanisms for NOx contamination have been proposed the NOx reduction mechanism and the NOx oxidation mechanism. Mohtadi et al. [11] proposed the reduction mechanism, suggesting that NO2 was electrochemically reduced and thereby competed with O2 for Pt sites. The product of NO2 reduction was NH4+ (via equation (3.8)). NH4+ has been reported to be an ionomer poisoning species that might affect the ionomer and/or catalyst-ionomer interface. 

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