Contamination negative

The Doctor Test, NF M 07-029 or ASTM D 484, is conducted in qualitative tests for free sulfur, free H2S, and mercaptans. The specifications require a negative Doctor Test for these three types of contaminants. 

Stemmier, E.A. and Hites, R.A., Electron Capture Negative Ion Mass Spectra of Environmental Contaminants and Related Compounds, VCH, Weinheim, Germany, 1988. 

The impact on negative-CA resists of airborne base contamination differs qualitatively from their positive tone counterparts. Suppression of acid-catalyzed chemistry at the surface of a negative resist results in some film erosion at the top of the exposed fields and in some cases an apparent loss of photosensitivity, but in general the reUef images formed exhibit the expected cross-sectional profile. This is in sharp contrast with the typical behavior seen with positive-tone CA resists, where suppression of acid-catalyzed chemistry at the surface causes an insoluble surface skin. 

Retention and drainage additives are vital to the use of recycled fibers. Papermakers consider recycled fibers to behave like virgin fines, while recycled fines behave like filler. Drainage on the paper machine can be impeded and first-pass retention reduced by the use of recycled fiber (9). Additionally, the negative impact of contaminants found in recycled fibers can be minimized by the appropriate use of dispersants and other pitch-control additives. 

Electrodialysis. Electro dialysis processes transfer ions of dissolved salts across membranes, leaving purified water behind. Ion movement is induced by direct current electrical fields. A negative electrode (cathode) attracts cations, and a positive electrode (anode) attracts anions. Systems are compartmentalized in stacks by alternating cation and anion transfer membranes. Alternating compartments carry concentrated brine and purified permeate. Typically, 40—60% of dissolved ions are removed or rejected. Further improvement in water quaUty is obtained by staging (operation of stacks in series). ED processes do not remove particulate contaminants or weakly ionized contaminants, such as siUca. 

Eor the negative electrolyte, cadmium nitrate solution (density 1.8 g/mL) is used in the procedure described above. Because a small (3 —4 g/L) amount of free nitric acid is desirable in the impregnation solution, the addition of a corrosion inhibitor prevents excessive contamination of the solution with nickel from the sintered mass (see Corrosion and corrosion inhibitorsCorrosion and corrosion control). In most appHcations for sintered nickel electrodes the optimum positive electrode performance is achieved when one-third to one-half of the pore volume is filled with active material. The negative electrode optimum has one-half of its pore volume filled with active material. 

Available Chlorine Test. The chlorine germicidal equivalent concentration test is a practical-type test. It is called a capacity test. Under practical conditions of use, a container of disinfectant might receive many soiled, contaminated instniments or other items to be disinfected. Eventually, the capacity of the disinfectant to serve its function would be overloaded due to reaction with the accumulated organic matter and organisms. The chlorine germicidal equivalent concentration test compares the load of a culture of bacteria that a concentration of a disinfectant will absorb and still kill bacteria, as compared to standard concentrations of sodium hypochlorite tested similarly. In the test, 10 successive additions of the test culture are added to each of 3 concentrations of the hypochlorite. One min after each addition a sample is transferred to the subculture medium and the next addition is made 1.5 min after the previous one. The disinfectant is then evaluated in a manner similar to the phenol coefficient test. For equivalence, the disinfectant must yield the same number of negative tubes as one of the chlorine standards. 

The important question, then, is not whether a substance is pure but whether a given sample is sufficiently pure for some intended purpose. That is, are the contaminants likely to interfere in the process or measurement that is to be studied. By suitable manipulation it is often possible to reduce levels of impurities to acceptable limits, but absolute purity is an ideal which, no matter how closely approached, can never be attained. A negative physical or chemical test indicates only that the amount of an impurity in a substance lies below a certain sensitivity level no test can demonstrate that a specified impurity is entirely al ent. 

Even when the building as a whole is maintained under positive pressure, there is always some location (for example, the outdoor air intake) that is under negative pressure relative to the outdoors. Entry of contaminants may be intermittent, occurring only when the wind blows from the direction of the pollutant source. The interaction between pollutant pathways and intermittent or variable driving forces can lead to a single source causing lAQ complaints in areas of the building that are distant from each other and from the source. 

FIGURE 7.17 When contaminants can be both negatively and positively buoyant, exhausts should be located both hl and low. 

An example of displacement ventilation with cold contaminants is shoun in Fig. 8.31. The contaminants, i.c., the fumes from the sewer, are colder than the room air, and thus tend to stay low due to the negative buoyancy. 

Heavy fumes or gases (i.e., negatively buoyant contaminants should be exhausted at floor level. Tbis applies to displacement ventilation as well as to mixing ventilation (Fig. 8.45). 

In several cases it is not certain it the contaminants are negatively or positively buoyant. In such cases one should place exhaust openings both below the ceiling and at floor level (Fig. 8.46). 

Gas storage cabinets consist of a box that encloses the tank(s) and all connections. Many include change out capabilities and an access door. The cabinets are exhausted to remove any contaminant that may leak into the cabinet and to maintain the cabinet under negative pressure relative to the... 

Class 1IB3 A Class IIB3 BSC (Fig. 10.98) is a ducted Type A cabinet having a minimum inward air velocity of 0.5 m s E All positive-pre.ssure contaminated plenums within the cabinet are surrounded by a negative air pressure plenum. Thus, leakage in a contaminated plenum will be into the cabinet and not into the environment. 

Zone, pressure A zone within a building or a process that is held at a given positive, negative, or neutral pressure in order to contain process contaminants. 

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