Contamination, cross

Cross-contamination by carry over of air-bome dusts is generally less important as a route for cross-contamination unless dust control measures are inadequate. However, efficient dust control is an essential part of good laboratory practice. 

Cross-contamination or carry-over, which Is the undesirable mixing of two successive samples, affects not only automatic, but also manual systems although it is easier to avoid In the latter. Carry-over Is a very Important aspect of autoanalysers, which differ essentially In the manner in which this undesirable phenomenon is avoided. In assessing the real significance of this source of error one should take Into account that It Is a major limiting factor of sample throughput. 

Carry-over effects can also be classified according to which Ingredients should not be mixed. Thus, in principle, there can be cross-contamination between samples, reagents or both (Table 3.2). 

TABLE 3.2 Causes of carry-over In automatic analysers 

The error made In the determination of bi arising from the contamination 

In order to avoid carry-over as far as possible in multi-determinations with batch analysers, the determinative sequence chosen should be that which results in the smallest effects possible from the preceding reagents. 

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Suitable inlets commonly used for liquids or solutions can be separated into three major classes, two of which are discussed in Parts A and C (Chapters 15 and 17). The most common method of introducing the solutions uses the nebulizer/desolvation inlet discussed here. For greater detail on types and operation of nebulizers, refer to Chapter 19. Note that, for all samples that have been previously dissolved in a liquid (dissolution of sample in acid, alkali, or solvent), it is important that high-purity liquids be used if cross-contamination of sample is to be avoided. Once the liquid has been vaporized prior to introduction of residual sample into the plasma flame, any nonvolatile impurities in the liquid will have been mixed with the sample itself, and these impurities will appear in the results of analysis. The problem can be partially circumvented by use of blanks, viz., the separate examination of levels of residues left by solvents in the absence of any sample. 

Special sample inlet devices such as nebulizers, furnaces, and gas inlets are commonly used to avoid cross-contamination and accidental fractionation of isotopes. 

When a sample of ca 100 g has been obtained, a representative sample for use in size characterization equipment must then be taken. Some of the more modem methods of size characterization require as Htde as 1 mg of powder, thus obtaining a representative sample can be quite difficult. If the powder flows weU and does not contain too many fines, a device known as the spinning riffler (Fig. 4c) can be used. A spinning riffler consists of a series of cups that rotate under the powder supply. The time of one rotation divided into the time of total powder flow should be as large a number as possible. Although this device has been shown to be very efficient, problems can be encountered on very small (1 mg) samples, and the powder must be processed several times. Moreover, in order to avoid cross-contamination, cleanup after each of the sampling processes can be quite difficult. Furthermore, if the powder is cohesive and does not flow weU, the equipment is not easy to use. A siUca flow agent can be added to the powder to enable the powder to flow... 

Another style tries to place process tanks such that those most sensitive to drippage problems and cross-contamination are placed in parts of the in-line layout where they would be least troublesome. The transport system is then programmed to take the work in a nonsequential order, skipping over stations and backtracking in order to complete the plating cycle. The carrying of work over other bars tends to leave salt buildup on the superstmcture of the work bars, and potential contamination of any and all tanks in the line depending on which station the salt buildup eventually drops into. 

The level of antibiotic resistance of the gut flora of pigs has been found to be influenced by factors other than the inclusion of antibiotics in the diet " " such factors include the herd environment, history and the opportunity for cross-contamination. 

Cross-contamination between animal species may be a factor in the spread of resistance. Flint and Stewart " showed that a rumen strain (46/5) of the Gram... 

Shared equipment Design to avoid or minimize use of (e.g. auxiliary process- common equipment for incompatible ing scrubbers ). Pos- materials sihility of incompatible, Implement proper cleaning procedure materials coming between incompatible uses to prevent together. cross contamination Prescrub or treat process streams before transfer to common equipment API RP 750 CCPS G-11 CCPS G-22 Kletz 1991 Lees 1996 NFPA-91... 

Identifying cross-contamination problems between previous and successive products and processes Receiving product contaminated by other products and processes... 

Consider common scrubbers, vents, sumps, drains, off-gas treatment and other opportunities for inadvertently mixing process materials. Cross-contamination potential at transfer stations should not be overlooked. 

Dedication of equipment stipulations concerning the installation of special equipment, equipment decontamination and cross contamination... 

Cross-contamination Mixing chemicals unintentionally, typically through the use of the same process equipment or support systems for concurrent or successive tolls. 

Minimize contamination via fewer cross-connections and fewer hose stations. Minimize the number of hoses required in loading/un-loading facilities. Cross-contamination, sometimes even from catalytic amounts of material, can result in undesired hazardous reactions. To prevent contamination due to rainwater and spills, consider storing a material that can react vigorously with water under a roof. 

Class U The Class U (Types A, Bl, B2, and biological safety cabinets provide personnel, environmental, and product protection. Airflow is drawn around the operator, through the hood opening and into the front grill of the cabinet, which provides personnel protection, in addition, the downward flow of HEPA-filtered air provides product protection by minimizing the chance of cross-contamination along the work surface of the cabinet. Because cabinet air has passed through the exhaust HEPA filter, it... 

Class HA in a Class IIA BSC, an internal blower (Fig. 10.9,St draws sui-ficient room air into the front grill to maintain a minimum calculated measured average velocity of at least 0.37 m s at the opening of the cabinet. The supply air flows through a HEPA filter and provides particulate-free air to the work surface. Laminar airflow reduces turbulence m the work zone and niim-mizes the potential for cross-contamination. 

Contaminant concentrations Dispersal of airborne contaminants such as odors, fumes, smoke, VOCs, etc. transported by these airflows and transformed by a variety of processes including chemical and radiochemical transformation, adsorption, desorption to building materials, filtration, and deposition to surfaces evolution of contaminant concentrations in the individual zones air quality checks in terms of CO2 levels cross-contamination evaluation of zones air quality evaluations in relation to perception as well as health. Methods ate also applicable to smoke control design. 

Contaminants Concentration in the zones, cross-contamination effects... 

The problem is to evaluate cross-contamination of the assembly hall by VOCs from the chemical cleaning and bonding process in the treatment hall. 

Comparative references are devices which are used to verify that an item has the same properties as the reference. They may take the form of materials such as chemicals which are used in spectrographic analyzers or those used in tests for the presence of certain compounds in a mixture or they could be materials with certain finishes, textures, etc. Certificates should be produced and retained for such reference materials so that their validity is known to those who will use them. Materials that degrade over time should be dated and given a use by date. Care should be taken to avoid cross contamination and any degradation due to sunlight. A specification for each reference material should be prepared so that its properties can be verified. 

Provide secure storage for reference materials and avoid cross contamination and degradation. 

Segregation is another important requirement and not only for nonconforming product as specified in clause 4.13. Segregation is vital in many industries where products can only be positively identified by their containers. It is also important to prevent possible mixing or exposure to adverse conditions or cross-contamination. Examples where segregation makes sense are ... 

The draft document address the issue of solvent recovered from a process and the use of these solvents in the same process or reused for different processes. It requires that recovery procedures be validated to ensure cross-contamination between recovered solvents and monitoring of the solvent composition at suitable intervals during the process. 

With the SMB system, solvents are recovered, continuously monitored, and analyzed by an automated software system. The chromatographic SMB process ensures that the solvents meet appropriate standards and cross-contamination is eliminated. 

A third motivation for studying gas solubilities in ILs is the potential to use compressed gases or supercritical fluids to separate species from an IL mixture. As an example, we have shown that it is possible to recover a wide variety of solutes from ILs by supercritical CO2 extraction [9]. An advantage of this technology is that the solutes can be removed quantitatively without any cross-contamination of the CO2 with the IL. Such separations should be possible with a wide variety of other compressed gases, such as C2LL6, C2LL4, and SF. Clearly, the phase behavior of the gas in question with the IL is important for this application. 

A co-solvent that is poorly miscible with ionic liquids but highly miscible with the products can be added in the separation step (after the reaction) to facilitate the product separation. The Pd-mediated FFeck coupling of aryl halides or benzoic anhydride with alkenes, for example, can be performed in [BMIM][PFg], the products being extracted with cyclohexane. In this case, water can also be used as an extraction solvent, to remove the salt by-products formed in the reaction [18]. From a practical point of view, the addition of a co-solvent can result in cross-contamination, and it has to be separated from the products in a supplementary step (distillation). More interestingly, unreacted organic reactants themselves (if they have nonpolar character) can be recycled to the separation step and can be used as the extractant co-solvent. 

For example, Novasina S.A. (www.novasina.com), a Swiss company specializing in the manufacture of devices to measure humidity in air, has developed a new sensor based on the non-synthetic application of an ionic liquid. The new concept makes simple use of the close correlation between the water uptake of an ionic liquid and its conductivity increase. In comparison with existing sensors based on polymer membranes, the new type of ionic liquid sensor shows significantly faster response times (up to a factor of 2.5) and less sensitivity to cross contamination (with alcohols, for example). Each sensor device contains about 50 pi of ionic liquid, and the new sensor system became available as a commercial product in 2002. Figure 9-1 shows a picture of the sensor device containing the ionic liquid, and Figure 9-2 displays the whole humidity analyzer as commercialized by Novasina S.A.. 

Figures 10-7, 10-7A, 10-7B, and 10-7C illustrate the general arrangements of most manufacturers, although several variations of plate flow pattern designs are available to accomplish specific heat transfer fluids temperature exchanges. Also, the gasket sealing varies, and some styles are seal welded (usually laser) to prevent cross-contamination. Note that Figure 10-7C has no interplate gaskets and is totally accessible on both sides, yet easy to clean.

Two factors militate against the universal use of water-based fluids. Very severe machining operations call for a lubrication performance that is beyond the capacity of such fluids, and the design of some machine tools means that water cannot be used because of the risk of cross-contamination with machine lubricants. In these instances, neat cutting oil is the only fluid that can provide the required performance. 

In certain processes (such as rubber tire manufacture and plastic injection molding operations), it is common for a valve sequencing system to first provide hot steam followed by cold cooling water and thus for cross-contamination to occur. The result may be the stripping of passivated cooling system surfaces by steam and for the condensate to be infiltrated by cooling water. 

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