Artifacts contaminants

The least resolved measurement is determination of the isothermal rate constant k(T), where T is the isothermal temperature. Although conceptually simple, such measurements are often exceedingly difficult to perform for activated process without experimental artifact (contamination) because they require high pressures to achieve isothermal conditions. For dissociative adsorption, k(T) = kcol (T) [S (Tg = TS = T)), where kcol(T) is simply the collision rate with the surface and is readily obtainable from kinetic theory and Tg and T, are the gas and surface temperatures, respectively [107]. (S ) refers to thermal averaging. A simple Arrhenius treatment gives the effective activation energy Ea for the kinetic rate as... 

Tentatively Identified Artifact Contaminants from Resin Fractionation Scheme... 

None of the alkyl-substituted aromatics identified as artifact contaminants from XAD-2 resin by USEPA-Battelle were identified in this experiment. 

Studies of the types described above must be re-evaluated to include control organisms which have body burdens of lead at natural levels (100-2000 times lower than present levels), which breathe air at natural levels, and which consume food and water containing lead at natural levels. Furthermore, since these studies would involve lead measurements at subnanogram levels, it is essential that they be carried out under ultra-clean conditions using the best clean-lab technology to avoid artifact contamination during sample handling and analysis. 

Sample preparation can also be conducted by direct online extraction, which offers advantages over offline extraction in that it involves minimized sample preparation, reduces loss of the analyte during extraction, and decreases the chance of artifacts, contamination, and dilution during the extraction [16,103]. A typical configuration for online extraction consists of an autosampler, two sets of binary pumps, an extraction column, an analytical column, and a divert valve. In the loading/ extraction stage, a sample is injected onto the extraction column, and the analytes are retained on the head of the extraction column, while the undesired matrix components such as proteins and salts are removed and... 

Benoit, G. Hunter, K. S. Rozan, T. F. Sources of Trace Metal Contamination Artifacts During Collection, Handling, and Analysis of Freshwaters, Anal. Chem. 1997, 69, 1006-1011. Keith, L. H., ed. Principles of Environmental Sampling, American Chemical Society Washington, DC, 1988. 

Artifacts may be introduced from the environment or through preparative techniques. When assessing individual tiny particles of material, the risk of loss or contamination is high, so that samples of this nature are handled and prepared for examination in a clean bench or a cleanroom (class 100 or better). 

In addition to cleanliness (contamination effects), surface morpholt and the alteration of composition during specimen preparation can cause serious artifacts in microanalysis. In some older instruments, the microscope itself produces undesirable high-energy X rays that excite the entire specimen, making difficult the accurate quantitation of locally changing composition. Artifacts also are observed in the EDS X-ray spectrum itself (see the article on EDS). 

The aspect of sample preparation and characterization is usually hidden in the smallprint of articles and many details are often not mentioned at all. It is, however, a very crucial point, especially with surface and interface investigations since there might be many unknown parameters with respect to surface contaminations, surface conformations, built-in stresses, lateral sample inhomogeneities, roughness, interfacial contact etc. This is in particular important when surfaces and interfaces are investigated on a molecular scale where those effects may be quite pronounced. Thus special care has to be taken to prepare well defined and artifact free specimens, which is of course not always simple to check. Many of these points are areas of... 

Similar to most Hg sampling methods, sampling sediments and soils require care in avoiding contamination artifacts due to improper sample handling. However, because Hg concentratiorrs are much higher in soUd matrices than in water, if corrrmonly accepted trace-metal protocols are used, substantial contamination artifacts should be exceedingly rare. Also, because sediment Hg concentration profiles... 

The use of urea must be approached with caution, because urea solutions often contain ammonium cyanate, the concentration of which increases with temperature and pH. This contaminant can react with the amino group of lysines and the amino terminus of the polypeptide chain, thus leading to artifact peaks. This effect is minimized by the presence of ampholytes, whose primary amines are cyanate scavengers, and by deionizing the urea solution with a mixed-bed resin prior to adding the ampholytes and detergent. 

An additional comphcating factor with respect to air samphng rate calibration is the uncertainty associated with determinations of vapor-phase and particle sorbed concentrations of analytes by HiVol sampling. These systems suffer from artifacts such as the volatilization of particle-bound contaminants, insufficient retention of small particles, and adsorption of vapor-phase contaminants on the GFFs (Ockenden et al., 1998 Lohman et al., 2001). These artifacts may cause concentrations in the vapor-phase to be overestimated or underestimated, which results in sampling rates that are too low or too high. 

---