Contamination methods

Contaminant Method limit of detection in Mean level (range)d in Percentage of seafood types with levels above MDL (%) Mean daily intake (pg kg 1 body weight/day) Oral RfDe Cancer benchmark concentrationf... 

Table 5.4 summarizes the acceptance criteria for instrument, calibration, and method blanks. No contaminants of concern should be present in method blanks above the laboratory PQL. Equally important is that instrument blanks show no memory effects. If these conditions are not met, a possibility for false positive sample results becomes real. For decision on sample data with contaminated method blanks the chemist may rely on the following rules of the Functional Guidelines ... 

NOTE A contaminated method blank is the only circumstance that may require more than one rerun per sample. 

All positive samples associated with a contaminated method blank and any samples which contain peaks that do not meet all of the qualitative identification criteria in Section 11 associated with a contaminated method blank must be reextracted and reanalyzed. Acceptable laboratory method blanks must not contain any chemical interference or electronic noise at the m/z of the specified unlabeled PCDD/PCDF ions that is greater than five percent of the signal of the appropriate internal standard quantitation ion. A peak that meets identification criteria in the method blank must not exceed two percent of the signal of the appropriate internal standard. 

Krogh T. E. (1971a) A low contamination method for the decomposition of zircon and the extraction of U and Pb for isotopic age determinations. Carnegie Inst. Wash. Yearb. 70, 258-266. 

Preliminary extraction of 5-HIAA may be used as an initial purification step before HPLC analysis. Organic solvents, anion-exchange resins, and other solid phase extraction procedures have aU been used. For many systems, direct injection of urine onto the analytical column is a common practice,and samples are often merely diluted with a buffer to protect the HPLC system from contamination. Methods that analyze 5-HIAA without prior sample cleanup rely on the selectivity of the HPLC separation combined with fluorescence or electrochemical detection to provide the requisite specificity. 

Polyethylene glycol [25322-68-3] Mr various, from PEG 200 to 35,000. PEG is available commercially as a powder or as a solution in various degrees of polymerization depending on the average molecular weight, e.g. PEG 400 and PEG 800 have average molecular weights of 400 and 800, respectively. They may be contaminated with aldehydes and peroxides. Solutions deteriorate in the presence of air due to the formation of these contaminants. Methods available for purification are as follows ... 

Special care is required to handle trialkyltin hydrides and the waste they generate, and standard laboratory purification techniques often leave toxic levels of tin compounds in the product [88]. The industrial application of these methods has been hindered by the need to remove these tin-containing contaminants. Methods catalytic in tin have been developed [89-91], and tin hydride reagents modified to make their removal easier [92-98], but the need for alternatives to tin... 

Comparison of the 140-h performance of two MEAs, one untreated and the other with mobile ruthenium removed in an electrochemical treatment before the life test, is shown in Fig. 5. The performance of the treated MEA is significantly better than that of the untreated one, yielding less than 1 mA cm" in unrecoverable performance loss and 67 mA cm in recoverable performance loss. By comparison, the recoverable and uiuecoverable performance losses of the untreated MEA are 17 and 81mA cm, respectively. It is worth remembering, however, that the treatment used in the case depicted in Fig. 5 is capable of limiting losses caused mostly by the currentless ruthenium contamination. Methods for limiting current-assisted contamination in DMFC systems with a Pt-Ru anode stiU need to be developed. 

The electrical testing of polyethylene materials seeks to determine the response of a sample to various types of electric fields. Variables include voltage, current (alternating and direct), contact or noncontact conditions, and various types of surface contamination. Methods of characterizing the electrical properties of polyethylene fall into two general categories those that determine electrical characteristics predictive of end use performance and those that seek to rank materials with respect to one another. The first type is exemplified by the measurement of relative permittivity, the second by the determination of arc resistance. 

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