Contamination in reagents

The presence of metal contaminants in reagents may sometimes affect the chemical or biochemical outcomes of an experiment. In these cases, it is necessary to purify the reagents used. 

As the conditions used in the fluorometer are specific for chlorophyll and this is not introduced as a contaminant in reagents we have never foimd a blank, per se, with this method. The output (probably from scatter) of a tube of 90% acetone is not negligible on door 10 and the instrument should be zeroed against a tube of 90% acetone with all doors immediately prior to use. 

A detailed procedure for the use of MCPBA recently appeared in Reagents for Organic Synthesis by Fieser and Fieser. The commercially available MCPBA (Aldrich) is 85% pure the contaminant, m-chlorobenzoic acid, can be removed by washing with a phosphate buffer of pH 7.5. The epoxidation is usually performed as follows a solution of 3 -acetoxy-5a-androst-16-ene (2.06 g, 6.53 mmoles) in 25 ml of chloroform (or methylene dichloride) is chilled to 0° in a flask fitted with a condenser and drierite tube and treated with a solution of commercial MCPBA (1.74 g, 20% excess) in 25 ml chloroform precooled to the same temperature. The mixture is stirred and allowed to warm to room temperature. After 23 hr (or until TLC shows reaction is complete) the solution is diluted with 100 ml chloroform and washed in sequence with 100 ml of 10% sodium sulfite or sodium iodide followed by sodium thiosulfate, 200 ml of 1 M sodium bicarbonate and 200 ml water. The chloroform extract is dried (MgS04) and evaporated in vacuo to a volume of ca. 10 ml. Addition of methanol (10 ml) followed by cooling of the mixture to —10° yields 0.8 gof 16a,17a-epoxide mp 109.5-110°. Additional product can be obtained by concentration of the mother liquor (total yield 80-90%). 

A pentane solution of terf-butyllithium (purchased from either Alfa Inorganics, Inc. or Lithium Corporation of America, Inc.) was standardized by one of the previously described titration procedures (Note 1). If possible, it is desirable to use a freshly opened bottle of lert-butyllithium since previously used bottles of this reagent often contain lithium ferf-butoxide which will lead to formation of a contaminant in the final product (Note 10). 

Note As in related series, the addition of pyridine or (better) A,A-dimethylani-line (free of A-methylaniline, a common contaminant in some grades of this reagent) to phosphoryl chloride, appears to improve the yield of chloroqui-noxaline, especially if electron-withdrawing passenger groups are present. 

Portable or fixed multi-point colorimetric detectors are available which rely on paper tape impregnated with the reagent. A cassette of the treated paper is driven electrically at constant speed over a sampling orifice and the stain intensity measured by an internal reflectometer to provide direct read out of concentration of contaminant in sample. Such instruments are available for a range of chemicals including the selection given in Table 10.20. 

Today it has become clear that the effect of trace elements in living systems, in food, and in the environment depends on the chemical form in which the element enters the system and the final form in which it is present. The form, or species, clearly governs its biochemical and geochemical behaviour. lUPAC (the International Union for Pure and Applied Chemistry) has recently set guidelines for terms related to chemical speciation of trace elements (Templeton et al. 2000). Speciation, or the analytical activity of measuring the chemical species, is a relatively new scientific field. The procedures usually consist of two consecutive steps (i) the separation of the species, and (2) their measurement An evident handicap in speciation analysis is that the concentration of the individual species is far lower than the total elemental concentration so that an enrichment step is indispensable in many cases. Such a proliferation of steps in analytical procedure not only increases the danger of losses due to incomplete recovery, chemical instability of the species and adsorption to laboratory ware, but may also enhance the risk of contamination from reagents and equipment. 

After preliminary evaluation of the discovery route (scheme 1), we concluded that the overall yield of RWJ-26240 should be improved and that the use of NaCNBH. should be eliminated, since it produces a waste stream containing HCN or NaCN. Replacement of the expensive silver reagent, silver acetate, would permit significant cost reduction. The copper-catalyzed palladium coupling step would lead to palladium as a contaminant in the final product.177 Since a drug substance containing palladium would not be acceptable, this step would also have to be revised. 

The residues from a cortisol assay procedure (5 cc dichloromethane, 2.5 cc of a fluorescent reagent in 15 85 ethanol-sulfuric acid) were added to a 500 cc bottle and screw capped. After a 90 s delay, the bottle burst violently and brown fumes were seen. It was surmised that a nitrate or nitrite contaminant in the bottle had... 

It is vital to know if the analyte is present in the laboratory environment, adsorbed on the glassware, in reagents or in the demineralized water used in the analysis. These are all potential sources of contamination. This is particularly important when carrying out new determinations and for trace analysis. It is also important to ensure that your colleagues working nearby are not using chemicals that could affect your determination. 

An effect which alters or obscures the behaviour of an analyte in an analytical procedure. It may arise from the sample itself, from contaminants or reagents introduced during the procedure or from the instrumentation used for the measurements. 

The Blank Value Chart is also very important. This is a special form of a Shewhart chart where the direct measurements (e.g. in Volts) from the analyses of blank samples are used. From this chart information can be received about contamination of reagents e g. from the enviromnent and the state of the analytical system. 

The APEG-PLUS process is a technology for the ex situ treatment of chlorinated contaminants in a number of media. It is a form of alkaline dechlorination that uses an alkaline metal hydroxide with polyethylene glycol (APEG) as a reagent. It has been used to treat dioxin-contaminated... 

Fluor Daniel GTl, Inc. (now part of the IT Corporation), has developed in situ geochemical fixation technology to immobilize metallic contaminants in soil, sediment, sludge, and groundwater. The technology uses a site- and contaminant-specific combination of reagents to convert ionic contaminants to less soluble forms. In situ geochemical fixation has been used to remediate sites contaminated with chromium, uranium, molybdenum, and copper. 

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