Contamination organic trace analysis

Weissberger A (ed.) (1956) Techniques of Organic Chemistry, 2nd edn., vol. Ill, part 1. New York Interscience. Zief M and Horvath J (1976) Contamination in trace analysis. Laboratory Equipment Digest, October 1976, p. 47. 

In the context of organic trace analysis, appropriate stabilization and storage precautions are a function of the nature of the analyte and its concentration, To avoid contamination by ambient air and dust, all operations should be conducted under clean-room conditions (clean-bench environment). It is of paramount importance to assure that contamination from vessels, covers, septa, and stabilizers is rigorously excluded. 

In your laboratory refrigerator (5°C) you store pure cyclohexane that you use for extracting organic trace contaminants from water samples for subsequent analysis by gas chromatography. Among the compounds of interest is tetrachloroethene (also called perchloroethene or PCE). One day you realize that somebody is using tetrachloroethene in the laboratory. In fact, you can even smell the compound in the air (odor threshold values 0.03 - 0.1 mg-L-1). You are worried that your cyclohexane is contaminated, particularly, because you have realized that the bottle was not well sealed in the refrigerator. Calculate the concentration of PCE in the air that, at 5°C, would be sufficient to produce an equilibrium PCE concentration in the cyclohexane of 1 jig-fiL l, which you would consider to be a problem for your analysis. 

Water. A laboratory engaged in careful electrochemical work with aqueous solutions or in trace analysis will need facilities for the preparation and storage of highly purified water. Water commonly is contaminated with metals in both dissolved cationic form and in the form of colloidal or particulate matter that is not ionized appreciably.70 Frequently it also is contaminated by bacteria and by organic impurities that cannot be removed by ordinary or oxidative distillation because of the steam volatility of the impurities.71... 

To better understand the step-wise approach for method development and validation, it is necessary to give examples. They are taken from organic and inorganic trace analysis of environmental matrices. Figure 2.2 illustrates the steps for the validation of the analytical procedure for the determination of polychlorobiphenyls (PCB) in industrially contaminated soil. Figure 2.3 shows the steps necessary to validate the determination of trace elements and particularly arsenic in a fish tissue. Each step of the procedure will provide the necessary information so that the next step can be done with confidence. In practice, the analyst will develop a procedure to quantify all primary and secondary method characteristics as defined in section 2.1.4. 

Purity of the carrier gas is very important in modern GC equipment designated for trace analysis. Consequently, it is essential that the gas purifiers, such as the traps containing various adsorbents, be inserted in the gas tine before the injection port. The same requirement usually applies for purification of the combustion gases for the flame ionization detector. The role of these adsorbent traps is to remove even the trace quantities of water, oxygen and organic impurities present in commercial gas cylinders, and thus minimize both the system contamination and chemical alteration of an injected sample. 

Another major source of contamination in an analysis can be the analyst. It depends on what kind of analytes are being measured, but when trace or ultratrace levels of elements or molecules are being determined, the analyst can be a part of the analytical problem. Many personal care items, such as hand creams, shampoos, powders, and cosmetics, contain significant amounts of chemicals that may be analytes. The problem can be severe for volatile organic compounds in aftershave, perfume, and many other scented products and for sihcone polymers, used in many health and beauty products. Powdered gloves may contain a variety of trace elements and should not be used by analysts performing trace element determinations. Hair, skin, and clothing can shed cells or fibers that can contaminate a sample. 

Materials and chemicals must be carefully checked for contamination before use, and if necessary cleaned and purified. Heating for several hours [L51] or several days [152] is absolutely essential and very effective in the trace analysis of volatile analytes. Precleaning with boiling nitric acid may be necessary, as may a rinse with an organic solvent like acetone. Equipment for use in the determination of nonvolatile analytes is most effectively cleaned by rinsing with solvents that will be used subsequently in the course of the preparation process [153]. 

Acetic acid is a reasonable solvent which doesn t cause interferences. Its solvent strength for PAH is comparable to the low boiling chlorinated hydrocarborts. An important disadvantage of acetic acid is the formation of salts from the mineral part of the fly ash. After evaporation of the solvent a salt residue remains, which has to be isolated from the organic part by water extraction. Since it is unsuitable to have too many steps of sample preparation in trace analysis, extraction with acetic acid is not recommended. It was fotmd that water is contaminated with PAH in a way that it might affect the determination without further puriftcation by preextraction. 

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