Chemical separation methods trace amounts

Solutions must be concentrated or the constituents must be isolated before trace amounts of the various organics present as complex mixtures in environmental water samples can be chemically analyzed or tested for toxicity. A major objective is to concentrate or isolate the constituents with minimum chemical alteration to optimize the generation of useful information. Factors to be considered in selecting a concentration technique include the nature of the constituents (e.g., volatile, nonvolatile), volume of the sample, and analytical or test system to be used. The principal methods currently in use involve (1) concentration processes to remove water from the samples (e.g., lyophilization, vacuum distillation, and passage through a membrane) and (2) isolation processes to separate the chemicals from the water (e.g., solvent extraction and resin adsorption). Selected methods are reviewed and evaluated. 

Next, characteristic properties of components are listed to select appropriate separation method (Table 3.7). Because the trace components belong to different chemical families, we eliminate gas-phase catalytic oxidation or hydrogenation. More specific chemical-based techniques remain. A first one is reversible chemical absorption. As solvents we may enumerate liquid redox systems (chelated iron), caustic washing solutions, amines or special formulations, as Selexol . Since H2S and C02 both have an add character, we may expect that a certain amount of C02 will pass in the off-gas stream. Dry chemical treatment could also be used, as reaction of H2S with iron-sponge or impregnated wood chips. 

The staphylococcal toxin must be separated from food constituents and concentrated to detect trace amounts. The toxin is then identified by specific precipitation with antiserum as follows (1) the selective adsorption of the enterotoxin from an extract of the food onto ion exchange resins and (2) the use of physical and chemical procedures for the selective removal of food constituents leaving the enterotoxin in solution. More recently rapid methods based on monoclonal antibodies (e.g., enzyme-linked immunosorbent assay, reverse passive latex agglutination) have been developed for detecting very low levels of enterotoxin in food. 

In spite of the successful proliferation of non-reaction methods of trace analysis, the application of chemical techniques in the GC determination of trace substances permits a much better accomplishment of such general problems as the determination of trace amounts of components in the zone of the main component, selective concentration of trace substances, improved separation of the latter and the main substance, decreased detection limits. 

The frontal chemical concentration method can also be used for the intermediate concentration trace amounts of heavy components [60] in the analysis of gaseous monomers (ethylene, propylene). The concentration was conducted on a short intermediate column containing diethanolamine and pure carbon dioxide was used as the carrier gas. The method permits three steps to be integrated into a single run preliminary separation, concentration of heavy trace components and analytical determination of the composition of the concentrate. The concentration of the heavy trace components to be determined was 10" %. Trace analytical methods based on selective retention of the main component are becoming more common in chromatography. 

In earlier work [14] a chemical method was proposed for concentrating trace amounts of carbon dioxide and hydrogen sulphide, based on the ability of substances with acidic properties to form unstable compounds with organic bases, such as ethanolamines, at room temperature. As the temperature is increased, these compounds readily dissociate into the starting components, and the concentrated zone of acidic gaseous trace components is separated into individual components in the chromatographic column. 

In the determination of transuranium elements (or nuclides), the most important step is separation of the elements from the sample matrix. Differences in redox properties are used for the separation of the first four elements in the series (neptunium, plutonium, americium, curium). Since the higher members exist primarily in the same oxidation state (III), separation by ion-exchange chromatography is commonly used. The lighter transuranic elements can be determined by common chemical methods, and trace amounts are usually determined by radiometric methods such as a-spectrometry. 

One of the main differences between radiochemical analytical procedures and classical analytical methods is that the element (and particularly its radioisotope) to be determined is present in the sample in minor to trace amounts. Separation of radionuclides is performed with the aid of a suitable carrier. Generally, the carrier is a stable isotope (or a suitable compound) that is added to the radioactive compound in a small but detectable amount and has identical chemical properties. An isotopic carrier, i.e., a stable isotope of the element in question, is most frequently used. Both the radioactive isotope and the carrier must be in the same chemical form. The isotopic carrier is irreversibly mixed with the radioactive compound and cannot be separated from it again by chemical means. Such a carrier can therefore be used only when a lower specific activity is sufficient for the subsequent operations. For example, barium or lead can serve as carriers when... 

Phosphorous. The presence of trace amounts of phosphorus in metals and semiconductors is known to affect material properties. The produced in the (n,y) reaction is a pure emitter and has to be separated and rigorously purified. Paul (1998,2000) developed a method for P determination in steels and other high-temperature refractory alloys of interest to the aircraft industry. Irradiated samples were dissolved passed through cation-exchange columns to remove Co, Ni, and Cr followed by repeated precipitations of magnesium ammonium phosphate and ammonium phosphomolybdate. One of the major advantages of this technique is the determination of the chemical yield by gravimetry. Phosphorus was determined by INAA in matrices other than metals, e.g., polymers. In this case, the beta spectrum was corrected for interferences and self-absorption (St-Pierre and Kennedy 1998). A modified version of this procedure has been used to certify implanted phosphorus in silicon (Paul et al. 2003). 

Iodine and bromine. Trace amounts of halogens were determined in silicate rock samples by Ozaki and Ebihara 2007. Activated samples were fused with NaOH and dissolved in water. Metal impurities were removed with hydroxide precipitate. Iodine was separated as Pdl2, and then a mixed AgCl-AgBr precipitate was formed. Chemical yields determined by the reactivation method varied between 70% and 80%. 

The main features of PC are low cost, need for small sample amount, high level of resolution, ease of detection and quantitation, simplicity of apparatus and use, difficult reproducibility (because of variation in fibres) and susceptibility to chemical attack. Identification of the separated components is facilitated by the reproducible Rj values. Detection methods in PC have been reviewed [368]. Fluorescence has been used for many years as a means of locating the components of a mixture separated by PC or TLC. However, also ATR-IR and SERS are useful. Preparative PC is unsuitable for trace analysis because filter paper inevitably contains contaminants (e.g. phthalate esters, plasticisers) [369]. For that purpose an acceptable substitute is glass-fibre paper [28]. 

Use of a radioactive tracer to determine a chemical yield is part of a broad suite of techniques known as isotope dilution The analyst wishes to measure the amount of a stable element X in a sample from which a pure chemical fraction of X can be only incompletely separated. A tracer aliquot containing a known mass of X (Mx), labeled with a radioactive isotope of X characterized by radioactivity of a known intensity (C), is added to the sample. A separation is performed to obtain a pure sample of X of mass Ms and a measured radioactive intensity of C which is less than C due to losses in the separation procedure Ms is determined by any suitable standard quantitative-analysis method (e.g., gravimetry of a stoichiometric compound of X). The specific activity of the chemical fraction, C /Ms, is equal to the specific activity of the element X in the mixture after tracing but prior to... 

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