Preconcentration, elemental analysis sample preparation

Most samples for analysis by plasma emission spectrometry are dissolved in acids, because metal elements are more soluble in acids. The acidic nature of the solution prevents elements from adsorbing onto the surface of the glassware. Some bases, such as tetramethyl ammonium hydroxide (TMAH), are used to prepare solutions but care needs to be taken to avoid the formation of insoluble metal hydroxides. Organic solvents may be used to dissolve organometallic compounds directly or to extract chelated metals from aqueous solution into the solvent as part of separation and preconcentration steps in sample preparation. 

The term PCNAA is used when preconcentration precedes the neutron activation while if epithermal neutrons are used to excite the sample the acronym given is ENAA. The monitoring of the delayed neutrons emitted after excitation is termed DNAA. All these NAA procedures are nondestructive techniques used for characterizing solid (and in some cases also liquid) samples. However, a neutron source and a suitable detector are required and the sample can become quite radioactive after irradiation. The sensitivity of NAA techniques varies widely among different elements and sample preparation and post-irradiation methods employed. Several specific examples of NAA application for analysis of uranium in different matrices will be presented in the appropriate chapters. 

Sample preparation is an extremely critical step in the course of a speciation analysis, as the original sample has to be transformed into a form which can be subjected to analysis, while the original distribution of an element over its various chemical forms may not be altered. In general, determination of Hg species involves the following steps (1) sample collection/pretreatment/ preservation/storage (2) extraction of Hg from the matrix/cleanup/preconcentration (3) separation of Hg species of interest (4) detection. 

An analytical procedure consists of proper sampling, sample storage, if necessary, sample preparation, which is different for total elemental analysis and species analysis, separation procedures, if speciation or preconcentration is required, the quantification step, and quality assurance. All this applies to arsenic analysis and thus will be treated subsequently and demonstrated by practical examples for all these steps. Since speciation for arsenic is of paramount importance, it will be described in some detail. 

In order to prepare thin-film samples, the desired elements must usually be chemically or physically separated from the host compound. The elements being determined are collected in a physical form suitable for x-ray analysis using such methods as ion exchange, solvent extraction, or precipitation. Metallic ions, for example, may be collected on resin-loaded paper, which also serves as the mechanical support in the x-ray spectrometer. The absolute sensitivity for elements isolated from the host compound is 0.01 to 1 /ug analysis of elements present in trace concentrations is possible with this preconcentration approach, if the total sample size is sufficiently large. 

Long-lived radioisotope analysis is based on the measurement of a very small isotopic ratio. Usually, small amounts of chemically prepared pure samples (in the form of elements or compounds) rather than the original samples are used in the ion source. The element-preconcentration is performed during the preparation of the AMS sample. Since the natural abundances of long-lived radioisotopes are usually very low in terrestrial materials, the contamination by the radioisotope itself from the environment is not a severe problem. The background may come from interferences of atomic and molecular species, stable isobars and isotopes, which can be discriminated effectively by AMS. Consequently, high sensitivities can be achieved in the AMS measurements of the radioisotopes ( °Be, Al, Si, Cl, Ca, etc.) for tracer studies. The... 

During these preliminary preparation steps, additional modification of the sample such as preconcentration of the analyte or separation from potentially interfering components in the sample can be achieved. Therefore, the sample introduction technique of choice can play an important role in the design of an optimum strategy for analysis. A summary of proven sample introduction techniques for use with ICP-MS is given in Table 5.1. This table shows the types of techniques that are useful for the determination of analyte elements in gas, liquid, or solid samples and mixtures thereof. 

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