Adsorption effects, trace losses

The collection and preparation of water samples requires individual approaches for different analytical tasks. If heavy metals or long-lived radionuclides at the trace and ultratrace concentration range are to be determined in water samples by ICP-MS, especially careful sampling is necessary to avoid possible contamination (using clean bottles and containers washed and cleaned before use, for example, with 2 % nitric acid and high purity water to stabilize traces in the samples), and the loss of analyte by adsorption effects or precipitation should be also considered. 

Trace losses are particularly common in the case of highly volatile analytes and as a consequence of adsorption effects, whereas trace contamination with otherwise prevalent elements and compounds may arise from laboratory air, vessels, chemicals, and various desorption effects. 

The properties of an organic tracing compound should minimize loss while in transit. There are two main sources of dye loss, non-adsorptive loss and adsorptive loss. Nonadsorptive losses can be due, among other reasons, to photochemical decomposition, chemical decay, pH effects, and biodegradation of the compound by microorganisms. Adsorption of the tracer onto both organic and inorganic substrates is often irreversible and can be a source of much loss. 

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. 

First, the solvents should be relatively free of trace impurities while dissolving the compounds of interest effectively. The solvent trace impurities, while not detectable in the bulk, can produce spurious peaks after the concentration of the volume of extracted sample. Whenever an extensive solvent clean-up is either impossible or impractical, appropriate sample blanks should frequently be run. With today s capabilities of high-resolution GC and spectral identification, occasional solvent impurities are tolerated as convenient markers . A good sample solubility is required to minimize possible losses due to sample adsorption on the glassware or, simply, its unnoticed precipitation. 

The loss of trace analytes can be minimized by conducting all operations in a system that is almost completely sealed hermetically from the ambient atmosphere, and by using vessel materials characterized by a small effective. surface area [3J. Surfaces should be free of fissures and preconditioned as necessary to minimize physical adsorption of the analyte (e.g., through ion exchange or hydro-phobic interactions). It is also important to consider the possibility of diffusion into or through the vessel walls. 

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