Element analytical pretreatment protocols

The choice of method for a particular application should take into account this categorisation, as well as the precision and accuracy required. This chapter mainly deals with bulk analysis and speciation. 

The most frequently used methods for elemental analysis in plastics (certainly in the past) deal with digestions of some kind. Also, some derivatisation methods (e.g. hydride generation for element analysis, or the equivalent TMAH treatment for molecular analysis) may be used to generate volatile species which are more easily separated from each other by chromatography. Derivatisation reactions are often far from being well controlled. 

For destructive measuring methods, a CRM would serve as a reference to check the recovery of a particular matrix removal procedure. This is especially important for open destructions at atmospheric pressure. Alternatively, isotope dilution methods may be used once isotopic equilibrium is established, loss of analyte does not affect the analysis result. Isotope dilution techniques are only available in a few specialised laboratories. Another type of problem is encountered in pressurised methods oxidising the matrix in a closed vessel or bomb. Due to the large amounts of gas (CO2, NO, SO2) evolving from samples with a high organic matrix content, an excessive pressure build-up occurs that prohibits the use 

After matrix removal, samples can be measured using various techniques, such as AAS, AES, ICP, etc. Traditional chemical analysis methods, involving separation and gravimetric, titrimetric or polarographic determination of the elements, are being replaced by a wide selection of instrumental methods. 

Nondestructive radiation techniques can be used, whereby the sample is probed as it is being produced or delivered. However, the sample material is not always the appropriate shape or size, and therefore has to be cut, melted, pressed or milled. These handling procedures introduce similar problems to those mentioned before, including that of sample homogeneity. This problem arises from the fact that, in practice, only small portions of the material can be irradiated. Typical nondestructive analytical techniques are XRF, NAA and PIXE microdestructive methods are arc and spark source techniques, glow discharge and various laser ablation/desorption-based methods. On the other hand, direct solid sampling techniques are also not without problems. Most suffer from matrix effects. There are several methods in use to correct for or overcome matrix effects  

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As illustrated by the examples given in Table 3, the application of labs-on-chips to real samples is still limited. This is partly due to the fact that the analytical assay is only the final step of the whole procedure, which includes sample pretreatment protocols such as filtration, analyte cleanup, or analyte preconcentration. However, also the integration of corresponding microfabricated elements is described. Filtration was achieved by porous membranes or arrays of thin channels preventing particulates to enter the analytical device. Analyte preconcentration in combination with removal of other sample constituents is achieved by solid-phase extraction modules, which are either capillaries or beads coated with a suitable adsorbent, such as a Cl 8 phase originating from coating with octa-decyltrimethoxysilane, from which the analyte is... 

It is not possible to prescribe specific pretreatment procedures here because these can only be decided upon when the system and the purpose of the experiments has been properly defined. However, a wealth of information exist in various biochemical reference books on how to isolate various biological compounds. The recommended techniques and methods could be used as part of the trace element speciation protocol often after slight modification, taking into consideration the following points First, the trace element blank levels have to be low, less than 10% of the total concentration in the sample. Second, the regents used should not interfere with subsequent analytical determinations. Third, the experimental conditions should not deviate markedly from those found in vivo, especially the pH and ionic strength of the medium. 

The main goal of the present chapter is to summarize the significant items of these analytical protocols, including the steps corresponding to sampling, storage and preservation, laboratory pretreatments and instrumental techniques to determine heavy metals, trace elements, and major metals in natural waters. 

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