Sample preparation for liquids
Genuine solution can mostly be analysed by AES without further treatment. For false solution mechanical steps are often necessary. Problems can arise in covering solid or powder samples into genuine solutions. Thus organic constituents in plant or animal samples must be removed by dry or wet ashing before the genuine solution is prepared. Dry ashing is combustion in an oxidizing-gas flow. Wet ashing is destruction of organic compounds by chemical reagents.
For trace analyses high demands must be made with respect to purity of the solvent since it is present in far higher concentrations than the trace to be analyses eg. 106 : 1, when the concentration of trace elements to be determined is 1 μg/g. Care must be taken to ensure that containers are clean. Container materials with static charges are unsuitable as dust settles on their surfaces.
Samples should have comparable density, temperature and viscosity. It is often possible to eliminate irregularities of this kind by using a reference.
One well-known technique is enrichment of traces by vaporization of the solvent . Enrichment can only be carried out for as long as all constituents remain in solution.
Another method is trace enrichment by sorption on activated charcoal. The principle is based on dissolving the analysis sample with suitable precipitation or complexing agents. In so doing, the traces, but not the interfering main constituents, form adsorbable compounds. When filtered in a filter coated with activated charcoal the traces are retained and, after removal with acid, analysis can be carried out. The efficiency of this method can be illustrated with a few examples:
A few μg Cu can be quantitatively separated from 101 water. Up to 10 elements can be quantitatively separated from high-purity metal such as Al, Mg, Mn, Tl and W.
In almost every case, analysis of liquid sample is preceded by sample preparation. This applies for samples which are already in the “liquid” physical state and which are brought into a specific standardisable initial state and which state eg. by adding acid. Other samples such as sludges or mineral samples must be brought into solution by means of a suitable fusion process before analysis. This also applies to the third group, organic and biological samples. Compared with the analysis process, fusion requires much more time and work, and the various techniques will now be discussed.
Liquid samples are subjected to analysis of their inorganic and organic analytes. Organic analysis is largely carried out using gas chromatographs and coupling these to various detectors. In the field of water analysis, also dust analysis, the aim of examinations is to control and monitor constituents which are toxic to people. Most analyses to be carried out in this field are performed in accordance with standard procedures described from sampling through sample preparation to the analysis method and analysis limit values.
There are corresponding provisions for analysis of inorganic constituents for the environmental field, basically regulating the monitoring of heavy metals.
Inorganic analysis of liquid samples is dominated by the analysis procedures such as AAS and AES-ICP since they meet requirements with respect to analyte diversity and LOD. The trend towards maximum automation capability , low LOD in the ng/ml range and maximum information with high sample throughput is best met by AES-ICP which gives simultaneous measurements.
The classical determination methods such as titrimetry, complexometric titration and voltammetry are still frequently used for individual analyses. These methods are, for the most part, fully automated and have been simplified down to a few manipulations. One standard analysis procedure from the field of water chemistry is photometric phosphate determination method with sufficient sensitivity Inorganic analysis also includes analysis of the anions which are determined by photometric or ion chromatography methods.
To give a general view of sample diversity and requirements connected with analysis, some typical problems will now be discussed in more detail.
Determination of metals is one of the types of analysis most frequently required. Concentration ranges from only a few ng/ml up to the percent range eg. for geological samples, must be determined.
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