Preconcentration technique

Compton, T. R. Direct Preconcentration Techniques. Oxford Science Publications Oxford, 1993. 

Theoretical and applied aspects of microwave heating, as well as the advantages of its application are discussed for the individual analytical processes and also for the sample preparation procedures. Special attention is paid to the various preconcentration techniques, in part, sorption and extraction. Improvement of microwave-assisted solution preconcentration is shown on the example of separation of noble metals from matrix components by complexing sorbents. Advantages of microwave-assisted extraction and principles of choice of appropriate solvent are considered for the extraction of organic contaminants from solutions and solid samples by alcohols and room-temperature ionic liquids (RTILs). 

The term direct TXRF refers to surface impurity analysis with no surface preparation, as described above, achieving detection Umits of 10 °—10 cm for heavy-metal atoms on the silicon surface. The increasit complexity of integrated circuits fabricated from silicon wafers will demand even greater surfrce purity in the future, with accordingly better detection limits in analytical techniques. Detection limits of less than 10 cm can be achieved, for example, for Fe, using a preconcentration technique known as Vapor Phase Decomposition (VPD). 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

At present, inductively coupled plasma mass spectrometry provides a unique, powerful alternative for the determination of rare earths in natural samples [638,639]. Nevertheless, its application to the determination of rare earths at ultratrace concentration level in seawater is limited, because highly saline samples can cause both spectral interferences and matrix effects [640]. Therefore, a separation of the matrix components and preconcentration of the analytes are prerequisites. To achieve this goal, many preconcentration techniques have been used, including coprecipitation with... 

Winge et al. [730] have investigated the determination of twenty or more trace elements in saline waters by the inductively coupled plasma technique. They give details of experimental procedures, detection limits, and precision and accuracy data. The technique when applied directly to the sample is not sufficiently sensitive for the determination of many of the elements at the low concentrations at which they occur in seawater, and for these samples preconcentration techniques are required. However, it has the advantages of being amenable to automation and capable of analyzing several elements simultaneously. 

Hiraide et al. [737] developed a multielement preconcentration technique for chromium (III), manganese (II), cobalt, nickel, copper (II), cadmium, and lead in artificial seawater using coprecipitation and flotation with indium hydroxide followed by ICP-AES. The metals are simultaneously coprecipitated with indium hydroxide adjusted to pH 9.5, with sodium hydroxide, ethano-lic solutions of sodium oleate and dodecyl sulfate added, and then floated to... 

Much work has been published on preconcentration techniques this work is reviewed below. 

Table 6.2. Preconcentration techniques. Cations in estuary and coastal waters...

With the aim of minimising the time taken in the preconcentration, and extending the chemical analysis of surfactants to more complex aqueous matrices in which very low detection limits are required, preconcentration techniques using solid-liquid extraction with various adsorbent materials, such as XAD [27] and anionic exchange resin [28] have been developed. 

The major anions and cations in seawater have a significant influence on most analytical protocols used to determine trace metals at low concentrations, so production of reference materials in seawater is absolutely essential. The major ions interfere strongly with metal analysis using graphite furnace atomic absorption spectroscopy (GFAAS) and inductively coupled plasma mass spectroscopy (ICP-MS) and must be eliminated. Consequently, preconcentration techniques used to lower detection limits must also exclude these elements. Techniques based on solvent extraction of hydrophobic chelates and column preconcentration using Chelex 100 achieve these objectives and have been widely used with GFAAS. 

Sample preconcentration techniques are used with two purposes (1) to increase concentration in order to achieve detection and (2) to eliminate disturbances of the electrophoretic system during hydraulic or electrokinetic sample introduction when the conductivity of the sample is significantly different from that of the analysis buffer. It is important to keep sample manipulations and modifications to a minimum, and a rule of thumb is to prepare the sample so that its composition is at the same pH as the analysis buffer. It is also advantageous... 

EPA. 1988g. Method T03. Method for the determination of volatile organic compounds in ambient air using cryogenic preconcentration techniques and GC with flame ionization and electron capture detection. Compendium of methods for the determination of toxic organic compounds in ambient air. Atmospheric Research and Exposure Assessment Laboratory, Office of Research and Development, U.S. Environmental Protection Agency, RTP, NC. EPA/600/4-89/017. 

Solvent extraction is a very widely used and simple preconcentration technique. After the sample is extracted with a suitable solvent (such as methylene chloride), the extract is concentrated by evaporation and subjected to analysis. One important requirement is extremely clean solvents fortunately these are now commercially available. Because of the evaporation step, solvent extraction cannot be used for the analysis of very volatile compounds. Depending on sample size, sensitivities of 0.1 ppb can easily be achieved. 

HSSPIVIE Headspace solid-phase microextraction a preconcentration technique that concentrates volatile analytes on a fiber than can be inserted directly into a GC... 

Atlas, E and S. Schauffler, Analysis of Alkyl Nitrates and Selected Halocarbons in the Ambient Atmosphere Using a Charcoal Preconcentration Technique, Environ. Sci. Technol., 25, 61-67 (1991). 

Pillonel, L Altieri, D Tabacchi, R Bosset, J.O. (2004) Comparison of efficiency and stability of two preconcentration techniques (SPME and INDEx) coupled to an MS-based electronic nose . Mitt. Lebensmittelunters. Hyg. 95 85-98. 

Driss MR, Hennion M-C, Bouguerra ML. 1993. Determination of carbaryl and some organophosphorus pesticides in drinking water using on-line liquid chromatographic preconcentration techniques. J Chromatogr 639 352-358. 

Since many metal chelates are strongly coloured they lend themselves to absorptiometric procedures (see Section 10.5) and since formally uncharged chelates are commonly insoluble in water but soluble in an immiscible organic solvent they lend themselves to liquid-liquid extraction and various preconcentration techniques the same is often true of ion-pairs formed from bulky chelated cations (or anions) with suitable anions (or cations). 

Due to the high sensitivity of the UV-detection and less problematic UV-absorption maximum values (around 365 nm), simpler sample cleanup and preconcentration techniques were developed, as compared to the other antibiotic groups. 

Determination of inorganic anions by capillary electrophoresis is critically compared with ion chromatographic determinations on the basis of recent literature in the field. After a very brief summary of the theoretical background, the selection and optimization of the running electrolyte system are discussed, especially in connection with modification of the electroosmotic flow. Preconcentration techniques are surveyed, as are the approaches to the sample introduction and analyte detection. The principal analytical parameters of the determinations are evaluated and illustrated on selected applications described in the literature. 1997 Elsevier Science B.V. 

Conventional Preconcentration Techniques for Explosive Vapors (1973) (AD A026117)... 

Coquart, V. and M.C. Hennion (1991). Interference removal in the organic trace-level analysis of aqueous environmental samples by online liquid chromatographic preconcentration techniques with two precolumns. J. Chromatogr., 553 329-343. 

Capillary electrophoresis (CE) coupled to MS has the advantage of high resolution and soft ionization for biomolecules, which may be used to differentiate post-translational modifications and variants of intact proteins and oligonucleotides. Different modes of CE (capillary zone electrophoresis, capillary isoelectric focusing, capillary electrochromatography, micellar electrokinetic chromatography, nonaqueous capillary electrophoresis) to MS as well as online preconcentration techniques (transient capillary isotachophoresis, solid-phase extraction, membrane preconcentration) are used to compensate for the restricted detection sensitivity of the CE methodology [77, 78]. 

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