Preconcentration, elemental analysis sample

Lum, T.-S., Tsoia, Y.-K., Leung, K.S.-Y. Current developments in clinical sample preconcentration prior to elemental analysis by atomic spectrometry a comprehensive literature review. J. Anal. At. Spectrom. 29, 234—241 (2014)... 

ICP-MS is very promising in the area of environmental studies. Many elements can be determined directly in drinking water. In waste water analysis sample decomposition by treatment with HNO3-H2O2 is often required and the most frequent isobaric interferences have been described [559]. For seawater analysis, the salt contents makes sample pretreatment necessary, which can be done by chelate extraction. Beauchemin et al. [560] obtained a preconcentration of a factor of 50 by sorption of the trace elements onto an SiC>2 column treated with 8-hydroxyquino-line and determined Ni, Cu, Zn, Mo, Cd, Pb and U in seawater. In river water Na, Mg, K, Ca, Al, V, Cr, Mn, Cu, Zn, Sr, Mo, Sb, Ba and U could be determined directly and Co, Ni, Cd and Pb after the above mentioned preconcentration procedure. For As, preconcentration by evaporation of the sample was sufficient. Isotope dilution delivers the highest accuracy [561] and the procedure has been applied to... 

Schramel P., Xu L., Knapp G. and Michaelis M. (1993) Multi-elemental analysis in biological samples by online preconcentration on 8-hydroxy-quinoline—cellulose microcolumn coupled to simultaneous ICP-AES, Fresenius J Anal Chem 345 600-606. 

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. 

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. 

ICPMS can be used for both qualitative and quantitative trace and ultra trace elemental analysis of inorganics and in isotope ratio determinations. Both cations and anions can be determined. Normally, the sample is introduced in the form of a solution into the plasma, but direct analysis of gaseous or solid sample is also possible. Hence, ICP-MS has grown into a referral technique for the ultra trace analysis of REE in electronic materials, and metallurgical samples. The individual REE concentrations in natural and sea waters are so low and require preconcentration techniques prior to determination by ICP-MS. Separation in addition to preconcentration is also needed as high salt matrix of sea waters results in irreproducible results in ICP-MS analysis of individual REE. 

Another method that has been employed with some success to preconcentrate elements in natural water samples is co-precipitation. This relatively simple method offers the advantage of giving a fairly uniform deposit that can be collected easily. One of the earlier applications of this technique involved the use of iron hydroxide as a co-precipitant for the determination of Fe, Zn, and Pb in surface waters [56]. One of the more popular co-precipitants in use today is ammonium pynolidine dithiocarbamate (APDC). In the application of this method to the analysis of natural waters, detection limits in the range 0.4-1.2 ppb have been claimed for the elements V, Zn, As, Hg, and Pb [57]. Other co-precipitants have been described, including the use of iron dibenzyl dithiocarbamate for the determination of U at the ppb level in natural waters [58], zirconium dioxide for the determination of As in river water [59], and polyvinylpyrrolidone-thionalide for the determination of Fe, Cu, Zn, Se, Cd, Te, Hg, and Pb in waste and natural water samples [60]. The use of... 

X-ray fluorescence XRF is one of the longest established techniques for trace elemental analysis. While XRF is not a very sensitive technique, its main advantages are the capability for direct solid sample analysis combined with multielement determinations. While sample pretreatment of solids can be substantially reduced or even omitted in some cases, perfect matching between standards and samples is required for accurate results, because of severe matrix effects. The main application field of XRF is, therefore, the analysis of solid materials, such as metallurgical and geological samples, where solid standards are readily available. Liquid samples can be analyzed either directly in special cells or by using preconcentration techniques with solid sorbents, which can be directly analyzed after sample loading. More modern methods, like total-reflection X-ray fluorescence, which is a multielement technique mainly for solutions, or particle-induced X-ray emission, which is a micromethod with some spatial resolution, have found limited application in some special areas. For speciation purposes, species separation has to be carried out in front in an offline mode. 

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. 

In order to do elemental speciation analysis, the original distribution of chemical spedes must be carefully preserved until the analysis is done. It can be very difficult to keep the sample stable up to the point where it is injected into the instrument. Since the analytes can be oxidized or sometimes reduced to related species within the sample, just handhng the sample may result in changes in the relative concentrations of the analytes. Preconcentration of the samples is not advised, and in many cases the samples must be treated to preserve the original state. Eor example, EDTA may be added to complex the metals in a sample, protecting them from oxidation or reduction reactions. Nitric add may not be used for speciation because nitric add is an oxidizing agent. 

Direct atomic absorption spectrometry (AAS) analysis of increasing (e 0,10 g) mass of solid samples is the great practical interest since in a number of cases it allows to eliminate a long-time and labor consuming pretreatment dissolution procedure of materials and preconcentration of elements to be determined. Nevertheless at prevalent analytical practice iS iO based materials direct AAS are not practically used. 

Fang et al. [661] have described a flow injection system with online ion exchange preconcentration on dual columns for the determination of trace amounts of heavy metal at pg/1 and sub-pg/1 levels by flame atomic absorption spectrometry (Fig. 5.17). The degree of preconcentration ranges from a factor of 50 to 105 for different elements, at a sampling frequency of 60 samples per hour. The detection limits for copper, zinc, lead, and cadmium are 0.07, 0.03, 0.5, and 0.05 pg/1, respectively. Relative standard deviations are 1.2-3.2% at pg/1 levels. The behaviour of the various chelating exchangers used was studied with respect to their preconcentration characteristics, with special emphasis on interferences encountered in the analysis of seawater. 

Berman et al. [735] have shown that if a seawater sample is subjected to 20-fold preconcentration by one of the above techniques, then reliable analysis can be performed by ICP-AES (i.e., concentration of the element in seawater is more than five times the detection limit of the method) for iron, manganese, zinc, copper, and nickel. Lead, cobalt, cadmium, chromium, and arsenic are below the detection limit and cannot be determined reliably by ICP-AES. These latter elements would need at least a hundredfold preconcentration before they could be reliably determined. 

The application of the Chelex 100 resin separation and preconcentration, with the direct use of the resin itself as the final sample for analysis, is an extremely useful technique. The elements demonstrated to be analytically determinable from high salinity waters are cobalt, chromium, copper, iron, manganese, molybdenum, nickel, scandium, thorium, uranium, vanadium, and zinc. The determination of chromium and vanadium by this technique offers significant advantages over methods requiring aqueous final forms, in view of their poor elution reproducibility. The removal of sodium, chloride, and bromide allows the determination of elements with short and intermediate half-lives without radiochemistry, and greatly reduces the radiation dose received by personnel. This procedure was successfully applied in a study of... 

A logical approach which serves to minimise such uncertainties is the use of a number of distinctly different analytical methods for the determination of each analyte wherein none of the methods would be expected to suffer identical interferences. In this manner, any correspondence observed between the results of different methods implies that a reliable estimate of the true value for the analyte concentration in the sample has been obtained. To this end Sturgeon et al. [21] carried out the analysis of coastal seawater for the above elements using isotope dilution spark source mass spectrometry. GFA-AS, and ICP-ES following trace metal separation-preconcentration (using ion exchange and chelation-solvent extraction), and direct analysis by GFA-AS. These workers discuss analytical advantages inherent in such an approach. 

For analysis of solutions, ICP-mass spectrometry (ICP-MS) is very promising (Houk et al., 1980 Houk, 1986 Bacon et al., 1990). Recent advances in separation and preconcentration techniques are discussed by Horvath et al. (1991). Bacon et al. (1990) report that although ICP-MS is a multi-element technique, recent papers tend to concentrate on a small number of target elements. With isotope dilution mass spectrometry (IDMS), detection limits are further reduced (Heumann, 1988) IDMS is also suitable for accurate speciation in very low concentration levels of elements (Heumann, 1990). For the direct analysis of solid samples, glow discharge mass spectrometry (GD-MS) (Harrison etal., 1986) is of interest. Tolg (1988) has suggested that a substantial improvement in the absolute detection power of GD-MS, as applied to micro analysis, can be expected, at least in comparison with the ICP as ion source. 

ICP-MS dominates the field of environmental assay most metallic and amphoteric elements are susceptible of analysis, and it is often a great convenience (to say nothing of being relatively economical) to be able to assay for all elements of interest in a single analysis. Another feature of ICP-MS, however, has been exploited perhaps even more tellingly than multielement analysis. This is that the sample introduction system lends itself to a wide variety of enhancement schemes, in part because the sample is introduced to the instrument at atmospheric pressure and in part because samples are most often in a water-based (dilute acid) medium. These attributes combine to allow various separation and preconcentration schemes to be implemented on-line or nearly so. 

Copper is invariably determined by AAS in a lean air-acetylene flame, using the main resonance line at 324.7 nm. The detection limit is generally around 10 ng ml-1, which is marginally better than that generally achievable by flame AFS, and comparable to that reported for AES using a carefully optimized nitrous oxide-acetylene flame.2 Provided samples are not excessively diluted, this value is adequate for many practical applications in environmental analysis, such as the measurement of plant copper concentration or EDTA- or DTPA-extractable copper in soils. Interferences are rare, and unlikely to be a problem from concomitant elements present in most environmental samples, but matrix matching is still advisable. The sensitivity is inadequate for the direct determination of copper in natural water samples, for which a suitable preconcentration technique must be employed.1,23,24... 

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