Precipitation preconcentration methods

Komjarova, I. and R. Biust. 2006. Comparison of liquid-liquid extraction, sohd-phase extraction and co-precipitation preconcentration methods for the determination of cadmium, copper, nickel, lead and zinc in seawater. Anal. Chim. Acta 576 221-228. 

Laboratory methods. Huorimetry has been used widely for selenium analysis in environmental samples but is being superseded by more sensitive instrumental methods. Some of the instmmental methods used for arsenic speciation and analysis can also be used for selenium. In particular, HPLC and HG can separate selenium into forms suitable for detection by A AS, AFS (Ipolyi and Fodor, 2000), or ICP-AES (Adkins et al, 1995). Only Se(IV) forms the hydride, and so Se(VI) must be prereduced to Se(IV) if total selenium is to be determined. This is normally achieved using warm HCl/KBr followed by co-precipitation with La(OH)3 if necessary (Adkins et al., 1995). KI is not used, since it tends to produce some Se(0), which is not reduced by HG. La(OH)3 collects only Se(IV), so the prereduction step to include the contribution from Se(VI) is required before co-precipitation. Other methods of preconcentration include co-precipitation of Se(IV) with hydrous iron oxide or adsorption onto Amberlite IRA-743 resin (Bueno and Potin-Gautier, 2002). 

A preconcentration method that bears some resemblance to SDE consists of isolating the volatile phenols by steam distillation, followed by freeze-drying of the distillate. End analysis was by HPLC with ELD. The method was applied for determination of such phenolic components in foodstuffs and packing materials. Determination of phenolic antioxidants in polyolefins was carried out by dissolving the polymer sample in a hep-tane-isopropanol mixture (1000/5, v/v), at 160-170 °C, in an autoclave. The polymer precipitated on cooling the solution, and the dissolved antioxidant could be determined by LC with UVD. The advantage of the method is the relatively short time of analysis (about 2 h) and its reproducibility (RSD 3-5%) . 

The present Section provides a discussion of the following separation and preconcentration methods solvent extraction, precipitation and co-precipitation with collectors, volatilization, and methods based on the use of ion-exchangers and other sorbents. These methods are used not only with spectrophotometry, but also in conjunction with other methods of determination. 

FI On-line Preconcentration Methods with Continuous Precipitation-dissolution... 

In principle, all effective FI continuous precipitation systems with dissolution may be used to achieve some degree of analyte preconcentration. The only prerequisite is that enough sample is available. Within limits defined by the capacity of the precipitate collector, the enrichment factors EF will be proportional to the amount of sample processed. The efficiencies of different preconcentration systems, however, may be quite different. A quantitative evaluation of the efficiencies of the systems may be made using the criteria originally proposed for on-line column preconcentration methods. i.e., in terms of CE and CL These are calculated for the various methods collected in Tables... 

Table 7.2 FI preconcentration methods for flame AAS with on-line precipitation-dissolution... 

A third technique which is somewhat less popular but still useful is application of surface adsorption reagents. Activated car n is generally used as the carrier after conversion of the metal ion(s) of interest to a suitable form. For example, iodide in ground water has been determined by this method after conversion to silver iodide [61]. Neutral 8-hydroxyquinoline complexes have been employed in which, again, the oxine complex is collected on activated carbon [62]. Use of activated carbon is particularly useful where PIXE is being employed as the analysis technique, since the resultant sample is ideal for direct presentation to the instrument [63]. Other preconcentration methods that have been employed in this area include electrodeposition [64], precipitation chromatography [65], liquid/liquid extraction, immobilized reagents [66], plus a variety of other techniques well known to the analytical chemist. 

Three methods for trace metal preconcentration were examined liquid-liquid extraction aided by a chelating agent, concentration on a synthetic chelating resin and reductive precipitation with NaBTLt. The latter method gave 1000-fold preconcentration factors with total recovery of Pb and other elements17. Preconcentration of nanogram amounts of lead can be carried out with a resin incorporating quinolin-8-ol (3)18. Enhancement factors of 50-100 can be achieved by such preconcentration procedures followed by determination in a FLA (flow injection analysis) system limits of detection are a few pg Pb/L19. 

If necessary a preconcentration was carried out on this solution to lower the detection limits of the method. Preconcentration was achieved by a method involving co-precipitation of the antimony with hydrous zirconium oxide in which the digest is stirred with 150mg zirconyl chloride and the pH adjusted to 5 with ammonia to coprecipitate antimony and hydrous zirconium oxide. The isolated precipitate is dissolved is 7M hydrochloric acid and 30% sulphuric acid. Antimony is then converted to the pentavalent state by successive treatment with titanium III chloride and sodium nitrite and excess nitrite destroyed by urea. 

Several strategies have been described for the preconcentration of sample components present at low concentrations. These techniques include zone sharpening,28-29 on-line packed columns,30 and transient capillary isotachophoresis (cITP).31-32 Other standard laboratory techniques are often used, including solid-phase extraction, protein precipitation, ultrafiltration, etc. Two important points to keep in mind when selecting a concentration protocol are the sample requirements of the method and the potential selectivity on relative concentrations of sample components. The latter point applies to purity and concentration analysis. 

Various methods ofachieving preconcentration have been applied, including Hquid -hquid extraction, precipitation, immobihzation and electrodeposition. Most of these have been adapted to a flow-injection format for which retention on an immobihzed reagent appears attractive. Sohd, sihca-based preconcentration media are easily handled [30-37], whereas resin-based materials tend to swell and may break up. Resins can be modified [38] by adsorption of a chelating agent to prevent this. Sohds are easily incorporated into flow-injection manifolds as small columns [33, 34, 36, 39, 40] 8-quinolinol immobilized on porous glass has often been used [33, 34, 36]. The flow-injection technique provides reproducible and easy sample handhng, and the manifolds are easily interfaced with flame atomic absorption spectrometers. 

A simpler procedure consisting of protein precipitation using MeCN and TCA following formaldehyde derivatization was used for the determination of AMP in milk samples. The use of MeCN and TCA described in this method resulted in a clear supernatant and the highest recoveries (93% with CV of 6.1%). The derivatization reaction was done at 100°C for 30 min. The fluorescence of AMP derivatives was at least 20 times higher than that of AMO derivative thus, no preconcentration step was needed, resulting in the simplicity of this assay. A coextracted interfering unknown compound was observed at or near the retention time of the AMP derivative. This did not affect the accuracy (less than 10%) of the determination (74). A similar procedure was reported for beef, pork, chicken, and catfish tissue samples (75). 

This method combines the advantages of liquid chromatography with the selective and sensitive receptor assay Charm H-Q test. Milk sample previously precipitated (with Mcllvaine buffer) and preconcentrated on a C8 cartridge was fractionated using an LC system. The fractions were collected according to the retention times and peak widths, and they were assayed directly with Charm II-Q test with small modifications. This method was suited for the AMO, AMP, PenG, CLO, CEF, and CEP residues in the milk samples and after small modifications for CFD, TIC, and NAF. A simple purification scheme gave recoveries from 50% for AMO to 80-90% for other /3-lactams (95). 

In this technique the analyte elements are concentrated on a carrier precipitate which is then dissolved in a much smaller quantity of solution. The main advantage of this method is that very high concentration factors can be achieved. There are, however, a number of drawbacks, (a) Separations are lengthy and time consuming, (b) The analytical solution after preconcentration contains a high level of carrier precipitate. This can give rise to non-specific scatter and matrix interference in the flame of the atomic absorption instrument, (c) Contamination from the carrier can be a problem. 

Tris(pyrrolidine dithiocarbamato-)—cobalt(III) chelate, the precipitate formed by adding ammonium pyrrolidine dithiocarbamate to cobalt(II) solutions, has been found to be a good matrix for preconcentrating lead and several other metals by co-precipitation. Concentration factors of 40 to 400 are available by the method. The analyte is co-precipitated on the Co-APDC from a litre of sample. The precipitate is filtered on a fine porosity glass sinter and redissolved in a small volume of 6 M nitric acid. The solution is then used for atomic absorption analysis. 

Despite these recent advances, for many determinations the level of interest is so low that the only feasible approach is to separate and perhaps preconcentrate the analyte prior to determination in a flame or furnace. Some of the possible methods of doing this for high purity mineral acids, such as volatilisation, precipitation and complexation, have been discussed by an IUPAC Commission [5]. An alternative way of categorising the possibilities and a discussion is presented below. 

The analysis of environmentally-relevant samples is a major field of application. Based on the work of Garbarino and Taylor [421], a method has been proposed by the US EPA (Environmental Protection Agency) [422] and later by DIN [423] for waste water analysis. The latter, standardized procedure describes the sample decomposition, the analytical range for 22 elements and frequent interferences of ICP-AES in waste water analysis. For the analysis of natural waters, hydride generation [424], preconcentration based on liquid-liquid extraction of the dithiocarbamate complexes [425], adsorption of trace elements onto activated carbon [426] and also co-precipitation [e.g. with In(OH)2] [427], etc. have been reported and special emphasis has been given to speciation (as given in the Refs, in [428]) and on-line preconcentration [134]. 

Extraction is equally useful in the preconcentration and separation of small amounts of elements, and in the separation of macrocomponents from traces. Extraction methods generally require less time than precipitation methods. The former give also purer separation of elements owing to the small area of phase contact. Co-extraction occurring in some cases [11] has not been widely used in extraction separations. 

Sample preparation (Chapter 9) includes simple procedures such as centrifugation or filtration. Other more complex sample preparation procedures include passive or active dialysis, preconcentration, combustion or precipitation of matrix ions. In some cases, choosing the correct eluent/column/detector system will negate the need for sample preparations. A selective detector will be able to detect a minor analyte in the presence of other ions. Ion exclusion allows the passage of strong acid anions prior to separation of weak acid anions. Methods that require the least sample preparation or minimal sample preparation directly prior to injection are the most desirable. 

At extremely low concentration, silver can be preconcentrated by precipitation-dissolution and determined using flow-injection air-acetylene flame AAS. This procedure concentrates the sample, thus extending the detection limits (San t Ana et al. 2002). In environmental and biological materials, multi-element determination by ICP-MS including silver is a very sensitive and straightforward method for this element (Krachler et al. 2002, Wappelhorst et al. 

Sequential injection has proven itself especially useful for a variety of separation and preconcentration schemes, It has been applied to analvtical procedures involving such methods as membrane separations, pi I adjusinienLs. solid-phase e.xiraction. precipitation, and titration. In addition to colorimetry, ion-selective elecirodes. amperometry. Huoresccncc, IR abs >rplion. chemiluminescence, and conductimeiry have been used as detection methods wi(h SlA. 

Coprecipitation is a method for separation and preconcentration based on the formation of mixed crystals thanks to isomorphic exchange or adsorption of microcomponents on the surface of ionic crystals. Microelements in solutions in concentrations below ng/dm can hardly be isolated by direct precipitation, therefore different reagent carriers are used (Hoste et al., 1971 Das et al., 1983 Mizuike, 1983 Toelgyessy and Kyrs, 1989 Stoeppler, 1992 Nickson et al., 1995). The off-line approach... 

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