Preconcentration by precipitation

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. 

Figure 5.11 Steps followed for analyte preconcentration by precipitation with a knotted reactor in ICP-MS.

Stripping voltammetry procedure has been developed for determination of thallium(I) traces in aqueous medium on a mercury film electrode with application of thallium preconcentration by coprecipitation with manganese (IV) hydroxide. More than 90% of thallium present in water sample is uptaken by a deposit depending on conditions of prepai ation of precipitant. Direct determination of thallium was carried out by stripping voltammetry in AC mode with anodic polarization of potential in 0,06 M ascorbic acid in presence of 5T0 M of mercury(II) on PU-1 polarograph. 

A wide variety of sample types, sample preparations, and processes have been used. Powdered rock samples were fused with K2B07 or K2C03, followed by precipitation of the potassium using perchloric acid, separation with methanol-perchloric acid, evaporation to a residue, and dissolution of the residue in dilute nitric acid. Detection limits in the solid were in the microgram per gram ( xg/g) to nanogram per gram (ng/g) levels in the solid without preconcentration. 

Determined by ICPAES after preconcentration by fluoride and oxalate precipitations. 

The strontium preconcentration by coprecipitation of calcium and strontium phosphates requires magnetic stirrers with heating plate and a centrifuge collecting the precipitate. A drying cabinet is used to gently dry the precipitate of calcium and strontium phosphates. 1220 Quantulus counter is routinely used for the determination of low-level radioactivities. 

Magnesium-induced coprecipitation (Karl and Tien, 1992) involves the precipitation of brucite (Mg(OH)2) from seawater by the addition of base. It was observed that phosphorus compounds, including organic phosphates, coprecipitate with the brucite, and this was used as the basis for preconcentration of phosphorus prior to analysis of total phosphorus in the collected precipitate. Nucleic acids have also been determined quantitatively by precipitation with cetyltri-methylammonium bromide, with DNA and RNA determined at xg/l levels by fluoromet-ric and colorimetric procedures (Karl and Bailiff, 1989). Iron and barium salts are also commonly used as precipitants to isolate inositol phosphates after selective oxidation of organic phosphorus (Cosgrove, 1980). 

As a result of the recent developments, separation and preconcentration procedures by precipitation, originally requiring hours of operation and a few hundred milliliters of sample and reagent in the conventional batch mode, may now be completed in less than a minute with an automated FI system, while consuming a few milliliters of sample and reagent. Risks of contamination are also drastically reduced in the FI systems, which in turn improves the reliability and precision of the determinations. 

An alternative method has been described by Hartmann et al. (1989). It is based on preconcentration by co-precipitation of manganese hydroxide with Mg(OH)2 (fi om the seawater magnesium) at a pH of 10. After separation from the bulk of the seasalts by centrifugation and redissolution of the precipitate with nitric acid, Mn is measured by ETAAS. When applying a concentration factor of 40, the authors reported a detection limit for Mn of 0.02-0.04 nmol/kg. 

The trace level bromide content of natural water samples was preconcentrated by coprecipitation as AgBr with AgCl in the work of Denis and Masschelein [72], The precipitate was oxidized to AgBrOs with NaClO at pH 7. After separation it was determined via DPP in 1 M MgCh solution using 50 mV pulse amplitude and —0.2 to 1.8 V sweep range. A 2 ng/cm detection limit could be achieved with this method. 

Yan, X.-P, Kerrich, R., and Hendry, M. J. (1999) Row injection on-line group preconcentration and separation of (ultra)trace rare earth elements in environmental and geological samples by precipitation using a knotted reactor as a filterless collector for inductively coupled plasma mass spectrometric determination. J. Anal. At. Spectrom., 14,215-21. 

Trace metals in sea water are preconcentrated either by coprecipitating with Ee(OH)3 and recovering by dissolving the precipitate or by ion exchange. The concentrations of several trace metals are determined by standard additions using graphite furnace atomic absorption spectrometry. 

Matthews and Riley [99] preconcentrated iodide by co-precipitation with chloride ions. This is achieved by adding 0.23 g silver nitrate per 500 ml of seawater sample. Treatment of the precipitate with aqueous bromine and ultrasonic agitation promote recovery of iodide as iodate which is caused to react with excess iodide under acid conditions, yielding I3. This is determined either spectrophotometrically or by photometric titration with sodium thiosulfate. Photometric titration gave a recovery of 99.0 0.4% and a coefficient of variation of 0.4% compared with 98.5 0.6% and 0.8%, respectively, for the spectrophotometric procedure. 

Holm et al. [74] used a spectrometry for the determination of 237neptunium in seawater. The actinides are preconcentrated from a large seawater sample by hydroxide precipitation. The neptunium was isolated by ion exchange, fluoride precipitation, and extraction with TTA. 238Neptunium or 235neptunium was used to determine the radiochemical yield. 

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. 

Hess GG, McKenzie DE, Hughes BM. 1986. Selective preconcentration of polynuclear aromatic hydrocarbons and polychlorinated biphenyls by in situ metal hydroxide precipitation. J Chromatogr 366.197-204. 

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. 

Many metals in seawater can be preconcentrated for analysis by coprecipitation with Ga(OH)3. A 200-p.L HC1 solution containing 50p,g of Ga3+ is added to 10.00 mL of the seawater. When the pH is brought to 9.1 with NaOH, a jellylike precipitate forms. After centrifugation to pack the precipitate, the water is removed and the gel is washed with water. Then the gel is dissolved in 50 p,L of 1 M HN03 and aspirated into an inductively coupled plasma for atomic emission analysis. The preconcentration factor is 10 mL/50 p,L = 200. The figure shows elemental concentrations in seawater as a function of depth near hydrothermal vents. 

Concentrations of chromium in natural waters are very low. Thus, preconcentration of chromium is usually necessary. Some of the techniques used for pre-concentration include co-precipitation, solvent extraction using a variety of reagents, ion-exchange and electrodeposition. Most procedures have involved the determination of Crm and total chromium Crvl was then calculated by difference. 

River, stream, and lake water groundwater as well as atmospheric precipitation are commonly analyzed by using ICP-MS [303], Often the samples can be run directly or after simple filtration or centrifugation to remove suspended particulates [304]. Typically samples can be preserved by the addition of 1% to 2% concentrated nitric acid by volume [305]. Nitric acid is preferable to hydrochloric or sulfuric acid because of the lack of molecular ion spectral overlaps from nitric acid or its reaction products in the ICP, as discussed earlier. In most cases preconcentration or separation is unnecessary. In some cases preconcentration and removal of chlorides from the sample are advantageous and can be done by using a simple flow injection approach [306]. 

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