Adsorbent-based preconcentration

One limitation of adsorbent-based preconcentration, which manifests itself during the sample collection step, is the poor retention of certain compounds on the adsorbent itself. For example, Tenax-GC retains nonpolar compounds more effectively than polar compounds, such as methanol or vinyl chloride (2), making the quantification of such poorly retained, polar materials difficult. Adsorbents that have higher affinities for polar compounds can help overcome this limitation. 

P. Hashemi, J. Boroumand, and M.R. Fat hi. A dual column system using agarose-based adsorbents for preconcentration and speciation of chromium in water. Talanta 64 578-583, 2004. 

Chromatographic method The determination of silicate based upon the adsorption of preconcentrated phosphomolybdic and silicomo-lybdic heteropoly acid (HPA) in the dynamic mode on a microcolumn packed with an Amberlite XAD-8 polyacrylate adsorbent is reported [49]. The method is based on the adsorption of HPAs followed by desorption with acetonitrile and determination by reversed-phase HPLC. Another highly sensitive HPLC method for the simultaneous determination of soluble silicate and phosphate in environmental waters is used in ion-pair liquid... 

Chemical preconcentration of the samples (e.g., extraction into chloroform and dithizone) was used in the past by several authors in order to determine the very low concentration of Pb in ancient Antarctic ice (6, 45, 64). Although this method gave reliable results, it needed a huge amount of sample (500-1000 g) and a long period of sample treatment with organic solvents. Another method is based on the preconcentration onto W wire loops that can be placed directly in a graphite furnace of instrument AAS for the atomisation of adsorbed metals for the analysis (44, 65). To minimise the contamination problems from the air of the laboratory the whole preconcentration procedure was carried out inside a vertical laminar flow clean bench. Detection limits of 0.01, 0.47, 0.22 and 0.24 pg/g were obtained for Cd, Cu, Pb and Zn respectively (44). These extremely low detection limits have recently enabled the concentrations of heavy metals in Antarctic ancient ice and recent snow to be determined (28, 72). 

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. 

Amberlite XAD polymeric adsorbents have been used for the preconcentration of trace elements in batch procedures followed by AAS or photometric measurements. The adsorbent series also shows promise in on-line preconcentration systems. Plantz et al.[24] used XAD-4 (polystyrene divinyl benzene based) to collect the bis(carboxymethyl)-dithiocarbamate complexes of Co, Cu, Cr, Ni, Mo, Pt, and V by reversed phase adsorption, and eluted the complexes with 0.1 M NH4OH, with the subsequent determination made by ICP-MS. Xu et al.[25] used the polyacrylate adsorbent XAD-8 to collect the chloro-complex of gold, which was subsequently eluted by ethanol and determined by flame AAS. 

An elegant example of SPE method based on ion-exchange retention was used for inline preconcentration of inorganic anions. A single capillary containing a preconcentration zone (adsorbed layer of cationic latex particles) and a separation zone (fused-silica modified by adsorption of a cationic polymer) was used. Analytes were retained in the preconcentration zone and eluted isota-chophoretically into the separation zone by means of an eluotropic gradient. This approach was used to determine nitrate in Antarctic ice cores at the 2.2-11.6 p,g L" level. 

Traditional adsorption preconcentration techniques are based aroimd the passage of filtered (and sometimes acidified or otherwise pretreated) water through a column of adsorbent. After passage of the sample the compounds of interest are eluted with an appropriate solvent or solvent mixture. Such techniques can be laboratory based, in situ (pumped systems or, more rarely, in passive towed units), or, in more recent systems, can be used in online applications. 

A new silica gel-based chelating sorbent, with thiourea as a functional group, was used for the FI online preconcentration and separation of trace levels of silver, gold, and palladium. The selected metal ions were adsorbed onto a column packed with thiourea-modified silica gel (TuSG). The sorbent exhibited excellent stability and the sorption properties of TuSG did not change after 1000 cycles of use. The selected metals in a secondary nickel alloy, an anode slim, an electrolytic solution, and three national certified ore samples were determined satisfactorily using the proposed method. ... 

Capitan-Vallvey et al. [92] proposed an integrated solid-phase spectrophotometry FIA method for the simultaneous determination of the mixture of saccharin and aspartame. The procedure is based on online preconcentration of aspartame on a Cjj silica gel minicolumn and separation from saccharin, followed by measurement at 210 nm of the absorbance of saccharin, which is transiently retained on the adsorbent Sephadex G-25 placed in the flow-through cell of a monochannel FIA setup using pH 3.0 orthophospho-ric acid-dihydrogen phosphate buffer, 3.75 x 10 M, as carrier. Subsequent desorption of aspartame with methanol enables its determination at 205 nm. 

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