Adsorptive Preconcentration

Figure 3 shows the operation of the preconcentrator. In Step 1, the preconcentrator processes the airflow from the sample collection subsystem. Minute amounts of explosive particles and vapors are entrained in this airflow. In Step 2, the preconcentrator adsorption surface is heated to vaporize the collected and concentrated explosives sample into the airflow of the detector inlet. 

Last time development of methods of iodine determination, which include preliminary sorption preconcentration of microcomponents and their subsequent determination in phase of concentrate get great practical significance. Silica gel (SG) with adsorptively modified quaternary ammonium salts (QAS) gets properties of anion-exchange resin. The sorbents modified in this way can be used successfully for determination of different anions. 

ETAAS, SPECTROSCOPIC AND TEST DETERMINATION OF HEAVY METALS IN NATURAL OBJECTS AFTER THEIR PRECONCENTRATION BY ADSORPTION... 

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. 

Adsorptive cathodic stripping voltammetry has an advantage over graphite furnace atomic absorption spectrometry in that the metal preconcentration is performed in situ, hence reducing analysis time and risk of contamination. Additional advantages are low cost of instrumentation and maintenance, and the possibility to use adapted instrumentation for online and shipboard monitoring. 

Electrochemical preconcentration can be achieved in the following two different ways, depending on whether differential pulse stripping voltammetry (differential pulse ASV) or adsorption differential pulse voltammetry has been applied. 

Cathodic stripping voltammetry has been used [807] to determine lead, cadmium, copper, zinc, uranium, vanadium, molybdenum, nickel, and cobalt in water, with great sensitivity and specificity, allowing study of metal specia-tion directly in the unaltered sample. The technique used preconcentration of the metal at a higher oxidation state by adsorption of certain surface-active complexes, after which its concentration was determined by reduction. The reaction mechanisms, effect of variation of the adsorption potential, maximal adsorption capacity of the hanging mercury drop electrode, and possible interferences are discussed. 

A variety of preconcentration procedures has been used, including solvent extraction of metal chelates, coprecipitation, chelating ion exchange, adsorption onto other solids such as silica-bonded organic complexing agents, and liquid-liquid extraction. 

Hicks and Riley [287] have described a method for determining the natural levels of nucleic acids in lake and seawaters, which involves preconcentration by adsorption onto a hydroxyapatite, elution of the nucleic acids, and then photometric determination of the ribose obtained from them by hydrolysis. 

Adsorption on XAD-2 and XAD-4 resins followed by solvent desorption and head space GS has been employed for the preconcentration and determination of volatile organosulfur compounds in estuary and seawater [330]. 

The adsorption behavior of the psychotropic drug flunitrazepam (256) at the hanging mercury drop electrode was studied by staircase voltammetry and by adsorptive stripping differential pulse voltammetry. 256 can be determined down to nanomolar levels by using adsorptive preconcentration prior to the differential pulse voltammetry scan. The method was applied to determination of 256 in human urine530. 

Analytical methods for detecting phenol in environmental samples are summarized in Table 6-2. The accuracy and sensitivity of phenol determination in environmental samples depends on sample preconcentration and pretreatment and the analytical method employed. The recovery of phenol from air and water by the various preconcentration methods is usually low for samples containing low levels of phenol. The two preconcentration methods commonly used for phenols in water are adsorption on XAD resin and adsorption on carbon. Both can give low recoveries, as shown by Van Rossum and Webb (1978). Solvent extraction at acidic pH with subsequent solvent concentration also gives unsatisfactory recovery for phenol. Even during carefully controlled conditions, phenol losses of up to 60% may occur during solvent evaporation (Handson and Hanrahan 1983). The in situ acetylation with subsequent solvent extraction as developed by Sithole et al. (1986) is probably one of the most promising methods. 

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. 

This improvement was due to an easy adsorption of ss- and dsDNA on the GC(ox) surface [46,47]. Regarding the nonconductive nature of graphite oxide film formed on the surface during anodization [15], the activation of GC woifld affect primarily the adsorption process but not the charge transfer of the G and A residues. The ssDNA was preconcentrated on GC(ox) surface... 

Analytes can be separated from complex matrices by sample preparation techniques that include liquid extraction, supercritical fluid extraction, and solid-phase extraction. Dilute ionic analytes can be preconcentrated by adsorption onto an ion-exchange resin. Nonionic analytes can be concentrated by solid-phase extraction. Derivatization transforms the analyte into a more easily detected or separated form. 

Figure 24.6 Adsorptive stripping voltammograms for 4 x 10 8 M daunorubicin following different preconcentration times (A) 0, (B) 90, (C) 180, and (D) 300 s. [From J. Wang, M. S. Lin, and V. Villa, Analyst 112 1303 (1987), reproduced with permission.]...

Figure 24.7 Catalytic-adsorptive stripping voltammograms for 0.2 pg/L (ppb) platinum after different preconcentration times 0, 60, 120, 180, and 300 s. Inset Current vs. preconcentration time plots for (A) 0.2 and (B) 0.6 pg/L platinum. [From Ref. 33, reproduced with permission.]...

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