Sample Preparation---Aqueous Samples

A number of techniques can be used to isolate analytes from water. The technique used will depend on the volatility of the analyte. Volatile compounds (i.e., more volatile than n-C12) can be analyzed using Purge and Trap techniques or by Headspace analysis. Semivolatile compounds are extracted using liquid—liquid or solid phase extraction techniques. 

In this technique, an aliquot of sample (10 ml) is placed in a septum vial (20 ml) to a maximum of 50% capacity. The vial is spiked with surrogates and then heated for a moderate period ( 30 min) to create an equilibrium for volatile organic compounds between the air phase (headspace) and the water. The headspace is sampled (20—100 pi) with an airtight syringe and injected into a GC. Analytes are similar to 

Headspace does not have problems with water, and memory effects are greatly reduced from those found with P T. Problems will be encountered if the sample contains matrix modifiers that change the analyte pKas. In this case, quantification should be based on standard additions methods. 

Samples are transferred to a separatory funnel, surrogates are added, and an immiscible solvent (dichloromethane, hexane, etc.) is added. The liquids are shaken vigorously for a few minutes and then allowed to rest until a separation between the two phases occurs. The solvent is removed and the extraction process is repeated twice more. The extracts are combined, dried over anhydrous sodium sulphate, and processed further (cleanup) as required. Some laboratories have automated this tedious procedure by performing extractions in bottles. In this case, solvent and water are placed in a bottle and rotated (windmill rotators) or shaken (platform shakers) for 1—2 h. The lack of vigorous shaking is replaced by an extended time for extraction. Liquiddiquid extraction is used for all semivolatile analysis (hydrocarbons C12, PAH, pesticides, PCB, dioxins). By lowering the pH, extraction of phenols (pentachlorophenol) and acidic compounds (2,4-dichlorophenoxyacetic acid—2,4-D) will be enhanced. Increasing the pH will increase extractability of basic (aromatic amines) and neutral compounds (PAH). 

Rather than extracting water with solvent, the water sample is poured through a column or filter containing an absorbent resin. The organics will preferentially adsorb to the resin, which is subsequently desorbed with solvent. This technique has been used for PAHs, pesticides, and PCBs and has been well characterized for drinking water. Laboratories should take proper steps to evaluate the efficiency of this technique for effluent samples or turbid samples and may refer to EPA method 3535A or to guideline documents from SPE suppliers (e.g., Supelco bulletin 910). 

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Despite the remarkable sensitivity of modern instrumental detection techniques, analysis of environmental water samples nearly always requires enrichment of the analytes. This, together with separation from the matrix, are the two main functions of sample preparation appropriate sample preparation techniques address both issues at the same time, while striving to impose as few restrictions as possible on the subsequent instrumental determination (separation and detection). Sample preparation is strongly dependent on the nature of the analyte and the matrix, particularly with regard to its volatility and polarity. Figure 13.8 gives a general overview of common sample preparation (enrichment) techniques for aqueous and other matrices. 

The fume cupboard is cleaned directly after sample preparation. Any samples, organic solvent waste, chlorinated solvent waste, and aqueous wastes that do not require decontamination are collected into separate, clearly marked waste containers. In the same way, paper and consumable wastes that do not require decontamination are collected in a clearly marked waste box. Materials requiring decontamination must be treated with a proper decontamination solution and disposed of in designated waste containers. Glassware and accessories are flushed with decontamination solution and soaked in potassium hydroxide solution and, if not destroyed, they are washed with alkaline nonphosphorus detergent before further cleaning. 

The reasons for postponing a discussion of the preliminary steps to this point are pedagogical. Experience has shown that it is easier to introduce students to analytical techniques by having them first perform measurements on simple materials for which no method selection is required and for which problems with sampling,. sample preparation, and. sample dissolution are either nonexistent or easily solved. Thus, we have been largely concerned so far with measuring the concentration of analytes in simple aqueous solutions that have few inteifering. species. 

Raman spectroscopy is a nondestructive tool and requires little or no sample preparation. A sample may be analyzed in solid or powder form or in an aqueous solution and placed in glass containers such as an NMR tube, GC vial, test tube, light-path cell, or glass bottle. Aside from structure elucidation and functional group analysis, FT-Raman may be used for quantitative determination of polymorphs in a preformulation study. 

The AOAC and Codex recommended methods for determination of potassium and sodium are by flame atomic emission spectrometry. These methods have the advantage of quick and simple sample preparation, aqueous dilution of sample and filtration, with rapid, reasonably accurate detection. The samples can be aspirated directly into the flame and the readings quantified by comparison with a range of known standards. Atomic absorption spectrometry can also be used, giving increased accuracy and low-level sensitivity and a far wider range of detectable... 

Sample preparation. Whole samples, aqueous homogenates, or alkaline digests of samples can be used for analysis. Biological materials that lend themselves to the three methods of sample preparation are listed below. 

Atomization The most important difference between a spectrophotometer for atomic absorption and one for molecular absorption is the need to convert the analyte into a free atom. The process of converting an analyte in solid, liquid, or solution form to a free gaseous atom is called atomization. In most cases the sample containing the analyte undergoes some form of sample preparation that leaves the analyte in an organic or aqueous solution. For this reason, only the introduction of solution samples is considered in this text. Two general methods of atomization are used flame atomization and electrothermal atomization. A few elements are atomized using other methods. 

Sample preparation is straightforward for a scattering process such as Raman spectroscopy. Sample containers can be of glass or quartz, which are weak Raman scatterers, and aqueous solutions pose no problems. Raman microprobes have a spatial resolution of - 1 //m, much better than the diffraction limit imposed on ir microscopes (213). Eiber-optic probes can be used in process monitoring (214). 

Glass-Transition Temperature. The T of PVP is sensitive to residual moisture (75) and unreacted monomer. It is even sensitive to how the polymer was prepared, suggesting that MWD, branching, and cross-linking may play a part (76). Polymers presumably with the same molecular weight prepared by bulk polymerization exhibit lower T s compared to samples prepared by aqueous solution polymerization, lending credence to an example, in this case, of branching caused by chain-transfer to monomer. 

One of trends of development of thin-layer chromatography implies that replacement of aqueous-organic eluents by micellar surfactants solution. This is reduces the toxicity, flammability, environmental contamination and cost of the mobile phases, reduce sample prepar ation in some cases. 

Raman spectroscopy is a very convenient technique for the identification of crystalline or molecular phases, for obtaining structural information on noncrystalline solids, for identifying molecular species in aqueous solutions, and for characterizing solid—liquid interfaces. Backscattering geometries, especially with microfocus instruments, allow films, coatings, and surfaces to be easily measured. Ambient atmospheres can be used and no special sample preparation is needed. 

Different samples of aqueous solution containing radionuclides of Co and Eu were prepared at different copper sulphate concentrations and constant polymer concentrations (pAM) of 15 mg/1. The addition of salt to the system was done to reduce both the repulsion forces between the radionuclides and the interaction between the polymeric chains [7]. The polymer efficiency for the prepared samples was determined, results are shown in Fig. 15. It is clear that the polymer efficiency for Eu " is higher than for Co. This can be explained by the difference in the tightly bound structured water associated with different cationic species [14,107]. On this basis, we expect that Co is more hydrated than Eu. This is due to the difference in the ionic size. The hydra-... 

Over the past 10 years it has been demonstrated by a variety of in situ and ex situ techniques187,188 485 487 488 534 that flame-annealed Au faces are reconstructed in the same way as the surfaces of samples prepared in UHV,526-534 and that the reconstructed surfaces are stable even in contact with an aqueous solution if certain precautions are taken with respect to the potential applied and the electrolyte composition 485,487,488 A comprehensive review of reconstruction phenomena at single-crystal faces of various metals has been given by Kolb534 and Gao etal.511,513... 

For quantitative work, it is necessary to estimate the concentration of 5-amino-l-(P-D-ribofuranosyl)imidazole in aqueous solution. It seems that the only available method is the Bratton-Marshall assay, which was originally developed for the estimation of arylamines in biological fluids. The principle of the method is the spectrometric estimation of a salmon-pink colored dyestuff obtained by diazotation in situ, followed by coupling with /V-( 1 -naphthyl)ethyl-enediamine.65 The only remaining problem then is to know the molar extinction of this dye because pure samples of AIRs are not available. A value of 16800 at 520 nM was obtained for the dyes prepared from a model compound, 5-amino-l-cyclohexylimidazole-4-carboxylic acid (54), which is crystalline. A comparable molar extinction can be expected for the dye prepared from imidazole 55, if the carboxyl group does not exert too much influence on the chromophore. Actually, its influence is perceptible even with the naked eye, the dyestuff prepared from 53 having a somewhat different, wine-red color, with max>520 nM. The molar extinction for 55 is 17400 at 500 nM. When the decarboxylation of 54 was conducted under mild acidic conditions (pH 4.8, 50°C, 1 hour), estimation of 5-aminoimidazole 55 by the Bratton-Marshall method led to the conclusion that the reaction was almost quantitative.66 Similar conditions for the final decarboxylation were adopted in the preparation of samples of AIRs labeled with stable isotopes.58... 

In this paper we report (i) the catalytic activity for SCR of VOx/Zr02 samples prepared by various methods (adsorption from aqueous metavanadate solutions at different pH values, dry impregnation, and adsorption from VO(acetylacetonate)2 in toluene), (ii) sample characterization (nuclearity, dispersion and oxidation state) by means of XPS, ESR and FTIR and (iii) the nature and reactivity of the surface species observed in the presence of the reactant mixture. Catalytic results are here reported in full. Characterization data relevant to the discussion of the catalytic activity will be given, whereas details on the catalysts preparation and... 

Alternatively, an aqueous solution of sodium diethyldithiocarbamate (3.5%, 2 ml) or freshly prepared solution of dithizone in chloroform (0.1%, 10 ml) was added to sample A. The metal diethyldithiocarbamates (termed sample B) or metal dithizon-ates (termed sample C) thus formed were extracted in chloroform. The volume of chloroform extract was reduced to 1.0 ml. Aliquots (10 pi) each of sample B and sample C were chromatographed on plates coated with 0.25-mm layer of silica gel G using benzene -t methyl isopropylketone (50 1) and toluene -r chloroform (50 1), respectively, as mobile phases. Metal dithizonates were self-detected. The namral colored metal diethyldithiocarbamates were converted into brown spots by spraying... 

Plant material. Weigh 25 g of the chopped and frozen sample into a blender jar. To confirm recoveries, prepare fortiflcation samples by spiking the matrix with the appropriate volume of metabolite standard. Add 200 mL of acetonitrile-water (4 1, v/v) solution to the jar, and blend the mixture at medium speed for 5 min. Filter the extract through a Buchner funnel fitted with a glass-fiber filter pad into a 500-mL round-bottom flask containing 10 drops of Antifoam B and 3mL of 10% aqueous Igepal CO-660 (nonionic surfactant). The flask is connected to the Buchner funnel by... 

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