Metal analysis water samples

There are two main types of proficiency testing scheme. First, there are those set up to assess the competence of a group of laboratories to undertake a very specific analysis, e.g. lead in blood or the number of asbestos fibres in air collected on membrane filters. Secondly, there are those schemes used to evaluate the performance of laboratories across a certain sector for a particular type of analysis. Because of the wide range of possible analyte/matrix combinations it is not practicable to assess the performance of laboratories when analysing all the possible sample types. Instead, a representative cross-section of analyses is chosen (e.g. determination of different pesticide residues in a range of foodstuffs or the determination of trace levels of metals in water samples). 

Batley, G.E. and Gardner, D., 1977. Sampling and storage of natural waters for trace metal analysis. Water Res., 11 745-756. 

Common to nearly all analyses is preservation with refrigeration at 2-6°C, a practice, which minimizes the volatilization of organic compounds with low boiling points and the bacterial degradation of most organic compounds. That is why we must place samples on ice immediately after they have been collected, ship them in insulated coolers with ice, and keep them refrigerated until the time of analysis. Water samples collected for metal analysis and preserved with nitric acid are an exception to this rule as they may be stored at room temperature. The addition of methanol or sodium bisulfate solution to soil collected for VOC analysis is the only chemical preservation techniques ever applied to soil samples. 

Preconcentration of metals from water samples prior to atomic absorption spectrophotometric analysis increases the sensitivity of the analytical technique. It is only necessary when the level of metal required to be determined in the unconcentrated sample is below that which can be confidently measured by direct aspiration. 

Electrodeposition onto solid electrodes or mercury cathodes is a long established pre-treatment capable of large concentration factors, and provided the cathode potential is carefully controlled it is also of considerable selectivity. When atomic absorption is used as the finish, selective deposition is not usually required. There have been recent reports of electrodeposition of trace metals from water samples directly onto special graphite furnace tubes [8] and this technique should prove to be just as applicable to the analysis of reagents, where the chemical conditions can be more carefully controlled. The utility of electrodeposition for electrothermal atomisation... 

A. Cavicchioli, I.G.R. Gutz, In-line TiCb-assisted photodigestion of organic matter in aqueous solution for voltammetric flow analysis of heavy metals in water samples, Anal. Chim. Acta 445 (2001) 127. 

The sample containers are then air dried in an organic and dust free environment. After drying, containers should be sealed with the clean caps and liners and stored in a clean area to prevent re-contamination before they are used. For trace metal analysis the sample bottle should be thoroughly washed with detergent and tap water rinsed with 1 1 nitric acid, tap water, 1 1 hydrochloric acid, tap water and finally deionized water... 

Among the spectrometric methods used to determine metal concentrations, FAAS is particularly useful to perform water analysis (Figure 11.1). It is a relatively inexpensive method, which presents an adequate sensitivity sufficiently high for the determination of major metals in most of aquatic systems. Considering that most of atomic absorption instruments are also equipped to operate in an emission mode, large number of alkali metals (i.e., Na, K) are typically determined by flame photometry or flame atomic emission spectrometry (FAES) due to their relatively low excitation and simplicity of the emission techniques. This technique is relatively free from spectral interferences, and considering its versatility and simplicity of operation, it has become the most extensively used method for the determination of metals within water samples. 

The determination of metals in water samples by neutron activation analysis (NAA) shows different sensitivities for different samples, including several cases where NAA sensitivity is better than all the analytical techniques. Several of the factors which can affect the sensitivity of the method are sample composition, neutron flux, irradiation time, decay time, coxmting time, and detector efficiency [328,329]. Different preconcentration methods have also been applied to NAA protocols for metal analysis. For instance, the use of coprecipitation method [330,331], chelating adsorbents [332], etc. One of the additional advantages of this methodology is that both the irradiation and neutral activation can be directly performed on the resin, without eluting the metals from the column. 

BIA in association with anodic stripping voltammetry has been applied in the determination of trace metals in water samples [107]. In this study, the authors used a solid sorbent fixed in the micropipette tip for on-line preconcentration of the metals. After the optimization of experimental parameters, it was possible to obtain peak currents about 10-15% higher than without preconcentration step and linear concentration ranges at low levels such as 10-100 nmol 1 for Cd analysis. 

Batley, G. E. Gardner, D. Sampling and Storage of Natural Waters for Trace Metal Analysis , Wat Res. 1977,11, 745-756. 

The variety of AES techniques requires careful evaluation for selecting the proper approach to an analytical problem. Table 4 only suggests the various characteristics. More detailed treatment of detection limits must include consideration of spectral interferences (191). AES is the primary technique for metals analysis in ferrous and other alloys geological, environmental, and biological samples water analysis and process streams (192). 

Although simple analytical tests often provide the needed information regarding a water sample, such as the formation and presence of chloroform and other organohaUdes in drinking water, require some very speciali2ed methods of analysis. The separation of trace metals into total and uncomplexed species also requires special sample handling and analysis (12). 

At the Taylor Road landfill (originally intended for the disposal of municipal refuse only), unknown quantities of hazardous wastes from industrial and residential sources were deposited. During the period when the landfill was active, soil and groundwater samples collected at the site were found to contain concentrations of volatile organic compounds and metals above acceptable safe drinking water standards. Analysis of samples collected from private drinking water wells indicated that contamination... 

In order to derive a quantitative relation between emission Intensity as measured by EMI and actual metal content, cell samples were subjected to graphite furnace atomic absorption (GFAA) analysis (14). Atomic absorption experiments were performed both on cells which had been stained with a fluorescent reagent and on cells not exposed to a lumlnophore. After EMI analysis, 50 fiL of cell suspension were withdrawn from the 0.30 mL of sample used for EMI and were digested In 150 iiL of concentrated HNO3 for 90 minutes at 85° . These solutions were then diluted to 1/10 of their concentration with deionized water, and the 150 liL of these diluted... 

Samples for mercury analysis should preferably be taken in pre-cleaned flasks. If, as required for the other ecotoxic heavy metals, polyethylene flasks are commonly used for sampling, then an aliquot of the collected water sample for the mercury determination has to be transferred as soon as possible into glass bottles, because mercury losses with time are to be expected in polyethylene bottles. 

Shipboard analysis for the sampling of trace metals in seawater has been discussed by Schuessler and Kremling [2] and Dunn et al. [3]. Teasdale et al. have reviewed methods for collection of sediment pore-waters using in situ dialysis samples [4]. Bufflap and Allen [5] compared centrifugation, squeezing, vacuum filtration, and dialysis methods for sediment pore-water sampling. 

Evaluation of Methods for the Determination of Fluoride in Water Samples. 2 Analysis of a Competitive Product. 3 The Assessment of the Heavy Metal Pollution in a River Estuary. 4 The Analysis of Hydrocarbon Products in a Catalytic Reforming Study. 

Environmental water samples to be analyzed for metals are best stored in quartz or Teflon containers. However, because these containers are expensive, polypropylene containers are often used. Borosilicate glass may also be used, but soft glass should be avoided because it can leach traces of metals into the water. If silver is to be determined, the containers should be light absorbing (dark colored). Samples should be preserved by adding concentrated nitric acid so that the pH of the water is less than two. The iron in well water samples, for example, will precipitate as iron oxide upon exposure to air and would be lost to the analysis if not for this acidification. 

Environmental water samples to be analyzed for phosphate are not stored in plastic bottles unless kept frozen, because phosphates can be absorbed onto the walls of plastic bottles. Mercuric chloride, used as a preservative and acid (such as the nitric acid suggested for metals above), should not be used unless total phosphorus is determined. All containers used for water samples to be used for phosphate analysis should be acid rinsed, and commercial detergents containing phosphates should not be used to clean sample containers or laboratory glassware. 

The performance of atmospheric pressure interfaces appears to vary widely from instrument to instrument. A variety of interface designs are available from the various manufacturers. Optimisation of operating parameters, such as cone voltage, temperature, and mobile-phase composition is always necessary prior to actual analysis of samples. A given optimised set of parameters is likely to change with changing matrices, and may also vary with local conditions, such as the alkaline metal content of water or tubing used. 

A significant proportion of the needs for reference materials for seawater trace metal studies would be addressed by the preparation of these materials. Although the total iron concentration of these reference materials should be provided, these materials clearly will be useful for studies of other important metals such as zinc, manganese, copper, molybdenum, cobalt, vanadium, lead, aluminum, cadmium, and the rare earth elements. With careful planning, such water samples should be useful for analysis of dissolved organic substances as well. The collection sites should be chosen carefully to provide both a high and a low concentration reference material for as many metals as possible. 

Most of our understanding of the marine chemistry of trace metals rests on research done since 1970. Prior to this, the accuracy of concentration measurements was limited by lack of instrumental sensitivity and contamination problems. The latter is a consequence of the ubiquitous presence of metal in the hulls of research vessels, paint, hydrowires, sampling bottles, and laboratories. To surmount these problems, ultra-clean sampling and analysis techniques have been developed. New methods such as anodic stripping voltammetry are providing a means by which concentration measurements can be made directly in seawater and pore waters. Most other methods require the laborious isolation of the trace metals from the sample prior to analysis to eliminate interferences caused by the highly concentrated major ions. 

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