Preconcentration evaporative

Liu, Y., Mou, S., and Heberling, S., Determination of trace level bromate and perchlorate in drinking water by ion chromatography with an evaporative preconcentration technique, J. Chromatogr. A, 956, 85-91, 2002. 

Much more common, however, is secondary minerogenesis associated with loss of ff O through evaporation. This process is sometimes called evaporative preconcentration, which is increase in salinity and change in water composition due to the removal of H O. This process often leads to the precipitation of least soluble salts. On the surface and in the aeration zone it is associated with moisture evaporation and at great depths, possibly, with groxmd water degassing processes. 

This quadratic equation allows the determination of for different n values, i.e., the way Ca + and concentration in the solution changes as the evaporative preconcentration goes. Calculation results are in Figure 2.67. 

Drying operations also involve the removal of liquids by evaporation. Preconcentration of slurries by mechanical means, e.g. press filtration and centrifuging, is normal practice in order to reduce the evaporative load in the dryer. The degree of diluent removal is determined by the particle size of the solid and feed characteristics for the selected dryer, e.g. pumpable slurry, preformable paste and hard lumps. 

It is useful to compare various sampling methods to quantitative chemical analysis and to list their respective advantages and limitations (Table 6.3). In fact, an analysis is only as good as the sample which has been introduced into the analytical instrument. The ideal way to carry out a quantitative analysis with a sampling technique is to transfer an analyte completely from the sample matrix to the analytical apparatus. This means that in principle quantitative analysis of an additive is well carried out by dissolution (100% recovery), especially when the procedure restricts additional handling (evaporation, preconcentration, redissolution, etc.). The routine application of )uSEC-GC is a case in point. For quantitative analysis, most instruments require a solution. On-line combinations of sample treatment and analytical systems are being studied intensively. The idea behind such systems is to perform sample extraction, clean-up and concentration as an integral part of the analysis in a closed system [14]. 

Production of KCl at the Wendover, Utah operation employs a large 7000 acre complex of solar ponds. Both shallow brine wells and deeper wells are used to pump brine into the pond complex. In the preconcentration ponds water is evaporated and sodium chloride is crystallized. Later the brine is transferred to production ponds where sylvinite is deposited. Brine is then transferred to other ponds where camaUite is crystallized. Sylvinite is removed from drained ponds with self-loading scrapers and taken to the plant were KCl is separated by flotation with an amine oil collector. The camaUite,... 

The possibility of preconcentration of selenium in form of SeO by evaporation of low alkali water solution (for 20-1000 J.g/L) has been investigated. Considerable losses of selenium have been observed during evaporation of acidic and neutral solutions owing to volatility of selenium compounds. During evaporation of low alkali solutions at ph 9-10 there are no losses of selenium. Relative error of selenium determination is 1-2% for 1000 P-g/L solution and 3-5% for 20-100 p.g/L. Concentration factor is 10. 

The pressurised dissolution/cooling procedure of Macko el al. [490], which uses a UV-transparent low-boiling point solvent, is fast and simple as no additional evaporation of the solvent, preconcentration or redissolution of the additive is necessary. Macko el al. [491] have given an extensive listing of HPLC analyses of aromatic antioxidants and UVAs which can be separated with n-heptane and n-hexane as the main component of the mobile phase. The method was also used for HPLC quantification of thioether antioxidants (Santonox R, Chimox 14 and Irganox PS 802) in MDPE [612],... 

Mykytiuk et al. [184] have described a stable isotope dilution sparksource mass spectrometric method for the determination of cadmium, zinc, copper, nickel, lead, uranium, and iron in seawater, and have compared results with those obtained by graphite furnace atomic absorption spectrometry and inductively coupled plasma emission spectrometry. These workers found that to achieve the required sensitivity it was necessary to preconcentrate elements in the seawater using Chelex 100 [121] followed by evaporation of the desorbed metal concentrate onto a graphite or silver electrode for isotope dilution mass spectrometry. 

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. 

Preconcentration will only give precise results for quantitative work if the initial extraction technique gives high, or at least known and reproducible, recoveries of the desired compounds from the initial sample. As typically, several cycles are needed, and the solvent containing the extracted compounds must be concentrated to a small volume. This is normally carried out by evaporation under reduced pressure. Volatile compounds may be lost in this procedure. However, for many applications the compounds of interest... 

Solvent extraction is a very widely used and simple preconcentration technique. After the sample is extracted with a suitable solvent (such as methylene chloride), the extract is concentrated by evaporation and subjected to analysis. One important requirement is extremely clean solvents fortunately these are now commercially available. Because of the evaporation step, solvent extraction cannot be used for the analysis of very volatile compounds. Depending on sample size, sensitivities of 0.1 ppb can easily be achieved. 

Finally, when ultratrace determinations are being performed it is often necessary to preconcentrate the sample or separate the analyte of interest from the matrix. The most commonly employed methods for preconcentration and separation of water samples include evaporation, chelation, coprecipitation, extraction, ion-exchange, chromatography, and electrochemistry. The procedure adopted will depend on the analyte, the form in which it exists, and the sample matrix. 

Thermex is a commercially available ex situ technology that is based on the use of membrane separation technology for preconcentrating plant radioactive wastewater and evaporation for drying the preconcentrates. The technology is designed to minimize the volume of waste that would require storage or disposal. 

Where the analyte is present in sufficient concentration, it may be determined directly. Otherwise it may need to be concentrated by evaporation before determination. Methods of concentration include evaporation, solvent extraction and preconcentration using ion exchange or chelating resins. 

Air (diazinon, diazoxon) Preconcentration using ORBO-42 pesticide adsorbent tubes (Supelco). Extraction with acetone, evaporation just to dryness and redissolution in 100 pL acetone containing internal standard. Capillary GC/NPD No data >90 at 0.1 and 1 pg/m3 (diazinon) Williams et al. 1987... 

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