Other preconcentration techniques

Solvent extraction, coprecipitation and ion-exchange techniques are the main concentration methods used for seawater analysis. Other interesting concentration techniques, such as electrodeposition, amalgam trap (for mercury), a cold trap-vaporization system for hydride generation, and recrystallization, are often used by marine and analytical chemists. The first three methods are briefly reviewed here. 

Electrodeposition is a unique concentration technique, because the separation of trace metals from interfering matrix species can be easily carried out at the same time. Thus, the detection limits of AAS can be improved remarkably without further chemical preconcentration steps and spectral and chemical interferences due to major components in seawater can be also eliminated easily. Thus, many applications of the technique to seawater analysis by AAS have been made, especially in flameless AAS [69—78]. The 

HMDE (hanging mercury drop electrode) [71, 72], gold-foil [73], copper-wire [74], tungsten-wire [75, 76] and pyrolytic graphite-coated tube [78] have been used as the electrodes for electrochemical deposition, and successfully applied to the determination of Cu, Cd, Pb, Zn, Hg and so forth. In atomic absorption analysis the electrodes are usually heated directly for atomization of metals. 

CONCENTRATIONS OF ARSENIC SPECIES IN NATURAL WATERS (ng As ml 1) Locality and sample type As(III) As(V) MMAAa DMAAb 

25 m below surface 50 m below surface 75 m below surface 100 m below surface Sacramento River, Red Bluff, Calif. Owens River, Bishop, Calif. Colorado River, Parker, Ariz. Colorado River, Slough near Topock, Calif. 

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In addition to the aforementioned methods, TLC in combination with other instrumental techniques have also been used for quantification of inorganic species. For example, two-dimensional TLC coupled with HPLC has been utilized for the separation and quantification of REEs in nuclear fuel fission products using silaiuzed silica gel as layer material [60]. In another interesting method, REEs in geological samples have been determined by ICP-AAS after their preconcentration by TLC on Fixion plates [32]. TLC in combination with neutron activation has been used to determine REE in rock samples on Eixion 50 x 8 layers with the sensitivity limit of 0.5 to 10 pg/g for 10- to 30-mg samples [41]. A combination of TLC and A AS has been utilized for the isolation and determination of zinc in forensic samples [27]. 

Chromatographic Techniques. These techniques have long been applied to the problems of separation and analysis of trace atmospheric species. For stable species, batch samples are usually collected as described in the preceding section and transported to the laboratory for subsequent analysis. However, some compounds are not sufficiently stable to survive transport intact. In situ chromatographic analyses have been used for these samples. Usually, chromatography is used on aircraft in a batch mode samples are collected, preconcentrated, and separated on a column, and the individual species are detected as they elute the process is then repeated for the next sample. Thus, as with other batch techniques, time resolution is limited. 

The combination of preconcentration by electrodeposition with stripping by voltammetry is probably the most sensitive electroanalytical method in common use today. Consequently, this popular technique is discussed in more detail in Chapter 24. Preconcentration of material by an electrode reaction has been used in sample preparation for atomic absorption, neutron activation, x-ray fluorescence, microprobe, and several other spectroscopic techniques. 

Due to the high sensitivity of the UV-detection and less problematic UV-absorption maximum values (around 365 nm), simpler sample cleanup and preconcentration techniques were developed, as compared to the other antibiotic groups. 

Samples may be prepared for AAS in a variety of ways (Hui-Ming and Yao-Han 1984 Johnson et al. 1983 Murata and Noguchi 1974 Pierce and Brown 1977 Renshaw 1973 Sharp and Knevel 1971 Sugiyama et al. 1984). Acid digestion with nitric acid is the most common method of preparation. Sample dilution with nitric acid or other agents to solubilize barium from the matrix can also be employed. If the concentration of barium in the dissolved sample is very low, preconcentration techniques such as chelation or extraction may be employed. 

In the book, much attention has been paid to separation and preconcentration methods, since they play an essential role in increasing the selectivity and sensitivity of spectrophotometric methods. The separation and preconcentration methods have been utilised also in other determination techniques. 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

Determination LC-PDA recoveries 1 to 56% preconcentration cartridges prepacked with 0.5 g of silica and 10 mg of antifluorene antibodies and OSP-2 cartridges prepacked with 80 mg of silica and 2 mg of antifluorene antibodies other preconcentration precolumns prepacked with C18 silica higher selectivity of the antifluorene immunosorbent compared to conventional cleanup (immunosorbents more reliable technique than use of alumina for cleanup of complex environmental samples) methodology using immunosorbents can be automated LOD 0.9 to 37 /ig/l... 

Since its introduction, SPME has found numerous applicahons in the analysis of different classes of compounds present in various matrices. Several analytical methods for the determination of OCPs in water samples which make use of SPME as extracting and preconcentrating technique have been described. Magdic and Pawliszyn analyzed environmental water samples for the determination of OCPs using a PDMS-coated hber (him thickness 100 /rm). PDMS was preferred to other commercially available coating, i.e., polyacrylate, the latter being... 

Two other techniques which warrant some mention are inductively-coupled plasma atomic emission spectrometry (ICP-AES) and inductively-coupled plasma mass spectrometry (ICP-MS). The former is limited in sensitivity. Bussiere et al. (1989) found a detection limit of 2 fig/L which was just sufficient to determine manganese in amniotic fluid directly when matrix-matched calibration and an internal standard of gallium was used. Concentrations in ten samples ranged from 14 to 16 /ig/L. Preconcentration techniques using a poly(dithiocarbamate) resin have been applied to the determination of manganese in urine (Barnes et al., 1983 Van Berkel and Maessen, 1988). Barnes et al. (1983) digested the... 

It is not within the scope of this book to provide the reader with a comprehensive treatment on the most basic aspects of flow-injection analysis (FIA). However, the main principles of FIA will be discussed briefly in this chapter, with special reference to separation and preconcentration techniques, to help the reader gain a deeper insight into the related techniques. For a more comprehensive treatment of the basics of FIA. the reader may consult the second edition of " Flow Injection Analysis , authored by Ruzicka and HansenI 1], and other monographs on the topic listed in the references [2.3]. 

The general merits of FI separation and preconcentration summarized in Chapter 1 are all well demonstrated by FI sorption column techniques. The technique is inherently easier to operate than other separation methods, and the equipment generally more robust. An additional benefit over other separation techniques is its extremely high versatility, owing to the availability of a broad range of choice for different sorbents, complexing systems, and eluents. However, there are some restrictive features which are characteristic... 

ITP, also known as displacement electrophoresis, was first performed in capillary tubes by Ever-aerts et al. [69] for the separation of strong anions using a thermocouple detector. Since then, ITP has been used for the analysis of various important chemical and biological species such as amino acids [70], peptides [71], nucleotides [72], proteins [73,74], heavy metal ions [75], and other organic/inorganic ions [76,77] on a variety of detection platforms such as UV absorbance, conductivity and fluorescence detection. Over the past 15 years, ITP has been used as a preconcentration technique in conjunction with CE [78]. This mode of ITP, referred to as transient isotachophoresis (tITP), has been implemented on microchip platform in the recent years to achieve improved sensitivity [22,79,80]. 

Liquid-liquid extraction (LLE) is among the oldest sample extraction and preconcentration techniques available in analytical chemistry. LLE is a method whereby two immiscible phases, generahy an organic solvent and an aqueous solution, are brought into contact in order to extract one or more analytes from one phase into the other. If the receiving phase has a smaller volume than the donor phase, preconcentration can be effected. The separation mechanism is, like SPE, based on partitioning. At equilibrium, the partition coefficient of analyte i (Ki) in a two-phase system is given by... 

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