Enrichment factor systems

Enrichment factors have been used in stndies with methyl tert-butyl ether (MTBE) that is a fuel additive and is a widespread contaminant in snbsnrface aquatic systems. 

Of the analytical procedures used for the determination of LAS in soils (Table 6.7.1), most methods rely on (Soxhlet) extraction with methanol, followed by clean-up on SPE cartridges (RP-C18 and/or SAX) and final quantitative measurements by HPLC—UV/FL. Applying this protocol, detection limits were achieved ranging between 0.05 and 5 mg kg-1 depending on the matrix, the enrichment factor and the optical detection system employed. 

Carbon nanotubes could be also used in a format of disc. A comparison smdy showed that the double-disk system (comprising two stacked disks with 60 mg of CNTs) exhibited extraction capabilities that were comparable to those of a commercial Cig disk with 500 mg sorbent for nonpolar or moderately polar compounds. Moreover, the former system was more powerful than the latter for extracting polar analytes. The triple-layered CNTs disk system showed good extraction efficiency when the sample volume was up to 3,000 mL. Katsumata et al. [136] obtained very high enrichment factor for preconcentration of atrazine and simazine (3,900 and 4,000, respectively, for 200 mL of sample solution when only 30 mg of MWCNTs was used in the format of disk. 

Figure 4. Enrichment factors (relative to Sc) vs. particle size curves for aerosols collected up- and downstream of the flue-gas desulfurization system show considerable concentration enhancement in the submicrometer size region.

A solid -phase preconcentration system was implemented. Several chemical and flow-related parameters potentially influencing the enrichment factor were studied, of which only three were revealed to be significant... 

Inorganic compounds may be more stable to radiation damage than organic compounds, and their decomposition products are less deleterious. Accordingly, a search was made for inorganic compounds which would contain these trivalent ions in a nonexchangeable state and, at the same time, allow recoil atoms ejected from their position in the structure to be separated. Szilard-Chalmers reactions have been reported for certain inorganic systems, for example clays ( ), but yields and enrichment factors were both low. 

Aldstadt et al. described the determination of U in groundwater using a FI system with a TRU-Resin column coupled on-line to an ICP-MS. The system used three 2-position valves and a flow-reversal scheme to load the sample in one direction and elute in the reverse direction. The system provided an enrichment factor on the order of 30-fold compared to direct groundwater analysis. Separate elution of Pu and Th in a group prior to eluting U was also discussed. 

Body fluids, such as serum, contain several different carotenoids in low amounts. The crucial point in the isolation and analysis of these samples is the enrichment factor. Serum samples can be directly analysed with hyphenated extraction-sample enrichment-separation systems, such as on-line SPE-HPLC employing tailored stationary phases [29]. By using special restricted access materials (RAMs) for sample enrichment, the carotenoids are retarded on the pre-column while the protein binding is broken and the macromolecules are eluted. The preparation of artifacts is hindered, as the whole analysis steps take place under conditions of light- and oxygen-exclusion. The scheme of on-line SPE-HPLC is presented in Figure 5.2.2. 

In rivers and streams heavy metals are distributed between the water, colloidal material, suspended matter, and the sedimented phases. The assessment of the mechanisms of deposition and remobilization of heavy metals into and from the sediment is one task for research on the behavior of metals in river systems [IRGOLIC and MARTELL, 1985]. It was hitherto, usual to calculate enrichment factors, for instance the geoaccumulation index for sediments [MULLER, 1979 1981], to compare the properties of elements. Distribution coefficients of the metal in water and in sediment fractions were calculated for some rivers to find general aspects of the enrichment behavior of metals [FOR-STNER and MULLER, 1974]. In-situ analyses or laboratory experiments with natural material in combination with speciation techniques are another means of investigation [LANDNER, 1987 CALMANO et al., 1992], Such experiments manifest univariate dependencies for the metals and other components, for instance between different metals and nitrilotriacetic acid [FORSTNER and SALOMONS, 1991], but the interactions in natural systems are often more complex. 

Other oxides of nitrogen are present in these systems, and they affect the single stage enrichment factor a = ( N/ N)iiquid/( N/ N)gas as well as the interphase transfer rate as measured by the height of the column equivalent to a theoretical plate (HETP). 

FIGURE 23-3 Enrichment factor for solute A from solute B as a function of number of extractions for a system in which = V, for various values of Df and Dg as shown. 

Denitrification causes the 6 N of the residual nitrate to increase exponentially as nitrate concentrations decrease, and causes the acidity of the system to decrease. For example, denitrification of fertilizer nitrate that originally had a distinctive 6 N value of A-0%c can yield residual nitrate with much higher 6 N values (e.g., -K5%o to - -30%o) that are within the range of compositions expected for nitrate from a manure or septic-tank source. Measured enrichment factors (apparent fractionations) associated with denitrification range from —40%o to —5%o hence, the fi N of the N2 is lower than that of the nitrate by about these values. The N2 produced by denitrification results in excess N2 contents in groundwater the fi N of this N2 can provide useful information about sources and processes (Vogel et al., 1981 Bohlke and Denver, 1995). 

With a large equilibrium distribution coefficient D (complete trapping) and a large phase ratio, Ee will increase linearly up to large values. In Eigure 12.4, which refers to an SLM flow system experiment, D in curve 1 is approximately 10,000 and the enrichment factor is linear at least up to at least 3000 times. In many cases, especially in flow systems, the extraction is not allowed to go to equilibrium, and an extraction efficiency E is defined as the fraction of the total amount of analyte that is transferred to the acceptor. Thus... 

Most of the discrimination between inorganic and methylmercury thus occurs during trophic transfer, while the major enrichment factor is between water and the phytoplankton. This also has been reported for the diatom Thalassiosura weissflogii in a marine food chain (Mason et al. 1996). Methylmercury was accumulated in the cell cytoplasm, and its assimilation by copepods was 4 times more efficient than the assimilation of inorganic mercury. Bioaccumulation has been demonstrated for predator fish in both freshwater and marine systems and in marine mammals (see Section 5.4.4). Bioaccumulation of methylmercury in aquatic food chains is of interest, because it is generally the most important source of nonoccupational human exposure to this compound (EPA 1984b WHO 1990, 1991). 

The enrichment factors Stj, together with the deviations from the experimental data of surface tension for the investigated systems are given in Table 6.4. 

Donnan dialysis of conjugate bases of weak acids presents an additional factor for control, namely pH. Cox and Cheng (34) investigated preconcentration of a number of anions in this category. Sample pH was adjusted to ensure that a major fraction of the weak acid system was in anionic form. Maximum enrichment factor, defined as ratio of receiver concentration to sample concentration after some given, fixed time (usually 30 minutes), was obtained when pH of the receiver solution was much less than the... 

Donnan dialysis has been exploited in flow systems with flame atomic absorption spectrometry and ICP-OES, for which remarkable analytical figures of merit, including high enrichment factors, have been reported [283—285]. This type of dialysis is less compatible with flow-based spectrophotometric determinations, as the inherent concentration gradients can give rise to the occurrence of the Schlieren effect. 

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