Enrichment factors

An adsorbent can be visualized as a porous soHd having certain characteristics. When the soHd is immersed in a Hquid mixture, the pores fill with Hquid, which at equilibrium differs in composition from that of the Hquid surrounding the particles. These compositions can then be related to each other by enrichment factors that are analogous to relative volatiHty in distillation. The adsorbent is selective for the component that is more concentrated in the pores than in the surrounding Hquid. 

Recently, SPE cartridges and disks have been widely and successfully used in preconcentration processes [1-3]. They reduce solvent usage, disposal costs, and extraction time for sample preparation and obtain large enrichment factors. 

Determination of C -C cai bonyl compounds, phthalic acid alkyl esters, mutagen MX and its geometric isomer E-MX, chlorophenols, organochlorine pesticides (OCPs) - a-, P-, J-, 5-HCH, DDE, DDD, DDT, total isomeric-specific content of polychlorinated byphenyls (PCBs) (tri-, tetra-, penta-, hexa-, hepta-, octachloroisomers) in SCAN and SIM mode was canned out. MDE of procedure for OCPs makes up 0.01-0.1 and PCBs - 0.17-0.38 ng/1 at enrichment factor K = (1.50-4.00)T0. ... 

A common approach for identifying the enriched elements in a material is to calculate the enrichment factor (EF). The EF of an element, M, is obtained by comparing its concentration with that of a reference element, R, such as cerium or... 

Fig. 5. The log (enrichment factor) for 44 elements in Antarctic atmospheric dust compared with mean crustal concentrations.

The first study of metal carbonyls was that of Toropova ° whose objective was isotope enrichment using Cr(CO)g. After dissolving the target compound in chloroform, she found nearly 90% of the Cr to be extracted further into 0.1 M HCl, with isotopic enrichment factors greater than 10". This implies retention values of the order of 10%. 

Target compound Isotope Enrichment factor References... 

Results have been expressed in a number of ways. In the Raleigh model that has been extensively used, the fractionation factor a is given by R/Rq = when the fraction of remaining substrate is/and where R is the isotopic composition of the substrate during degradation and Rq is the initial value. The enrichment factor e where e = 1000 (a - 1) has also been used. There are certain conditions that must be fulfilled for the Raleigh model to be applicable ... 

For aerobic degradation by Pseudomonas sp. strain P51, which carried out degradation by dioxygenation, the valnes were not significant. In contrast, the isotope enrichment factor (e) for anaerobic dechlorination by Dehalococcoides sp. strain CDBl, which produced 1,3-dichlorobenzene from 1,2,3-trichlorobenzene, was -3.4 ppm, and for 1,2,4-trichlorobenzaene, which produced 1,4-dichlorobenzene, was -3.2 ppm. 

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. 

Under laboratory methanogenic and sulfidogenic conditions, only low values of enrichment factors were observed, although enrichment of toluene was found at late stages of transformation (Ahad et al. 2000). 

Enrichment factors during the anaerobic degradation of o-xylene, m-xylene, m-cresol, and p-cresol by pure cultures of sulfate-reducing bacteria that use the fumarate pathway ranged from -1.5 to -3.9 ppm (Morasch et al. 2004). It was therefore proposed that this could be applied to evaluating in situ bioremediation of contaminants that use this pathway for biodegradation. 

A sulfate enrichment culture prepared from a contaminated site gave enrichment factors (e) of-1.1 for naphthalene and -0.9 for 2-methylnaphthalene (Griebler et al. 2004). These values combined with literature values from analogous laboratory experiments were used to quantify degradation of toluene, xylenes, and naphthalene at the site. Additional evidence for degradation of BTEX was derived from analyses of established metabolites produced by anaerobic degradation. 

During the aerobic degradation of chloroethene (vinyl chloride [VC]) by strains of Mycobacteria and Nocardioides, enrichment factors (e) lay within the range -8.2 0.1 to... 

Carbon isotope fractionation was examined during the aerobic degradation of TCE by Burkholderia cepacia strain G4 that possesses toluene monooxygenase activity (Barth et al. 2002). There were substantial differences in values of isotope shifts during degradation, from 57 to 17 ppm, and when the data were corrected to correspond to the same amount of substrate reduction the Releigh enrichment factor was 18.2. 

A methanogenic enrichment culture was used to determine 8 C emichment factors during the dechlorination of TCE, ci -l,2-dichloroethene (cw-DCE), and chloroethene (VC) (Bloom et al. 2000). Using the Raleigh model, enrichment factors (e) were 6.6 and... 

Where c, is the concentration of an element measured at the surface, Cbt is the concentration of a stable element due to batch melting and a, is the enrichment factor due to transport. The concentrations in the melt are given by the batch melting equation. 

Uranium is not a very rare element. It is widely disseminated in nature with estimates of its average abundance in the Earth s crust varying from 2 to 4 ppm, close to that of molybdenum, tungsten, arsenic, and beryllium, but richer than such metals as bismuth, cadmium, mercury, and silver its crustal abundance is 2.7 ppm. The economically usable tenor of uranium ore deposits is about 0.2%, and hence the concentration factor needed to form economic ore deposits is about 750. In contrast, the enrichment factors needed to form usable ore deposits of common metals such as lead and chromium are as high as 3125 and 1750, respectively. 

From the expression provided above it may be pointed out that to have a high enrichment it is necessary to have not only the separation factor, S, high but that the volume of organic phase must also be as small as possible in comparison to aqueous phase. It can be left as a simple exercise to the reader to examine the fact that enrichment factor, F, enhances with enhanced VJV0. 

Generally, performance of a method is often judged in one of two ways. The first is the enrichment factor, enrichment for short, which is the ratio of the cumulative number of actives in the top N% of the total number in the dataset to random retrieval rate. Many early studies focused on the enrichment obtained when the top 10% of the dataset was screened. However, this is operationally unrealistic if compound collections exceed 100,000 compounds, which is common in mid- to large-sized companies. A more realistic test is the enrichment obtained in the top... 

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