Fractionation method

Miscellaneous Fractionation Methods.—Turbidimetric titration by non-solvent addition has virtually vanished since the advent of GPC, but Hay et have recently shown that the method of turbidimetric titration by temperature decrease can give useful information about the polydispersity of polymers. Nonsolvent induced turbidimetry of fractions eluted from a GPC column has been used by Hoffmann and Urban to examine composition distribution in copolymers. 

Klesper and Hartman have shown that chromatography using a supercritical pressurized gas as the mobile phase is an effective way of separating polystyrene oligomers. Good resolution is achieved for chains with up to 35 repeat units and preparative separation is possible up to 15 units. The apparatus for this technique has been described.  

Giddings et have examined a new method for polymer analysis, termed Field Flow Fractionation (FFF). In this technique a flow profile of parabolic shape is set up in a thin flow channel by pumping solvent through it. At the same time some form of field (usually a thermal gradient) is applied across the flow channel. If a polymer is injected into the field with no flow then the field causes it to concentrate at one side of the flow channel. This concentration is opposed by diffusion so that a concentration profile is set up across the field, in which the molecules are resolved according to their thermal diffusion coefficients and hence their molar masses. When the flow is applied the polymer is eluted with molar mass resolution. As yet the method has not shown anything like its theoretical resolution but preliminary results are very interesting. 

In GPC of polymers it is desirable to avoid specific interactions between polymer and substrate. In the past few years there has been a very rapid increase in the use of thin layer chromatography (t.l.c.) for polymer analysis. In this technique it is possible to separate polymers either by size differences as in GPC or by chemical differences, thus making it extremely useful for analysis of polymer mixtures and copolymers. Applications of t.l.c. have been thoroughly reviewed. - Kamide et al. report studies of cellulose nitrate showing that the polymer can be dispersed on a t.l.c. plate according to either molar mass or nitrogen content. 

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For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

The foremnner of the modern methods of asphalt fractionation was first described in 1916 (50) and the procedure was later modified by use of fuller s earth (attapulgite [1337-76-4]) to remove the resinous components (51). Further modifications and preferences led to the development of a variety of fractionation methods (52—58). Thus, because of the nature and varieties of fractions possible and the large number of precipitants or adsorbents, a great number of methods can be devised to determine the composition of asphalts (5,6,44,45). Fractions have also been separated by thermal diffusion (59), by dialysis (60), by electrolytic methods (61), and by repeated solvent fractionations (62,63). 

Various fractionation methods have been used to study the heterogeneity of LCBs and SCBs of PEs [13]. The average values of LCBs in the commercial grades of HP LDPE range from 0.07-12.9 per 1000 C atoms [13,20,21], and their concentration increases with the increase in MW [15]. The content of SCBs (mainly from 1-6 carbon atoms) ranges from 8.5-22.4 per 1000 C atoms in HP LDPE [22,23]. It is reported that paired and/or branched branches are also found in HP LDPE [23]. 

This expression, however, is not correct. The correct one is obtained by eliminating the cyclical pair, k2k-2[X ], from the denominator. Doing so, one obtains Eq. (4-63). Since the success fraction method is so useful in general, it is advantageous to learn how to extend it to such cases rather than abandon it in favor of much tedious algebra. Once one learns to recognize the extraneous term as the one with the pair of cyclical rate constants, it is easy to apply a correction. Problem 4.5 illustrates this. The number of denominator terms is equal to the number of sequential steps in the reaction scheme, which is three in the scheme to which Eq. (4-63) applies. 

Separation methods The methodology for their separation often consists of fractionation methods (see Section 3.3.4) and chromatographic methods of the soluble species. Common detection methods are AAS, ICP-OES, and fluoride electrode methods. 

Using the data given below, draw an information flow diagram of the process and calculate the process stream flow-rates and compositions for the production of 600 t/d ammonia. Use either the Nagiev split fraction method, with any suitable spreadsheet or manual calculations. 

Shuman L.M. Fractionation method for soil microelements. Soil Sci 1985a 140 11-22. 

O Farrell PH et al. Two-dimensional polyacrylamide gel electrophoretic fractionation. Methods Cell Biol 1977 16 407-420. 

We also remark that Eq. (5.44) may be decomposed into separate sets of equations for the odd and even ap(t) which are decoupled from each other. Essentially similar differential recurrence relations for a variety of relaxation problems may be derived as described in Refs. 4, 36, and 73-76, where the frequency response and correlation times were determined exactly using scalar or matrix continued fraction methods. Our purpose now is to demonstrate how such differential recurrence relations may be used to calculate mean first passage times by referring to the particular case of Eq. (5.44). 

There is an extensive literature on concentration, separation, and fractionation methods. References [1-5] give general reviews. 

No class reaction has been proposed for the measurement of total sterols. Instead, various fractionation methods, usually derived from the biochemical litreature, have been adapted to the concentrated materials collected from seawater. Certain of the more important sterols, particularly those used in the evaluation of water quality, have been determined by the use of a compound-specific reaction, after concentration from solution. Thus Wann et al. [405,... 

Table 1. Partial digestions and selective extractions applied to Talbot soil samples (<250 pm fraction) Method (ICP-MS) Target Phases Laboratory... 

Fractionation. See also Plasma fractionation foam, 12 22 physical, 10 813-814 Fractionation methods, for particle size measurement, 18 139, 140-146 Fractionation process, in paper recycling, 21 441... 

Fractionation Methods. Ultrafiltration and gel filtration are nondestructive methods which, based on limited experience, can be used for fractionation of mineral complexes from digests. In earlier studies mineral absorption on the gel material was a problem. Lonnerdal (30) introduced a method of treating dextran gels with sodium borohydride in order to eliminate the mineral-binding sites on the gel. In preliminary studies we have recovered more than 90 of Ca, Mg, Fe, Zn and P in samples applied to a borohydride-treated gel column (Sephadex G-50, Pharmacia Fine Chemicals, Piscataway, NJ). Recovery of Ca (Table IV) and Mg, Fe and Zn from ultrafiltration was also good. 

Quintanilla-Lopez JE, Lebron-Aguilar R, Polo-Diez LM. 1992. Comparative study of clean-up and fractionation methods for the determination of organochloride pesticides in lipids by gas chromatography. J Chromatogr 591 (1 -2) 303-311. 

During the development of sample-fractionation methods, the main goal is to achieve the highest resolution possible between the molecule of interest and the contaminants. Fractionation by SEC is used both as an analytical tool and as a... 

Standard Method 5520E Sludge Suitable for biological lipids and mineral hydrocarbons. Not suitable for low-boiUng fractions. Method E is a modification of Method D. 

Rather than quantifying a complex total petroleum hydrocarbon mixture as a single number, petroleum hydrocarbon fraction methods break the mixture into discrete hydrocarbon fractions, thus providing data that can be used in a risk assessment and in characterizing product type and compositional changes such as may occur during weathering (oxidation). The fractionation methods can be used to measure both volatile and extractable hydrocarbons. 

In contrast to traditional methods for total petroleum hydrocarbons that report a single concentration number for complex mixtures, the fractionation methods report separate concentrations for discrete aliphatic and aromatic fractions. The petroleum fraction methods available are GC-based and are thus sensitive to a broad range of hydrocarbons. Identification and quantification of aliphatic and aromatic fractions allows one to identify petroleum products and evaluate the extent of product weathering. These fraction data also can be used in risk assessment. 

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