Thermal fractionation

Thermal fractionation employs a temperature routine (either programmed step cooling or a series of heating and cooling cycles) that is designed to produce a distribution of lamellar crystals whose sizes reflect the distribution of methyl sequence length (MSL) that are present in the ethylene/a-olefin copolymer [33]. The experiment is performed in a conventional DSC. 

Step crystallization (SC), where a programmed step cooling is applied [33,38 0], and SSA, where a series of heating and cooling cycles are employed [33,41 8], are the two most frequently employed thermal fractionation techniques, and their comparative advantages and shortcomings have been reviewed [33]. The polymer chains are never physically separated during thermal fractionation and therefore the technique 

Muller et al. designed and implemented SSA [33,41,42, 46-48]. After thermal conditioning of the sample, a final DSC heating run reveals the distribution of melting points induced by SSA as a result of the heterogeneous nature of the chain stmcture of the polymer under analysis. SSA is performed at substantially shorter times than SC and with better resolution [33,41 8]. 

The first two steps are identical to the SN protocol described above (Fig. 5.4) 

The effects of changing A7, the fractionation time, and the initial have been previously discussed in the literature [33]. 

---

The mobility of gases in the firn column leads to temperature-dependent changes in the composition of gas trapped in ice at the base of the fim (Severinghaus et ah, 1998). If there is a rapid change in climatic temperature at the ice-sheet surface, a steep temperature gradient will temporarily exist throughout the firn. This will temporarily cause thermal fractionation of gas-... 

Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice. Nature 391,141-146. 

In practice, the analyst monitors the bias correction through analysis of a reference standard on a routine, often daily, basis. This value comes to be known very well and makes insignificant contributions to overall precision. Even though it may not be truly applicable to the sample being analyzed, using it is far better than applying no correction it is the best that can be done in an imperfect world. A model of thermal fractionation on mass spectrometer filaments has been developed by Habfast [58]. 

Thermally fractionated Mg isotopes See 24Mg. Isotopic deviations from thermal chemistry are known to exist in the FUN (Fractionated and Unknown Nuclear)... 

The choice of the term oil or fat usually is based on tradition and the physical state of the material. Generally, oils are liquid at ambient temperatures, and fats are semisolid mixtures of crystals in oil. Fats often are of animal origin (beef tallow, pork lard, and butter fat) or hardened (hydrogenated, interester-ified, or thermally fractionated) vegetable oils, whereas oils are extracted from plant seeds or tissues or fish. In English-speaking countries outside the United States, oils liquid at room temperature sometimes are called soft oils, and those hard or pasty are called hard oils. Nutritionists generally use fats for solids or liquids. 

Thermal fractionation technology is most developed in the palm oil industry, where most oils are fractionated before sales. Solids profiles of stearins that have been fractionated from crude palm oil by chilling to different temperatures are shown in Fig. 34.21.118 Thermal fractionation, and double fractionation can be useful tools in obtaining fat fractions with the specific desired characteristics (Fig. 34.22).119... 

Severinghaus J. P., Sowers T., Brook E. J., Alley R. B., and Bender M. L. (1998) Timing of abrupt climate change at the end of the Younger Dryas interval from thermally fractionated gases in polar ice. Science 391, 141 -146. 

Our ThFFF channel is similar to the model TlOO Thermal Fractionator (FFFractionation, Inc., Salt Lake City, UT), with a channel thickness of 0.10 mm. When the carrier liquid was tetrahydrofuran (THF) or cyclohexane, a UV monitor set at 254 nm was used for sample detection when toluene was the carrier liquid, a refractive index monitor was used instead. The temperature difference was 60.0 K and the cold wall temperature was 298.2 K. Intrinsic viscosities were measured with a CannonFenske ASTM-25 viscometer obtained from Fisher Scientific (Santa Clara, CA). Viscosities were measured in a thermostated temperature bath set at T g. All solvents were high-performance liquid chromatography grade. 

Another intriguing point is that non-exponential damping of impurity motional states in a BEC provides an experimentally feasible example of manifestation of non-mean-field effects in a BEC. Up to now, only few experimental works on non-mean-field effects in BECs are available influence of a large thermal fraction of a bosonic system not far below the condensation point on damping and frequency shifts of elementary excitations was studied in Ref. [Jin... 

Now can estimate the influence of the finite BEC temperature T on the nonexponential decay of impurity wave packets. Let us consider [i/ki> < T Boltzmann constant, T = 3.31 nf/ /(m2 k/j), [Isihara 1971], and rit = n I v is the total number density of the degenerate atomic sample. The number density of the above-condensate (thermal) fraction is na = nt (T/Tc)3/2, [Isihara 1971], The characteristic lime scale Ty/, on which an impurity atom experiences a collision with the atom belonging to the abovecondensate fraction, is given by... 

Severinghaus et al. (1998) demonstrated that thermal fractionation of the air in polar fim layers can be detected and separated from the gravitational effect by high precision analysis of and " Ar/ Ar ratios. Rapid temperature changes at the surface induce... 

Severinghaus et al. (2001) directly observed thermal fractionation of the present-day fim air in response to seasonal temperature gradients. Elemental and isotopic ratios of N2, O2, Ar, Kr, and Xe analyzed in the air of the top 15 m of the fim at two Antarctic sites were found to match a model without adjustable parameters reasonably well. Relative thermal diffusion sensitivities for different gas pairs could be derived. 

Analytical methods. Detection of the small thermal fractionation signals requires... 

Severinghaus JP, Grachev A, Battle M (2001) Thermal fractionation of air in polar fim by seasonal temperature gradients. Geochem Geophys Geosyst 2 2000GC000146... 

A review on thermal fractionation methods has been presented by Muller and Amal [73], who recall that DSC methods are sensitive to both intra- and intermolecular defects whereas solution crystallization methods, where separation takes place according to crystallizability, are more sensitive to inter- than intramolecular heterogeneity. 

---