Homologous Series, Fractionation

Irreversible processes are mainly appHed for the separation of heavy stable isotopes, where the separation factors of the more reversible methods, eg, distillation, absorption, or chemical exchange, are so low that the diffusion separation methods become economically more attractive. Although appHcation of these processes is presented in terms of isotope separation, the results are equally vaUd for the description of separation processes for any ideal mixture of very similar constituents such as close-cut petroleum fractions, members of a homologous series of organic compounds, isomeric chemical compounds, or biological materials. 

Fractions rejected by 1.0 KDa (C>i) and permeated through 0.5 KDa (C0 5) membranes were also subjected to TLC analysis. In Figure 3 are reported the values relevant to the various spots detected in the two samples as a function of an arbitrary polymerisation degree (DP). The good linear correlation between these parameters allows to hypothesise a difference of one monomer units between the subsequent spots [32]. Consequently, C0.5 would correspond to the monomer, Cj to an homologous series fi om the monomer to the hexamer. 

The major components have been identified tentatively as phenolic and fatty acids. At this time, seven phenolics have been identified in only four of the fractions. These are shown in Table III. A measure of the magnitude of the confidence level (cc) with a spectrum of standards is given. The first three entries are from the sunflower the last, from the Jerusalem artichoke. In all fractions isolated, both from the sunflower and the Jerusalem artichoke, a homologous series of fatty acids ranging from Cjo to Ci8 have been identified also by GC-MS. Even-chain, Cj6 to Cjs saturated and Cxs mono- and di-unsaturated, predominated. This is not surprising, since fatty acids are major constituents of plant... 

In recent years the application of electrospray ionization (ESI) mass spectrometry, quadrupole time-of-flight (QqTOF) mass spectrometry, and Fourier transform ion cyclotron resonance (FT-ICR) are used for further structural characterization of DOM (Kujawinski et al., 2002 Kim et al., 2003 Stenson et al., 2003 Koch et al., 2005 Tremblay et al., 2007 Reemtsma et al., 2008). MS/MS capabilities provide the screening for selected ions, and FT-ICR allows exact molecular formula determination for selected peaks. In addition, SEC can be coupled to ESI and FTICR-MS to study different DOM fractions. Homologous series of structures can be revealed, and many pairs of peaks differ by the exact masses of -H2, -O, or -CH2. Several thousand molecular formulas in the mass range of up to more than 600 Da can be identified and reproduced in element ratio plots (O/C versus H/C plots). Limitations of ESI used by SEC-MS are shown by These and Reemtsma (2003). 

Figure 16. FIMS data for an Athabasca maltene fraction. The peaks marked with an asterisk correspond to a homologous series of benzothiophenes, 0S...

Figure 17. FIMS data for an Athabasca maltene fraction. The prominent series of peaks at 212, 226 and 240 daltons correspond to a homologous series of dibenzothiophenes, Cnl 2n-16 with n = 14, 15 and 16, respectively. (Reproduced with permission from Ref. 5. Copyright 1985, Alberta Oil Sands Technology and Research Authority.)...

The above equation is, however, invalid. In homologous series in which Eq. 2.2 holds, a is essentially the fraction value, fcm.%- But it is also well established that/( H/) is a variable quantity in different solvent systems. Log P values in octanol and cyclohexane have been determined by Taft and coworkers [21, 22] for a series of non-am-phitropic, monofunctional aliphatics. The partition coefficient obtained could be described and correlated by the following equations ... 

The difficulties presented by log Kp vs log of standard M are obviated by alternatively plotting log[r ]M vs log wel. For each fraction of a polysaccharide homologous series, log[r ]M vs log we, is superimposable on a so-called universal calibration curve (Grubisic et al., 1967) from which an unknown M2 may be estimated, after substitution in the equation... 

Studies on the chemical structure of olive wax esters have shown that the homologues present in olive-pomace oil are almost entirely esters of oleic acid with long chain alkanol constituting the homologous series C40, C42, C44, C46. Odd-chain esters identified in the oil were esters of oleic acid with C23, C25, C27 alcohols. Gas chromatography and mass spectrometry analysis has shown that each carbon chain of the esters in made up of a single isomer in which the acyl moiety is that of oleic acid (Bianchi el al., 1994). Thus, for example, ester C44 was found to be made up of the couple acid-alcohol Cl8 1 and C26, whereas other possible isomers such as C16 l-C28 were not detected. This is unusual if it is compared with the composition of epicuticular ester fractions of oil seeds for which, in cases studied, each ester chain was composed of several positional isomers of the ester group. 

---