Chemical class fractionation

Chemical Class Fractionation of Fossil-Derived Materials for Biological Testing... 

Many investigations of relationships between composition and properties take into account only the concentration of the asphaltenes, independendy of any quality criterion. However, a distinction should be made between the asphaltenes which occur in straight mn asphalts and those which occur in blown asphalts. Remembering that asphaltenes are a solubiUty class rather than a distinct chemical class means that vast differences occur in the make-up of this fraction when it is produced by different processes. 

In the case of citrus essential oils, LC pre-fractionation can be used to obtain more homogeneous chemical classes of compounds for analysis by GC without any problems of overlapping peaks. 

Extracts which demonstrate sufficient activity in the bloassay are next fractionated by a simple partitioning procedure (Figure 1) which separates the components into gross chemical classes. The various fractions are re-assayed and those showing activity are then further fractionated and purified by appropriate chromatographic procedures. 

Figure 5.1 Key aspects of the SPMD sampling and residue analysis process. Often class fractionation is required following SEC when extracts contain complex mixtures of chemicals. Reprinted with permission from the American Petroleum Institute (Huckins et al., 2002).

Chemical enrichment and class fractionation procedures vary from laboratory to laboratory. Thus, no universal method for further enrichment and fractionation exists. However, the following provides some specific examples of commonly used... 

Lewis SC, King RW, Cragg ST, et al. 1984. Skin carcinogenic potential of petroleum hydrocarbons n. Crade oil, distillate fractions and chemical class subfractions. In MacFarland HN, Holdsworth CE, MacGregot JA, et al., eds. Advances in modem environmental toxicology. Vol. VI Applied toxicology of petroleum hydrocarbons. Princeton, NJ Princeton Scientific Publishers, Inc. 

An alternative to identifying organic chemicals in water and then determining their biological activity is the direct bioassay of mixtures accumulated from drinking water. This approach can provide part of the data for the preliminary assessment of health risks. The data can also be used to select those water sources on which the most strenuous identification efforts should be directed. Mixtures exhibiting considerable activity can be separated into chemical classes and bioassays can then be used to identify the active fractions. Ultimately the individual culprit chemicals would be identified. In this manner, efforts are directed towards the more detrimental components. 

The advantage of fractionation includes the separation of active constituents from inactive or otherwise toxic components. Disadvantages include the limited amount of sample available for testing following processing, the likelihood of spillover of chemical classes between fractions, and the possible loss or modification of components with fractionation. 

The list of separations given here cannot be regarded as exhaustive. Many separations done in RPC are reported in such a fashion that it is difficult to discover them as RPC separations using standard methods of searching the literature. Furthermore, the reports of separations of certain chemical classes, e.g. amino acids or catecholamines, are so prevalent that only a fraction of the references were used in the interest of keeping the reference list to manageable size. 

Illustrative calculations appear below for estimating the particulate fraction (c()) of p,p -DDT in urban air at 20°C using the Junge-Pankow adsorption model (Equation 3), the Mackay adsorption model (Equation 15), and the octanol-air partition coefficient model (Equation 25). Table 10.3 lists values of P3 and Koafor p,p -DDT and other POPs of different chemical classes. All model calculations are for an urban air TSP = 80 pg/m3 (Shah et al., 1986). 

The Pankow absorption model supposes that gaseous POPs partition into a liquid-like organic film on the aerosol. It uses P,s, the fraction of organic matter in the particle (fom), and the activity coefficient of the solute in the liquid film (yorJ. The latter is apt to vary substantially among different chemical classes of POPs. 

A conceptual DOM pie diagram (Fig. 1) represents the chemical view of the DOM pool of a typical riverine sample. The specific chemical classes or wedges can be separated based upon several different operational fractionation schemes (Thurman and Malcolm, 1981 Aiken, 1985 Aiken et al., 1992 Benner, 1998). For typical natural waters, the dominant fraction comprising DOM is fulvic acid, which is defined as the yellow, moderate molecular weight organic acid fraction of aquatic humic substances that is soluble at all pH values (Aiken et al., 1985). In surface waters receiving... 

The materials boiling above 850°F, were subsequently separated into four fractions by a preparative scale chemical class separation in our laboratory. This was done as follows 2 gms of the residual material were ground and dissolved in tetrahydrofuran (HPLC grade, Fisher Products, Fairlawn NJ). Ten grams of 30-40pm silica gel ("Sepralyte", Analytichem Int., Harbor City, CA), which was previously washed with methanol and dried, was added to the THF... 

QSARs for aquatic oxidation reactions are specific for chemical classes and the respective oxidant. Such models have been reviewed thoroughly by Canonica and Tratnyek (2003). Their application to the estimation of environmental persistence — aside from engineered remediation — is limited because of insufficient knowledge regarding the fractional contributions of these processes to the total degradation. 

The extracts were fractionated by a Preparative Liquid Chromatography method - PLC-8 [2], in eight distinct chemical classes FI-saturated hydrocarbons (HC), F2-monoaromatics, F3-diaromatics, F4-triaromatics, F5-polynuclear aromatics, F6-resins, F7-asphaltenes and F8-asphaltols. This method, proposed by Karam et al. as an extension of SARA method [4], was especially developed for coal-derived liquids. It combines solubility and chromatographic fractionation, affording discrete, well-defined classes of compounds which are readable for direct chromatographic and spectroscopic analysis. 

More detailed aspects of hydrocarbon bouquets acting as pheromones and various problematic aspects in revealing their true biological function have been discussed.2, 29° In many cases, however, only extracts of the insect cuticle are used for bioassays, sometimes roughly separated by chromatography into different chemical classes. Tests with synthetic blends have been less frequently performed only such approaches would allow unequivocal assignment of activity to the compounds tested. Actually, in whole extracts or fractions thereof... 

In this paper we outline techniques used to isolate two specific classes of compounds as well as a general class separation method suitable for large scale production of fractions. The specific isolation schemes are devised to produce an alkane or a PAH-enriched fraction. The general scheme provides for rapid fractionation of whole samples into several broad chemical classes. 

---