Compound class fractionation

Nishioka, M. G., C. C. Chuang, and B. A. Petersen, Development and Quantitative Evaluation of a Compound Class Fractionation Scheme for Bioassay-Directed Characterization of Ambient Air Particulate Matter, Environ. Ini., 11, 137-146 (1985). 

Compound class fractionation. E achedier extract was subjected to hi vacuum distillation as previous described 17). The volatile extract (distillate) was concentrated to 20 mL under a gentle stream of nitrogen gas (N2). It was then washed with aqueous sodium carbonate solution (Na2C03) (5 %w/v 3 x 20 mL). The upper ether phase, containing the neutral/basic volatiles, was collected and concentrated to 10 mL under N2. It was tiien dried over anhydrous Na2S04 (2 g) and concentrated to 0.2mL under N2. The pooled aqueous phase (bottom layer) was acidified with aqueous HCl (10% w/v) to pH 3 and the acidic volatiles extracted with ether (3 x 15 mL). The pooled ether extract was then concentrated to 10 mL under N2, dried over anhydrous Na2S04, and then further concentrated to 0.2 mL under N2. 

The residual sediment after organic solvent extraction was extracted repeatedly with 0.2N NaOH to isolate the fulvic/humic acid fraction. Humic acid was then precipitated with hydrochloric acid to separate it from fulvic acid in the aqueous phase. Fulvic acid was purified by adsorption and subsequent elution from a column of Amberlite resin. Humic acid was purified by redissolving in NaOH and reprecipitation with hydrochloric acid. The residual sediment was then treated sequentially with hydrofluoric acid of increasing concentration to remove the silicates, washed several times with water and then dried to recover protokerogen (Stuermer et al, 1978). Procedure blank for each compound class fraction was dso combusted for carbon dioxide measurement. 

Carbon dioxide from ffie procedure blanks for all ffie individual compound class fractions including those from humic and fulvic acid fraction was collected. Background reading of the monometer in ffie vacuum line was 0.65 /rmol which was consistent over ffie 1 month period during gas collection. None of the procedure blank had carbon dioxide more than 1-1.2 /rmol (inclusive of ffie background reading) which was too low to give any stable or radiocarbon... 

Fig. 2. Radiocarbon age of the organic fractions from the sediments of Santa Monica Basin. For explanation of abbreviations of compound class fractions, refer to Fig. 1. Faa fatty acid.

Separation according to compound class, fractionation of vinylogous series, separation according to degree of unsaturation, separation of cis4rans isomers and of positional isomers. 

Figure 8.13 Class fractionation of polycyclic compounds on colunns of alunina and silica gel. (Ksproduced with pemission from reference 127. Copyright Anerican Chenical Society).

Principles and Characteristics A sample can contain a great number of compounds, but analysts are usually interested only in the qualitative presence (and the quantitative amount) of a small number of the total compounds. Selectivity is an important parameter in analytical separations. The total analytical process clearly benefits from selectivity enhancement arising from appropriate sample preparation strategies. Selective separation of groups or compound classes can simplify a mixture of analytes before analysis, which in turn enhances analytical precision and sensitivity. Selective fractionation, in some cases, allows easier resolution of the compounds of interest, so analysts can avoid the extreme conditions of high-resolution columns. 

Recall from Chapter 23.2.4 that humic substances are isolated from seawater by adsorption on a hydrophobic resin followed by elution using solvents of varying pH. The desorbed compounds are fractionated into two classes, humic acids fulvic acids based on their solubility behavior. A model structure for a humic acid is illustrated in Figure 23.10a in which fragments of biomolecules, such as sugars, oligosaccharides. 

Figure 14.13. Proportions of compound classes in organic-mineral particle-size fractions of long-term fertilization experiments in Germany. The gray areas indicate the largest and smallest proportions of these compound classes found by Py-FIMS of other particle-size fractions.

Gas chromatograms of three saturated hydrocarbon fractions, roughly representing the different facies (Figure 2), show that some relative and absolute variations exist between the three facies. n-Alkanes, pristane, phytane and the extended hop-17(21)-enes are indicated. The most prominent changes within different compound classes in the saturated hydrocarbon fraction are highlighted below. 

Molasses. A large number of volatile and nonvolatile compounds have been identified in the flavor fractions of various types of molasses (51-621. Compound classes identified include aliphatic and aromatic acids, aldehydes, phenols, lactones, amines, esters, furans, pyrazines, and sulfides. Most of these compounds can arise from carbohydrate degradation through a number of traditional pathways especially because residual nitrogen-containing sources are present. 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

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