Hydrocarbon matrices, extraction

The most critical decision to be made is the choice of the best solvent to facilitate extraction of the drug residue while minimizing interference. A review of available solubility, logP, and pK /pKb data for the marker residue can become an important first step in the selection of the best extraction solvents to try. A selected list of solvents from the literature methods include individual solvents (n-hexane, " dichloromethane, ethyl acetate, acetone, acetonitrile, methanol, and water ) mixtures of solvents (dichloromethane-methanol-acetic acid, isooctane-ethyl acetate, methanol-water, and acetonitrile-water ), and aqueous buffer solutions (phosphate and sodium sulfate ). Hexane is a very nonpolar solvent and could be chosen as an extraction solvent if the analyte is also very nonpolar. For example, Serrano et al used n-hexane to extract the very nonpolar polychlorinated biphenyls (PCBs) from fat, liver, and kidney of whale. One advantage of using n-hexane as an extraction solvent for fat tissue is that the fat itself will be completely dissolved, but this will necessitate an additional cleanup step to remove the substantial fat matrix. The choice of chlorinated hydrocarbons such as methylene chloride, chloroform, and carbon tetrachloride should be avoided owing to safety and environmental concerns with these solvents. Diethyl ether and ethyl acetate are other relatively nonpolar solvents that are appropriate for extraction of nonpolar analytes. Diethyl ether or ethyl acetate may also be combined with hexane (or other hydrocarbon solvent) to create an extraction solvent that has a polarity intermediate between the two solvents. For example, Gerhardt et a/. used a combination of isooctane and ethyl acetate for the extraction of several ionophores from various animal tissues. 

Accelerated solvent extraction is a new technique for the extraction of a range of organic pollutants from soils and related material. The technique is based on the use of a solvent or combination of solvents to extract organic pollutants at elevated pressure and temperature from a solid matrix. The range of organic pollutants for which the technique is proposed includes semivolatile compounds, organochlorine pesticides, organophosphorus pesticides, chlorinated herbicides, polychlorinated biphenyls and polycyclic aromatic hydrocarbons [53-56],... 

Soxhlet, sonication, supercritical fluid, subcritical or accelerated solvent, and purge-and-trap extraction have been introduced into a variety of methods for the extraction of contaminated soil. Headspace is recommended as a screening method. Shaking/vortexing is adequate for the extraction of petroleum hydrocarbons in most environmental samples. For these extraction methods, the ability to extract petroleum hydrocarbons from soil and water samples depends on the solvent and the sample matrix. Surrogates (compounds of known identity and quantity) are frequently added to monitor extraction efficiency. Environmental laboratories also generally perform matrix spikes (addition of target analytes) to determine if the soil or water matrix retains analytes. 

Although these methods measure different petroleum hydrocarbon categories, there are several basic steps that are common to the analytical processes for all methods, no matter the method type or the environmental matrix. In general, these steps are (1) collection and preservation—requirements specific to environmental matrix and analytes of interest (2) extraction so that separations of the analytes... 

As regards a contaminated soil, this type of analysis may not be possible because the various hydrocarbons cannot be extracted from the sample with equal efficiency. Volatile organic compounds require special procedures to achieve satisfactory recovery from the soil matrix. It thus becomes important to distinguish between those compounds that are considered to be volatile and those that rank as semi- or nonvolatile compounds. 

Extraction can be nsed for separation or isolation of the analyte from the sample matrix or vice versa as well as a preconcentration method. Extraction of metal ions is based on the reaction of weak organic acids with metal ions that give nncharged complexes that are highly solnble in organic solvents as ethers, hydrocarbons, ketones and polychlorinated species (generally chloroform and carbon tetrachloride). The efficacy of the extraction is mainly dependent on the extent to which solntes distribnte themselves between two immiscible solvents. The amonnts of analyte can be determined spectrophotometrically as well as with other available analytical methods. 

Further analyses of representative extracts of each of the Amberlite resins employing GC-MS indicated the presence of significant concentrations of a variety of aromatic hydrocarbons, including alkylated derivatives of benzene, styrene, naphthalene, and biphenyl. A more comprehensive listing of these contaminants, including their approximate concentrations in the sorbent matrix, is provided in Table I. 

Polycyclic aromatic hydrocarbons (PAHs) have been extracted from contaminated land samples by supercritical fluid extraction jSFE) with both pure and modified carbon dioxide. Removing an analyte from a matrix using SFE requires knowledge about die solubility of the solute, the rate of transfer of the solute from the solid to the solvent phase, and interaction of the solvent phase with the matrix. These faclors collectively control the effectiveness of the SFF process, if not of the extraction process in general. The range of samples for which SFE has been applied continues to broaden. Applications have been in the environment, food, and polymers. 

In this paper, the supercritical fluid extraction (SFE) of organic compounds from sand spiked with 36 nitroaromatic compounds, 19 haloethers, and 42 organochlorine pesticides, and from a standard reference material certified for 13 polynuclear aromatic hydrocarbons (PAH), dibenzofuran, and pentachlorophenol was examined using a two- and a four-vessel extractor. Although the results achieved by SFE for the sand and the standard reference soil samples were very encouraging, previous data obtained in our laboratory on the standard reference soil and a few other standard reference marine sediments were less favorable. It was therefore decided that an investigation of seven variables for their influence on the analyte recoveries from the standard soil sample would be useful. Two tests were conducted in which these variables were investigated. In Test 1, the seven variables selected were pressure, temperature, moisture content, cell volume, sample size, extraction time, and modifier volume. In Test 2, the seven variables were pressure, temperature, volume of toluene added to the matrix, volume of solvent in the collection vessel,... 

Replicability. Table IV shows replicability data for five extractions of a hydrocarbon mixture from a simple matrix (filter paper). This was an early application run on preliminary instrumentation (note footnote at bottom of Table IV) in order to establish on-going tracking of the performance of developing instrumentation. Even under these conditions the precision (as indicated by the relative standard deviation) is good, ranging from 5% for to 11.5% for C12. 

Lopez-Avila et al. [8] published a study in 1993 that evaluated the Soxtec extraction of 29 target compounds (seven nitroaromatic compounds, three haloethers, seven chlorinated hydrocarbons, and 12 organochlorine pesticides) from spiked sandy clay loam and clay loam. Among the five factors investigated (matrix type, spike level, anhydrous sodium sulfate addition, total extraction time, and immersion/extraction time ratio), matrix type, spike level, and total extraction time had the most pronounced effects on method performance at the 5% significance level for 16 of the 29 target compounds. The two solvent mixtures, hexane-acetone (1 1) and methylene chloride-acetone (1 1), performed equally well. Four compounds were not recovered at all, and apparently were lost from the spike matrix. Limited experimental work was performed with 64 base-neutral-acidic compounds spiked onto clay loam, and with three standard reference materials certified... 

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