Polar substances, extraction

Soil extracts are usually very complex. In water samples, humic and fulvic acids make analysis difficult, especially when polar substances are to be determined. Multidimensional chromatography can also make a significant contribution here to this type of analysis. 

A common approach for personal dosimetry is collection of pollutant on, e.g., silica gel, organic resins or activated charcoal in small tubes worn on the operator s lapel (Table 9.2). Silica gel is useful for polar chemicals charcoal finds wide use for non-polar substances. The pollutant is then solvent-extracted or thermally desorbed for subsequent analysis by, e.g., chromatography. 

Figure 28-16 shows steps in the solid-phase extraction of 10 ng/mL of steroids from urine. First, a syringe containing 1 mL of C 8-silica is conditioned with 2 mL of methanol to remove adsorbed organic material (Figure 28-16a). Then the column is washed with 2 mL of water. When the 10-mL urine sample is applied, nonpolar components adhere to the C18-silica, and polar components pass through (Figure 28-16b). The column is then rinsed with 4 mL of 25 mM borate buffer at pH 8 to remove polar substances (Figure 28-16c). Then rinses with 4 mL of 40 vol% methanol/60% water and 4 mL of 20% acetone/80% water remove less polar substances (Figure 28-l6d). Finally, elution with two 0.5-mL aliquots of 73% methanol/27% water washes the steroids from the column (Figure 28-16e).

The extraction result is further influenced by the moisture content. For spices and herbs this concentration should be in general between 8 and 15 %. At higher values the water is extracted preferentially and/or the co-extraction of polar substances is at a low level. Low... 

Many undesired substances such as chlorophyll or other polar substances, which are extracted in the alcohol extraction, are only slightly soluble in CO2. In case of necessity, the aroma components can be removed from the extract by a further purification step. 

Processes of such kinds can be found in the literature. E. Stahl, et al. [1] describe the decontamination of drugs, in particular, the reduction of pesticides from senna leaves by means of supercritical dry carbon dioxide [2]. The patents EP 0382116 A2 [4] and DE 4342874 Al [5] specify processes to remove non-polar substances from ginseng with dense carbon dioxide, in which the ginseng is moisturized prior to extraction, and the second method also uses an entrainer. The European Patent Application EP 0925724 A2 from NATEX [3] explains a process for the removal of plant-protection agents and/or undesired substances from cereals. 

The excellent procedures for dehydration of N-alkyl- and N-arylformamides developed by Hertler and Corey4 and by Ugi and co-workerss are unsuccessful with low-molecular-weight isocyanides. This common failure is probably due to poor efficiency in extraction of these very polar substances from water. The present method also has been successfully employed for the preparation of smaller quantities of methyl (50%), ethyl (45%), 5-butyl (35%) and cyclobutyl (24%) isocyanides.6 The procedure is less laborious than that reported earlier for ethyl isocyanide.7... 

Extraction with supercritical CO2 is a technical process of increasing importance. It provides a mild and rapid technique for the extraction of low- or medium-polarity substances. Supercritical CO2 is used for supercritical fluid extraction (SFE) in important technical processes such as the decaffeination of coffee and the extraction of hops, as well as the extraction of naturally occurring compounds from biomaterials. As many applications are performed in the pharmaceutical, polymer, environmental and nutritional fields, direct on-line SFE-NMR would be an ideal tool to monitor the various extraction processes. 

The selection of adsorbent packing material is based on the polarity of pollutants to be analyzed. The nonpolar hydrophobic adsorbents retain the nonpolar analytes and allow the polar substances to pass through the column. The hydrophilic adsorbents adsorb the polar components, allowing the nonpolar materials to pass through. Various stationary phases for solid phase extraction are listed below in Table 1.5.1. 

Because C02 has weak dissolving capabilities, it is suitable as an extraction medium in SFE only for compounds of small and medium molecular mass and of low polarity. As a result, suitable modifiers must be added in order to extract polar substances. Modifiers are polar organic solvents, that is, with a nonzero dipole moment (methanol, acetonitrile, tetrahydrofuran, or water are the most commonly used) that enhance the diffusibility of polar analytes in nonpolar extraction media such as C02. 

Because water is a polar substance that can be heated by microwave irradiation, it can often improve analyte recovery. In a study of focused MAE of PAHs from soil and sediments [6], sample moisture level showed significant influence on extraction efficiency, and 30% water in the sample provided the highest recovery. Similarly, the maximum recovery of phenyl-urea herbicides was obtained with 10% water in soils [75], In the extraction of triazines from soil, water content in the range 10 to 15% yielded the highest recovery [7],... 

A MicroPak MCH-10 reverse phase column was chosen for separation of the hexane and ether extracts. The monomolecular bonded phase provides efficient separation of both polar and non-polar substances and rapid equilibration to initial activity after gradient elution programs. The reverse phase column provides symmetrical, narrow peaks for the cannabinoic acids, which tend to tail on polar, normal phase columns (e.g. silica). 

Work has recently been undertaken at the Home Office Central Research Establishment at Aldermaston on a procedure for the specific measurement of THC and its metabolites. Following an initial HPLC separation stage, the RIA is used as a sensitive quantatative method for screening column eluants. HPLC separation was performed on a reverse phase column system - highly polar compounds being eluted first followed by substances of decreasing polarity. Methanol extracts of plasma or hydrolyzed urine samples were chromatographed. 

The like dissolves like is a useful guide for selecting solvents to use in the extraction, Non-polar substances are usually successfully extracted into non-polar solvents like hexane or methylene chloride. Polar and ionic substances are often extracted with water. 

Cocaine gives a bright blue colour with acidified iodoplatinate solution, and has the same Rf value as methadone in System TE the two are well separated in System TA. In practice, cocaine is rarely detected in urine samples by this technique. The drug is rapidly metabolised to benzoylecgonine, a polar substance which extracts poorly into organic solvents. Immunoassay kits which are highly specific for benzoylecgonine are available, and this method should be used if it is essential to detect cocaine abuse. 

Coupled supercritical CO,-subcritical water systems may facilitate the class-selective extraction of non-polar substances and families of compounds of different polarity by adjusting the temperature of the water. 

The distillate of the extractive distillation described in chapter 16.6.1 had been found to contain 62.3 % diacetyl, 9.0 % 2,3-pentanedione, and minor quantities of water, methanol, ethanol, acetone, and acetaldehyde dimethyl acetal. Contrary to the diketones, the latter more or less polar substances all form low-boiling azeotropes with the extremely nonpolar hexane. The respective data are compiled in Table 3. 

When the hydrophobic fraction was analyzed by silica 60 TLC, quite apolar substances were found at the front of the developing solvent (chloroform/methanol/watep=65 25 4). Therefore, we examined whether or not the strain HD-1 produced hydrocarbons from C02. HD-1 cells were collected from 10 liter BM culture. Non-polar substances contained in the chloroform extracts of the disrupted cells were directly analyzed by GC/MS(E1) (Fig. 3a). The characteristic fragmentation patterns and each mass number of the molecular ion peak of these... 

The isotopic characterisation of organic compounds by classical elemental analysis with isotope ratio mass spectrometry (EA-IRMS) demands milligram amounts of pure compounds, which can not always easily be provided. Most analytes in flavour characterisation are volatile, and therefore, after their extraction, coupling of (capillary) gas chromatography (cGC) with IRMS would be an ideal tool for the isotope ratio analysis of the individual substances. For C- ]127, 128] and N-analysis ]179, 180] this has been realised for a long time by combining GC to IRMS via a combustion (C) unit (GC-C-IRMS), even for polar substances after their derivatisation ]181, 182]. 

CO solvent that is fully miscible with supercritical carbon dioxide can provide a more polar solubilizing environment needed for the extraction of polar substances. For example, solubility of fish liver oil in supercritical carbon dioxide increases by addition of small amounts of ethanol (Fig. 6). Depending on the amount of the cosolvent added, critical temperature and critical pressure of the binary mixture also change. 

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