Other Extractions from Solids

An analyte may be present in one material phase (either a solid or liquid sample) and, as part of the sample preparation scheme, be required to be separated from the sample matrix and placed in another phase (a liquid). Such a separation is known as an extraction—the analyte is extracted from the initial phase by the liquid and is deposited (dissolved) in the liquid, while other sample components are insoluble and remain in the initial phase. If the sample is a solid, the extraction is referred to as a solid-liquid extraction. In other words, a solid sample is placed in the same container as the liquid and the analyte is separated from the solid because it dissolves in the liquid while other sample components do not. 

The process can take one of two forms. In one, the sample and liquid are shaken (or otherwise agitated) together in the same container, the resultant mixture filtered, and the filtrate, which then contains the analyte, collected. In the other, fresh liquid is continuously cycled over a period of hours through the solid sample via a continuous evaporation-condensation process (that usually does not require an extra filtration step), and the liquid is collected. This latter method is known as a Soxhlet extraction. Soxhlet extraction will be discussed in more detail in Chapter 11. 

Tai Van Ha of the Nebraska State Agriculture Laboratory inspects a sample of fertilizer that is obviously not homogeneous. There are various light- and dark-colored granules of fertilizer in the bag. 

Because their solvent properties are very good and their viscosities are very low, supercritical fluids can be used for very efficient extraction of analytes from solid phase samples. The solid phase sample is held in a tube or cartridge and the supercritical fluid made to flow through (minimal pressure required). The fluid with the analyte is then made to flow through a trap solvent. The analyte dissolves in this solvent and the fluid reverts back to the gas phase. 

Finally, extractions from solids can be performed by heating followed by solvent trapping. Such a procedure is known as thermal extraction. 

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In Section 2.5, we described separation procedures in which analytes are extracted from solid samples via contact with liquid solvents that selectively dissolve the analyte and leave other components undissolved or unextracted. There are several methods by which analytes can be extracted from liquid matrices as well. 

SCFs may be used in the same way as other ordinary solvents taking into account their different properties and behaviors. Supercritical fluids can replace liquids solvents in many processes, such as extractions from solids (leaching), countercurrent multistage separations, chromatographic separations, and others, provided the solvent properties of the SCFs are adequate. 

In this method, CO2 gas at the critical temperature and pressure is u.sed to extract pesticides and other cliemicals from solid samples or liquid samples soaked in an inert powder. CO2 and other gases become fluid (neither gas nor liquid) when temperature and pres.surc reach a critical point calk d the supercritical phase. In SEE, this fluid is used to extract target chemicals OPs and CMs) from solid samples. Instruments connecting the SEE apparatus with online gas chromatography (GC) have been developed in which the serie.s of steps from extraction to analysis have been... 

Extraction From Solids and Semi-solids Other Than Biomass... 

For organometallic species, the analytical challenge involves quantitative extraction of the species from the solid phase without appreciable transformation to other forms. Various liquid extractants have been used such as methanolic hydrochloric acid (extraction of organotins). Strong alkali digests (KOFI or tetramethylammonium hydroxide) will dissolve most tissue without breakdown or transformation of arsenic species. As an alternative to liquid extraction, methylmercury species may be extracted from solids using steam distillation. Specific reaction conditions will affect the recovery of various species and rigorous validation of extraction procedures is therefore recommended. 

Other analytical procedures, such as supercritical fluid extraction (SFE) and microwave-assisted extraction (MAE), are gaining significant attention. SFE is commonly applied to many analyses that require extraction from solid matrices and has found important applications in the... 

Size of solid particles Mass transfer in gas extraction from solid substrates in most cases depends heavily on the transport rate in the solid phase. The length of the transport path determines mass transport in the solid phase. In general the extraction rate increases with decreasing particle size. On the other hand, mass transfer has to be achieved into the fluid phase. If the smaller particles hinder fluid flow in the fixed bed, then mass transfer rate decreases with smaller particles. 

Multistage countercurrent contacting is the most effective mode. It reduces the amount of solvent and makes possible continuous production of extract. Real countercurrent contact is not easily established for solids, since special effort is necessary for moving the solid, with increased difficulties at elevated pressure. Therefore, it is easier to not move the solid material and to achieve countercurrent contact by other measures. For the purposes of extraction from solids with supercritical solvents, several fixed beds in countercurrent contact with the solvent are the best configuration (Fig. 11). 

Volatile analytes can be separated from a nonvolatile matrix using any of the extraction techniques described in Ghapter 7. Fiquid-liquid extractions, in which analytes are extracted from an aqueous matrix into methylene chloride or other organic solvent, are commonly used. Solid-phase extractions also are used to remove unwanted matrix constituents. 

The materials of construction, from the cupboard to the fan, should be inorganic and resistant to attack by perchloric acid. For the cupboard itself suitable materials include stainless steel of types, 316 or 317, solid epoxy resin, and rigid PVC. Stainless steel has been popular for this application as it is easy to form, weld, and polish. It is, however, attacked by the acid, which causes discoloration of the metal surface and the formation of iron(III) perchlorate, which can be explosive. Ductwork, separate from other extract systems, is usually made from stainless steel or plastic materials. Fire regulations may preclude the use of plastic ductwork or require it to be sheathed in an outer casing of metal or GRP. The fan casing and impeller can both be made of plastic. 

High performance liquid chromatography (HPLC) has been by far the most important method for separating chlorophylls. Open column chromatography and thin layer chromatography are still used for clean-up procedures to isolate and separate carotenoids and other lipids from chlorophylls and for preparative applications, but both are losing importance for analytical purposes due to their low resolution and have been replaced by more effective techniques like solid phase, supercritical fluid extraction and counter current chromatography. The whole analysis should be as brief as possible, since each additional step is a potential source of epimers and allomers. 

Carmine extracted from cochineal insects is one of the most used natural colorings for beverages and other foods. Some representative articles refer to isolation and spectrometric analysis or the use of HPLC or capillary electrophoresis (CE) to separate and characterize all cochineal pigments. Its active ingredient, carminic acid, was quantified by rapid HPLC-DAD or fluorescence spectrometry. Carminic acid, used as an additive in milk beverages, was separated within 9 min using a high-efficiency CE separation at pH 10.0 after a previous polyamide column solid phase extraction (SPE), ... 

In liquid-solid extraction (LSE) the analyte is extracted from the solid by a liquid, which is separated by filtration. Numerous extraction processes, representing various types and levels of energy, have been described steam distillation, simultaneous steam distillation-solvent extraction (SDE), passive hot solvent extraction, forced-flow leaching, (automated) Soxh-let extraction, shake-flask method, mechanically agitated reflux extraction, ultrasound-assisted extraction, y -ray-assisted extraction, microwave-assisted extraction (MAE), microwave-enhanced extraction (Soxwave ), microwave-assisted process (MAP ), gas-phase MAE, enhanced fluidity extraction, hot (subcritical) water extraction, supercritical fluid extraction (SFE), supercritical assisted liquid extraction, pressurised hot water extraction, enhanced solvent extraction (ESE ), solu-tion/precipitation, etc. The most successful systems are described in Sections 3.3.3-3.4.6. Other, less frequently... 

Selection of a suitable ionisation method is important in the success of mixture analysis by MS/MS, as clearly shown by Chen and Her [23]. Ideally, only molecular ions should be produced for each of the compounds in the mixture. For this reason, the softest ionisation technique is often the best choice in the analysis of mixtures with MS/MS. In addition to softness , selectivity is an important factor in the selection of the ionisation technique. In polymer/additive analysis it is better to choose an ionisation technique which responds preferentially to the analytes over the matrix, because the polymer extract often consists of additives as well as a low-MW polymer matrix (oligomers). Few other reports deal with direct tandem MS analysis of extracts of polymer samples [229,231,232], DCI-MS/MS (B/E linked scan with CID) was used for direct analysis of polymer extracts and solids [69]. In comparison with FAB-MS, much less fragmentation was observed with DCI using NH3 as a reagent gas. The softness and lack of matrix effect make ammonia DCI a better ionisation technique than FAB for the analysis of additives directly from the extracts. Most likely due to higher collision energy, product ion mass spectra acquired with a double-focusing mass spectrometer provided more structural information than the spectra obtained with a triple quadrupole mass spectrometer. 

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