Extractants disadvantages, ether

Discuss the advantages and disadvantages of this type of column separation (specific crown ether extractant) for strontium. Why not use four times the column size to accommodate a normal amount of carrier (20 mg) ... 

Atomic absorption spectrometry (AAS), atomic emission spectrometry (AES) [11], infrared (IR), Fourier transform infrared (FTIR) and Raman spectroscopy have all been studied at various times for the determination of silicon compounds. AAS has been used to determine silicon in methylisobutylketone, chloroform or petroleum ether extracts of packaging materials and foodstuffs [12-17]. However, these methods suffer from the disadvantage that they do not distinguish between organic and inorganic silicon compounds, similarly inductively coupled plasma AES measures total silicon [11]. 

The ether extraction technique is the official method of fat analysis. It consists of extracting oil using petroleum ether, followed by removal of the solvent and weighing of the oil. The main disadvantage of ether extraction is that it takes several hours to perform the test. 

Borane—dimethyl sulfide complex (BMS) (2) is free of these inconveniences. The complex is a pure 1 1 adduct, ca 10 Af in BH, stable indefinitely at room temperature and soluble in ethers, dichioromethane, benzene, and other solvents (56,57). Its disadvantage is the unpleasant smell of dimethyl sulfide, which is volatile and water insoluble. Borane—1,4-thioxane complex (3), which is also a pure 1 1 adduct, ca 8 Af in BH, shows solubiUty characteristics similar to BMS (58). 1,4-Thioxane [15980-15-1] is slightly soluble in water and can be separated from the hydroboration products by extraction into water. 

By this method of extracting the esters from their hydrochlorides, neither that of tyrosine, which remains behind combined with alkali, nor those of the diamino acids, which are soluble with difficulty in ether, are obtained. This is advantageous for the subsequent process of separation, but the method has the disadvantage that the whole quantity of esters is not taken up by the ether on account of their destruction by the alkali. In order to avoid their loss, the mass of carbonate is treated with excess of hydrochloric acid and evaporated down, the potassium... 

Procedures for isolation and measurement of lipids in foods include exhaustive Soxhlet extraction with hexane or petroleum ether (AOAC, 1995 see Basic Protocol 1), chloro-form/methanol (Hanson and Olley, 1963 Ambrose, 1969), chloroform/methanol/water (Folch et al., 1957 Bligh and Dyer, 1959 see Basic Protocol 2 and Alternate Protocol 2), acid digestion followed by extraction (see Basic Protocol 4), or, for starchy material, extraction with n-propanol-water (e.g., Vasanthan and Hoover, 1992 see Basic Protocol 3). Each method has its own advantages and disadvantages and successful measurement of lipid content is often dictated by the type of sample and extraction medium employed. Commercial extraction and preparation of edible oils are explained in the literature (Williams, 1997). 

The most suitable solvents for extracting SPA from fats are acetonitrile (113,125,126, 139,142) and water-alcohol mixtures. The fat is usually dissolved in hexane or petroleum ether, and SPA are extracted into acetonitrile (105,110-113,125,126,128). The disadvantages of acetonitrile extraction are that (a) BHT recovery is low and (b) moderately high levels of interfering compounds are coextracted. The advantage of aqueous methanolic extraction of SPA from nonpolar solvent is that the fat is mostly excluded (99,114). Hammond (99) described a methanolic extraction of a melted fat sample, heated to 40-50°C, followed by transfer of the mixture to a deep freeze for a few hours to aid the solidification of any excess fat from methanol. The methanol layer was then decanted and filtered prior to the addition of an internal standard and direct injection. 

BTF is not miscible with water and hence, it can be used for extraction purposes. Like dichloromethane, it is heavier than water. We observed, however, that phase separation between water and BTF is sometimes difficult. BTF can be used as solvent for chromatography as well. However, it has a typical intense aromatic odor, which is not convenient when using large quantities. In addition,because of the high boiling point, the solvent removal process is relatively slow (compared to solvents like ether and dichoromethane) and loss of relatively volatile products can occur. Because of these practical disadvantages, we have limited the use of BTF as a chromatography solvent to the purification of fluorous compounds (see 4.0). 

The ability of diethyl ether to extract uranyl nitrate from aqueous solution has been known for a hundred years and was the method chosen by the Manhattan Project to purify the uranium used in the first nuclear chain reactors. This solvent has numerous disadvantages. It is very volatile, very flammable, and toxic, and it requires addition of sodium, aluminum, or calcium nitrate to the aqueous phase to enhance extractions. When solvent extraction was first applied to recovery of uranium and plutonium from irradiated fuel, other oxygenated solvents less volatile than diethyl ether that were first used were methyl isobutyl ketone, dibutyl... 

The common disadvantage of crown ethers as extractants of amino compounds is the interference from hard metal ions, such as alkali and alkaline earth cations. This may cause problems in extraction-based analytical and technological applications, as these metals, particularly sodium and potassium, are often present in (bio)media of interest. For illustration. Fig. 1 shows the influence of alkali metals on the extraction-photometric determination of benzylamine with DC18C6 and picrate [56,70]. 

---