Ion-pair extraction method

Reactions of trident and ambident anions of hydroxy compounds with alkylating reagents were also studied.121 The ion-pair extraction method was found to be superior alkylating agents were methyl iodide (soft) and dimethyl sulfate (hard). Upon alkylation by ion-pair extraction the 5-methyl-substituted selenolene-2-one system gives mainly C-alkylation with the soft acid, methyl iodide, and mainly O-alkylation with the hard acid, dimethyl sulfate. 

The potentially tautomeric side-chain thiol systems exist mainly in the thiol form in liquid solution and in the gas phase, as found by IR and NMR spectroscopy and by a study of ionization potentials.126 Upon alkylation using the ion-pair extraction method, only the S-alkylated compounds were obtained. The synthesis, reactions, and properties of some selenides of thiophene, furan, and selenophene have been reviewed.127... 

The ion-pair extraction method (phase-transfer catalysis) can offer improved syntheses of a-substituted cyanomethylphosphonates. It is effectively employed in the alkylation of dialkyl cyanomethylphosphonate to smoothly achieve specific monoalkylation. °5 ° This technique is also well suited to the fc/v-alkylation and cycloalkylation of cyanomethylphosphonates. 

The ion-pair extraction method (phase-transfer catalysis) was rather disappointing it can be used for the specihc monoalkylation of diethyl 1-(ethoxycarbonyl)methyIphosphonate but has low yield (30%). °... 

This salt can be prepared by the ion-pair extraction method from tetra-n-butylammonium bromide and sodium p-toluenesulfinate (equation 1). 

A more convenient method of alkylation is through an ion-pair extraction method using the tetrabutylammonium salt, which is better than the alkylation of thallium(i) salts. Also, in these cases dimethyl sulphate gives predominantly O-methylation. Oxidative coupling of the 2,5-dimethyl-3-hydroxythiophen system with potassium ferricyanide yields a mixture of the racemic and meso-foims of (112). 2,5-Di-t-butyl-3-hydroxythiophen, on the other hand, gives (112a). Treatment of 2-t-butoxy-3-thienylmethanols (113) with toluene-p-sulphonic acid at elevated... 

Smutny. The t-butyl ester was used in order to achieve hydrolysis of (30), which required reaction with trifluoroacetic acid at 0 °C for 4—5 h. Treatment of (30) with warm trifluoroacetic acid for 10 min yielded (31). The reaction of (29) with ethyl a-chloroacetate gave (32). A-Phenyl phenylpropynthioamide (33) reacts in a similar manner with active bromomethylene derivatives, such as bromo-acetonitrile, bromonitromethane, and /j-nitrobenzyl bromide, in the presence of triethylamine to give the thiophen (33a). Instead of enamine thioesters such as (29), 2-aminovinyl thioketones (34) react with a-bromo-ketones in the presence of triethylamine to give 2-acyl-thiophens (35). By the ion-pair extraction method, tetrabutylammonium salts of acetylacetone were allowed to react with carbon disulphide to give (36), which upon reaction with chloroacetone gave a 45% yield of the thiol (37), in addition to a thieno[2,3-A]thiophen derivative. ... 

The ion pair extraction by flow injection analysis (FIA) has been used to analyze sodium dodecyl sulfate and sodium dodecyl ether (3 EO) sulfate among other anionic surfactants. The solvating agent was methanol and the phase-separating system was designed with a PTFE porous membrane permeable to chloroform but impermeable to the aqueous solution. The method is applicable to concentrations up to 1.25 mM with a detection limit of 15 pM [304]. 

The measurement of log and log Pi by dual-phase potentiometric titration has been described by Avdeef [33], Briefly, the method is based on the shift in the apparent pK upon addition of the partition solvent In absence of ion-pair extraction, the apparent pK (PoK) is related to the aqueous pK by the equation below ... 

Thomas et al. [31] used a colorimetric method for the estimation of miconazole nitrate in creams. The method is based on the ion-pair extraction with bromocresol purple solution and measurement of the solution at 410 nm. 

There is a recent trend towards simultaneous CE separations of several classes of food additives. This has so far been applied to soft drinks and preserved fruits, but could also be used for other food products. An MEKC method was published (Lin et al., 2000) for simultaneous separation of intense sweeteners (dulcin, aspartame, saccharin and acesulfame K) and some preservatives (sorbic and benzoic acids, sodium dehydroacetate, methyl-, ethyl-, propyl- and isopropyl- p-hydroxybenzoates) in preserved fruits. Ion pair extraction and SPE cleanup were used prior to CE analysis. The average recovery of these various additives was 90% with good within-laboratory reproducibility of results. Another procedure was described by Frazier et al. (2000b) for separation of intense sweeteners, preservatives and colours as well as caffeine and caramel in soft drinks. Using the MEKC mode, separation was obtained in 15 min. The aqueous phase was 20 mM carbonate buffer at pH 9.5 and the micellar phase was 62 mM sodium dodecyl sulphate. A diode array detector was used for quantification in the range 190-600 nm, and limits of quantification of 0.01 mg/1 per analyte were reported. The authors observed that their procedure requires further validation for quantitative analysis. 

Method A ( ion-pair extraction technique) TBA-HS04 (38.4 g, 0.11 mol) is added to aqueous NaOH (2 M, 100 ml) and the mixture stirred at room temperature for 5 min. The aqueous solution is added to the methylene compound (0.1 mol) in CHC1, (100 ml) and the two-phase system is stirred until the aqueous phase is neutral. The organic phase is separated and evaporated. 

The fact that the EP wants to replace old TEC methods with more selective, efficient, and sensitive separation methods provides the chance for the introduction of more CE methods. The continuous development of analytical methods is reflected in the national and international pharmacopoeias. This might be demonstrated for atropine sulfate. Whereas the Deutsches Arzneibuch, 7th Edition (DAB 7) only limits the tropic acid by extraction and titration with NaOH and phenolphthalein indication, the 4th edition of the EP looked for foreign alkaloids and decomposition products by means of TEC with a potassium iodobismuthate for detection. By intensity comparison of the obtained spots, it was possible to limit these impurities to 0.5%. The EP 5 utilizes an ion-pair HPLC method that is able to limit most of the impurities to less than 0.2%. To make the method more robust, an HPLC method using a polar embedded was applied, which might be the next step for the EP. However, recently the same authors have reported on a MEEKC method being as robust and precise as the latter HPLC method (see Eigure 6) but far more sensitive and, therefore, a future perspective for the EP. 

Ylinen et al. [53] developed an ion-pair extraction procedure employing tetrabutylamonium (TBA) counter ions for determination of PFOA in plasma and urine in combination with gas chromatography (GC) and flame ionisation detection (FID). Later on, Hansen et al. [35] improved the sensitivity of the ion-pair extraction approach using methyl tertiary butyl ether (MTBE) and by the inclusion of a filtration step to remove solids from the extract making it amenable to liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS) determination. Ion-pair extraction procedure has been the basis of several procedures for biota [49,54-58] and food samples [50,59,60]. However, this method has shown to have some limitations, such as (1) co-extraction of lipids and other matrix constituents and the absence of a clean-up step to overcome the effects of matrix compounds and (2) the wide variety of recoveries observed, typically ranging. 

Ion pair extraction provides a standard method for estimating ionic surfactants either colorometrically or titrimetrically. For example a cationic surfactant such as cetrimide can be estimated by pairing it with a lipophilic anionic dye such as bromocresol purple. The ion pairing creates a coloured lipophilic ion pair, which can be extracted into an organic solvent such as chloroform and a quantitative measurement of the colour extracted can be made spectrophotometrically. This type of assay is described in the BP for Clonidine Injection and Benzhexol Tablets. 

An ion-pairing spectrophotometric method has also been described [14]. This method involves the use of bromphenol blue, bromcresol purple, bromcresol green, bromthymol blue, or methyl orange, and extracting the ion-pair species into chloroform. The absorbance of the yellow product is determined at 420 nm. 

In methods of tissue analysis in which ion-pair extraction procedures are not used, long extraction times have been shown to be necessary for extraction... 

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