Pair Extraction Systems

Ion-pair formation can also result in an uncharged species having solubility characteristics suitable for their extraction into organic solvents. Reactions involving ion-pair formation are widely used for the extraction of metal ions as well as nonmetallic inorganic and organic ions. Typical 

It has been assumed that sufficient HCl is present in the aqueous phase to permit the neglect of unprotonated tribenzylamine. 

For this system the distribution ratio of iron is given by which formally resembles the expression shown earlier for the distribution of acetic acid 

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Ethoxylated alkyl amines in pesticide formulations were separated using two different columns a cyano-modified silica column to determine the alkyl distribution and an amino-modified column to determine the ethylene oxide distribution [88]. The detection, specific for ethoxylated amine, was performed with a post-column ion-pair extraction system and fluorescence detection. 

A reversed-phase HPLC post-column ion-pair extraction system was developed by Kim and Stewart [71, 72] for the analysis of carboxylic acid drugs and their salts (sodium formate, sodium acetate, 3-bromopropionic acid, 6-aminocaproic acid, 11-bromoundecanoic acid, 1-heptanesulfonic acid, / -n i t rophcny 1 acetic acid, sodium benzoate, sodium salicylate, valproic acid, probenecid, naproxen, ketoprofen, ibuprofen, mefenamic acid, flufenamic acid, and cefuroxime sodium) using a-(3,4-dimethoxy-phenyl)-4,-trimethylammoniummethylcinnamonitrile methosulfate... 

Since NB has a relatively high permittivity ( ° = 34.8), the ion-pair formation in NB is usually of less significance. Thus, in the ion-pair extraction systems using NB, the formation of ion pairs is not necessarily required to be considered for low overall concentrations of the salts. According to the proposed theory [30-33], the distribution equilibrium in such a system can be described in terms of the standard ion-transfer potential [A"0° = —(AGt° ° /zF) z, the charge number of the ion F, the Faraday constant] as a measure of extractability (or hydrophobicity) of an individual ion. When m kinds of ions are distributed at equilibrium between the 0/W interface, the Galvani potential difference A (p s (j) — (j)° across the 0/W interface is given by [30-32]... 

The ion-pair extraction system with pyrogallol and tetraphenylarsonium is an appropriate system for the selective sybstoichiometry of As(V) Tetraphenylarsoni-... 

Table 42 Ion association-ion pair extraction systems. (Source G. H. Morrison and H. Freiser, Solvent Extraction in Analytical Chemistry, John Wiley, New York, 1957)...

The effects of various additives on the sensitivity and selectivity ofTSP-LC-MS of thiocyanates and anilines were studied to optimize ionisation conditions. Trialky-lammonium formates were found to increase the selectivity and sensitivity of the TSP process [242]. TSP-LC-MS was also used for the characterization of the quaternary amine pesticides paraquat, difenzoquat, diquat, mepiquat and chlorme-quat from water and soil samples. Base peaks were [M-i-H] and [M-CH3-i-H] [243]. Difenzoquat, a difficult-to-determine quaternary ammonium pesticide was analysed using a post-column ion-pair extraction system [244]. 

Shoester, M., G. Kloster, Chromatographic post-column ion pair extraction system for the determination of cationic surfactants. Vom Wasser, 1991, 77,13-20. 

Schoester, M., G. Kloster, HPLC separation and quantification of anionic surfactants using an automated on-line ion pair extraction. system, Fresenius Z. Anal. Chem., 1993,345,767-772. 

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]. 

What became known as the tetrad effect was first observed in the late 1960s during lanthanide separation experiments [25]. Fig. 1.3 shows a plot of log K, where is the distribution ratio between the aqueous and organic phases in a liquid-liquid extraction system. There are four humps separated by three minima, first at the f /f pair, secondly at the f point, and thirdly at the pair. 

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 physical as well as chemical interactions of a solute is capable of changing its apparent partition coefficient between a pair of solvents. Therefore, it is absolutely necessary to take this into consideration while selecting an appropriate extraction-system. Craig and Craig have advocated that the partition coefficients... 

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 reaction to form the palladium complex is similar to that reported for amine salts, although here, because a bidentate chelating ligand is used, no chlorine atoms are retained in the complex, and the system is easy to strip. Also, as both reactions involve initial ion pair extraction, fast kinetics are observed with 3-5 min contact time to reach equilibrium at ambient temperature. The extraction conditions can be easily adjusted in terms of acidity to suit any relative metal concentrations and, because the reagent is used in the protonated form, good selectivity over base metals, such as iron and copper,... 

The selection of solvents for LLPC is similar to the selection of solvents in liquid-liquid extraction systems. The solid support has little effect upon the selection of the solvent pair, except for the obvious fact that a hydrophilic support for a polar stationary phase requires a hydrophobic... 

Ion-pair extraction, or extractive alkylation, is essentially identical to PTC in liquid-liquid systems, but the R4N+ salts are used in stoichiometric quantities. The intermediate R4N + salts of the required anions can be prepared and isolated.14... 

Precipitation/homogenization with hydrochloric acid and MeCN was presented for the milk, muscle, liver, and kidney samples. The extract was partially evaporated, and TCs were analyzed in separate ion-pair chromatographic systems, depending on the kind of sample and analyzed compound (47). 

Deproteinization with hydrochloric acid and MeCN was reported for OTC, TC, and CTC residue determination in milk samples. The filtrate was extracted with hexane and dichlorometh-ane, followed by partial evaporation of the aqueous layer. An addition of f-butyl alcohol to the aqueous layer during evaporation prevented foaming and bumping. Recoveries greater than 90% were achieved, and TCs were separated using the ion-pair chromatographic system (48). 

A multiresidue procedure for /3-lactams in milk (67) was adapted for the determination of free CEF metabolites—DFC dimer and DFC cysteine conjugate. The extraction and deprotein-ization procedure was modified for tissue samples. For LC analysis, an ion-pair chromatographic system gave satisfactory separation from the interference in the LC fractions. The recoveries of this procedure were 54-61% with CV < 10% (71). 

The sample homogenization with an MeCN THF mixture was used for the simultaneous determination of SMM, miloxacin, and oxolinic acid. The supernatant was filtered and injected directly into the ion-pair chromatographic system using a shielded hydrophobic phase. This method did not require time-consuming and complex extraction procedures moreover, the use of a restricted-access-material column prevented both column clogging and peak broadening throughout the analysis. On the other hand, no preconcentration of the sample affected the LOD... 

We showed direct measurements of the mass transfer rate of a simple ion-pair extraction in the single microdroplet/water system and discussed the transfer mechanism on the basis of the kinetic data. 

Tetraalkylammonium salts are the most common electrolytes for the non-aqueous solvent systems and high molecular weight ammoniums have been used for the ion-pair extractant. The thermodynamic properties of the salts such as partition equilibria, solubility, ion-pair formation etc. have been studied in a variety of solvents. The detailed equilibria or structure of ion pair, however, are not fully elucidated. 

Two basic approaches have been described to enhance the affinity of ionic compounds for the stationary phase in reversed-phase systems. The first approach, sometimes inappropriately referred to as ionic suppression involves the adjustment of the pH mobile phase so that the compound of interest is in the neutral or unionized form. The second approach to enhance the retention of ionic compounds on reversed-phase supports involves the application of ion-pair extraction techniques. 

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