Back-extraction method

Thiopental has UV absorption maxima at 290 nm and 305 nm in acidic and alkaline media, respectively. In contrast, the oxobarbitnrates have absorption maxima at 220 nm and 255 tun, respectively [46]. These differences provided a means of distinguishing thiopental from its major metabolite, pentobarbital, and as a consequence most determinations were carried out at around 280 nm [33,35,45]. In one case greater selectivity was provided by a change in extraction solvent, which permitted the determination of the carboxylic acid metabolite of thiopental [35]. Additional sensitivity could be provided by the use of back-extraction methods (vide supra). 

A much more forgiving yet limited extraction method can be used to isolate phenol species such as eugenol and chavicol. You see farther back in this chapter where one can use dilute NaOH to remove eugenol from sassafras oil Well, why not use it to isolate the damn things for further research. It works like a charm ... 

The apparent rate constant kapp depends on the concentration of hydroxide ion as is shown in Fig. 1. The absorption maxima of TcCl2(acac) 2 in chloroform appear at 281,314(sh), 340(sh), 382 and 420 nm. On the other hand, the spectrum of the aqueous phase exhibits absorption maxima at 292,350 and 540 nm. The absorbances at 350 and 540 nm increase with time, but decrease after reaching maxima. This suggests that the chemical species which is formed by the back-extraction of TcCl2(acac)2 decomposes with time. In order to clarify the behavior of chloride ion liberated from the complex, an electrochemical method was introduced for the homogeneous system. In acetonitrile, no detectable change in the spectrum of TcCl2(acac)2 was observed. On the addition of an aqueous solution of hydroxide, however, the brown solution immediately turned red-violet, and exhibited absorption maxima at 292,350 and 540 nm. The red-violet... 

Szathmary and Luhmann [50] described a sensitive and automated gas chromatographic method for the determination of miconazole in plasma samples. Plasma was mixed with internal standard l-[2,4-dichloro-2-(2,3,4-trichlorobenzyloxy) phenethyl]imidazole and 0.1 M sodium hydroxide and extracted with heptane-isoamyl alcohol (197 3) and the drug was back-extracted with 0.05 M sulfuric acid. The aqueous phase was adjusted to pH 10 and extracted with an identical organic phase, which was evaporated to dryness. The residue was dissolved in isopropanol and subjected to gas chromatography on a column (12 m x 0.2 mm) of OV-1 (0.1 pm) at 265 °C, with nitrogen phosphorous detection. Recovery of miconazole was 85% and the calibration graph was rectilinear for 0.25 250 ng/mL. 

Klinkhammer [432] has described a method for determining manganese in a seawater matrix at concentrations ranging from about 30 to 5500 ng/1. The samples are extracted with 4 nmol/1 8-hydroxyquinoline in chloroform, and the manganese in the organic phase is then back-extracted into 3 M nitric acid. The manganese concentrations are determined by graphite furnace atomic absorption spectrophotometry. The blank of the method is about 3.0 ng/1, and the precision from duplicate analyses is 9% (1 SD). 

The theoretical yield of the method is less than 100%, as only 80 - 90% of the aqueous phase is removed after back-extraction. The actual yield obtained by 54 Mn counting was 69.5 7.8%, and this can be allowed for in the calculation of results. Environmental Protection Agency standard seawater samples of known manganese content (4370 ng/1) gave good manganese recoveries (4260 ng/1). 

In a method for the determination of copper, nickel, and vanadium in seawater, Shijo et al. [840] formed complexes with 2-(5-bromo-2 pyridylazo)-5-(N-propyl-N-sulfopropylamino) phenol and extracted these from the seawater with a xylene solution of capriquat. Following back-extraction into aqueous sodium perchlorate, the three metals were separated on a C is column by HPLC using a spectrophotometric detector. 

Bartlett et al. [55] used the method of Uthe et al. [70] for determining methylmercury. Sediment samples of 2-5g were extracted with toluene after treatment with copper sulphate and an acidic solution of potassium bromide. Methylmercury was then back extracted into aqueous sodium thiosulphate. This was then treated with acidic potassium bromide and copper sulphate following which the methylmercury was extracted into pesticide grade... 

The oldest application of solvent extraction in spectrophotometric determinations uses extraction from the original aqueous solution and subsequent back-extraction into a second aqueous phase. Here the extractant provides only separation or concentration, as in the case of Np(IV) and Pu(IV) determination [16]. However, as only a few element species (e.g., Mn04, Cr04 ) are capable of absorbing light in the UV-VIS range, usually all spectrophotometric methods are based on reactions of analytes with... 

Counterion extraction Due to the relative slowness of back extraction based on the methods above, the back-extraction of proteins encapsulated in AOT reverse micelles was evaluated by adding a counterionic surfactant, either TOMAC or DTAB, to the reverse micelles [33]. This novel backward transfer method gave higher backward extraction yields compared to the conventional method. The back-extraction process with TOMAC was found to be 100 times faster than back-extraction with the conventional method, and as much as three times faster than forward extraction. The 1 1 complexes of AOT and TOMAC in the solvent phase could be efficiently removed using adsorption onto montmorillonite so that the organic solvent could be reused. 

The fermentation broth typically contains 20-30 mg/L of antibiotics, which is to say 30 parts per billion, and must be extracted into concentrated form using solvent extraction. The solvent extraction method was developed by Shell Oil and by Podbielniack and is based on the principle that penicillin is hydrolyzed in aqueous medium to H+ and RCOO ions. Thus, equilibrium in an acidic medium (i.e., one with low pH or high H+ concentration) is favored by the neutral RCOOH form, whereas equilibrium in an alkaline medium (i.e., one with high pH or low H+ concentration) is favored by the RCOO ionic form. The neutral form is more soluble in an organic medium, and the ionic form is more soluble in an aqueous medium. Thus, with amyl acetate as the organic solvent the partition coefficient of penicillin between solvent and water is about 100 at pH 3 and about 1 at pH 6. In the industrial process, the aqueous broth was acidified to pH 3 for the extraction into the organic solvent, and alkalized to a pH 6 for reverse extraction back into an aqueous medium. 

Li and coworkers synthesized the novel IL l-butyl-3-trimethylsilylimid-azolium hexafluorophosphate and demonstrated its utility for liquid/ liquid extraction of inorganic mercury. Using o-carboxyphenyl diazoamino p-azobenzene as a chelator to form a stable neutral complex with the metal ion, the authors demonstrated selective extraction into the hydrophobic IL phase [19]. When sodium sulfide was added to the IL phase, the mercury ion was back-extracted into the aqueous layer, providing an avenue for recycling the IL. The authors report extraction and back-extraction efficiencies of 99.9 and 100.1%, respectively, for a 5.0 pg/L aqueous mercury standard. The mercury detection limit was 0.01 ng/mL in water and the method was successfully applied to detecting trace mercury in natural water samples. 

Using the described extraction system, we developed methods of amino acid recovery from pharmaceutical samples and fermentation broth. Amino acids were extracted efficiently from the diluted solution of fermentation broth into [C4Cilm][PFg] in the presence of DC18C6 and may be well back-extracted by the alkaline aqueous solution (pH > 9). These methods served as a basis for the corresponding analytical procedures. 

Liquid-liquid extraction is used extensively and successfully (6). If the analytes are acidic or basic, as is often the case when HPLC is the analytical method selected, appropriate ionization suppression can be employed to affect the desired extraction. Back extraction of the analytes into an appropriately buffered aqueous volume can then serve to isolate and concentrate. Anionic and cationic surfactants, or so-called ion-pairing reagents, can be added prior to extraction to increase the partition coefficients of the trace organic ionic compounds. 

Tanaka et al. [ 16] have described a spectrophotometric method for the determination of nitrate in vegetable products. This procedure is based on the quantitative reaction of nitrate and 2-sec-butylphenol in sulfuric acid (5 + 7), and the subsequent extraction and measurement of the yellow complex formed in alkaline medium. The column reaction is sensitive and stable and absorbances measured at 418 nm obey Beer s law for concentrations of nitrate-nitrogen between 0.13 and 2.5 xg/ml. In this procedure, the vegetable matter is digested at 80 °C with a sodium hydroxide silver sulfate solution, concentrated sulfuric acid and 2-sec-butylphenol are added, and after 15 minutes of standing time the nitrated phenol is extracted with toluene. Finally, the toluene layer is back-extracted with aqueous sodium hydroxide and evaluated spectrophotometrically at 418 nm. The standard deviation of the whole procedure was 1.4%, and analytical recoveries ranged between 91 and 98%. 

Ultraviolet Spectrophotometry Pentazocine can be determined in pentazocine hydrochloride tablets by a UV spectral method. The sample is extracted with sulfuric acid, basified, extracted with ether and back-extracted into 1 in 70 dilute sulfuric acid. The absorbance of the solution in 0.5 N H2SO1L is determined at 278 nm and compared to that of the standard preparation. A cation exchange column isolation method can be utilized to determine pentazocine in pentazocine lactate injection. Pentazocine is eluted from the column with methanol 6 N HC1 (1 1). Sample absorbance is read at 278 nm and compared to a pentazocine standard at approximately 120-w.g/ mL (3). 

Other fluorescence methods for pentazocine have been used on plasma (59) and for TLC quantitation ( 7). A fluorescent 2-p-chlorosulfophenyl-3-phen-ylindone (DIS-C1) derivative of pentazocine has been measured by TLC (60) while another extraction method using heptane with 0.5% isoamyl alcohol on basified plasma has been developed. In the latter, fluorescence was measured after back-extraction into 0.1 N HC1 (61). 

Total dissolved MMHg can be analyzed by aqueous-phase ethylation after separating MMHg from the interfering chloride matrix by extraction with methylene chloride.88 For a 200-mL sample a detection limit of 0.075 pM is achieved. An alternative method for the simultaneous extraction of Hg(II) and MMHg in natural waters at fM levels is to extract both into toluene as dithiozonates after acidification of the water sample, followed by back extraction into an aqueous solution of Na2S and removal of H2S by purging with N2.89... 

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