Separation of acids

Fig. 59. Flow-sheet of washing nitroglycerine at Gyttorp —separator of acid wash-water, B—separator of alkaline wash-water, C—separator of warm wash-water, D—storage tanks, with. wash-equipment, E— storage tanks with refrigerating coils, F— nitroglycerine waggon on a balance, /, 4, 7—air separators, 2, 5—injectors, 3, 6, 9—wash-columns.

Mixing the additive in the eluent used as a mobile phase can also modify the chromatographic system (dynamic modification), but the use of modified adsorbents has led to an improvement of resolution. Example works include that by Armstrong and Zhou [11], who used a macrocyclic antibiotic as the chiral selector for enantiomeric separations of acids, racemic drugs, and dansyl amino acid on biphenyl-bonded silica. 

Figure A4.1 Separation of acidic, basic, and neutral molecules pH 2 (0.1% TFA modifier). Note Peak 1 = aniline peak 2 = benzylalcohol peak 3 = phenol peak 4 = acetophenone peak 5 = nitrobenzene. Aniline elutes first, but has very small response at pH2.

Alkaline conditions are used so frequently in carbohydrate separations on CarboPac columns that it should be pointed out that acidic conditions are suitable for separation of acidic sugars. Figure 24 shows the separation of sialic acid containing oligosaccharides.256 Alkaline conditions were used for neutral milk oligosaccharides and mucin oligosaccharide alditols were characterized similarly.257 The carbohydrates released from yeast mannopro-tein with N-acetyl-P-D-glucosaminidase were also fractionated on CarboPac ... 

Invented by H. E. Benson in 1952 and then developed with J. H. Field at the U.S. Bureau of Mines. First licensed by the Benfield Corporation of Pittsburgh, subsequently acquired by the Union Carbide Corporation, and now licensed by UOP. The current UOP version includes new solution activators and incorporates zeolites or membrane processes for complete separation of acid gases and minimal loss of product gases. More than 650 plants were operating in 1996. Variations include the Benfield HiPure process and the Benfield LoHeat process. See also Carsol, CATACARB, Giammarco-Vetrocoke, HiPure. 

The pH value also affects the ionization of acidic and basic analytes and their electromigration. Since this migration can be opposite to that of the electroos-motic flow, it may both improve and impair the separation. This effect is particularly important in the separation of peptides and proteins that bear a number of ionizable functionalities. Hjerten and Ericson used monolithic columns with two different levels of sulfonic acid functionalities to control the proportion of EOF and electromigration. Under each specific set of conditions, the injection and detection points had to be adjusted to achieve and monitor the separation [117]. Another option consists of total suppression of the ionization. For example, an excellent separation of acidic drugs has been achieved in the ion-suppressed mode at a pH value of 1.5 [150]. 

The retention and the peak asymmetry of benzoic acid also indicate the inertness of the bonded phase. If basic compounds remain on the surface or are used as reagents, the peak asymmetry of benzoic acid is poor. The peak height is lower than that of the same quantity of o-toluic acid.3,4 This phenomenon is observed if the basic catalyst that was used in the synthesis process has not been completely washed off the stationary phase or if active amino groups remain. This type of column is not suitable for the separation of acidic compounds. 

Maase, M. Massonne, K. Halbritter, K. et al. Method for the separation of acids from chemical reaction mixtures by means of ionic fluids. World Patent WO 03, 062171 (2003). 

The broad and nearly universal applicability of the cinchonan carbamate CSPs for chiral acid separations is further corroborated by successful enantiomer separations of acidic solutes having axial and planar chirality, respectively. For example, Tobler et al. [124] could separate the enantiomers of atropisomeric axially chiral 2 -dodecyloxy-6-nitrobiphenyl-2-carboxylic acid on an C-9-(tert-butylcarbamoyl)quinine-based CSP in the PO mode with a-value of 1.8 and Rs of 9.1. This compound is stereolabile and hence at elevated temperatures the two enantiomers were interconverted during the separation process on-column revealing characteristic plateau regions between the separated enantiomer peaks. A stopped-flow method was utilized to determine the kinetic rate constants and apparent rotational energy barriers for the interconversion process in the presence of the CSP. Apparent activation energies (i.e., energy barriers for interconversion) were found to be 93.0 and 94.6 kJ mol for the (-)- and (-l-)-enantiomers, respectively. 

Fanah, S., Catarcini, P., and Presutti, C., Enantiomeric separation of acidic compounds of pharmaceutical interest by capillary electrochromatography employing glycopeptide antibiotic stationary phases, J. Chromatogr. A, 994, 227, 2003. 

New brush-type phases (donor-acceptor interactions) are appearing all the time. " Examples are stationary phases comprising quinine derivatives and trichloro-dicyanophenyl-L-a-amino acids as chiral selectors. Quinine carbamates, which are suitable for the separation of acidic molecules through an ionic interaction with the basic quinine group, are also commonly used but in general they are classified with the anion-exchange type of chiral selectors (see further) because of their interaction mechanism, even though r-donor, r-acceptor properties occur. (Some separations on Pirkle-type CSPs are shown in Table 2.)... 

Tanaka, Y., Kishimoto, Y., and Terabe, S. (1998). Separation of acidic enantiomers by capillary electrophoresis-mass spectrometry employing a partial filling technique. /. Chromatogr. A 802, 83-88. 

Y Tanaka, S Terabe. Enantiomer separation of acidic racemates by capillary electrophoresis using cationic and amphoteric f -cyclodextrins as chiral selectors. J Chromatogr A 781 151-160, 1997. 

If the HPLC mobile phase is operated close to the pA of any solute or if an acidic or basic buffer is used in the mobile phase, the effects of temperature on retention can be dramatic and unpredicted. This can often be exploited to achieve dramatic changes in the separation factor for specific solutes. Likewise, the most predictable behavior with temperature occurs when one operates with mobile phase pH values far from the pA s of the analytes [10], Retention of bases sometimes increase as temperature is increased, presumable due to a shift from the protonated to the unprotonated form as the temperature increases. As noted by Tran et al. [26], temperature had the greatest effect on the separation of acidic compounds in low-pH mobile phases and on basic compounds in high-pH mobile phases. McCalley [27] noted anomalous changes in retention for bases due to variations in their pA s with temperature and also noted that lower flow rates were needed for optimal efficiency. 

Setlow et al.83 have studied the photolysis of poly dl poly dC (polydeoxyinosinic acid polydeoxycytidylic acid, see Glossary) and poly dA dT. The photochemical changes were estimated by following absorbance changes, by chromatographic separation of acid hydrolysates, and by chromatographic separation of products from enzymatic hydrolysates. 

Miller, J. L., Shea, D., and Khaledi, M. G., Separation of acidic solutes by nonaqueous capillary electrophoresis in acetonitrile-based media. Combinated effects of deprotonation and heteroconjugation, ]. Chromatogr. A, 888,251-266, 2000. 

The data in the upper panel of Figure 9 show the elution profile obtained from the separation of acid extracts of uncooked, cooked, and cooked/stored meat by size exclusion chromatography. Uncooked meat has the greatest... 

Hata R, Nagai Y (1972) A rapid and micro method for separation of acidic glycosaminoglycans by two-dimensional electrophoresis. Anal Biochem 45 462-468... 

Wessler E (1968) Analytical and preparative separation of acidic glycosaminoglycans by electrophoresis in barium acetate. Anal Biochem 26 439-444... 

The mixture leaving the reaction zone is in the form of a hydrocarbon-acid emulsion and passes to an acid settler for separation of acid and hydrocarbon phases. This acid settler is usually a separate vessel from the reactor itself, although it is an integral part of one type of system. The hydrocarbon-free acid from the acid settler recirculates to the reactor. The hydrocarbon layer, which consists of alkylate, excess isobutane, and the inert diluents introduced with the feed, receives a caustic treatment and goes to the fractionating section of the plant. Caustic treatment is necessary at this stage of the process to neutralize acidic components, such as sulfur dioxide, which are formed in small quantities by catalyst degeneration. 

As in sulfuric acid alkylation, the hydrocarbon-acid emulsion passes from the contactor into an acid settler for separation of acid and hydrocarbon phases and the acid layer recirculates to the reactor. Unlike sulfuric acid, however, hydrofluoric acid is appreciably soluble in hydrocarbons, and as much as 1% by weight may be retained in the hydrocarbon layer. The necessity of recovering this acid from the hydrocarbon phase results, in another difference between hydrofluoric and sulfuric acid processing in that a hydrofluoric acid stripper is required. This stripper is ordinarily packed with aluminum rings which serve not only as tower packing but also as a catalyst for the decomposition of organic fluorides into hydrocarbons and free hydrofluoric acid. 

The XAD-4 quaternary resin used in these studies was prepared by the Ames Laboratory in Ames, Iowa. This resin had been used in studies by the Ames group for the adsorption and selective separation of acidic material in waste waters. For this study, the resin was chosen for its effectiveness in concentrating anionic material from solution. At the same time, it was thought that sufficient sites would be available to effectively adsorb neutral organic compounds from water. The resin was basically an XAD-4 macroreticular cross-linked polystyrene into which a trimethylamine group was introduced. The resin was stored in the chloride form but was converted to the hydroxide form before use in the resin sorption experiments. 

The gas chromatographic separation of acids present in plasticizers, apart from identifying volatile aliphatic carboxylic acids up to Ce, deals mainly with methyl esters. Carboxylic acids, present either as free acids or as alkali salts after saponification of the plasticizers, must be esterified. Conversion with methanol in presence of boron trifluoride (2) is recommended. But even better suited for plasticizer analysis is direct re-esterification of the plasticizers writh methanolic hydrochloric acid (2). 

Separation of acids on cation-exchange column. [From V T. Turkelson and M. Richards. 

Optimizing a separation of acids. Benzoic acid containing, 60 can be separated from benzoic acid containing lsO by electrophoresis at a suitable pH because they have slightly different acid dissociation constants. The difference in mobility is caused by the different fraction of each acid in the anionic form, A. ... 

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