Carboxylic Acid Eluents

Figure 4.7 Anion exchange separation of carboxylic acids in red wine. Column, Shodex C811, 100 cm x 7.6 mm i.d. eluent, 3 mM perchloric acid flow rate, 0.9 ml min-1 temperature, 60 °C detection, reaction detection using chloro-phenol red at 430 nm. Peaks 1, citric acid 2, tartaric acid 3, malic acid 4, succinic acid 5, lactic acid 6, formic acid and 1, acetic acid.

One fact to keep in mind with such phases is that weak acid cation-exchange materials based on carboxylic acid functional groups are subject to esterification in the presence of alcohol containing eluents. Even thongh typical eluent conditions (i.e., weakly acidic aqneous eluents containing alcohol) do not favor ester formation, such stationary phases typically exhibit slowly declining capacity when operated in the presence of alcohol-containing eluents. Consequently, such columns are normally operated with acetonitrile, tetrahydrofuran or acetone rather than with methanol, in order to avoid this problem. 

In nonsuppressed anion chromatography, the conductivity of the analyte anion is higher than that of the eluent, so conductivity increases when analyte emerges from the column. Detection limits are normally in the mid-ppb to low-ppm range but can be lowered by a factor of 10 by using carboxylic acid eluents instead of carboxylate salts. 

A solution is prepared from 3.3 g (8.5 mmol) of 6-benzyloxy-4-methoxymethyl-l,2,3,4-tetrahydro-p-carboline-3-carboxylic acid isopropyl ester in 150 ml of methylene chloride, combined under argon with 3.9 ml of triethylamine, and cooled to -15°C. At this temperature, a solution of 3.2 ml (25.6 mmol) of t-butyl hypochlorite in 50 ml of methylene chloride is added dropwise without delay to this solution. After the adding step is completed, the mixture is stirred for another 10 min, combined with 2.6 ml of triethylamine, and agitated for 2 hours at room temperature. Subsequently, the mixture is concentrated to one-half thereof and extracted once by shaking with dilute ammonia solution. The organic phase is dried, filtered, and concentrated. The residue is chromatographed over silica gel with methylene chloride acetone=4 l as the eluent. Recrystallization from ethyl acetate gives 1.1 g (35% yield) of 6-benzyloxy-4-methoxymethyl-p-carboline-3-carboxylic acid isopropyl ester, m.p. 150-151°C. 

To a stirred solution of [4-(2-pyrimidinyl)piperazino]ethylamine (2.0 g, 0.01 mol) in 50 ml of methylene chloride, adamantane-l-carboxylic acid chloride (3.6 g, 0.018 mol) and triethylamine (2.9 g, 0.015 mol) were added. Stirring was continued at room temperature overnight. The methylene chloride solution was washed with water, dried over anhydrous sodium sulfate and evaporated under reduced pressure. The remaining residue was subjected to preparative HPLC. The residue was dissolved in ethyl acetate (10 ml) and subjected to flash chromatography using a 9 inch column of silica gel and ethyl acetate as the eluent. The N-[2-[4-(2-pyrimidinyl)-l-piperazinyl]ethyl]tricyclo[3.3.1.1(3,7)] decane-l-carboxamide was separated. 

Okada [1] has described a redox suppression for the ion exclusion chromatography of carboxylic acids with conductiometric detection. The reaction between hydriodic acid (the eluent) and hydrogen peroxide (the precolumn reagent) is used as the redox suppressor for ion exclusion chromatography of carboxylic acids. The suppressor is useful with highly acidic eluents and reduces background conductance more effectively than a conventional ion exchange suppressor. 

Adamic and Bartak [6] used high pressure aqueous size exclusion chromatography with reverse pulse amperometric detection to separate copper(II) complexes of poly(amino carboxylic acids), catechol and fulvic acids. The commercially available size exclusion chromatography columns were tested. Columns were eluted with copper(II) complexes of poly(aminocarboxylic acids), citric acids, catechol and water derived fulvic acid. The eluent contained copper(II) to prevent dissociation of the labile metal complexes. Reverse pulse electrochemical measurements were made to minimise oxygen interferences at the detector. Resolution of a mixture of DTP A, EDTA and NTA copper complexes was approximately the same on one size exclusion chromatography column as on Sephadex... 

The second direct method depends on the ability of aqueous potassium chloride, adjusted to pH 4 with carbon dioxide, to selectively elute hydrogen ions from sulfonic acid groups in pulps that first have been converted to their hydrogen form with 0.1 M hydrochloric acid (Cappelen and Schoon 1966). Carbon dioxide is removed from the eluent by sparging with nitrogen and the remaining acid is titrated with 0.1 M sodium hydroxide. Again, a correction factor for interference from carboxylic acids is required. This factor, as before, is based on the protons eluted from bleached pulps by the eluent. As the results depend on the concentration of potassium chloride used, the letter is adjusted so that the sulfonate content corresponds to the sulfur content of pulps assumed to contain only acidic sulfur. 

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