Improvement of Extractive Alkylation Gas Chromatography of Short-Chain Carboxylic Acids in Aqueous Solution
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Analytical Sciences Advance Publication by J-STAGE Received February 25, 2021; Accepted April 26, 2021; Published online on May 7, 2021 DOI: 10.2116/analsci.21P073 Original Papers Improvement of extractive alkylation gas chromatography of short-chain carboxylic acids in aqueous solution Shigemi UEDA,*† Noriyuki TAKEMOTO,** Risa ONODERA,** Shinji TSUNOI,*** and Ikuya SHIBATA*** * CMC Analysis laboratory, Toray Research Center, Inc.,9-1, Oe-cho, Minato, Nagoya, Aichi 455-8502, Japan ** Organic Analysis laboratory, Toray Research Center, Inc.,3-3-7, Sonoyama, Otsu, Shiga 520-8567, Japan *** Research Center for Environmental Preservation, Osaka University, 2-4, Yamadaoka, Suita, Osaka 565-0871, Japan † To whom correspondence should be addressed. E-mail: [email protected] 1 Abstract In analysis of short-chain carboxylic acids such as formic acid and acetic acid in aqueous solution using the extractive alkylation gas chromatography, tetrahexylammonium bromide (THAB) as phase transfer catalyst (PTC) causes the high intensity and broad peaks in the gas chromatogram, and interfere with detection of carboxylic acid derivatives. By easy treatment of the extractive alkylation solution with perchloric acid and n-hexane as, it is possible to remove more than 95 % of THAB, and to provide good gas chromatogram with a little admixture of carboxylic acid derivatives. The desensitization was 16 % at the maximum, the contamination of glass insert in gas chromatograph and liquid phase in column by THAB was minimized, and trouble in continuous measurement can be avoided. Keyword: extractive alkylation, gas chromatography, pentafluorobenzylation, short-chain carboxylic acids, formic acid, acetic acid, removal of tetrahexylammonium bromide, perchloric acid, 2 Introduction Carboxylic acids are essential substances in the field of industrial chemistry. Since they have active hydrogen in the molecule, they are used as pH moderator. Further, they are important monomers in the field of polymer materials which undergo condensation with alcohols and amines to give polyesters and polyamides, respectively. Conventionally, analysis of carboxylic acids in resins, immersion liquids of organic material and wastewater has been required. In oxidation process of organic compounds, because the carboxylic acids such as formic acid and acetic acid are produced, quantification of carboxylic acids is an indicator of the oxidation. For analytical methods of carboxylic acids in aqueous solution, ion-exchange chromatography and liquid chromatography are generally used. However, in liquid chromatography, interaction of carboxylic acid with column stationary phase is weak, so it cannot be separated enough with the solvent. In ion-exchange chromatography, owing to inorganic anions of the high-sensitivity interfering, enough sensitivity is not often provided. Therefore, the analysis of short-chain carboxylic acids such as formic acid and acetic acid is particularly difficult. As a solution to these, some carboxylic acids such as acetic acid and propionic acid are analyzed by liquid chromatography - mass spectrometry after amidation with condensation agent and amine.1,2 On the other hand, in gas chromatography, the carboxylic acid analysis is carried out using derivatizations such as methylation with diazomethane,3 and trimethylsilylation with silylating reagent.4,5 However, when an analytical sample is an aqueous solution, these derivatizations are invalid because derivatizing reagent easily reacts with water, and extraction of short-chain carboxylic acids such as formic acid and acetic acid is difficult due to their high water solubility. Therefore, these derivatizations cannot be applied to aqueous solution. In this case, special derivatization method called the extractive alkylation is a very 3 effective method wherein a phase transfer catalyst (PTC) such as tetraalkylammonium halide and alkylating reagent such as alkyl bromide are used.6-15 Tetrahexylammonium bromide (THAB) as PTC and pentafluorobenzyl bromide (PFBB) as alkylating reagent are particularly effective even when analysis target is formic acid and an acetic acid.11 The feature of this method is that derivatization to hydrophobic ester followed by extraction to hydrophobic solvent can be simultaneously performed. However, there is a serious problem that PTC is extracted into the hydrophobic solvent, and is injected into the GC together with the carboxylic acid ester. As a result, thermal decomposition products such as hexyl bromide and trihexylamine are detected over a wide range of the gas chromatogram and interfere with the detection of carboxylic acid derivatives. In addition, PTC causes contamination of GC inlet and column. To avoid interference from PTC, selective detection methods and PTC removal methods have been developed. The former method uses PFBB and PFB esters formed are selectively detected using a gas chromatograph - electron capture detector (GC-ECD),6,7,11 or GC-mass spectrometer (GC-MS).12-14 The analytical method using selective detection does not prevent contamination of the liquid phase in the column or the glass insert at the inlet of the GC, and causes trouble in continuous measurement. Further, the large amount of alkylamine derived from THAB acts as the liquid phase of the column, and the peak shape becomes asymmetric or broad. The latter method uses a solid-phase column or a saturated aqueous solution of silver sulfate to remove THAB.12,15,16 These removal methods require extraction solvent of several milliliters and concentration by nitrogen stream or evaporator, leading to volatilization of low boiling point derivative. Therefore, it is difficult to apply these conventional methods to short-chain carboxylic acids. Therefore, it is necessary to efficiently remove PTC from the extractive alkylation solution including carboxylic acid derivatives. The purpose of this study is to develop a method for removing THAB as PTC for GC analysis of short-chain carboxylic acids with good 4 reproducibility. Experimental Reagents and chemicals Formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid and capric acid were purchased from FUJIFILM Wako Pure Chemical Corporation (Osaka, Japan) abbreviated as WAKO and Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan) abbreviated as TCI. THAB and PFBB were obtained from WAKO. Sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium hydroxide, perchloric acid (60 wt%, 9.2 mol/L), toluene, n-hexane and methanol were obtained from WAKO. Purified water was processed with Milli-Q water purification system (Millipore. Bedford, MA, USA). Sulfuric acid (97 wt%, 18.2 mol/L) and silver sulfate obtained from WAKO. All reagents and chemicals were of the highest grade available. Solutions Standard stock solution A was prepared by dissolving each 0.01 mL of formic acid, acetic acid, propionic acid, butyric acid and valeric acid in 100 mL of 0.25 mol/L sodium hydroxide aqueous solution. The standard stock solution A was diluted to 200 times with 0.25 mol/L sodium hydroxide aqueous solution. The standard stock solution A, the diluted solutions of standard stock solution A and 0.25 mol/L sodium hydroxide aqueous solution were regarded as the standard solution A. Standard stock solution B of carboxylic acids was prepared by dissolving each 0.05 mL or 0.05 g of formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelargonic acid and capric acid in 5 ml of methanol. The standard 5 stock solution B was diluted to 200 times with methanol. 0.01 mL of the diluted standard stock solution B or methanol was mixed with 1 ml of 0.025 mol/L sodium hydroxide aqueous solution, and these solutions were regarded as the standard solution B. THAB toluene solution of 0.1 mol/L and 0.2 mol/L were prepared by dissolving 0.002 mol and 0.004 mol respectively in 20 mL of toluene. 2 mol/L phosphate buffer of pH 7 was prepared by dissolving 0.02 mol of sodium dihydrogen phosphate and 0.03 mol of disodium hydrogen phosphate with 25 mL of purified water. 2 mol/L perchloric acid was prepared by mixing 2 mL of perchloric acid (60 wt% aqueous, 9.2 mol/L) and 7.2 mL of purified water. 2 mol/L sulfuric acid was prepared by mixing 1 mL of sulfuric acid (97 %, 18.2 mol/L) and 8.1 mL of purified water. Saturated aqueous solution of silver sulfate was prepared by dissolving 2 g of silver sulfate with 20 mL of purified water as much as possible. Standard procedure (without PTC removal) for extractive alkylation One milliliter of the standard solution A or the standard solution B, 0.1 mL of the phosphate buffer, 1 mL of the 0.025 mol/L THAB toluene solution, 0.035 μL (0.25 mmol) of PFBB and magnetic stir bar were put in a vial, and sealed it tightly. The vial was heated up at 100 ℃ by dry bath heater with stirring by magnetic stir for one hour. After cooling and centrifuged for phase separation, the upper toluene solution was moved to GC automatic liquid sampler vial. Optimum PTC removal procedure for extractive alkylation One milliliter of the standard solution A or the standard solution B, 0.1 mL of the phosphate buffer, 0.5 mL of the 0.05mol/L THAB toluene solution, 0.035 μL (0.25 mmol) of PFBB and magnetic stir bar were put in a vial, and sealed it tightly. The vial was heated at 100 ℃ by dry bath heater with stirring by magnetic stirrer for one hour. After cooling, magnetic 6 stir bar was taken out, and 0.5 mL of n-hexane was added. The vial was shaken for 30 minutes and centrifuged for phase separation. After the aqueous solution (lower layer) was removed, 1 mL of the 2 mol/L perchloric acid was added. The vial was shaken for 30 minutes and centrifuged for phase separation. The upper hydrophobic solution of toluene and n-hexane was moved to GC automatic liquid sampler vial. After the treatment with perchloric acid, THA perchlorate precipitated as a white solid.