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 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 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 . 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 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 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 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, , , , , , and 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. THA perchlorate is sensitive to heating, impact and friction and should be treated with caution.

Apparatus and conditions

The extractive alkylation solutions were measured by gas chromatography. All analyses were performed with the Agilent GC 6890 (Santa Clara, California, USA) equipped with split / splitless injection, frame ionization detector (FID) and automatic liquid sampler. The column was DB-WAX fused silica column (30 m long, 0.25 mm I.D., 0.25 µm film thickness) from

Agilent (Santa Clara, California, USA). The carrier gas was helium (99.995 vol%) with a constant column flow of 2 mL/minute. Aliquots (1 μL) of the extractive alkylation solution were injected in the split mode of 1/20 at the injector temperature of 250 ℃. FID was kept at 250 ℃.

The GC oven temperature was programed as follows: 1 minute at 50 ℃, to 250 ℃ at

20 ℃/minute, 1 minute at 250 ℃.

Results and Discussion

We investigated addition of perchloric acid and sulfuric acid to remove PTC. It was predicted that these anions show a good PTC removal efficiency because they have stronger adsorption ability to quaternary cations than bromide ion and the resultant ion pairs have high water solubility. The use of n-hexane as a solvent was also tried at the same time as the

7 treatment with these anions. The addition of silver sulfate was also tested for comparison.

Interference of THAB on detection of formic acid to capric acid

Extractive alkylation under the standard procedure using the standard solution B and subsequent GC measurements were performed. The concentration of carboxylic acids in the standard solution B was ca. 100 μg/mL each. The gas chromatogram was shown in Fig. 1.

The peaks detected from THAB were mainly hexylbromide (peak A), heptylbromide (peak B), trihexylamine (peak C), dihexylheptylamine (peak D), hexyldiheptylamine (peak E) and triheptylamine (peak F). Amines containing heptyl group were impurities in THAB. These peaks showed large tailing because of highly basic property of amines.

PFB esters of carboxylic acids shorter than valeric acid could be separated from thermal decomposition products derived from THAB. However, the detection of carboxylic acids longer than caproic acid was interfered with by thermal decomposition products of PTC. Particularly, caproic acid could not be detected because large peak of trihexylamine eluted in the same retention time.

Removal effects of various additives

Extractive alkylation using the standard solution A under the standard procedure and the

PTC removal procedure using THAB and various additives, and subsequent gas chromatography measurements were carried out. The concentration of carboxylic acids in the standard solution A was ca. 100 μg/mL each. The relative intensity of admixtures derived from

THAB and the carboxylic acid derivatives were calculated using GC peak area. These effects were summarized in Table 1.

The removal efficiency was evaluated by relative peak area of thermal decomposition products of PTC in Table 1. The removal effects under the procedure No.2 to No.5 using

8 aqueous solution of various additives were observed. Perchloric acid was the highest efficiency, and subsequently were sulfuric acid, silver sulfate and purified water.

The removal efficiency was higher under the procedures No.6 to No.10 using aqueous solution and n-hexane than under the procedures No.2 to No.5 only using aqueous solution. The turn of removal efficiency under the procedures No.6 to No.10 using aqueous solution and n-hexane was the same as under the procedures No.1 to No.5 using aqueous solution. The transfer of tetrahexylammonium salt to the aqueous layer was promoted by mixing n-hexane to reduce the polarity of the organic layer.

The removal efficiency under the procedures No.3 and No.8 using perchloric acid, the procedures 4 and 9 using sulfuric acid, and the procedures No.5 and No.10 using silver sulfate was greatly different from the efficiency of the procedures No.2 and No.7 using purified water.

THAB became perchloriate and sulfate (and/or hydrogen sulfate) by reacting with perchloric acid, sulfuric acid and silver sulfate. And their solubility to hydrophobic organic solvent

(toluene and/or n-hexane) decreased, and the removal effect became higher.

The PTC removal procedures having removal efficiency more than 95 % were three of the procedure No.3 using perchloric acid, the procedures No.8 using perchloric acid and n-hexane and the procedure No.9 using sulfuric acid and n-hexane. The removal efficiency 98.6 % of the procedure No.8 was the highest. From above consequence, it is concluded that the PTC removal procedure No.8 was the most effective in the point removing tetrahexylammonium salt from the hydrophobic extractive alkylation solution.

In only the procedure No.8, the white solid substances precipitated in the interface between the hydrophobic solution and the aqueous solution was supposed with tetrahexylammonium perchlorate (reference, specific gravity 1.0 g/cm3 of tetrabutylammonium perchlorate). On the other hand, under the procedure No.3 only using perchloric acid same as additive, this solid substances were not precipitated in the interface. The residual rate of THAB

9 by gas chromatography of the procedures No.3 and No.8 were 2.7 % and 1.4 % respectively.

These facts were considered to be due to the property that tetrahexylammonium perchlorate showed high solubility in toluene and low solubility in an aqueous solution and a mixed solvent of toluene and n-hexane, and not to be pyrolyzed at 250 ℃ of GC injection temperature. The except for n-hexane as linear saturated hydrocarbon solvent were not valid, because tetrahexylammonium perchlorate was dissolved. Therefore, the use of n-hexane was essential to

PTC removal procedure using perchloric acid.

Influence of the additive in the detection of carboxylic acids

After comparing the peak intensity of the carboxylic acid derivatives, the procedures No.2 to No.5 only using aqueous solution decreased 0 to 12 % in comparison with the procedure

No.1 (no removal), and the procedures No.6 to No.10 using aqueous solution and n-hexane 10 to 25 % decreased. PFB ester as the carboxylic acid derivative was hydrophobic, but, by adding the acidic solution and n-hexane, a small amount of them moved to the aqueous layer. When compare the decreasing rate between the carboxylic acid derivatives, that of formic acid was the highest, and mobility became smaller as the hydrocarbon chain became longer. The cause was because the water-solubility (polarity) of the derivative rose if hydrocarbon shortens. However, it was supposed that the reduction of the peak intensity of this level was supplemented in the modification of GC measurement conditions. In addition, since there is no difference in the peak intensities of carboxylic acid derivatives between the procedures No.2 to No.5 and the procedures No.7 to No.10, no hydrolysis of PFB ester occurred.

Other conditions

Among some quaternary ammonium salts examined as PTC, THAB was particularly effective against formic acid and acetic acid.

10 About perchloric acid concentration in THAB removal, sufficient effect was obtained in the concentration range of 0.5 to 2 mol/L. At the concentration above 0.5 mol/L, efficiency was slightly improved. The perchloric acid of 0.5 to 2 ml/L did not break down the derivative.

Therefore, a removal concentration of 2 mol/L was assumed to be most effective in the concentration range tested.

About the pH of the aqueous phase in the extractive alkylation, formic acid derivative was not obtained stably under alkaline conditions (0.5 mol/L sodium hydroxide aqueous solution). It is reported that formic acid and acetic acid react effectively with PFBB under weak acid to neutral and neutral to weak base respectively.17 Therefore, the optimum pH of aqueous solution was assumed to be 7.

Regarding the extraction operation after derivatization, the aqueous layer was removed once and then perchloric acid treatment was performed. Since the precipitated tetrahexylammonium perchlorate is an oxidizing solid and is designated as hazardous material

Class 1 under the Fire Service Act in Japan. With an emphasis on safety, the aqueous layer was removed once to reduce the ammonium salt concentration, and perchloric acid treatment was performed. Solid tetrahexylammonium perchlorate should be handled with care, avoiding heating, impact and friction.

Optimization of PTC removal procedure

Finally, the PTC removal procedure No.8 using 2 mol/L perchloric acid and n-hexane was most suitable to remove THAB from the extractive alkylation solution in this study. Extractive alkylation using standard solution B under this procedure and GC measurement were performed.

The concentration in the standard solution B was assumed ca. 100 μg/mL. The gas chromatogram was shown in Fig. 2.

The peak intensities of trialkylamine and alkylbromide as a mixture derived from THAB

11 significantly smaller than in Fig. 1, which makes it very easy to detect derivatives of caproic acid to pelargonic acid.

The recoveries (= derivatization yield × extraction efficiency) measured using benzyl carboxylate reference standards were good: 82% for formic acid, 98% for acetic acid, 98% for propionic acid, 95% for butyric acid, and 97% for valeric acid.

About GC analysis of short-chain carboxylic acids with the pentafluorobenzylation in double-phase system like this work, the application to formic acid and acetic acid is slightly seen,11 but the application to the other lower carboxylic acids is not found. On the other hand,

GC-MS analysis using pentafluorobenzylation in single-phase system of acetone and water has

17,18 been reported, but it was supposed that acetone (KOW = -0.24) contained in aqueous phase reduced extraction efficiencies of PFB ester. Therefore, this technique, which is applicable to many target carboxylic acids and has a higher recovery, is more effective than the reported methods.

Calibration curves under the optimum PTC removal procedure using the standard solution B

(formic acid to capric acid)

The extractive alkylation using the standard solution B under the standard procedure and the optimum PTC removal procedure, and GC measurements were carried out. The concentration in the standard solution B was assumed 0 to ca. 100 μg/mL. The typical gas chromatograms were shown in Fig. 3. Data of the calibration curves and the removal efficiency of THAB were shown in Tables 2 and 3 respectively.

Under the standard procedure, the high intensity admixtures derived from THAB were appeared over retention time 7 to 9 minutes in the upper gas chromatogram of Fig. 3. On the other hand, under the PTC optimum removal procedure, these admixtures became very small in the lower gas chromatogram of Fig. 3. The effect of PTC removal was remarkable, and the

12 detectivity of the carboxylic acids in this range became higher. The correlation coefficient of the calibration curves of the carboxylic acids under the standard procedure and the PTC optimum removal procedure was more than 0.998 in Table 2. The desensitization by PTC remove was

16% of formic acid at the maximum from the ratios of slope shown in Table 2. This desensitization did not become the serious problem. Table 3 shows the results of examining the reproducibility of PTC removal. PTC was efficiently removed regardless of the carboxylic acid concentration (0 to 100 μg/mL). The reproducibility was also very good with RSD 4.2%.

The blank of formic acid, propionic acid, caproic acid and pelargonic acid was relatively high intensity. It was estimated that the blank of formic acid and propionic acid came from methanol, and the blank of caproic acid and pelargoic acid came from THAB. Therefore it was guessed that the blank was reduced by preparing the standard solution without methanol when analyzed formic acid and propionic acid. In addition, the blank of caproic acid and pelargonic acid was reduced by using tetrabutylammonium bromide, tetrapentylammonium bromide, or tetraoctylammonium bromide instead of THAB.

Calibration curves under the optimum PTC removal procedure using the standard solution A

(formic acid to valeric acid)

For reduction of the blank of formic acid and propionic acid, the extractive alkylation using the standard solution A without methanol under the optimum PTC removal procedure and

GC measurements were carried out. The concentration in the standard solution A was assumed 0 to ca. 100 μg/mL. Data of the calibration curves were shown in Table 4.

The blank of formic acid and propionic acid were able to largely decrease in the absence of methanol. Therefore, when analyze of formic acid to valeric acid, it is necessary to prepare standard solution without using methanol.

13 Conclusions

THAB remained in the extractive alkylation solution including carboxylic acid derivatives was able to removed more than 95% by using perchloric acid and n-hexane as removal additive.19

This high removal efficiency is considered to be because perchlorate ion has a higher adsorption capacity for quaternary ammonium cations than bromide ion, and the resulting ion pair has remarkably low solubility to water and hydrophobic organic solvent including n-hexane. This result was superior to that of silver sulfate or sulfuric acid. The amount of carboxylic acid derivative introduced into GC was slightly lower than the standard procedure without removing, however, the detection sensitivity was largely improved because the interference by thermal decomposition products PTC was eliminated. In addition, the contamination of glass insert in

GC and liquid phase in column by THAB was minimized.

14 References

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1723.

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8. A. Arbin, J. Chromatogr., 1979, 170, 25.

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10. N. A. Adinolfe, and M. K. L. Bicking, J. Chromatogr. Sci., 1985, 23, 407.

11. S. Jacobsson, A. Larsson, and A. Arbin, J. Chromatogr., 1988, 447, 329.

12. H. Kataoka, K. Tamura, M. Maeda, and M. Makita, Bunseki Kagaku, 1989, 38, 327.

13. C. H. Lindh, and B. A. G. Jönsson, J. Chromatogr. B, 1997, 691, 331.

14. Y. C. Fiamegos, C. G. Nanos, J. Vervoort, and C. D. Stalikas, J. Chromatogr. A, 2004, 1041,

11.

15. H. Ehrsson, Anal. Chem., 1974, 46, 922.

16. H. H. Maurer, and J. W. Arlt, J. Chromatogr. B, 1998, 714, 181.

17. S. Kage, K. Kudo, H. Ikeda, N. Ikeda, J. Chromatogr. B, 2004, 805, 113.

18. K. Tomcik, R. A. Ibarra, S. Sadhukhan, Y. Han, G. P. Tochtrop, GF Zhang, Anal. Biochem.,

2011, 410, 110.

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2017, 202331

15 Table 1 Effects of various additives on PTC removal efficiency and peak areas of carboxylic acid derivatives.

Additive Relative GC peak THAB toluene solution Relative GC peak area of PFB ester Aqueous solution Volume of area of thermal Procedure Solute Volume n-hexane Concentration Volume decomposition Formic Acetic Propionic Butyric Valeric

(concentration) / mL / mL / mol/L / mL products of PTC acid acid acid acid, acid

No.1 - - - 0.1 1 100.0 100 100 100 100 100

No.2 Purified water 1 - 0.1 1 85.2 91 89 89 89 102

No.3 Perchloric acid (2 mol/L) 1 - 0.1 1 2.7 94 97 96 97 98

No.4 Sulfuric acid (2 mol/L) 1 - 0.1 1 13.7 88 98 98 99 100

No.5 Silver sulfate (saturated) 1 - 0.1 1 34.1 98 97 96 97 98

No.6 - - 0.5 0.2 0.5 23.3 79 79 80 80 84

No.7 Purified water 1 0.5 0.2 0.5 19.5 79 81 82 83 84

No.8 Perchloric acid (2 mol/L) 1 0.5 0.2 0.5 1.4 75 80 81 81 81

No.9 Sulfuric acid (2 mol/L) 1 0.5 0.2 0.5 2.0 77 84 90 87 87

No.10 Silver sulfate (saturated) 1 0.5 0.2 0.5 5.9 82 84 84 85 85

16

Table 2 Data of the calibration curves of formic acid to capric acid under the standard

procedure and the optimum PTC removal procedure.

Standard procedure Optimum PTC removal procedure Ratio

of a b R C a b R C slope

Formic acid 1948 18940 0.999 11.3 1642 18993 0.999 10.7 0.84

Acetic acid 2047 2180 1.000 0.6 1956 2484 1.000 0.6 0.96

Propionic acid 1878 15860 0.998 6.7 1838 6998 0.999 2.5 0.98

Butyric acid 1828 1251 0.999 0.6 1802 1265 1.000 0.3 0.99

Valeric acid 1732 3762 1.000 1.8 1710 3506 1.000 1.6 0.99

Caproic acid - - - - 1663 11606 1.000 6.6 -

Enanthic acid 1469 139 0.998 1.3 1602 1237 1.000 0.8 1.09

Caprylic acid 1468 2507 0.999 1.3 1463 1218 0.999 0.04 1.00

Pelargonic acid 1469 9909 0.998 6.7 1508 9232 1.000 5.2 1.03

Capric acid 1428 2042 0.999 1.3 1484 486 1.000 0.1 1.04

y=ax+b : calibration curve, x : concentration, y : peak area, a : slope, b : intercept, R :

correlation coefficient

C : Concentration of 0 μg/mL standard = (Peak area of 0 μg/mL standard solution) / Slope

Ratio of slope = (Slope under removal) / (Slope under no removal)

17

Table 3 Removal efficiency of THAB under the standard procedure and the optimum PTC removal procedure.

Total GC peak area of hexyl bromide and Rough concentration of trihexylamine carboxylic acid in aqueous Optimum PTC solution Standard procedure removal procedure

0 27978012 151555

1 28470739 150969

2 28256491 156975

5 28216348 157116

10 28100485 154745

20 28183733 167983

50 28235656 159266

100 28445517 168602

Mean 28235873 158402

RSD / % 0.6 4.2

Residual rate / % 100.0 0.6

18

Table 4 Data of the calibration curves of short-chain carboxylic acid under the optimum PTC removal procedure without methanol.

a b R C

Formic acid 1869 2049 1.000 0.5

Acetic acid 1893 114 1.000 0.2

Propionic acid 1723 13 1.000 0.1

Butyric acid 1565 -82 1.000 0.1

Valeric acid 1493 565 1.000 0.5 y = ax+b: calibration curve, x: concentration, y: peak area, a: slope, b: intercept, R: correlation coefficient

C : Concentration of 0 μg/mL standard solution = (Peak area of 0 μg/mL standard solution) /

Slope

Ratio of slope = (Slope under removal)/(Slope under no removal)

19 Figure Captions

Fig. 1 Gas chromatogram of the carboxylic acids (ca. 100 µg/mL each) under the standard procedure. 1, formic acid; 2, acetic acid; 3, propionic acid; 4, butyric acid; 5, valeric acid; 6, caproic acid; 7, enanthic acid; 8, caprylic acid; 9, pelargonic acid; 10, capric acid; A, hexylbromide; B, heptylbromide; C, trihexylamine; D, dihexylheptylamine; E, hexyldiheptylamine; F, triheptylamine.

Fig. 2 Gas chromatogram of the carboxylic acids (ca. 100 µg/mL) under the optimum PTC removal procedure. 1, formic acid; 2, acetic acid; 3, propionic acid; 4, butyric acid; 5, valeric acid; 6, caproic acid; 7, enanthic acid; 8, caprylic acid; 9, pelargonic acid; 10, capric acid; A, hexylbromide; B, heptylbromide; C, trihexylamine; D, dihexylheptylamine; E, hexyldiheptylamine; F, triheptylamine.

Fig. 3 Gas chromatograms of the carboxylic acids (ca. 10 µg/mL) under the standard procedure and the optimum PTC removal procedure. 1, formic acid; 2, acetic acid; 3, propionic acid; 4, butyric acid; 5, valeric acid; 6, caproic acid; 7, enanthic acid; 8, caprylic acid; 9, pelargonic acid; 10, capric acid; A, hexylbromide; B, heptylbromide; C, trihexylamine; D, dihexylheptylamine; E, hexyldiheptylamine; F, triheptylamine.

20

A C

6

B D E 1 2 3 4 5 7 8 9 F 10

0 2 4 6 8 10 12 RetenTitmioen (m tiinm) e / min

Fig. 1

21

1 2 3 4 5 6 7 C 8 9 10 A

0 2 4 6 8 10 12 RetenTitmioen (m tiinm) e / min

Fig. 2

22

(Th e standard procedure) A B 1 C D E F

8 7 9

2 10 3 5 4 6

0 2 4 6 8 10 12 (Th e optimum PTC removal proceTdimuer e(m) in) Retention time / min C 1

6 7 2 3 9 5 8 4 10

A

0 2 4 6 8 10 12 Time (min ) Retention time / min

Fig. 3

23 Graphical Index

24