In paper chromatography (PC), paper fibers act as the carrier and adsorbed water acts as the stationary phase. Separation is achieved based on differences among the distribution coefficients of the samples. PC is flexible, fast, and efficient [1]. However, few studies of chiral separation using PC have been reported [2]. At present, in academic research and industrial applications chiral separation is generally achieved by liquid chromatography, using a chiral separation column [3, 4]. This method requires expensive equipment, which hinders the use of chiral separation in general teaching and research activities. Furthermore, the lack of visual effects makes the results less convincing in independent analytical processes. In this study, PC was used to separate racemic products, without the use of any supplementary equipment. The process is efficient and visible. We developed a dialdehyde-based filter paper (DaFP) via oxidation of the raw material, i. e., filter paper, with sodium periodate [5, 6]. A chiral amino acid was then anchored on the DaFP to produce a chiral filter paper (CFP) [7, 8]. This material gave good results in chiral separation tests.
Qualitative filter paper from Hangzhou Whatman-Xinhua Filter Paper Ltd. (Hangzhou, China) was used. Reagents were all purchased from the Sinopharm Group Chemical Reagent Co., Ltd. (Shanghai, China).
Filter paper oxidation: a qualitative filter paper sample of dimensions 10 cm×30 cm was immersed in a reactor containing 100 mL 5% (mass percentage) sodium hydroxide solution for 2 h. The filter paper was removed from the reactor and the base remaining on it was washed off. The treated filter paper was then placed in another reactor with sodium periodate solution at pH 2; the water bath temperature was adjusted and the reaction was performed in the dark for a specified time. The treated filter paper was then washed with ethanol and ultrapure water until no IO4- was detected, and then dried to give a DaFP.
CFP: a DaFP sample of dimensions 30 cm×4 cm was immersed in a reactor containing an ethanol solution of sodium L-glutamate. After microwave irradiation for 1.5 min, the mixture was removed and cooled to room temperature. The filter paper was then washed with water and dried to obtain the CFP.
The carbonyl content was determined using a semi-trace hydroxylamine method [9, 10]. The dried oxidized filter paper carbonyl content was accurately determined as follows. The oxidized filter paper was filtered into a flask. Hydroxylamine hydrochloride methanol solution and then thymol blue methanol indicator were added. The mixture was heated in a bath at 70 ℃ for 2 h. After cooling, titration was performed with standard sodium hydroxide methanol solution, until the color of the solution changed from pink to yellow. A blank test was performed at the same time.
Table 1 shows the aldehyde group contents and morphologies of filter papers oxidized using sodium periodate mass percentages ranging from 2% to 10%.
The data in Table 1 show that the aldehyde group content of the filter paper increased with increasing concentration of sodium periodate, and the maximum aldehyde group content obtained was 85.06% (amount of substance percentage). However, when the sodium periodate mass percentage was higher than 4%, the filter paper became yellowish. Further increases in the sodium periodate concentration led to an increase in the aldehyde group content of the filter paper, but the filter paper became yellowish and even shrank, i. e., a well-functioning filter paper was not obtained. The optimal oxidation condition was therefore less than 4% (mass percentage) NaIO4.
Fig. 1 shows the aldehyde group contents of filter papers oxidized using 4% (mass percentage) sodium periodate at pH values ranging from 1 to 5.
Fig. 1 shows that the pH significantly affected the oxidation capacity of sodium periodate. At pH 2, the oxidizing ability of sodium periodate was strongest, and the aldehyde group content of the filter paper was highest, reaching 58.53% (amount of substance percentage). At pH 1, reduction of sodium periodate can easily occur, therefore the content of aldehyde groups decreased. When pH > 2, because of the less acidic environment in the solution, the oxidizing capacity of sodium periodate was less strong, therefore the aldehyde group content was low. The optimal pH for oxidation using sodium periodate was therefore 2, based on the percentage of aldehyde group content.
Table 2 shows the aldehyde group contents and morphologies of filter papers treated under the optimal conditions, i. e., 4% (mass percentage) sodium periodate solution and pH 2, at reaction temperatures ranging from 25 to 65 ℃.
The data in Table 2 show that when the reaction temperature was less than 45 ℃, the oxidation rate with sodium periodate was low, resulting in a low aldehyde group content. When the reaction temperature was greater than 45 ℃, sodium periodate became unstable and decomposed because of the high temperature; this resulted in a decrease in the aldehyde group content produced by the oxidation reaction. The experimental results showed that the aldehyde group content of the oxidized filter paper reached a maximum at 45 ℃, and the appearance of the oxidized filter paper showed that it remained in good condition. The optimal temperature for the oxidation reaction was therefore 45 ℃.
Table 3 shows the aldehyde group contents and morphologies of DaFP obtained using various oxidation times. The other reaction conditions were the optimal conditions, i. e., sodium periodate 4% (mass percentage), pH 2, and temperature 45 ℃.
The data in Table 3 show that the aldehyde group content increased with increasing reaction time up to 4 h, finally reaching 57.93% (amount of substance percentage). When the reaction time was prolonged, the aldehyde group content decreased. This is because the hydroxyl and aldehyde groups can be condensed under acidic conditions to produce acetals and hemiacetals. As the reaction progressed, the aldehyde groups in the filter paper reacted with the hydroxyl groups that did not participate in the oxidation reaction, leading to a lower aldehyde group content. The optimal reaction time for the synthesis of the oxidized filter paper was therefore considered to be 4 h.
Four factors and three levels of experiments were used for the orthogonal design. The four factors were mass percentage of sodium periodate, pH value, temperature, and oxidation time, and the three corresponding levels are shown in Table 4.
The data in Table 5 show that the mass percentage of sodium periodate significantly affected the amount of aldehyde groups in the oxidized filter paper. The degree of impact of each factor on the aldehyde group content was mass percentage of sodium periodate > pH > temperature > time. Under the optimal conditions, i. e., A3, B2, C2, and D2, the maximum aldehyde group content of the oxidized filter paper was 57.93% (amount of substance percentage).
The CFP was produced by a microwave-assisted Schiff-base reaction using the DaFP and sodium L-glutamate as the raw materials. The infrared spectra a, b, c, and d in Fig. 2 show carbonyl to amino group ratios of 1:1, 1:2, 1:3, and 1:4 (amount of substance ratio), respectively. The carbonyl groups were derived from the oxidized filter paper and the amino groups were derived from sodium L-glutamate. The peaks at 1 630.66, 1 630.81, 1 631.02, and 1 630.89 cm-1 in the CFP are all attributed to C=N; these peaks confirm the formation of imino groups. The absorption peak for C=N at 1 630.90 cm-1 indicates the reaction of oxidized cellulose with the -NH2 groups on the α-carbon of sodium L-glutamate.
The amount of substance ratios of carbonyl to amino groups were 0.5:1, 1:1, 1:2, 1:3, and 1:4, respectively. The product was synthesized using a microwave-assisted Schiff-base reaction. The nitrogen contents of different CFP samples were determined quantitatively using the Kjeldahl method. The results are shown in Table 6.
In this study, an improved PC expansion method [1] was used for the separation of tartaric acid. The quantities of developer and coloring agent were 100 mL of n-butanol and 50 mL of acetic acid, and 0.100 0 g of bromophenol blue, and the developing time was 4 h. The rate of flow (Rf) value was fixed at about 0.35 under laboratory conditions. Spots developed as a result of the separation were yellow, whereas the background was blue because of the coloring agent used. The spots were clear and stable. No smearing was observed, which made the results easy to interpret by visual inspection.
Fig. 3 shows the separation of tartaric acid using the improved PC expansion method. Column a shows the separation of racemic tartaric acid on untreated filter paper. Column b shows the separation of 2.5% (mass percentage) racemic tartaric acid after development; its Rf values correspond to 0.52 and 0.40. Column c shows the separation of 5% (mass percentage) racemic tartaric acid after development; its Rf values correspond to 0.54 and 0.41. Column d shows the results for 2.5% (mass percentage) racemic tartaric acid after development; its Rf values correspond to 0.51 and 0.40. Column e shows the result for 2.5% (mass percentage) L-tartaric acid after development; its Rf value corresponds to 0.52. Column f shows the result for 2.5% (mass percentage) D-tartaric acid after development; its Rf value corresponds to 0.40. The spot with Rf=0.40 in column e indicates that the purity of the L-tartaric acid sample was not high and it contained enantiomers. Racemic tartaric acid was separated into two clearly defined yellow spots at the bottom of the CFP after development. Single-enantiomer L-tartaric acid developed only one clearly defined yellow spot. The position and Rf value of the L-tartaric acid spot correspond to those of the top yellow spots after D-expansion. The single enantiomer of D-tartaric acid was also clearly defined as one yellow spot with an Rf value corresponding to the bottom yellow spot after D-expansion. These results show that the racemic compound can be separated using the CFP.
In this study, we oxidized filter paper with sodium periodate to develop a DaFP. A paper-based chiral separation material, CFP, was synthesized using a microwave-assisted Schiff-base reaction. A modified PC method was used to separate racemic tartaric acid on the CFP; the known single enantiomers, i. e., L-tartaric acid and D-tartaric acid, were used as reference samples. Under the same conditions, the CFP successfully separated racemic tartaric acid.
2017,
Vol. 35 Issue (9): 912-917
PDF
2017,
Vol. 35 Issue (9): 912-917
PDF