FULL PAPER

* E-mail: analchem@jlu.edu.cn; Tel./Fax: 0086-0431-85168399; 0086-0431-85112355 Received May 22, 2010; revised June 11, 2010; accepted July 12, 2010.

Chin. J. Chem. 2011, 29, 147—152 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 147

Ultrasonic Extraction Coupled with Capillary Electrophoresis for the Determination of Azo Dyes in Lipsticks Using Ionic

Liquid as Dynamic Coating and Background Electrolyte

Li, Dan(李丹) Wang, Ziming(汪子明) Wang, Lu(王璐) Xu, Xu(许旭) Zhang, Hanqi*(张寒琦)

Department of Chemistry, Jilin University, Changchun, Jilin 130012, China

A simple and rapid method based on ultrasonic extraction and capillary electrophoresis using 1-butyl-3-methylimidazolium tetrafluoroborate as dynamic coating and background electrolyte was developed. The method was applied to the separation and determination of three azo dyes in two lipsticks. To increase extraction rate and yield, lipstick samples were coated on a glass slide before ultrasonic extraction. The dyes were extracted by ultrasonic extraction for 30 min, and then determined by capillary electrophoresis. Several experimental factors, such as ionic liquid concentration, pH value of background electrolyte and applied voltage, were examined and op-timized. Under the optimal conditions, three azo dyes were completely separated within 12 min, and the detection limits for the three azo dyes ranged from 0.33 to 0.88 µg/mL. The recoveries were in the range of 96.8% to 108.8%.

Keywords capillary electrophoresis, ionic liquids, dyes, lipstick

Introduction

Azo dyes are the compounds of a kind and charac-terized by one or more N＝N groups in their structures. Nearly half the usual dyes are azo dyes owing to their superlatively coloring capacity, stability to light and heat, color uniformity and possibility of mixing differ-ent dyes to give a variety of coloration.1 They are widely used in different kinds of products, such as tex-tile, food, cosmetic and pharmaceutical. However, about 15% of dyes are released into the wastewater,2 which brings a serious pollution to the environment, because these compounds and their degradation products are toxic.3,4 Furthermore, they are harmful not only to the environment, but also to human health. The anaerobic reduction and cleavage of azo-bonds by mammalian intestinal microorganisms can generate degradation products. The mainly representative degradation prod-ucts are aromatic amines,5,6 which are potentially mutagenic and carcinogenic to the human body. In re-cent years, the researches on azo dyes were mainly fo-cused on food,7 textile,8,9 and wastewater,10-13 and did not pay much attention to cosmetics. As far as lipsticks are concerned, azo dyes can be taken into the body through eating or drinking inadvertently. In the hygienic standard for cosmetics (GB7916-87), only 67 categories of dyes are allowed to be used in cosmetic temporarily, and some of them are azo dyes. The area and the maxi-mum content of these azo dyes which can be used are strictly restricted. So qualitative and quantitative analy-

sis of these azo dyes in cosmetics are necessary. Generally, separation of azo dyes can be performed

by gas chromatography (GC),14,15 high performance liquid chromatography (HPLC),16,17 and capillary elec-trophoresis (CE).18,19 However, some azo dyes (e.g., sulfonated dyes) are non-volatile and thermally unstable, so they can not be determined by GC. As for HPLC, samples need to be cleaned up before injection and it requires a long time for column reconditioning after sample injection, which leads to a costly and time-con- suming analysis. CE is an alternative technique and can overcome the disadvantages in GC and HPLC. The separation of azo dyes in CE is based on their relative electrophoretic mobilities under the applied electrical field, and samples can be directly injected into the cap-illary column after simple pretreatment. Furthermore, only a small quantity of sample and solvent are needed, which is economical and generates little waste. There-fore, CE has been successfully applied to the separation and determination of azo dyes.

Ionic liquids (ILs) are the compounds of a kind which are molten salts at room temperature. ILs are composed of bulky organic cations and inorganic anions. ILs have a lot of properties superior to conventional organic solvents, such as negligible vapour pressure, incombustibility, high conductance, ability to dissolve organic, inorganic and polymeric materials and a wide temperature range.20 They are considered to be envi-ronmentally benign due to their negligible vapour pres-sure. In recent decades, ILs attract more and more atten-

Li et al.FULL PAPER

148 www.cjc.wiley-vch.de © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2011, 29, 147—152

tion of analytical chemists. An exponential growth in the number of publications on ILs shows that many re-search groups have developed their research dealing with many aspects of ILs, such as excellent solva-tion,21-23 liquid-liquid extraction,24 electrochemistry,25 catalysis for clean technology and separation.26,27 The application range of ILs also refers to chromatographic separations. They can be used as stationary phases in GC and organic modifiers, mobile phase additives, sta-tionary phases in HPLC. ILs can not be directly used in CE because of their high viscosity and conductivity, so the application of ILs in CE should be carried out through some appropriate ways. They can be used as capillary wall covalent coating,28 background electrolyte (BGE) and dynamic wall coating in aqueous CE,29 ionic additives,30 and in non-aqueous CE.31 Furthermore, chiral separation can be achieved by the use of chiral ILs in CE.32

In our previous study, HPLC was applied to the de-termination of two azo dyes in lipsticks. The results in-dicated that the retention time of azo dyes was too short, and the resolution was not very satisfactory.33 Besides, ILs was applied in CE as BGE for the separation of 11 amino acids, and the results were satisfactory.34 In this study, an attempt to separate three azo dyes by CE using ionic liquid as dynamic coating and BGE was made. The target azo dyes were temporarily allowed to be used in cosmetics. Ultrasonic extraction was applied to fast and efficient extraction of azo dyes from lipsticks. The influences of ILs concentration, pH value of BGE, and applied voltage were investigated and optimized. Sev-eral analytical performances, including linearity and sensitivity, were reported. The method was applied to the determination of azo dyes in two lipsticks.

Experimental

Apparatus

The separation and determination were performed with a LUMEX CAPEL-105 CE system (LUMEX Co., Ltd, Moskovsky prospect, St. Petersburg, Russia) equipped with a UV detector (wavelength range: 190 nm to 380 nm). A bare fused-silica capillary of 60 cm (50 cm to the detector)×75 µm I.D. (Yongnian Photo-conductive Fiber Factory, Hebei Province, China) was used. The data acquisition was carried out with a Chrom&Spec version 1.5 Chromatography Data System. Samples were introduced into the capillary by pressure injection for 10 s at 30 mbar. The absorbance was measured at wavelength 254 nm. The separation tem-perature was maintained at (25±0.5) with a circ℃ u-lating water cooling system. Prior to first use, capillary was washed successively with 0.1 mol/L NaOH for 20 min, water for 20 min, 0.1 mol/L NaOH for 10 min and water for 10 min. Before sample injection the capillary was rinsed with water for 2 min, 0.5 mol/L HCI for 2 min, water for 2 min, 0.1 mol/L NaOH for 2 min, water for 2 min and BGE for 2 min. A DELTA-320 acidity

meter (Mettler-Toledo Instruments Co., Ltd, Shanghai, China) was used for pH measurement. The BGE was adjusted to the desired pH value with 0.1 mol/L NaOH. Ultrasonic extraction was carried out with a KQ2200E Ultrasonic cleaner (Kunshan Ultrasonic Instrument Co. Ltd. Kunshan. China).

Reagents and materials

Doubly distilled water and analytical reagents were used for the preparations of the standard and sample solutions. Lithol Rubin BCA (Color Index No.15850), Orange II (Color Index No.15510), and Ponceau SX (Color Index No.14700) were purchased from TCI. The chemical structures and forbidden areas of uses of the three azo dyes are shown in Table 1. The standard stock solutions of azo dyes were prepared by dissolving 50 mg of each azo dye in 50% methanol, in which 0.02 mol/L HCl was added to increase the solubility of Lithol Rubin BCA, and the total volume was 100 mL. The work solutions were prepared by the appropriate dilu-tion of the stock solutions in 50% methanol. [BMIM]BF4 was purchased from Shanghai Chengjie Chemical Company. BGE was prepared by diluting the ILs with doubly distilled water to make the concentra-tions of ILs range from 20 to 80 mmol/L. Methanol was used as the electroosmotic flow (EOF) maker. All solu-tions were stored at 4 ℃ before using.

Sample preparation

The lipsticks were purchased from local market. Because the lipsticks were ropy and ceraceous, extrac-tion of dyes from bulky lipstick samples was somewhat difficult. A glass slide was used in the ultrasonic extrac-tion. After the sample was weighed the first time (W1), the surface of a glass slide was coated with the sample (2 cm×3 cm). After coating, the sample was weighed the second time (W2). The sample amount was obtained by subtracting W2 from W1. The amount of sample on the surface was about 0.01 g. The glass slide coated with sample was put into a 50 mL beaker, and 25 mL of methanol was added in it. The beaker was sealed and immersed in the water bath in ultrasonic cleaner. The ultrasonic extraction was performed for 30 min. The resulting extract was diluted with water to the final volume of 50 mL. The sample solution was filtered fi-nally through a 0.45 µm membrane before it was in-jected into the CE system.

Calculation

The mobility of the analyte was calculated by the equation:

eff m eo

1 1Ll

V t tµ

⎛ ⎞⎜ ⎟ ⎜ ⎟⎝ ⎠

＝ －

Ultrasonic Extraction Coupled with Capillary Electrophoresis for the Determination of Azo Dyes

Chin. J. Chem. 2011, 29, 147—152 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 149

Table 1 Chemical structures and application range of the three azo dyes studied

Color index Name Chemical structure Frobidden areas of uses

15850∶1 Lithol Rubin BCA

(D&C Red No.7)

Forbid to be use in eye cosmetics

15510 Orange II

(D&C Orange No.4)

Forbid to be used in eye, mouth, and lip cosmetics

14700 Ponceau SX

(FD&C Red No.4)

Forbid to be used in mouth and lip cosmetics

where µeff is the electrophoretic mobility of the analyte tested, tm and teo are the migration times of analyte and EOF marker, respectively. L is the total length of capil-lary, l is the length of the capillary between sample in-troduction part and detector, and V is the applied volt-age.

Results and discussion

Ultrasonic extraction of azo dyes from lipstick

In our previous study, microwave-assisted extraction was applied to the extraction of azo dyes from lipstick.33 In this experiment the ultrasonic extraction was applied and methanol was used as the extraction solvent. How-ever, when methanol was used as the solvent for sample injection, the peaks became wide. After ultrasonic ex-traction, the extract was diluted with water to make the concentration of methanol be 50%.

Capillary electrophoresis separation of azo dyes

Capillary zone electrophoresis (CZE) was selected as the separation mode. To achieve satisfactory separa-tion of azo dyes, the effects of [BMIM]BF4 concentra-tion, pH value of BGE and applied voltage were inves-tigated.

Mechanism of separation using [BMIM]BF4 as BGE

A bare fused-silica capillary was used for the separa-tion. The silanol groups on the surface of the capillary are weakly acidic with average pKa values assessed at 5.3.35 When the pH value of BGE is higher than 7, there will be a high dissociation of silanols, which results in negative charges on capillary wall and induces an EOF towards the cathode. In this experiment [BMIM]BF4

was used as BGE, and the capillary wall could be coated by the positively charged imidazolium groups of [BMIM]BF4. The azo dyes were negatively charged, which could be confirmed by the phenomenon that the migration time of the dyes was longer than that of the EOF. The azo dyes may associate with either the free

imidazolium group on the capillary wall or the free imidazolium group in the solution. Considering the structures of azo dyes and imidazolium group, the asso-ciation could be mainly driven by the electrostatic and hydrogen bonding interaction between the azo dyes and the [BMIM]＋.

Effect of the [BMIM]BF4 concentration

To evaluate the effect of [BMIM]BF4 concentration on the separation of azo dyes, the effects of the concen-tration of [BMIM]BF4 on the electrophoretic mobility (µeff) and migration time of azo dyes are shown in Fig-ure 1a and Figure 1b, respectively. From Figure 1a, it can be seen that electrophoretic mobilities of azo dyes increase with the increase of [BMIM]BF4 concentration from 20 to 80 mmol/L. The negative sign of the elec-trophoretic mobility indicates that the direction of elec-trophoretic mobilities of azo dyes is towards anode. Figure 1b indicates that the migration time of both EOF and azo dyes increases when the concentration of [BMIM]BF4 increases. The effects of the concentration of [BMIM]BF4 on the peak areas of azo dyes were in-vestigated, and the results are shown in Figure 1c. The peak areas increase slightly with the increase of [BMIM]BF4 concentration in the range of 20 to 70 mmol/L. When the concentration of [BMIM]BF4 is higher than 70 mmol/L, the peak areas increase sharply. Based on the compromise between sensitivity and analysis time, 60 mmol/L [BMIM]BF4 was chosen as the BGE.

Effect of pH value of BGE

The effect of pH value of BGE on the resolution was investigated, and the results are shown in Figure 2. When the pH value is lower than 12.00, the three azo dyes can not be separated completely. With the increase of pH value, the resolution improves. However, both the migration time and peak areas of azo dyes change slightly with the increase of the pH value. Though high pH value makes resolution be improved, it also causes

Li et al.FULL PAPER

150 www.cjc.wiley-vch.de © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2011, 29, 147—152

Figure 1 Effect of concentration of [BMIM]BF4 on electro-phoretic mobility (a), migration time (b) and peak area (c). pH: 12.50. Capillary: 60 cm (50 cm to detector)×75 µm i.d. Applied voltage: 15 kV. Temperature: (25.0±0.5) . Detection℃ wave-length: 254 nm.

large current, which can produce much more Joule heat that influences the resolution. Therefore, 12.50 was ap-propriate to the work.

Effect of the applied voltage

At a fixed capillary length and buffer, the elec-tric-field intensity that can influence the migration time and resolution would be determined by applied voltage. In order to find an appropriate applied voltage for the

Figure 2 Effect of pH value of BGE on the resolution BGE: 60 mmol/L [BMIM]BF4. The other parameters are the same as in Figure 1.

separation of azo dyes, the effect of applied voltage in the range of 11 to 19 kV was studied, and the experi-mental result is shown in Figure 3. When the applied voltage increased, the migration time of azo dyes short-ened and the peaks narrowed, which leads to the de-crease of the peak areas. Furthermore, too high applied voltage can induce large current. Based on these ex-perimental results, 15 kV was selected as the optimal applied voltage.

Figure 3 Effect of applied voltage BGE: 60 mmol/L [BMIM]BF4. pH: 12.50. The other parameters are the same as in Figure 1.

According to the experimental results mentioned above, the selected optimal conditions are obtained as follows: the concentration of [BMIM]BF4 is 60 mmol/L, pH value is 12.50 and the applied voltage is 15 kV. Un-der these conditions, the three azo dyes were completely separated within 12 min. Figure 4 shows the typical electropherogram of azo dyes.

Linearity, reproducibility, and detection limit

Under the optimum conditions, the linear relation-ship between the concentrations of three azo dyes and the corresponding peak areas were obtained by a series

Ultrasonic Extraction Coupled with Capillary Electrophoresis for the Determination of Azo Dyes

Chin. J. Chem. 2011, 29, 147—152 © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.cjc.wiley-vch.de 151

Figure 4 Electrophoregram of azo dyes 1. Lithol Red BCA; 2. Orange II; 3. Ponceau SX

of injections of mixed standard solutions. The corre- sponding regression equations are listed in Table 2. The reproducibility of the method was determined with a mixed standard solution containing 7.50 µg•mL－1 Lithol Red BCA, 3.25 µg/mL Orange II, and 7.50 µg•mL－1 Ponceau SX. The RSD values (n＝10) of the peak areas for Lithol Red BCA, Orange II, and Ponceau SX were 3.58%, 2.49% and 2.10%, respectively. Those of the migration time for the analytes were 1.34%, 1.35% and 1.12%, respectively. The detection limits shown in Ta-ble 2 were determined as the lowest concentration of analytes that yielded a signal to noise ratio of 3. The detection limits for Lithol Red BCA, Orange II, and Ponceau SX were 0.88, 0.33, and 0.58 µg/mL, respec-tively.

Analysis of samples

The proposed method was applied to the determina-tion of azo dyes in two kinds of lipsticks. The electro-phoregrams of samples are shown in Figure 5. The identification of the chromatographic peaks for the ana-lytes were based on the migration time and peak areas obtained by standard addition method. The contents of azo dyes found in the lipsticks together with RSDs (n＝5) are listed in Table 3. It is seen from Table 3 that only one kind of dye, Lithol Red BCA, in the two kinds of lipsticks was detectable. The recoveries were deter-mined by spiking the work solution containing three azo dyes into the extracts of lipstick. The obtained recover-ies of analytes are in the range of 96.8% to

108.8% and given in Table 3. The results indicate that the proposed method is suitable for the extraction, separation and determination of azo dyes in real sam-ples.

Figure 5 Electrophoregrams of sample 1 (a) and sample 2 (b).

Conclusion

Three azo dyes were separated and determined by CZE using [BMIM]BF4 as dynamic coating and BGE after ultrasonic extraction. The experimental conditions were examined and optimized. Under optimal condi-tions, the analytes were completely separated from one another within 12 min. Due to the good reproducibility, linearity, and recovery, the proposed method is suitable for the fast determination of azo dyes in lipsticks. It is especially significant that the method extends the appli-cation of ILs in CE separation and azo dyes determina-tion.

Table 2 The results of regression analysis on calibration curves and detection limitsa

Azo dye Linear regression equation Linear range/(µg•mL－1) r Detection limit/(µg•mL

－1)

Lithol Red BCA Y＝1.73783X＋0.04824 2.94—100 0.9992 0.88

Orange II Y＝3.67330X＋2.29072 1.10—100 0.9976 0.33

Ponceau SX Y＝2.14997X＋0.53162 1.95—100 0.9999 0.58 a BGE: 60 mmol/L [BMIM]BF4. pH: 12.50. Applied voltage: 15 kV. Temperature: (25.0±0.5) . Detection℃ wavelength: 254 nm.

Li et al.FULL PAPER

152 www.cjc.wiley-vch.de © 2011 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chin. J. Chem. 2011, 29, 147—152

Table 3 Recoveries of the three azo dyes in samples

Lithol Red BCA Orange II Sample Content/

(mg•g－1) RSD/% (n＝5)

Added/ (mg•g－1)

Found/ (mg•g－1)

Recovery/% Content/ (mg•g－1)

RSD/% (n＝5) Added/

(mg•g－1) Found/

(mg•g－1) Recovery/%

1 55.05 2.65 50 107.96 105.8 — — 50 48.41 96.8

2 30.50 2.27 50 84.54 108.8

— — 50 49.62 99.2

Ponceau SX Sample Content/

(mg•g－1) RSD/% (n＝5)

Added/ (mg•g－1)

Found/ (mg•g－1)

Recovery/%

1 — — 50 52.86 105.7

2 — — 50 49.92 99.8

References

1 Zhao, X. H.; Lu, Y. P.; Hardin, I. Biotechnol. Lett. 2005, 27, 69.

2 Zhao, X. H.; Lu, Y. P.; Phillips, D. R.; Hwang, H. M.; Har-din, I. R. J. Chromatogr. A 2007, 1159, 217.

3 Gomathi Devi, L.; Girish Kumar, S.; Mohan Reddy, K.; Munikrishnappa, C. J. Hazard. Mater. 2009, 164, 459.

4 Lucas, M. S.; Dias, A. A.; Sampaio, A.; Amaral, C.; Peres, J. A. Water Res. 2007, 41, 1103.

5 Pinheiro, H. M.; Touraud, E.; Thomas, O. Dyes Pigm. 2004, 61, 121.

6 Bhaskar, M.; Gnanamani, A.; Ganeshjeevan, R. J.; Chandrasekar, R.; Sadulla, S.; Radhakrishnan, G. J. Chro-matogr. A 2003, 1018, 117.

7 Dossi, N.; Piccin, E.; Bontempelli, G.; Carrilho, E.; Wang, J. Electrophoresis 2007, 28, 4240.

8 Sasaki, K.; Sakai, M.; Matusita, K.; Masuda, Y.; Sato, K. Bunseki Kagaku 2008, 57, 833.

9 Sahin, S.; Demir, C.; Güçer, S. Dyes Pigm. 2007, 73, 368. 10 Jain, R.; Sharma, N.; Radhapyari, K. J. Appl. Electrochem.

2009, 39, 577. 11 Pazdzior, K.; Smolka, A. K.; Ledakowicz, S.; Ledakowicz, J.

S.; Mrozinska, Z.; Zylla, R. Chemosphere 2009, 75, 250. 12 Arslan-Alaton, I.; Tureli, G.; Hanci, T. O. J. Photochem.

Photobiol., A 2009, 202, 142. 13 Lu, X. J.; Yang, B.; Chen, J. H.; Sun, R. J. Hazard. Mater.

2009, 161, 241. 14 Bilgi, S.; Demir, C. Dyes Pigm. 2005, 66, 69. 15 Rehorek, A.; Plum, A. Anal. Bioanal. Chem. 2007, 388,

1653. 16 Vanerkova, D.; Sakalis, A.; Holcapek, M.; Jandera, P.;

Voulgaropoulos, A. Rapid Commun. Mass Spectrom. 2006, 20, 2807.

17 Plum, A.; Rehorek, A. J. Chromatogr. A 2005, 1084, 119.

18 Perez-Urquiza, M.; Ferrer, R.; Beltran, J. L. J. Chromatogr. A 2000, 883, 277.

19 Jager, A. V.; Tonin, F. G.; Tavares, M. F. M. J. Sep. Sci. 2005, 28, 957.

20 Greaves, T. L.; Drummond, C. Chem. Rev. 2008, 108, 206. 21 Jiang, X. C.; Nie, Y.; Li, C. X.; Wang, Z. H. Fuel 2008, 87,

79. 22 Schleicher, J. C.; Scurto, A. M. Green Chem. 2009, 11, 694. 23 Fan, J.; Fan, Y. C.; Pei, Y. C.; Wu, K.; Wang, J. J.; Fan, M.

H. Sep. Purif. Technol. 2008, 61, 324. 24 Heitzman, H.; Young, B. A.; Rausch, D. J.; Rickert, P.;

Stepinski, D. C.; Dietz, M. L. Talanta 2006, 69, 527. 25 Li, J. W.; Fan, C.; Xiao, F.; Yan, R.; Fan, S. S.; Zhao, F. Q.;

Zeng, B. Z. Electrochim. Acta 2007, 52, 6178. 26 Shen, J. H.; Wang, H.; Liu, H. C.; Sun, Y.; Liu, Z. M. J. Mol.

Catal. A: Chem. 2008, 280, 24. 27 Wong, H. T.; Han, S. J.; Livingston, A. G. Chem. Eng. Sci.

2006, 61, 1338. 28 Borissova, M.; Vaher, M.; Koel, M.; Kaljurand, M. J.

Chromatogr. A 2007, 1160, 320. 29 Wu, X. P.; Wei, W. P.; Su, Q. M.; Xu, L. J.; Chen, G. N.

Electrophoresis 2008, 29, 2356. 30 Gao, Y.; Xiang, Q.; Xu, Y. H.; Tian, Y. L.; Wang, E. K.

Electrophoresis 2006, 27, 4842. 31 Vaher, M.; Koel, M.; Kaljurand, M. J. Chromatogr. A 2002,

979, 27. 32 Francois, Y.; Varenne, A.; Juillerat, E.; Villemin, D.; Gareil,

P. J. Chromatogr. A 2007, 1155, 134. 33 Wang, Z. M.; Zhou, X.; Zheng, J.; Zhang, H. Q.; Liu, L.; Li,

Y.; Liu, Z. H.; Wang, H. J.; Zeng, H.; He, H. Chem. J. Chin. Univ. 2005, 26, 1623 (in Chinese).

34 Li, D.; Wang, Z. M.; Wang, L.; Qu, C. L.; Zhang, H. Q. Chromatographia 2009, 70, 825.

35 Schwer, C.; Kenndler, E. Anal. Chem. 1991, 63, 1801.

(E1005221 Ding, W.; Fan, Y.)