in situ ionic-liquid-dispersive liquid-liquid microextraction of sudan dyes from...

J. Sep. Sci. 2014, 37, 1967–1973 1967

Bo XuDaqian SongYuanpeng WangYan GaoBocheng CaoHanqi ZhangYing Sun

College of Chemistry, JilinUniversity, Changchun, P. R.China

Received March 22, 2014Revised April 24, 2014Accepted May 6, 2014

Research Article

In situ ionic-liquid-dispersive liquid–liquidmicroextraction of Sudan dyes from liquidsamples

In situ ionic-liquid-dispersive liquid–liquid microextraction was introduced for extractingSudan dyes from different liquid samples followed by detection using ultrafast liquid chro-matography. The extraction and metathesis reaction can be performed simultaneously, theextraction time was shortened notably and higher enrichment factors can be obtained com-pared with traditional dispersive liquid–liquid microextraction. When the extraction wascoupled with ultrafast liquid chromatography, a green, convenient, cheap, and efficientmethod for the determination of Sudan dyes was developed. The effects of various experi-mental factors, including type of extraction solvent, amount of 1-hexyl-3-methylimidazoliumchloride, ratio of ammonium hexafluorophosphate to 1-hexyl-3-methylimidazolium chlo-ride, pH value, salt concentration in sample solution, extraction time and centrifugationtime were investigated and optimized for the extraction of four kinds of Sudan dyes. Thelimits of detection for Sudan I, II, III, and IV were 0.324, 0.299, 0.390, and 0.655 ng/mL,respectively. Recoveries obtained by analyzing the seven spiked samples were between 65.95and 112.82%. The consumption of organic solvent (120 �L acetonitrile per sample) was verylow, so it could be considered as a green analytical method.

Keywords: Dispersive liquid–liquid microextraction / Ionic liquids / Liquid samples/ Sudan dyes / Ultrafast liquid chromatographyDOI 10.1002/jssc.201400317

� Additional supporting information may be found in the online version of this articleat the publisher’s web-site

1 Introduction

Since Sudan dyes are harmful to humans, illegal additivesof Sudan dyes in foodstuffs have attracted great attention.The addition of Sudan dyes in foodstuffs may afford signif-icant benefits from the economic point of view. The Sudandyes are forbidden from using in food the world over [1]. Re-cently, Sudan Red events in many countries caused panic ofconsumers, the Sudan dyes were unlawfully added in chillisauce, chilli powder, and fried chicken by unscrupulous mer-chants to restore the natural colors lost during processing [2].

Sudan dyes are a family of synthetic industrial colorantsand widely used as coloring agents or polishing agents inpetrol, plastics, waxes, printing inks, shoes, and flooring fortheir fresh color, colorfastness, wide availability, and low cost[3]. Sudan II is the dimethyl derivative of Sudan I. The dyesand their metabolites may cause liver, spleen, and bladdercancer. Therefore, Sudan I is categorized by the International

Correspondence: Ying Sun, College of Chemistry, Jilin University,Qianjin Street 2699, Changchun 130012, P. R. ChinaE-mail: [email protected]: +86-431-85168399

Abbreviations: DLLME, dispersive liquid–liquid microextrac-tion; IL, ionic liquid; UFLC, ultrafast liquid chromatography

Agency for Research on Cancer (IARC) as a class 3 carcinogen[4]. Many studies have already demonstrated that both SudanI and Sudan II are carcinogens [3, 5]. On account of theircarcinogenicity they are strictly prohibited for use as additivesat any level in foodstuffs in most countries. However, theRapid Alert System for Food and Feed (European Regulation2002/178/EC) reported the fact that Sudan dyes can still befound in kinds of food products [6]. Since the concentrationsof Sudan dyes in food matrices, especially in liquid samples,are very low, accurate trace analysis with a high enrichmentfactor preconcentration step is of huge importance.

Detection of Sudan dyes in various food matrices hascaused increasing attention all over the world during thepast few decades [7]. HPLC coupled with different detec-tors; such as MS [8], UV/Vis [1], and diode array detec-tion [9] are often applied for the determination of Sudandyes in food matrices. Meanwhile, various extraction meth-ods have been established for preparing sample, such asSPE [10], molecularly imprinted solid-phase extraction [11],magnetic solid-phase extraction [12], supercritical fluid extrac-tion [13], cloud point extraction [14], and dispersive liquid–liquid microextraction (DLLME) [15]. However, these meth-ods always have many deficiencies, such as expenditure of

Colour Online: See the article online to view Figs. 1 and 2 in colour.

C© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.jss-journal.com

1968 B. Xu et al. J. Sep. Sci. 2014, 37, 1967–1973

time and organic solvent, demanding conditions, and the re-quirement of multi-step procedures. So it seems to be verynecessary to develop a convenient, time saving, cheap, lowtoxic, efficient, and green extraction method. Ionic-liquid-dispersive liquid–liquid microextraction (IL-DLLME) hassuperior advantages of high enrichment factor and perfectrecovery [16–18].

The room temperature ionic liquids (ILs), which are com-posed of organic cations and various anions, are a family of airstable salts with wide temperature-stable liquid ranges [19].Due to their low melting point, negligible vapor pressure,miscibility, and ability to dissolve a wide range of inorganicand organic compounds, noncombustible and high thermalstability as well as environmentally friendly nature [20], theyare used as green extraction solvents in pretreatment of thesamples. Several excellent reviews have compiled the appli-cations of ILs in analytical chemistry [21–23].

In the current work, an in situ IL-DLLME coupled withultrafast liquid chromatography (UFLC) was applied for thedetermination of Sudan dyes in variety liquid samples. Thein situ IL-DLLME has some advantages over conventionalDLLME. The metathesis reaction and extraction can be simul-taneously performed in a single vessel. The extraction timeis very short. Moreover, this method avoids ultrasound, heat-ing, or cooling of the sample solution, which simplifies theexperiment procedures greatly. And in situ IL-DLLME doesnot require a dispersive organic solvent normally required inconventional DLLME. To the best of our knowledge, this isthe first study about in situ IL-DLLME coupled with UFLC–UV for the identification and quantification of Sudan dyes inliquid samples up to now.

2 Materials and methods

2.1 Chemicals and samples

The standards of Sudan I–IV were obtained from the NationalInstitutes for the Control of Pharmaceutical and BiologicalProducts (Beijing, China). The chemical information of theSudan dyes is shown in Supporting Information Fig. S1.The stock solutions of Sudan dyes (100 �g/mL) were respec-tively prepared by dissolving appropriate amount of analytesin HPLC-grade acetonitrile and stored at 4�C in darkness.Mixed working standard solutions at desirable concentrationlevels were freshly prepared by diluting the stock solutionswith HPLC-grade acetonitrile.

HPLC-grade methanol and acetonitrile were obtainedfrom Fisher (New York, USA). 1-Alkyl-3-methylimidazoliumionic liquids (purity > 99%), including [C4MIM]Cl,[C5MIM]Cl and [C6MIM]Cl, and ammonium hexafluorophos-phate ([NH4][PF6]) were purchased from Chengjie Chemi-cal (Shanghai, China). Analytical-grade NaCl was purchasedfrom Beijing Chemical Factory (Beijing, China).

Deionized water was prepared with Milli-Q water purifi-cation system (Millipore, New York, USA).

Seven real samples, including tap water, lake water,red wine, vinegar, soy sauce, and two kinds of fruit juiceswere collected for validating the present method. Tap waterwas taken from our own laboratory after continual flow for10 min. The lake water was collected in Donghu (Changchun,China). Red wine and juices in different brands were pur-chased from local supermarkets in Changchun. The spikedsamples were prepared by adding the mixed working standardsolution in the samples and stored in glass containers at 4�Cin the dark before used. The samples were filtered through0.45 �m micropore membranes and directly analyzed.

2.2 Apparatus

A UFLC–UV system (Shimadzu, Kyoto, Japan) equippedwith two LC-20AD pumps, an SIL-20A autosampler, a CTO-20A thermostatted column compartment and an SPD-20AUV/Vis detector was used for analysis. The chromatographicseparation of the analytes was carried out on a shim-packVP-ODS column (150 mm × 4.6 mm, 4.6 �m particle size).Data acquisition and processing were performed with LC-solution software (Shimadzu, Kyoto, Japan).

A pH meter (DELTA 320, METTLER-TOLEDO, Shang-hai, China) was used to measure the pH value of solution.A TDL 80-2B centrifuge (Shanghai Anting Scientific Instru-ment Factory, China) was used for centrifugation.

2.3 In situ IL-DLLME

Eight milliliters of the spiked sample and 0.56 g NaCl wasplaced into the 10 mL centrifuge tube. Then, 0.05 g of[C6MIM]Cl was added and dispersed completely into thesample after gentle shaking. An aqueous [NH4][PF6] solution(1 mL, 0.30 g/mL) was added into the tube resulting in theformation of a turbid solution. After shaking for 1 min, theturbid solution was centrifuged for 10 min at 3000 rpm. TheIL sediment was deposited at the bottom of the tube and thenthe supernatant was removed with a pipette. The IL sedi-ment was dissolved in acetonitrile and the final volume was150 �L. Then the resulting solution was filtered through a0.22 �m syringe filter before injecting into the UFLC system.All experiments were performed in triplicate.

2.4 UFLC–UV conditions

In this study, isocratic elution was adopted for the chromato-graphic separation of the Sudan dyes with the mobile phasescontaining acetonitrile and water (99:1, v/v) at a flow rate of1.0 mL/min. The temperature of the column was kept at 30�C.Injection volume was 10 �L. The monitoring wavelength was478 nm for Sudan I, Sudan II and 520 nm for Sudan III,Sudan IV. The Sudan dyes can be separated from each otherabsolutely within 10 min.


J. Sep. Sci. 2014, 37, 1967–1973 Liquid Chromatography 1969

Figure 1. The effect of extraction solvent volume on extraction recoveries (A) and enrichment factors (B).

3 Results and discussion

In this study, the spiked water samples at a concentration levelof 65 ng/mL were employed for the optimization. Some sig-nificant conditions, including type and amount of IL, amountof ion-exchange reagent, pH value of sample solution, saltconcentration, extraction time, and centrifugation time wereinvestigated in detail to obtain high extraction recovery (R)and enrichment factor (EF). The equations of the two param-eters are shown in Eqs. (1) and (2).

EF = Csed

C0(1)

where Csed and C0 are the analyte concentration in the sed-iment and the initial analyte concentration in the samplesolution, respectively.

R = Csed × Vsed

C0 × Vaq× 100% = EF × Vsed

Vaq× 100% (2)

where Vsed and Vaq are the volume of sediment phase and thevolume of sample solution before adding [NH4][PF6] aqueoussolution, respectively.

3.1 Optimization of the in situ IL-DLLME conditions

3.1.1 Type of IL

The structures of ILs, especially the length of the alkyl chain,have a significant influence on their physicochemical prop-erties [24], which greatly decide the partition coefficients ofinteresting analytes between the IL and sample phase. Soselecting an appropriate solvent should be put in the firstplace. The IL used as the extraction solvent should be able tomeet several demands, such as high capability to extract theanalytes, dissolving completely in sample, good chromato-graphic behavior under the selected UFLC conditions and

forming a turbid solution after adding [NH4][PF6] aqueoussolution. In this work, three kinds of hydrophilic ILs, in-cluding [C4MIM]Cl, [C5MIM]Cl, and [C6MIM]Cl, were used.When [C4MIM]Cl was used, there was no obvious turbid so-lution forming after adding [NH4][PF6] aqueous solution. Ac-cording to the experimental results, the enrichment factorsobtained with [C5MIM]Cl were higher and the extraction re-coveries were lower than those obtained with [C6MIM]Cl, re-spectively. So considering both enrichment factor and extrac-tion recovery, [C6MIM]Cl was selected as dispersive solventin this study.

3.1.2 Amount of IL

In order to estimate the influence of the ionic liquid amounton extraction efficiency, sample solutions containing differ-ent amounts of [C6MIM]Cl (0.041, 0.045, 0.050, 0.055, and0.060 g) were subjected to the same DLLME. When theamount of [C6MIM]Cl was 0.041, 0.045, 0.050, 0.055, and0.060 g, the volumes of the sediment phase were 25, 30, 35,40, 45 and �L, respectively. The extraction recoveries and en-richment factors are shown in Fig. 1. The extraction recoveriesshow an increase with increasing the amount of [C6MIM]Cland when the amount exceeds 0.050 g, the increase of therecoveries is not obvious. On the contrary, the enrichmentfactors decrease gradually because the volume of sedimentincreases with increasing the amount of [C6MIM]Cl. Con-sidering both enrichment factor and extraction recovery,0.050 g of IL was selected in the following experiments.

3.1.3 Mass ratio of [NH4][PF6] to IL

The influence of the amount of ion-exchange reagent on theextraction efficiency was investigated by varying the massratio of [NH4][PF6] to [C6MIM]Cl from 1:1 to 8:1. When theratio was increased beyond 6:1, the volume of sediment inthe bottom was almost unchanged. Therefore, the mass ratioof 6:1 was chosen in this study.


1970 B. Xu et al. J. Sep. Sci. 2014, 37, 1967–1973

Figure 2. Effect of extraction conditions: effect of pH of sample solution (A); effect of concentration of salt (B); effect of extraction time (C);and effect of centrifugation time (D).

3.1.4 pH value of sample solution

The pH value of sample solution plays an important role inthe extraction of organic compounds. The influence of pHvalue in the range of 2–12 on the extraction recovery wasinvestigated. It can be seen from Fig. 2A that the effect of pHvalues on extraction recoveries is not significant. To simplifythe experiment procedures, no extra solution was added forpH adjustment in this work.

3.1.5 Salt concentration

NaCl (0–11%) was added to evaluate the influence of the ionicstrength of the solution. As apparently shown in Fig. 2B, theextraction recoveries increase first, and then decrease withthe increase of NaCl concentration. The salting out effectplays the dominant role at certain salt concentration range.However, when concentration of salt is too high, due tothe ion exchange between [PF6]− in [C6MIM][PF6] and Cl−,[C6MIM][PF6] was transformed into [C6MIM]Cl, which is sol-uble in water as mentioned above [25]. This phenomenon

leads to the decrease in the volume of sediment and the poorextraction recoveries. So, in further experiments, the NaClconcentration was chosen as 7%.

3.1.6 Extraction time

In this work, extraction time is defined as interval time be-tween the injection of the [NH4][PF6] solution and the cen-trifugation. In the in situ extraction the contact surface be-tween extraction solvent and sample solution is very large andthe time to obtain the extraction equilibration is greatly short-ened. It can be seen from the experimental results shown inFig. 2C that no obvious variation of the recovery is observedwith the extraction time in the range of 1–30 min. Therefore,the extraction time selected was 1 min.

3.1.7 Centrifugation time

To achieve a good separation result of the phases, the effectof centrifugation time (3, 5, 7, 10, 15, 20, and 40 min) was



Table 1. Analytical performances of the present method

Analyte Linear range (ng/mL) Regression equation Correlation coefficient LOD (ng/mL) LOQ (ng/mL)

Sudan I 1.5–1000 A = 706.17 C + 4171.9 0.9969 0.324 1.068Sudan II 1.5–1000 A = 786.06 C + 5398.5 0.9979 0.299 0.989Sudan III 1.5–1000 A = 990.82 C − 4680.3 0.9973 0.390 1.285Sudan IV 2.5–1000 A = 1003.2 C − 9242.3 0.9956 0.655 2.162

Table 2. Intraday and interday precision

Analyte Concentration(ng/mL)

Intraday (n = 5) Interday (n = 5)

Recovery(%)

RSD(%)

Recovery(%)

RSD(%)

Sudan I 30 105.05 4.68 99.71 5.27100 108.64 2.56 103.17 3.98

Sudan II 30 94.86 2.53 96.90 4.99100 104.60 2.87 102.90 2.67

Sudan III 30 95.91 4.39 97.38 6.41100 102.73 2.63 93.59 2.11

Sudan IV 30 105.23 2.60 110.53 4.58100 105.58 1.29 100.67 3.47

examined. When the centrifugation time is not long enough,the IL phase cannot be entirely deposited in the bottom of thetube. The experimental results are shown in Fig. 2D, whichindicated that the recoveries reach the maximum value at10 min and no obvious variation was observed when thecentrifugation time exceeded 10 min. Therefore, 10 min wasselected as the centrifugation time in the following experi-ments.

3.2 Evaluation of the method

3.2.1 Working curve and detection limit

The working curves were constructed by analyzing the spikedwater samples under the optimal conditions and plotting thepeak areas versus the concentrations of Sudan dyes. Underthe optimal experimental conditions, linear range, correlationcoefficient (r), and detection limit were obtained for evaluat-ing the performances of the present method. The LODs andLOQs were determined based on the signal to noise (S/N) ra-tio of 3 and 10, respectively. The results are shown in Table 1.The obtained LODs and LOQs of analytes were 0.299–0.655and 0.989–2.162 ng/mL, respectively.

3.2.2 Precision and recovery

To evaluate the precision of this present method, the intra-day and interday repeatability were tested. The intraday andinterday precision of the method were evaluated by analyz-ing the spiked samples at two concentrations levels (30 and100 ng/mL) on the same day and the five consecutive days,respectively. As shown in Table 2, the intraday and interdayRSDs for all Sudan dyes were in the range of 1.29–4.68 and2.11–6.41%, respectively. The recoveries were 94.86–108.64and 93.59–110.53% for intraday and interday, respectively.

Table 3. Analytical results for real samples

Sample Spiked (ng/mL) Sudan I Sudan II Sudan III Sudan IV

Recovery (%) RSD (%) Recovery (%) RSD (%) Recovery (%) RSD (%) Recovery (%) RSD (%)

Tap water 25 102.31 5.57 94.71 1.71 95.86 0.97 100.38 1.5480 109.21 3.23 93.71 1.85 98.50 3.38 94.02 1.04

River water 25 100.36 4.23 95.61 3.06 97.62 1.85 96.85 2.8680 104.52 2.12 97.18 1.65 94.71 3.21 97.37 4.36

Juice A 25 109.89 1.35 93.65 2.10 85.01 2.18 82.65 3.8580 100.21 3.75 97.67 2.64 86.16 1.90 88.66 1.71

Juice B 25 107.85 4.23 105.02 2.18 87.74 2.32 83.33 3.1280 106.58 3.97 95.59 1.60 78.55 2.25 83.35 3.55

Red wine 25 104.30 1.91 85.77 3.53 75.19 3.28 83.75 7.8580 112.82 3.32 92.80 3.22 82.46 1.61 82.43 2.95

Vinegar 25 96.16 3.46 81.15 2.59 65.95 3.28 70.74 4.8580 99.86 4.56 79.63 3.69 67.59 5.69 71.23 3.86

Soy sauce 25 99.70 5.35 70.56 4.78 68.13 6.81 69.51 5.4680 97.23 4.63 69.38 2.69 66.82 5.21 70.69 4.83


1972 B. Xu et al. J. Sep. Sci. 2014, 37, 1967–1973

Table 4. Comparison of the present method with other methods

Method Sample (amount) Solvent (amount) Extractiontime (min)

LODs (ng/mL) Recoveries (%) RSDs (%) Reference

Magnetic solid-phaseextraction

Red wine, juice,vinegar (100 mL)

Acetonitrile (4 mL) 15 0.0039–0.017 76.3–96.6 2.6–9.4 [1]

Magnetic solid-phaseextraction

Environmental water(75 mL)

Acetonitrile (4 mL) 6 0.082–0.12 87.1–111.4 0.59–7.94 [12]

Liquid–liquidmicroextraction

Red wine, fruit juice (4mL)

[C6MIM][PF6] (50 �L) 10 0.428–1.454 68.54–108.28 1.42–6.19 [26]

Molecularly imprintedsolid-phaseextraction

Water (100 mL) Methanol (8 mL) 1.5 0.01–0.05 88.5–101.2 1.9–4.6 [27]

Solid-phaseextraction

Environmental water(750 mL)

Methanol (4 mL) 22 0.05–0.2 91.9–98.1 1.1–4.6 [28]

In situ IL DLLME Water, juice, redwine, vinegar, soysauce (8 mL)

[C6MIM]Cl (0.050 g) 1 0.299–0.655 65.95–112.82 0.97–7.85 This work

3.2.3 Analysis of real samples

In order to evaluate the applicability of the present method,seven real samples were pretreated under the optimized con-ditions. No Sudan dyes were detectable in these real samples.Supporting Information Fig. S2 shows the chromatograms ofthe red wine. As listed in Table 3, the recoveries of the Sudandyes are in the range of 65.95–112.82% with RSDs between0.97 and 7.85%, which indicate that the present method issatisfactory for determining Sudan dyes in liquid samples.However, due to the complex matrices, the recoveries of theanalytes in liquid food samples are apparently lower thanthose of the analytes in water samples. Because ethanol inred wine and acetic acid in vinegar are not beneficial to thedeposition of IL, the recoveries decrease. A good deal of saltin soy sauce can affect the recoveries as mentioned in Section3.1.5.

3.2.4 Comparison of in situ IL-DLLME with other

methods

In order to evaluate the performances, the present methodwas compared with other methods. It can be seen from Ta-ble 4 that the present method has the advantages of smallamount of sample, short extraction time, and low solventconsumption over other methods with acceptable recoveriesand detection limit.

4 Conclusion remarks

In this study, in situ IL-DLLME was successfully applied forthe extraction of Sudan dyes from different liquid samples.The recoveries achieved by the present method were accept-able. The LODs and LOQs of the Sudan dyes were in therange of 0.299–0.655 and 0.989–2.162 ng/mL, respectively.Recoveries obtained by analyzing spiked real samples at two

concentration levels were between 65.95 and 112.82%. Theintraday and interday RSDs were lower than 6.41%. It is in-dicated that this method is feasible and should be applied tothe determination of Sudan dyes in other complex samplesby varying the extraction conditions.

The authors have declared no conflict of interest.

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