Vortex-Assisted Liquid–Liquid Microextraction Combinedwith HPLC for the Simultaneous Determination of FivePhthalate Esters in Liquor Samples

Yuanpei Lian & Xiaoyan Qiu & Yaling Yang

Received: 14 January 2013 /Accepted: 14 June 2013# Springer Science+Business Media New York 2013

Abstract A vortex-assisted liquid–liquid microextraction(VALLME) method using hexanoic acid as extractantfollowed by high-performance liquid chromatography–di-ode array detection was developed for the extraction anddetermination of five phthalate esters (PAEs) including bis-methylglycol ester (DMEP), benzyl butyl phthalate (BBP),dicyclohexyl phthalate (DCHP), di-n-butyl phthalate (DBP),and di-n-octyl phthalate (DNOP) from liquor samples. In thismethod, hexanoic acid was employed as extraction solvent,because its density is lower than water. And vortex mixingwas utilized as a mild emulsification procedure to reduceemulsification time and improve the effect of extraction.Under the studied conditions, five phthalate esters were suc-cessfully separatedwithin 20min and the limits of detectionwere2.3 ng mL−1 for DMEP, 1.1 ng mL−1 for BBP, 1.9 ng mL−1 forDCHP, 1.2 ng mL−1 for DBP, and 1.5 ng mL−1 for DNOP,respectively. Recoveries of the PAEs spiked into liquor sampleswere ranged from 89 to 93 %. The precisions of the proposedmethod were varied from 1.6 to 2.6 % (RSD). The VALLMEmethod has been proved to have the potential to be applied to thepreconcentration of the target analytes. Moreover, the method issimple, high sensitivity, consumes much less solvent than tradi-tional methods and environmental friendly.

Keywords Vortex-assisted liquid–liquid microextraction .

Hexanoic acid . Phthalate esters . Liquor samples . High-performance liquid chromatography

Introduction

Phthalate esters (PAEs) are mainly used as plasticizers forpolyvinyl chloride resins, cellulose film coating, and adhe-sives, in order to increase their flexibility via weak secondarymolecular interactions with polymer chains (Prokupkov´aet al. 2002; Liang et al. 2008). Manufacturers produce about400,000 tons of PAEs every year worldwide (Stanley et al.2003). PAEs are the physically bound to polymer chains and“embedded” in the polymeric materials. Use and incinerationof the polymeric materials may release PAEs to the environ-ment or food (Staples et al. 1997; Balafas et al. 1999). PAEs,as well as their metabolites and degradation products, cancause liver cancer, infertility, and fetal malformation (Arcadiet al. 1998). Potential endocrine disruption was also reported(Tyler et al. 1998). Recently, many commercial liquors sold inChina have been reported containing overdosed phthalateesters above the national standard (Guo 2012). General fer-mentation process does not produce PAEs. PAEs detected inChinese liquors come from plastic materials during fermenta-tion and other treatments, including plastic barrel, plastic tube,caps for the final products, and the packaging procedure, etc.Contamination by PAEs in alcoholic liquors and food hasbecome a big threat to human health. Thus developing ana-lytical methods to detect PAEs has recently increased.

Gas chromatography (GC) and high performance liquidchromatography (HPLC) are the two most widely used tech-niques to quantify PAEs. Due to complexity of matrices andlow concentration of PAEs (Yan et al. 2010), separation andconcentration of samples are significant steps in the analyt-ical procedure, which is critical for accuracy and sensitivity

Practical Application This paper established a new method to extractphthalate esters, which uses less experimental steps, less toxic organicsolvent, and more sensitive. This method would save time, energy, andcosts in industrial manufacture, and be more environmental friendly.

Y. Lian :Y. Yang (*)Faculty of Life Science and technology, Kunming Universityof Science and Technology, Kunming, Yunnan Province 650224,Chinae-mail: yilyil8@163.com

X. QiuYunnan Institute of Standardization Kunming, Kunming 650228,China

Food Anal. MethodsDOI 10.1007/s12161-013-9666-9

To date, several techniques have been used to extract PAEs,including solid-phase extraction (SPE) (Kato et al. 2003; Jaraet al. 2000), liquid–liquid extraction (LLE) (Ke et al. 2000),solid-phase microextraction(SPME) (Luks-Betle et al. 2001;Kayali et al. 2006), and liquid-phase microextraction(LPME) (Psillakis and Kalogerakis 2003; Rasmussen andPedersen-Bjergaard 2004).

Traditional LLE uses large volumes of toxic organic sol-vents. The procedure is tedious and may hurt both the

environment and operators. Recently, a new microextractionmethod termed vortex-assisted liquid–liquid microextraction(VALLME) was introduced by Yiantzi et al., whereby dis-persion of low-density extraction solvent into water isobtained through using vortex mixing, a mild emulsificationprocedure(Yiantzi et al. 2010). The fine droplets could rap-idly extract target analytes from solution samples due to theshorter diffusion distance and larger surface area. After cen-trifugation, the extraction solvent containing target analytes

Table 1 The structures of five PAEs

DMEP C14H1806

O

O

O

O

OCH3

OCH3

BBP C19H20O4

O

O

O

O

DCHP C20H26O4 O

O

O

O

DBP C16H22O4 O

O

O

O

DNOP C22H34O4

O

O

O

O

Fig. 1 Micrographs (×400) ofthe liquid mixtures emulsifiedby either: a manually assisted,b vortex mixer, c ultrasound

Food Anal. Methods

in the upper surface of the aqueous sample was used for next-step instrumental analysis after the aqueous phase was re-moved. This method has been successfully applied to quan-tify organochlorine pesticides in tap and snow water (Zhaoet al. 2010), perfluorooctane sulfonate in tap, well, and riverwater (Papadopoulou et al. 2011) and organophosphoruspesticides in wine and honey (Yang et al. 2011), for theextraction procedure was simple and highly efficient.

To date, there are no reports that long-chain fatty acid or long-chain fatty alcohol is used as extractant for preconcentration ofsamples in VALLME. The aim of this study is to investigate anovelmethod to prepare aVALLME sample using hexanoic acidas extractant of PAEs in liquor samples prior to analysis byHPLC. Several factors affect the extraction efficiency of theVALLME, including the methods used for emulsification, sol-vent extracting process, volume of the extraction solvent, vortexpower, and time, as well as centrifuging rates and time; pH wasoptimized and the performance of the method was then assessedhere.

Materials and Methods

Food Samples

Liquor samples were purchased from local supermarkets inKunming City (Yunnan Province, China).

Chemicals and PAEs Standards

Standards of bis-methylglycol ester (DMEP; ≥99.3 %), ben-zyl butyl phthalate (BBP; ≥99.0 %), dicyclohexyl phthalate(DCHP; ≥98.5 %), di-n-butyl phthalate (DBP; ≥99.5.0 %),and di-n-octyl phthalate (DNOP; ≥99.0 %) were purchasedfrom Sigma (St. Louis, Mo, U.S.A.). Structures of five PAEsare shown in Table. 1. Acetonitrile and methanol (HPLCgrade) was purchased from Merck (Darmstadt, Germany).Hexanoic acid, n-caprylic acid, heptanoic acid, and nonanoicacid were purchased from Alading(Guangzhou, China).

Table 2 The extraction efficiency by the different methods of emulsification

Extraction efficiency recovery (%)

Analyte Emulsificationby manual-assisted

Emulsificationby vortex mixer

Emulsificationby ultrasound

DMEP 68 90 40

BBP 60 89 34

DBP 70 93 47

DCHP 67 91 44

DNOP 64 89 43

0 2 4 6 8 10 12 14 16

0

250

500

750

7

3

5

2000

1750

1500

1250

1000

6

a

b

c

0 2 4 6 8 10 12 14 160

250

500

750

d1750

1500

1250

1000

Fig. 2 HPLC–UV chromatograms: a standard (10 μg mL−1), b liquorsample spiked with PAEs after VALLME (10 μg mL−1), c blank liquorsample after VALLME, and d blank liquor sample without VALLME.Peak no· 1 DMEP, 2 BBP, 3 DBP, 4 DNOP, 5 DCHP, and 6, 7 hexanoicacid. HPLC conditions-gradient separation using acetonitrile and water;injection volume, 20 μl; flow rate, 1 mL min−1; wavelength, 225 nm

Food Anal. Methods

Octanol, butanol, iso-octyl alcohol, butyl alcohol, and isobutylalcohol were purchased from Zhiyuan Chemical ReagentCompany (Tianjing, China). Ultra-pure water was preparedwith a Milli-Q Water Purification System (MilliporeCorporation, Bedford, MA, USA) throughout the experiment.All reagents were at or above the analytical reagent grade.

Preparation of PAEs Standards

Stock standard solutions of 1 mg·mL-1 PAEs (DMEP, BBP,DCHP, DBP and DNOP) were dissolved in methanol. The

flask was shaken until a homogenous clear solution wasformed. The stock solution was covered with aluminum foil,stored at 4 °C and in the darkness, which can be used for1 month. Standard working solutions were prepared by dilu-tion of the stock solution in methanol before use.

Instruments

Chromatographic separation and quantification wereperformed on a HPLC system (Agilent 1200 Series,Agilent Technologies, USA). This HPLC system include avacuum degasser, auto sampler, quatpump, and diode arraydetector, which is equipped with a reversed-phase C18 ana-lytical column of 150× 4.6 mm (Agilent TC-C18).Empowered software was employed to acquire and analyzechromatographic data. A vortex mixer (Shanghai hanuo in-strument Co.Itd, XH-B, Shanghai, China) was used tovortex-mixed solution samples. A centrifuge (Shanghai sur-gical instrument factory, 80-2, Shanghai, China) wasused to complete phase separation. A microscope(Xinhai zhongfa instrument company, Olympus BX53,Beijing, China) was used to obtain the microstructure.

Micrograph Procedure

Hexanoic acid (150 μL) was diluted with 5 mL of ultrapurewater in a centrifugal vial and then emulsified for 1 min byone of three methods; (a) manual assisted, (b) vortex mixer,

n-ca

pryli

c acid

hept

anoic

acid

hept

anoic

acid

isooc

tyl a

lcoho

l

octa

nol

isobu

tyl a

lcoho

l

hexa

noic

acid

0

20

40

60

80

100

Rec

over

y(%

)

Extraction solvent

DMEP BBP DBP DNOP DCHP

Fig. 3 Effect of types of extrac-tion solvent

40 60 80 100 120 140 160 180 200 220

40

50

60

70

80

90

100

Rec

over

y(%

)

Volume of hexanoic acid µL

DMEP BBP DBP DNOP DCHP

Fig. 4 Effect of extraction solvent volume

Food Anal. Methods

or (c) ultrasound. Microstructure of the emulsion wasobtained under a microscope at field of vision of ×400.

Vortex-Assisted Liquid–Liquid Microextractionfrom Aqueous Samples

Aliquots of the solution (2.0 mL) containing different con-centrations of PAEs (DMEP, BBP, DCHP, DBP, and DNOP)were placed in centrifugal vials, to which hexanoic acid(150 μL) was added. The mixtures were then diluted withultra-pure water o 5 mL. pH of the mixture was adjusted to7.0. Centrifugal vials were vortex-mixed for 1 min, and cen-trifuged for 5 min at 4,000 rpm (RCF=2,130×g) to completephase separation. The water phase was carefully removed,using a syringe with a long needle, methanol was added into

the remaining organic phase up to 0.1 mL. Aliquot of the richphase solution (20 μL) was injected into HPLC for detection.

Preparation of Liquor Samples

A 5-mL liquor sample was transferred into a centrifugal tubeand the volume reduced to 2 mL by evaporation at 80 °C. Thesample was then diluted back to 5 mL with distilled water andthe samples were subjected to the VALLME procedure.

HPLC Conditions

Separation and quantification of PAEs (DMEP, BBP, DCHP,DBP, andDNOP) were carried out by directly injecting extractsin HPLC-UV (DAD). HPLC separation was performed on a

DMEP BBP DBP DNOP DCHP0

20

40

60

80

100

Rec

over

y(%

)

50Hz 55Hz 60Hz

Fig. 5 Effect of vortex power

DMEP BBP DBP DNOP DCHP0

20

40

60

80

100

Rec

over

y(%

)

3S 60S 90S 120S 180S 300S

Fig. 6 Effect of mixing time

Food Anal. Methods

reversed-phase system with gradient elutions composed withacetonitrile and water. The gradient elution was performed asfollows: 45 % acetonitrile (0 min), ramped to 80 % acetonitrile(10 min), and then ramped to 100 % acetonitrile (12–20 min).Afterwards, the system was allowed to stabilize for 1–2 minunder the initial conditions. The prepared mobile phase wasfiltered and degassed using ultrasonic agitation. The flow ratewas set at 1.0 mL min−1. The column temperature wasmaintained at 30 °C and the injection volume was set to20 μL. PAEs were recorded at the wavelength of 225 nm.

Result and Discussion

Study of Emulsification

Emulsification is an extreme factor to extraction efficien-cy. Figure 1 shows the micrographs of the liquid sampleof emulsification. Ultrasound was over-emulsified andvortex mixer was well-distributed. The manual-assistedmethod could not get enough drive force to emulsifica-tion. The effects of the method used for emulsificationon extraction efficiency were shown in Table 2. Table 2shows the highest extraction efficiency (≥89 %) isemulsification by vortex mixer. Ultrasound could causeexcessive emulsification, decrease the extraction effi-ciency, and result in incomplete phase separation aftercentrifugation. Therefore, the vortex mixed was chosenfor emulsification.

HPLC of the Extracted PAEs in Liquor Samples

The typical HPLC chromatograms of the extracted andpreconcentrated PAEs were showed in Fig. 2. Figure 2a isthe chromatogram of standard PAEs without VALLME.Figure 2b is the chromatogram of PAEs after its VALLMEfrom liquor sample spiked with it; Fig. 2c is the chromato-gram of blank liquor sample after VALLME and Fig. 2d isthe chromatogram of blank liquor sample without VALLME.The preconcentration effect of VALLME is clearly demon-strated in Fig. 2c and d. Although hexanoic acid has high UVabsorbance at the wavelength of 225 nm, it does not interferewith the determination of PAEs.

Fig. 7 Effect of pH

Fig. 8 Effect of centrifugationrate RCF: 2,000 rpm=523×g;3,000 rpm=1,198×g;4,000 rpm=2,130×g;5,000 rpm=3,328×g;6,000 rpm=4,792×g

Food Anal. Methods

Optimization of the VALLME Conditions

Hexanoic acid was chosen as the VALLME extraction sol-vent due to its excellent extraction efficiency. Several differ-ent parameters that can influence the extraction efficiencywere investigated in our experiments by the spiked PAEs atconcentration of 10 μg mL−1 in aqueous solutions. Threereplicates were performed to obtain a mean value. The opti-mization procedure was done as described in Vortex-AssistedLiquid-Liquid Microextraction from Aqueous Samples.

Selection of Extraction Solvent

Selecting an appropriate extraction solvent is quite importantto VALLME because the extraction solvent should havesome properties to extract the analytes efficiently comprise:lower density than water, low solubility in water, extractioncapability of interested compound, and good chromato-graphic behavior. Therefore, hexanoic acid, n-caprylic acid,heptanoic acid, nonanoic acid, octanol, iso-octyl alcohol,

butyl alcohol, and isobutyl alcohol were investigated in thiswork. The effect of the extraction solvents was showed inFig. 3. No emulsification was observed in the system whenbutanol was used as the extraction solvents. The recovery inFig. 3 indicated that hexanoic acid was the best extractionsolvent for the extraction of PAEs. Based on these results,hexanoic acid was adopted as the extraction solvent in thefollowing experiments.

Effect of Extraction Solvent Volume

Because the preconcentration factor depends on the volume ofhexanoic acid, so the effect of the hexanoic acid volume wasstudied. Aqueous solutions containing a mixture of standardPAEs (10 μg mL−1 each) were extracted by VALLME withdifferent volumes of hexanoic acid in the range of 50 to220 μL. Recovery is in inverse proportion to enrichmentfactor. To ensure the recovery, the enrichment factor shouldbe as low as possible. As can be seen in Fig. 4, recoverygradually increased with an increase in hexanoic acid volumeand showed the highest response at 150 μL. Below the vol-ume, analyzes cannot be extracted absolutely and very diffi-cult to make phases separate. Above the volume, the enrich-ment factor was sharply decreased. So, 150 μL of hexanoicacid was used in subsequent experiments.

Effect of Vortex Power and Mixing Time

To accelerate the dispersion of water-immiscible organicsolvent in aqueous phase, the vortex mixer was applied asan emulsifier. Thus, the vortex power and mixing timeshould be optimized. On the optimized condition, the influ-ence of vortex power was studied in a range of 50 to 60 Hz,

DMEP BBP DBP DNOP DCHP0

20

40

60

80

100

Rec

over

y(%

)

3min 5min 10min 20min 30min

Fig. 9 Effect of centrifugationtime

Table 3 The performance characteristics of the proposed method

Analyte LR (ng mL−1) Regressionequation

r LOD(ng mL−1)

DMEP 10–1,000 y=325.57x+259.36 0.9932 2.3

BBP 10–1,000 y=416.58x+159.78 0.9958 1.1

DCHP 10–1,000 y=363.96x+108.35 0.9902 1.9

DBP 10–1,000 y=399.35x+99.96 0.9978 1.2

DNOP 10–1,000 y=368.24x+258.65 0.9931 1.5

Food Anal. Methods

and the time was studied from 30 to 300 s. The effect ofvortex power and mixing time were shown in Figs. 5 and 6.Since emulsification time and the formation of homogeneousphase are as short as possible, a mixing time of 60 s at 60 Hzwas selected for the procedure.

Effect of pH

A pH is an important factor during the extraction procedure.The effect of pH was evaluated in the range of 3.0–11.0 andthe results demonstrated that the best recovery of the targetanalytes were obtained at the pH 7.0 (Fig. 7). The reason maybe that PAEs is a kind of lipoid substance which is very easyhydrolysis under strong alkali or acid condition. Therefore,pH 7.0 was chosen for subsequent study.

Effect of Centrifuging Rate and Time

Under the optimized condition, the influence of centrifugetime was studied in the range from 5 to 30 min. In order to

keep analysis time as short as possible, the turbid solution wascentrifuged immediately. The effect of centrifugation rate andtime was shown in Figs. 8 and 9. From the Fig. 8 can be foundthat the increase of centrifuge rate has no significant effect onthe extraction efficiency and analytical signal beyond4,000 rpm (RCF=2,130×g). And the best centrifugation timewas 5 min. A centrifugation time of 5 min at 4,000 rpm(RCF=2,130×g) was selected for the whole procedures.

Analytical Performance of the Method

In order to investigate the recovery, repeatability, and repro-ducibility of the proposed method, PAEs were spiked intofree liquor samples at three concentrations (50, 500, and1,000 ng mL−1), and analyzed five times per concentrationlevel in two independent analytical runs under the optimizedconditions. Linear calibration curves were obtained by plot-ting the peak area against the concentration of the respectivecompounds and found to be linear over the range of 10–1,000 ng mL−1 for all the PAEs. The correlation coefficients

Table 4 Recovery and RSD ofliquor spiked with PAEs

a Represents means of per con-centration level in ten timesb Represents means of per con-centration level in each day

PAEs Day 1 (n=5) Day 2 (n=5) CVR (%)a Recovery (%)Added (ng mL−1) Found (ng mL−1)

and RSD (%)Found (ng mL−1)and RSD (%)b

DMEP 50 44 (1.3) 45 (1.9) 1.6 89

500 442 (2.2) 440 (2.1) 2.1 88

1,000 892 (2.4) 893 (2.1) 2.2 89

50 46 (1.5) 46 (2.3) 1.9 92

BBP 500 458 (2.4) 455 (2.0) 2.2 91

1,000 921 (2.4) 914 (2.0) 2.2 92

50 45 (1.8) 46 (2.4) 2.1 91

DCHP 500 458 (1.4) 455 (2.0) 1.7 91

1,000 911 (2.1) 908 (2.0) 2.0 91

50 46 (1.8) 45 (2.1) 1.9 92

DBP 500 466 (2.6) 465 (2.2) 2.4 93

1,000 936 (2.4) 931 (2.3) 2.3 93

50 45 (1.6) 45 (2.0) 1.8 90

DNOP 500 454 (2.2) 451 (2.3) 2.3 90

1,000 904 (2.7) 901 (2.6) 2.6 90

Table 5 Comparison of the proposed method with other sample preparation techniques for the determination of PAEs

Method LRa (ng mL−1) LOD (ng mL−1) RSD (%) Extraction time References

DLLME-CG/FID 6.9–444 1.0–1.1 <3.7 >15 min H. Y. Yan et al. 2010

SPME-GC/FID 0.1–1,000 0.006–0.084 <10 >2 h X.J. Li et al. 2004

SPE-GC/MS 25–5,000 0.025–0.150 <15 >2 h M.D Carlo et al. 2008

CPE-HPLC/UV 5–200 1.0–3.8 <3.89 =65 min L. Wang et al. 2007

LPME-HPLC/UV 5–50,000 1.2–2.2 <7.4 >40 min J.R. Yao et al. 2008

VALLME-LC/DAD 10–1,000 1.1–2.3 <2.6 ≤20 min This work

Food Anal. Methods

for PAEs were all more than 0.990. The limit of detection(LOD) was taken when the signal/noise was 3, the LOD were2.3 ngmL−1 for DMEP, 1.1 ngmL−1 for BBP, 1.9 ng mL−1 forDCHP, 1.2 ng mL−1 for DBP, and 1.5 ng mL−1 for DNOP,respectively. The recovery and precisions (RSD) varied from89 to 93% and from 1.6 to 2.6 %, respectively. The results arelisted in Tables 3 and 4.

Comparison of VALLME with Conventional Methods

The extraction efficiency of VALLMEwas compared with otherreported techniques such as SPE, SPME, LPME, CPE, andDLLME (Yan et al. 2010; Li et al. 2004; Carlo et al. 2008;Wang et al. 2007; Yao et al. 2008). The LOD, RSD, linearity, andextraction time are listed in Table 5. It is observed that theextraction time of VALLME was 20 min which is greatly short-ened, the LOD and RSD varied from 1.1 to 2.2 ng mL−1 andfrom1.6 to 2.6% relativelywhich are lower comparedwith othermethods. The range of LODs of LPME, CPE, andDLLMEwerehigher than VALLME and were given in Table 5.

Conclusions

A new sensitive method to determine PAEs in liquor sampleswas established. This method has been shown to be simple,rapid and reliable for PAEs assay in liquor samples and will beapplicable for routine quality control of PAEs. Furthermore,liquor samples preparation with VALLME is a potential fieldof investigation. The method can be successfully employedfor the quantification of PAEs in all types of food.

Acknowledgments This work was supported by National InnovationFund for Small and Medium-sized Enterprises (12C26215306449) and

Analysis Test Research Center of Kunming University of Science andTechnology, Yunnan Province, China.

Compliance with Ethics Requirements

Conflict of Interest Yuanpei Lian declares that he has no conflict ofinterest. Xiaoyan Qiu declares that he has no conflict of interest. YalingYang declares that he has no conflict of interest. This article does notcontain any studies with human or animal subjects.

References

Arcadi FA, Costa C, Imperatore C et al (1998) Food Chem Toxicol36:693

Balafas D, Shaw KJ, Whitfield FB (1999) Food Chem 65:279Carlo MD, Pepe A, Sacchett G et al (2008) Food Chem 111:771Guo WJ (2012) China Food 23:14Jara S, Lysebo C, Greibrokk T et al (2000) Anal Chim Acta 407:165Kato K, Shoda S, Takahashi M et al (2003) J Chromatogr B 788:407Kayali N, Tamayo FG, Polo-Dez LM (2006) Talanta 69:1095Ke J, Yancey M, Zhang S et al (2000) Chromatogr B 742:369Li XJ, Zeng ZR, Chen Y et al (2004) Talanta 63:1013Liang P, Xu J, Li Q (2008) Anal Chim Acta 609:53Luks-Betle K, Popp P, Janoszka B et al (2001) Chromatogr A 938:93Papadopoulou A, Roman IP, Canals A et al (2011) Anal ChimActa 691:56Prokupkov´a G, Holadov´a K, Poustka J et al (2002) Anal Chim Acta

457:211Psillakis E, Kalogerakis N (2003) TrAC Trends Anal Chem 22:565Rasmussen KE, Pedersen-Bjergaard S (2004) TrAC Trends Anal Chem 23:1Stanley MK, Robillard KA, Staples CA (2003) Handb Environ Chem

3:1Staples CA, Peterson DR, Parkerton TF et al (1997) Chemosphere 35:667Tyler CR, Jobling S, Sumpter JP (1998) Crit Rev Toxicol 28Wang L, Jiang GB, Cai YQ et al (2007) J Envi Sci 19:874Yan HY, Liu BM, Du JJ et al (2010) Analyst 135:2585Yang ZH, Lu YL, Liu Y et al (2011) J Chromatogr A 1218:7071Yao JR, Xu H, Lv LL et al (2008) Anal Chimi Acta 616:42Yiantzi E, Psillakis E, Tyrovola K, Kalogerakis N (2010) Talanta

80:2057Zhao EC, Jia CH, Zhu XD et al (2010) J Chromatogr A 1217:5868

Food Anal. Methods