Recent advances in dispersive liquid-liquid microextraction for pesticide analysis

Trends in Analytical Chemistry 72 (2015) 181–192

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Trends in Analytical Chemistry j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / t r a c

Recent advances in dispersive liquid-liquid microextraction for pesticide analysis W. Ahmad, A.A. Al-Sibaai, A.S. Bashammakh, H. Alwael, M.S. El-Shahawi *,1 Department of Chemistry, Faculty of Science, King Abdulaziz University, P. O. Box 80203, Jeddah 21589, Saudi Arabia

A R T I C L E

I N F O

Keywords: Complex matrix Dispersive liquid-liquid microextraction Emerging pesticide Green aspects Nanotechnique Pesticide analysis Surface-modified electrode Thin-layer stripping voltammetry Trace analysis Water analysis

A B S T R A C T

Dispersive liquid-liquid microextraction (DLLME) techniques have attracted considerable interest because they are cost effective, easy to operate, and reliably preconcentrate trace levels of analytes in complex matrices. This comprehensive review is concerned with principles, applications and developments of DLLME techniques for analysis of trace emerging pesticides in water. DLLME techniques have had few couplings to spectrofluorimetric methods and relatively none with electrochemical techniques. We highly recommend thin-layer stripping voltammetric techniques at surface-modified electrodes and spectrofluorimetric techniques coupled and implemented with DLLME. Great attention should be focused on developing low-cost, precise methods for analysis of trace concentrations of pesticides in various biological and environmental samples. We describe milestones and the combination of nanotechniques in the DLLME field, green aspects, advantages and shortcomings of known DLLME protocols. © 2015 Published by Elsevier B.V.

Contents 1.

2.

3. 4. 5. 6.

Introduction ........................................................................................................................................................................................................................................................ 1.1. Theory and fundamentals of DLLME ............................................................................................................................................................................................. 1.2. Analytical parameters affecting extraction efficiency of DLLME ......................................................................................................................................... 1.3. Calculations in DLLME ....................................................................................................................................................................................................................... 1.4. Classification in DLLME ..................................................................................................................................................................................................................... Analytical applications of DLLME in the analysis of pesticides ........................................................................................................................................................ 2.1. DLLME combined with gas chromatography (GC) ................................................................................................................................................................... 2.2. DLLME combined with high-performance liquid chromatography (HPLC) .................................................................................................................... 2.3. DLLME combined with other techniques .................................................................................................................................................................................... Advantages of DLLME ...................................................................................................................................................................................................................................... Milestones, green aspects, shortcomings and developments in DLLME ........................................................................................................................................ Limitations and outlook on the future trends of DLLME .................................................................................................................................................................... Conclusion ........................................................................................................................................................................................................................................................... References ............................................................................................................................................................................................................................................................

182 182 182 183 183 183 183 184 187 187 187 190 190 191

Abbreviations: AS-DLLME, Auxiliary solvent dispersive liquid-liquid microextraction; CPE, Cloud-point extraction; DLLME, Dispersive liquid-liquid microextraction; DLLMESFOD, Dispersive liquid-liquid microextraction based on solidification of floating organic droplet; EF, Enrichment factor; ER, Extraction recovery; GC, Gas chromatography; Gas GC-FID, chromatography-flame-ionization detection; GC-MS, Gas chromatography-mass spectrometry; GC-TMS, Gas chromatography-tandem mass spectrometry; HFME, Hollow-fiber-protected microextraction; HPLC, High performance liquid chromatography; HPLC-DAD, High performance liquid chromatography-diode array detection; LLE, Liquid-liquid extraction; LLME, Liquid-liquid microextraction; LLLME, Liquid-liquid-liquid microextraction; MA-DLLME, Microwave assisted dispersive liquid-liquid microextraction; OCP, Organochlorine pesticide; OPP, Organophosphorus pesticide; PCP, Personal-care products; RR, Relative recovery; SDME, Single-drop microextraction; SME, Surfacemodified electrode; SPE, Solid-phase extraction; SPME, Solid phase microextraction; SFE, Supercritical fluid extraction; USA-DLLME, Ultrasound-assisted dispersive liquidliquid microextraction; WHO, World Health Organization. * Corresponding author. Tel.: +966 12 6952000 Ext 64422; Fax: +966 12 6952292. E-mail address: [email protected]; [email protected] (M.S. El-Shahawi). 1 On sabbatical leave from Department of Chemistry, Faculty of Science, Damiatta University, Damiatta, Egypt. http://dx.doi.org/10.1016/j.trac.2015.04.022 0165-9936/© 2015 Published by Elsevier B.V.

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1. Introduction Advancement in sample pre-treatment techniques in the past decade was mainly focused on miniaturization, simplification and automation in order to lower costs of both materials and personnel. Hence, the main aim of sample preparation is to clean up and to concentrate the target analyte and finally to execute it in a form that is well-matched with the desired analytical instrument. Liquidliquid extraction (LLE), Soxhlet extraction, chromatography, distillation, and absorption [1] are conventional practices for sample preparation that suffer from the different drawbacks, such as being time consuming, tedium, consumption of large amounts of toxic solvents, and, to some extent, complications in automation. The trend has resulted in several microextraction techniques [e.g., solidphase microextraction (SPME), single-drop microextraction (SDME) and dispersive liquid-liquid microextraction (DLLME)]. Most of these techniques are simple, fast and consume less extraction solvents than conventional sample-preparation techniques [1]. Interest in miniaturizing sample pre-treatment techniques started with the introduction of SPME by Pawliszyn and co-worker [2]. Since then, several other microextraction methods have been developed. DLLME was introduced by Rezaee et al. in 2006 [3] as a consequence of the demands for rapid, economical and environmentally benign sample-pretreatment techniques. DLLME was originally developed for water samples, but was afterwards also applied to other matrices, such as soil and foodstuffs. The extraction mechanism is based on the different affinities of the analytes to the aqueous sample and the organic extractant. The major advantages include simplicity, minimal use of harmful solvents, rapid extraction and low cost. This technique is one of the most outstanding due to the large number of publications since its inception. The main aim of this technique is:

• •

to overcome and to overpower the demerits of conventional techniques in order to reduce both personnel and material expense; and, to achieve promising results in terms of recovery and enrichment factor (EF).

DLLME, as a powerful sample-preparation and preconcentration technique, has attracted great attention due to its wide range of applications to organic and inorganic contaminants in different matrices. Among the most inspected analytes employing DLLME are pesticides. A number of reviews on DLLME have been published [4–20]. Few of these reviews were focused on analysis of pesti-

cides in complex matrices by DLLME [6,15,16,18]. Hence, an independent comprehensive study on pesticides was very much needed. Most of the reported pre-treatment methods suffer from many drawbacks (e.g., high cost, multiple steps and time consuming, costly HPLC solvents), while DLLME offers a stable system with short analysis time, good reproducibility, ruggedness and cost effectiveness. This review presents various chemical classes of pesticides analyzed by a variety of DLLME protocols grouped by analytical technique. In this article, we cover the best practices for pesticides with special emphasis on further advancement and future trends in the microextraction techniques for pesticide analysis. 1.1. Theory and fundamentals of DLLME DLLME uses a ternary solvent system in which a small amount of a blend of extraction and disperser solvents is rapidly injected into an aqueous sample containing the analyte. After shaking the mixture, a cloudy solution is obtained and tiny fine droplets of the extraction solvent are formed. The surface area between water and the extraction solvent becomes infinitely large, so rapid, effective mass transfer occurs. The mixture is then centrifuged and the sedimented phase is collected with a micro syringe for subsequent analysis. Fig. 1 shows the process. 1.2. Analytical parameters affecting extraction efficiency of DLLME Development of strategies and techniques allowing the selection of representative samples continues to be the main focus of research in pre-treatment and clean-up. In DLLME, several factors influence the extraction efficiency, including type and volume of the extraction solvent, type and volume of the disperser solvent, pH of the sample, effect of salt, extraction time, centrifugation time, and sample volume. These parameters have to be optimized for high extraction efficiency. The most important task in the process is the selection of an appropriate extraction solvent, based on several requirements. It must be immiscible in water and should form tiny droplets in it. It must have higher affinity towards the analyte. Its volume should be carefully optimized and its compatibility with the desired instrument is also considered. Rezaee et al. [3] used carbon tetrachloride, carbon disulfide and 1,1,2,2- tetrachloroethylene in their very first experiment. Optimizing the disperser-solvent type and volume is as important as optimizing the extraction solvent. The function of the disperser solvent is to empower the extracting solvent to

Fig. 1. Steps in dispersive liquid-liquid microextraction protocols.

W. Ahmad et al./Trends in Analytical Chemistry 72 (2015) 181–192

183

Fig. 2. Microextraction protocols.

partition itself uniformly in the aqueous sample, in order to achieve good extraction efficiency. The ratio of volume of extracting solvent to disperser solvent should be carefully considered. The volume of the sedimented phase is significantly influenced by disperser type and volume. Acetone, methanol and acetonitrile are commonly used as disperser solvents. Optimal pH and salt concentration should be established. Extraction and centrifugation times should be carefully optimized. Extraction time is defined as the time interval between the injection of the mixture of disperser and extraction solvent, and centrifugation. Centrifugation is important for phase separation and is time consuming compared to other parameters. It is usually 5–10 min. A long centrifugation time causes phase separation to dissolve [21]. 1.3. Calculations in DLLME In DLLME, the analyte EF and extraction recovery (ER) should be taken into consideration. Rezaee et al. [3] defined EF in the following equation:

EF = Csed C0

(1)

where Csed is the analyte concentration in the sedimented phase and C0 is the initial analyte concentration in the sample. ER is defined as the ratio of the amount of analyte in the sedimented phase to the initial concentration in the sample:

ER = (nsed n0 ) × 100 = Csed × Vsed C0 × V0

(2)

where nsed is the amount of the analyte in the sedimented phase, n0 is the initial analyte amount in the sample, Vsed is the volume of the sedimented phase and V0 is the volume of the aqueous phase:

ER = Vsed V0 × EF × 100

(3)

The relative recoveries (RR) can be calculated from the equation:

RR = Cfounded − Creal Cadded

(4)

where Cfounded is the analyte concentration measured from the sample after analyte addition, Creal is the native analyte concentration and Cadded is the amount of the analyte that was added to the sample.

1.4. Classification in DLLME A large number of papers have been published since the inception of the DLLME technique in 2006. Several new advances occurred in time to overcome possible disadvantages and drawbacks of the process, hence leading to different modifications in DLLME, and, most often, for each modification a different acronym was assigned by the researcher. Sometimes, there are more than two or three acronyms for the same DLLME method, so it is often difficult to differentiate them or it leads to complications. We have made an effort to arrange all those acronyms in four general groups, as shown in Fig. 2. The four bases of classification are: (i) (ii) (iii) (iv)

mixed mode extraction; extraction based on assisting dispersion; extraction based on use of ionic liquids (ILs); and, extraction based on solvent density in its acronym, and any other type not fitting into the other three groups.

2. Analytical applications of DLLME in the analysis of pesticides Classification of applications of DLLME continues to play a key role in searching for structure in data. Thus, it allows meaningful generalization about large amounts of data to be performed by recognizing a few basic patterns among them. The techniques coupled with DLLME techniques can be summarized as follows. 2.1. DLLME combined with gas chromatography (GC) It is evident from the large number of publications that DLLME is mainly used for the analysis of pesticides. Fig. 3 shows the relative percentages (%) of articles published each year with respect to pharmaceuticals and other analytes. Moreover, the most favorable analytical technique is GC, since it has rapidly developed in a short time. The extraction solvent in DLLME should be less soluble or immiscible with water in order to achieve adequate phase separation, followed by direct injection into GC. Table 1 shows applications of DLLME in combination with GC for pre-concentration and determination of pesticides. As can be seen, pesticides are mostly analyzed in an aqueous matrix because

184

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Fig. 3. Evolution of the percentage (%) of publications devoted to DLLME for separation and subsequent determination of pesticides, pharmaceutical and other chemicals per year (2010–14).

they are directly introduced to water for agricultural purposes and with personal-care products (PCPs), such as shampoos and sprays [15]. Due to their continual supply, pesticides have become persistent in the environment, causing adverse effects to health. The World Health Organization (WHO) and various national governmental institutions have established residue limits and published guidelines and policies for quantification of pesticide residues in different kinds of waters, including environmental, drinking and irrigation waters [16]. Xiong et al. [22] developed a DLLME method based on solidification of floating organic droplet (DLLME-SFOD) for the determination of chlorpyrifos in environmental water samples followed by GC-flame-photometric detection (GC-FPD). The ER was 79.02% and the EF was 232.42. Alves and co-workers [23] determined six organophosphorus pesticides (OPPs) in water utilizing DLLME prior to GC-mass spectrometry (GC-MS). In their method, chloroform was used as the extraction solvent and 2-propanol as disperser. The limit of detection (LOD) of the method was 1.5–9.1 ng L−1. The method was validated for tap, well and irrigation waters with relative recoveries 46.1–129.4%. A new method, ultrasound-vortex-assisted DLLME (US-VADLLME) for OPPs and triazines, resulted in excellent LODs of 0.007–0.07 ng mL−1 [24]. Similarly OPPs were analyzed by combining supercritical fluid extraction (SFE) with DLLME in marine sediment samples [25]. This method has been applied to real soil and marine samples with satisfactory results. OPPs were analyzed in fruit, vegetables, dried herbs and water [26,27]. Table 1 summarizes analysis of organochlorine pesticides (OCPs) [28–32]. Mudiam et al. [28] analyzed endosulfan and its metabolite in soil and urines. OCPs were also analyzed by low-density magnetofluid DLLME (LMF-DLLME) in water with good LODs [29]. In peach and pulps, OCPs were investigated by DLLME-SFOD as dodecanol with a density lower than water was used with EFs of 409–1089 [30]. Sousa and co-workers [33] developed a methodology for the analysis of carbamates and carbofuran in water using DLLME-GCMS. Chlorobenzene was used as extracting solvent and methanol as disperser solvent. A total sample volume of 10 mL satisfactorily obtained LODs of 0.04 μg L−1 for carbamates and 0.02 μg L−1 for organophosphorus pesticides. Similarly, Chen et al. [34] proposed a low-density extraction solvent-based solvent-terminated DLLME (ST-DLLME) combined with GC-tandem MS (GC-TMS) for

determination of carbamate pesticides in water samples with good recoveries of 94.5–104% at all spiked levels for all carbamates used. Triazoles are one of the major classes of pesticides having properties, such as high chemical and photochemical stability, and low biodegradability, which make them persist in soil, food and water for a long time with some endocrine-disrupter properties. Triazoles are preconcentrated by coupling stir-bar sorptive extraction with DLLME (SBSE-DLLME) with LODs up to 0.53–24 ng mL−1 [35]. A new air-assisted LLME technique, similar in principle, has been developed for analysis of triazoles in water, vegetables and juices with EFs of 713–808 [36]. Pyrethroids have been analyzed by DLLME-GC [37–41], including analysis of cypermethrin in rat tissues by low-density solvent DLLME (LDS-DLLME [37] followed by GC-ECD with good recoveries and EFs. With tomato as the matrix, pyrethroids were analyzed by Li et al. [38] with recoveries of 89–109%, and in soil [39] with recoveries 83.6–98.5%. 2.2. DLLME combined with high-performance liquid chromatography (HPLC) Table 2 shows that DLLME coupled with HPLC is also a popular separation and determination technique for pesticide analysis. However, in contrast to GC, in most cases, the extraction solvent containing the analyte after extraction has to be reconstituted in a solvent more compatible with HPLC columns, whereas, in some cases, it can be directly injected into the column. Table 2 summarizes the analysis of pesticides utilizing DLLME-HPLC. Carbamates have been successfully studied with DLLME techniques [42–45]. Among them, ultrasound-assisted emulsification microextraction (USAEME) has been successfully used for analysis of six carbamates in water with excellent EFs of 170–246 [42]. In watermelon and tomatoes, five carbamates were determined by DLLME with LODs of 0.5–1.5 ng g−1 [43]. OPPs are widely found in water due to their application in agriculture and other areas. Mostly toxic in nature, their presence in water is a threat to humans and wild-life. DLLME seems to be the most promising method for pre-concentration of OPPs because of their presence in trace concentrations [46–49], so it is analyzed in various matrices, including an UA-DLLME-SFOD for the determination of organophosphorus pesticides in environmental water [46]. The EFs were 215–557. This method has been successfully

Table 1 DLLME combined with GC for the analysis of pesticidesa Analyte Chlorpyrifos

Matrix Water

Extraction Solvent 1-Dodecanol (40 μL)

Disperser Solvent Methanol (1.5 mL)

EF 232.42

LOD −1

0.02 μg L

L−1

Method

Others

Ref

DLLME-SFO

Centrifugation time 3 min and 0.5 g NaCl, – Sample volume 10 mL, pH 8, vortex time 30 sec, ultrasonication time 10 min Sample volume 5 mL, pH 6, optimum pressure 150 bar No salt added, centrifugation time : 5 min Sample volume 5 mL, centrifugation time 3 min Na2SO4, (7% w/v)

[22]

NaCl added (3% m/v), extraction time 4 min No salt added, sample volume 5 mL – Centrifugation time 5 min, no salt added Sample volume (10 mL), no salt added Sample volume 5 mL, extraction time 10 min and pH 7. NaCl (30 % m/v)

[29]

Centrifugation time 2 min pH not significant, NaCl (10% w/v), No salt added, pH 4 – pH 6.3, ultrasonication time 2 min NaCl (16% m/v), extraction time: 90 s Centrifugation for 5 min, extraction for 2 min

[36]

Water Wine

CHCl3 1,2-Dichloroethane (250 μL)

2-Propanol –

– 210–232

1.5–9.1 ng 0.007– 0.07 ng mL−1

DLLME USVA-DLLME

7 OPPs

Soil &marine sediment sample Fruit, vegetables, dried herbs Water

CCl4 (17 μL)

Acetonitrile (1.0 mL)

67–144

0.001–0.009 mg kg−1

SFE-DLLME

CCl4 (80 μL)

Acetonitrile (1.0 mL)

51–99

0.12–4.92 ng kg

Carbon Disulfide (30 μL)

Methanol (1.0 mL)

–

0.047–0.201 μg m L−1

DLLME

Soil and Urine

Tetrachloro ethylene (58 μL)

Acetone (1.27 mL)

–

UA-DLLME

Water

n-Octane Magnetofluid (50 μL)

–

156–196

0.316–2.494 ng g−1 (soil) 0.049–0.514 ng g−1 (urine) 1.8–8.4 ng L−1.

LMF-DLLME

L−1

DLLME-SFO

10 OPPs 3 OPPs Endosulfan & its metabolite OCPs OCPs 14 OCPs 20 OCPs

−1

DLLME

Peach, pulps & peels River water Water

1-Dodecanol (8 μL)

Acetone (0.4 mL)

409–1089

2.8–18.5 ng

Carbon disulfide (13.5 μL) CCl4 (10 μL)

Acetone (0.50 mL) Acetone (1.5 mL)

647–923 –

0.05–0.001 μg L−1 0.21–11.65 ng L−1

Water

Chlorobenzene (80 μL)

Methanol (500 μL)

–

0.04 μg L−1, 0.02 μg L−1,

DLLME DLLME

Carbamates & Organophosphorus 4 Carbamates

Water

Toluene (50 μL)

Acetonitrile (1 mL)

–

0.001–0.50 ng mL

Triazole

Water

–

282–1792

0.53–24 ng mL−1

SBSE-DLLME

Triazole

–

713–808

0.53–1.13 ng mL−1

AALLME

Cypermethrin 3 Pyrethroids 3 Pyrethroids

Water, vegetables and juices Biological matrix Tomato Soil

1,1,2,2-Tetrachloro-Ethane (25 μL) Toluene (35 μL) n-Hexane (100 μL) CHCl3 (40 μL) Tetrachloro ethylene (50 μL)

Acetone (300 μL) Acetonitrile (1.0 mL) Acetone

477–659 – 128–138

0.043–0.314 ng 0.3–0.5 μg kg−1 0.45–1.13 ng g−1

LDS-DLLME DLLME MSPD-UA-DLLME

4 Pyrethroids

Water

1-Dodacanol (8 μL)

Methanol (500 μL)

475–790

1.4–2.9 ng L−1

DLLME-SFO

−1

USA-DLLME

9 Pyrethroids

Water

Chlorobenzene (15 μL)

Acetone (0.3 mL)

728–1725

−1

mg−1

0.2–0.7 μg L

DLLME ST-DLLME

[23] [24]

[25] [26] [27] [28]

[30] [31] [32] [33] [34]

[35]

[37] [38] [39]

W. Ahmad et al./Trends in Analytical Chemistry 72 (2015) 181–192

6 OPPs OPPs & Triazine

[40] [41]

a Abbreviations: AALLME, Air-assisted liquid-liquid microextraction; [D(i-C )IM][PF ], 1,3-Diisooctylimidazolium hexafluorophosphate; DLLME-SFO, Dispersive liquid-liquid microextraction based on solidification of floating 8 6 organic droplet; LDS-DLLME, Low-density solvent dispersive liquid-liquid microextraction; LMF-DLLME, Low-density magnetofluid dispersive liquid-liquid microextraction; ME-VADLLME, Matrix-extraction-vortex-assisted dispersive liquid-liquid microextraction; MSPD-UA-DLLME, Miniaturized pre-treatment procedure combining matrix solid-phase dispersion with ultrasound- = assisted dispersive liquid-liquid microextraction; SBSE-DLLME, Stirbar sorptive extraction dispersive liquid-liquid microextraction; SFE-DLLME, Supercritical fluid extraction coupled with dispersive liquid–liquid microextraction; ST-DLLME, Solvent-terminated dispersive liquid–liquid microextraction; UA or USA–DLLME, Ultrasound-assisted dispersive liquid-liquid microextraction; USVADLLME, Ultrasound-vortex-assisted dispersive liquid–liquid microextraction.

185

186

Table 2 DLLME combined with HPLC for analysis of pesticidesa Analyte

Matrix

Extraction Solvent

Disperser solvent

LOD

CHCl3 (126 μL)

Acetonitrile (1.5 mL)

–

Carbamates OPPs

Fruits and vegetables Environmental water

Trichlormethane (35.0 μL) 1-Dodecanol (15 μL)

Acetonitrile (1.0 mL) Methanol (200 μL)

5400–7650 215–557

Dichlorvos

Water

[BMIM][PF6] (65 μL)

THF (260 μL)

OPPs and Carbamates

Tea

1-Octanol (50 μL)

3 OPPs

Water

DDT and Its Metabolites

5 Carbamates

Water

170–246 80–177

0.1–0.3 ng

mL−1

0.5–1.5 ng g

−1

Method

Other

Ref

UASEME

Water sample 5 mL, no salt addition, 3 min extraction time NaCl (5% w/v), extraction time not significant Sample volume 5 mL, NaCl (4.7%w/v), extraction time 1 min, natural pH Sample volume 5.0 mL, NaCl (20% w/v) Sample volume 20 mL, 1g NaCl, extraction time 1 min NaCl (25% w/v), Sample volume, 8 mL pH 5 Sample volume, 12 mL NaCl (20% m/v), pH not significant No Salt added, pH not significant

[42]

pH 7 extraction time 5 min, no Salt added No salt added, vortex extraction time 90 s Sonication time 30 s, No salt added, pH not significant –

[50]

pH 5, Microwave conditions, 200 W and 60 s Extraction time 3 min, vortexed for 3 min Sample volume 5 mL, centrifugation 5 min pH 5, no salt added, extraction time 10 min Sample volume 5 ml, ultra sonication for 10 min, no salt added, extraction time 6 min – NaCl (10% w/v)

[54]

pH 2, sample volume 10 mL centrifugation time 5 min, extraction time few seconds, no salt added Sample volume (5 mL), extraction time for 1 min, no salt added, Tween 80 (10 mmol L−1) as emulsifier, Sample volume (5 mL) Surfactant 0.015 mg mL−1 Tween 80,ultrasonic time 1 min, NaCl (1% w/v) Fruit juice sample 5 mL

[61]

DLLME

0.0001 and 0.0005 μg mL−1

DLLME

5–60 pg kg−1 0.1–0.3 ng mL−1

SPE-DLLME DLLME-SFO

215

0.2 μg L−1

RTIL–DLLME

–

130–185

0.13–0.61 μg L−1

MSA-DLLME

CHCl3 (250 μL)

Methanol (1.5 mL)

20.7–26.4

DLLME

Water

[C8MIM][ PF6] (50 μL)

[C4MIM][BF4] (300 μL)

2 ng mL−1 for Fenitrothion, 3 ng mL−1 for others 0.21–0.49 μg L−1

MILs-DLLME

Benzoylurea

Water

[C6MIM][PF6] (70 μL)

Acetonitrile (300 μL)

0.05–0.15 μg L−1

MR-IL-DLLME

Urea derivatives

Water

CH2Cl2 (200 μL)

–

0.04–0.4 μg L−1

US-DLLME

– 261–302 78–160

Benzoylurea

Waste water

[C8MIM][PF6]

Methanol

–

0.5–1.0 ng

Pyrethroid

[N8881][Tf2N]

Methanol

–

–

4 pyrethroids

Complex environmental matrices Water and vegetables

1-Octanol (200 μL)

6 Pyrethroid

Fruit juice

CHCl3 (300 μL)

APTS magnetic nano particles Methanol (1.25 mL)

3 Pyrethroids

Tomato

[Bmim][PF6] (80 μL)

Acetonitrile (300 μL)

Imidacloprid

Tomatoes

Tetrachlo ethane (30 μL)

–

6 Neonicotinoid Neonicotinoids Insecticides

Soil Honey

Dichloromethane CHCl3 (100 μL)

Acetonitrile Acetonitrile

2,4-Dichlorophenoxyacetic acid and 4-chloro-2methylphenoxyacetic 6 fungicide residues

Water

20 mg DeA in 1.0 mL THF

–

148–157

Juices and red wine

1-Dodecanol (30 μL)

–

–

Strobilurin fungicides

Fruit juice

1-Undecanol (30 μL)

–

5 Fungicides

Fruit juice

1-Dodecanol

–

a

L−1

TCIL-DLLME Vs US-IL-DLLME MA-DLLME

62–84

0.05–2 ng mL−1 (water) 0.02–2.0 ng g−1 (vegetables) 2–5 μg L−1

DLLME

42–48

8.1–14.3 mg kg−1

IL-DLLME

0.045 mg kg−1

UDLLME

0.0005–0.003 μg mL−1 0.2 − 1.0 ng g−1 for DAD and 0.02 − 0.13 ng g−1 for MS/MS 0.5–0.8 μg L−1

DLLME SPE-DLLME

0.4 -1.4 μg L−1.

UASEME-SFO

95–135

2–4 ng mL−1

UASEME-SFO

25–56

5–50 μg L−1

UA-DLLME

51–108

375

– –

DLLME-D-μ-SPE

RM-DLLME

[43] [44] [45] [46] [47]

[48] [49]

[51] [52] [53]

[55] [56] [57] [58]

[59] [60]

[62]

[63]

[64]

Abbreviations: APTS, Aminopropyl triethoxysilane; [BMIM][PF6], 1-Butyl-3-methylimidazolium hexafluorophosphate; [C4MIM][BF4], 1-Butyl-3-methylimidazolium tetrafluoroborate; [C8MIM][PF6], 1-Octyl-3methylimidazolium tetrafluorophosphate; [C6MIM][PF6], 1-Hexyl-3-methylimidazolium hexafluorophosphate; DDT, Dichlorodiphenyltrichloroethane; DeA, Decanoic acid; D-μ-SPE- DLLME, Dispersive micro-solid-phase extraction combined with dispersive liquid-liquid microextraction; IL-DLLME, Ionic liquid dispersive liquid-liquid microextraction; MA-DLLME, Microwave-assisted dispersive liquid-liquid microextraction; MILs-DLLME, Mixed ionic liquids dispersive liquid-liquid microextraction; MR-IL-DLLME, Magnetic retrieval of the ionic liquid dispersive liquid-liquid microextraction; MSA-DLLME, Magnetic stirring-assisted dispersive liquid-liquid microextraction; [N8881][Tf2N], Trioctylmethylammonium bis(trifluoromethylsulfonyl)imide; RM-DLLME, Reverse micelle-mediated dispersive liquid-liquid microextraction; RTIL-DLLME, Room-temperature ionic liquid dispersive liquid-liquid microextraction; TC-IL-DLLME, Temperature-controlled ionic liquid dispersive liquid-liquid microextraction; THF, Tetrahydrofuran; UDLLME, Ultrasonic dispersion liquid-liquid microextraction; USAEME, Ultrasound-assisted emulsification microextraction; US-IL-DLLME, Ultrasound-assisted ionic liquid dispersive liquid-liquid microextraction.

W. Ahmad et al./Trends in Analytical Chemistry 72 (2015) 181–192

Acetonitrile (1.0 mL)

N-Methyl Carbamates

Water melon and tomatoes Water

CHCl3–C6H5Cl (1:1, v/v) (150 μL) CHCl3 (40 μL)

6 Carbamate

–

EF

W. Ahmad et al./Trends in Analytical Chemistry 72 (2015) 181–192

validated in real water samples. Wang et al. [47] also developed room-temperature IL DLLME (RTIL-DLLME) for the analysis of dichlorvos in water with an LOD of 0.2 μg L −1 . In tea, OPPs and carbamates were analyzed by magnetic stirringassisted DLLME (MSA-DLLME) with LODs of 0.13–0.61 μg L −1 [48]. Dichlorodiphenyltrichloroethane (DDT) and its metabolites have been determined by mixed ILs DLLME (MILs-DLLME) in environmental water using hydrophobic IL [C8MIM][PF6] as extractant and hydrophilic IL [C4MIM][BF4] as disperser solvent [50]. Under the optimum experimental conditions, the LODs reached 0.11–0.49 μg L−1, the linear range was 1–100 μg L−1 and recovery percentages of the spiked samples were 85.7–106.8%. Substituted ureas, especially benzoylurea, are powerful insect regulators. They are widely used for controlling numerous pests by inhibiting the synthesis of cuticle chitin. Due to its high consumption, its presence in the environment and foodstuffs is one of the risks faced by the general population. Zhang et al. [51] introduced a novel method magnetic retrieval of the IL DLLME (MR-IL-DLLME) for analysis of five benzoylurea (BU) insecticides in environmental water samples and it was successfully applied in real water samples with good recoveries and RSDs. BU was also studied in natural water [52] and wastewater [53] by two DLLME techniques, namely temperature-controlled IL (TCIL-DLLME), and USA-IL-DLLME. Pyrethroids are synthetic insecticides widely used in agriculture, households, forestry, horticulture and some other fields. The pyrethroids class of pesticides has low toxicity towards mammals and birds and low environmental persistence. However it possesses high toxicity towards aquatic arthropods, fish and honey bees, even at low concentrations. The low toxicity of pyrethroid compared to organochlorine, organophosphorus and carbamate pesticides does not rule out the environmental pollution associated with it. The microextraction techniques considered a bottleneck of analytical chemistry, especially DLLME, have a wide range of applications towards pyrethroids in aqueous and non-aqueous matrices [54–57]. Four pyrethroids were successfully determined by a microwave-assisted DLLME (MA-DLLME). In addition to solvent volume, pH and salt addition, microwave conditions are also carefully optimized [54]. Pyrethroids were also successfully analyzed by combing dispersive micro-solid-phase extraction with DLLME (D-μ-SPE-DLLME) in water and vegetables with EFs of 51–108 and LODs of 0.05.2 ng mL−1 and 0.02–2 ng g−1, respectively [55]. Imidacloprid has been investigated by ultrasonic dispersion LLME (UDLLME) in a sample of tomatoes [58]. A UD process was applied to achieve a cloudy solution quickly. Under optimum extraction conditions, the EF was 375. In another study, six neonicotinoids in soil were processed by DLLME giving recoveries of 55.3–95.6% [59]. Neonicotinoids were also investigated in honey by combining SPE with DLLME [60]. Chlorophenoxy-acid herbicides are considered potential pollutants. They are persistent, polar in nature, and have high water solubility, so they are widely found in water. A new DLLME technique, named reverse micelle-mediated DLLME (RM-DLLME), was applied for the analysis of water-soluble pesticides 2,4dichlorophenoxyacetic acid and 4-chloro-2-methylphenoxyacetic acid [61]. The LODs of the method were 0.5–0.8 μg L−1 and the repeatability of the proposed method, expressed as relative standard deviation (RSD), varied in the range of 2.5–3.2%. Linearity was found to be 1–200 μg L−1. Fungicides analyzed in different matrices are listed including six fungicides in juices and red wine [62] with recoveries of 79.5– 113.4%, strobilurin fungicides in fruit juice [63] with recoveries of 82.6–97.5% and other fungicides in fruit juice [64] with recoveries of 71.8–118.2%.

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2.3. DLLME combined with other techniques Other techniques, such as sweeping micellar electrokinetic chromatography (MEKC) [65–75] and spectrophotometry [76–78], have been used for analysis of pesticides (Table 3). Six carbamates in apple [65], 12 carbamates in juice samples [66], 17 N-methyl carbamates in water [67], five sulfonylurea herbicides in soil matrix [68], neonicotinoid in cucumber [69], phenoxyacetic acids in drinking and environmental water [70,71], five organophosphorus pesticides [72] and some other pesticides [73–75] were analyzed by DLLME coupled with MEKC. The EFs were 87–10,000 under the optimized conditions (Table 3). Carbaryl [76] was analyzed by DLLME and thiram [77] by DLLMED-μ-SPE coupled with spectrophotometric techniques. The LODs of carbaryl and thiram were 8.0 ng mL−1 and 11.5 ng mL−1, respectively, with RSDs of 1.1–2.7 %. DLLME with a new disperser, Aliquat 336, was introduced recently for analysis of carbendazim fungicides [78]. Aliquat 336 offered better extraction efficiency than conventional disperser solvents [78]. 3. Advantages of DLLME In analysis of pesticides, the problems of contamination and loss of analytes by classical LLE are significantly higher than DLLME, since DLLME requires only one operational step. DLLME offers rapid, low cost, short time, reliable, simple operation, high pre-concentration and recovery factors and environment friendliness. LODs in DLLME are much better and feasible compared to other liquid-phase microextraction techniques [7,8]. Table 4 compares the advantages and the drawbacks of different microextraction techniques, such as SDME, HFME and DLLME. In DLLME, a syringe is employed for collection and injection of the extract, so problems are avoided, whereas, in SDME, the problem of drop dislodgment is common due to the use of a syringe as the drop holder during extraction [11]. The performance of DLLME is much faster than cloud-point extraction (CPE) because, in many cases, CPE requires heating of the aqueous solutions for long periods to achieve the cloud-point temperature [7]. The extraction efficiency of CPE decreases in the presence of more than 3% of a water-miscible organic solvent (e.g., THF). The THF solvent is usually used to decrease the viscosity of surfactantrich phase and it facilitates sample handling due to dissolution of the surfactant-rich phase and decrease in the volume of this phase. The injection of a mixture of solvents, which already contain the target analytes in an aqueous solution, may be developed successfully (recovery values are maintained and cleaner extracts are obtained) [10–14]. 4. Milestones, green aspects, shortcomings and developments in DLLME This review mainly focuses on recent developments in DLLME in analysis of pesticides and its applications in conjunction with different analytical techniques for pre-concentration and subsequent determination of pesticides in complex matrices (Tables 1–3). Conventional DLLME utilizes solvents heavier than water [3]. The number of solvents with density greater than water is limited, and they are mostly halogenated and hazardous. This drawback leads to many modifications, such as avoiding the centrifugation step, solidifying the floating organic drop (DLLME-SFOD), and adjusting the solvent density (AS-DLLME) [9]. ILs were also introduced and are now extensively used in DLLME as extractants. These developments mainly focused on enhancing the extraction process leading to different modifications over time. Fig. 4 summarizes the milestones.

188

Table 3 DLLME combined with other techniques for analysis of pesticides Extracting solvent

Disperser solvent

EF

LOD

Method

Other

Ref

Sweeping micellar electrokinetic chromatography 6 Carbamates Apple

Matrix

CHCl3 (60 μL)

Acetone (1 mL)

491–1834

2–3 ng g−1

DLLME

[65]

12 Carbamates

Juice samples

CHCl3 (600 μl)

Methanol (1500 μl)

–

1–7 μg L−1

DLLME

17 N-Methylcarbamate

Toluene (636 μl)

Acetonitrile (940 μl)

–

1–144 ng L−1

Sulfonyl urea herbicides

Environmental and drinking water Soil

Sample volume 5 mL, pH 2.5, no salt added, extraction time 1 min Extraction time 5 min, pH 7.5, no salt added No salt added pH 2.0,

Chlorobenzene (60 μL)

Acetone (1.0 mL)

3000–5000

0.5–1 ng g

[68]

Neonicotinoid

Cucumber

CHCl3 (100.0 μL)

Acetonitrile(0.8 ml)

4000–10000

0.8–1.2 ng g−1

DLLME

Phenoxyacetic acids 3 Phenoxy-acid herbicides

– Chlorobenzene (180 μL)

– Acetonitrile (1000 μL)

– 151–216

0.002–0.005 mg L−1 1.56–1.91 ng mL−1

DLLME DLLME

Organophosphorus

Drinking water Environmental water Water

Sample volume 5 mL pH 2, no salt added, Sample volume 5 ml, no salt added, vortexed for 1 min –Sample volume 5 mL, no Salt added,

Dichloromethane (300 mL)

Acetonitrile (2.0 mL)

477–635

3–15 ng mL−1

DLLME

[72]

17 herbicides

Natural water

CHCl3 (175 μL)

1,4–Dioxane (2 mL)

–

0.5–3.0 μg L−1

3 Pesticides Triazine Spectrophotometry Carbaryl

Apple Water

Carbon tetrachloride (125 μL) Chlorobenzene (60 μL)

Acetone (1.5 mL) Acetonitrile (1 mL)

87–95 1750–2100

50–80 ng mL−1 0.05–0.10 ng mL−1

DLLME-On–line preconcentration DLLME DLLME

Sample volume 10 mL Centrifugation for 5 min, KI (1.5%) Centrifugation for 10 min, pH 2 No salt added, extraction time 3 min NaCl (2% w/v), extraction time 1 min

[74] [75]

Water, Fruits Juice

1-Octanol (150 μl)

Acetonitrile

2730

8 ng mL−1

DLLME-D-μ-SPE

[76]

Thiram Carbendazim fungicides

Water Water and soil

CCl4 (200 μL) CCL4

Ethanol (1 mL) Aliquat 336

58.8 –

11.5 ng mL−1 2.1 ng mL−1

DLLME DLLME

Vortex speed 3200 for 2 min, Sonication for 4 min Centrifugation for 5 min Centrifugation for 5 min

−1

DLLME DSPE-DLLME

[66] [67]

[69] [70] [71]

[73]

[77] [78]

W. Ahmad et al./Trends in Analytical Chemistry 72 (2015) 181–192

Analyte

W. Ahmad et al./Trends in Analytical Chemistry 72 (2015) 181–192

189

Table 4 Comparison of advantages and drawbacks of DLLME with other microextraction techniques SDME

HFME

DLLME

Advantages

Inexpensive, simple, easy to operate, nearly solvent free, more suitable for volatile and semivolatile analytes, environmental friendliness, various extraction modes, ease of automation, high extraction efficiency

Inexpensive, simple, environmentally friendly, high versatility and selectivity, headspace and immersion modes

Drawbacks and limitations

Problem of drop dislodgment, time consuming, incomplete equilibrium

Poor reproducibility, time consuming, formation of air bubble

Simple, rapid, inexpensive, high enrichment factors, environmentally friendly, enormous contact area between acceptor phase and sample, fast reaction kinetics, instantaneous extraction, complete analyte recovery, DLLME can be coupled with SPE,SFE,SBSE, nanotechniques Minor restrictions in solvent selection and automation

DLLME has been successfully combined with extraction techniques {e.g., SPE [45], SFE [25], SBSE [35] and some nanotechniques, such as D-μ-SPE [55]}. Combination of D-μ-SPE with DLLME leads to higher levels of selectivity and sensitivity, and extends the analytical utility of DLLME in complex matrices [55]. Fig. 5 shows the general protocol of DLLME with D-μ-SPE. The analytical applications of DLLME in analysis of pesticides are limited, since the main disadvantage of DLLME is the consumption of relatively large volumes (i.e., mL) of disperser solvents, which usually decrease the partition coefficient of analytes into the extractant solvent [12–18]. This problem is avoided by using ultrasonic energy or cationic surfactant to disperse the extraction solvent instead of disperser solvent [14–16]. The EF was improved to be more environment friendly with new techniques based on DLLME with little solvent consumption (DLLMELSC) [14] by combining some ILs in a binary mixture of disperser and extraction solvents [50]. Recently, some interesting modifications were introduced to avoid the problems and to expand the analytical utility of DLLME, and there is an increasing number of applications of DLLME techniques.

In some cases, extraction solvents with densities and toxicities lower than those of classical solvents for DLLME have been reported. Despite the feasibility of applying a low-toxicity solvent with a density lower than that of water, the use of a high-density solvent is still more desirable due to the simple, convenient collection of the sedimented phase. However, there is a limited number of applications of DLLME combined with spectrochemical and electrochemical techniques for analysis of pesticides in complex matrices. This hot research area will therefore attract considerable interest due to the suitability of combining DLLME with voltammetry at a surface-modified electrode. Voltammetric techniques (e.g., stripping analysis) have no applications with DLLME. Most probably, analytical application of voltammetric techniques to monitoring trace and ultra-trace concentrations of pesticides in different matrices would provide an efficient, low-cost, selective and excellent approach. However, further work continues on the possible application of on-line DLLME combined with stripping voltammetric analysis of such chemicals in various biological and environmental samples. Further research in this area is still needed to eliminate interferences, and to reduce time and cost.

Fig. 4. Milestones in DLLME.

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Fig. 5. General procedure for coupling dispersive micro solid-phase extraction (D-μ-SPE) with DLLME.

5. Limitations and outlook on the future trends of DLLME DLLME usually requires phase separation by centrifugation. Microcolumns packed with suitable sorbents may be used as an alternative to centrifugation and they may open the door to automation and on-line coupling of DLLME to analytical instruments. Amyl acetate-CCl4-acetonitrile mixture systems are generally used as carrier and aspirated at high flow rate, resulting in formation of a cloudy state and the extraction of analytes. Despite the apparent advantages of DLLME, it also has some problems [e.g., it uses high volumes (i.e., mL) of a polar solvent, it is not completely suitable for the analysis of analytes in complex matrices, centrifugation must be applied and the extraction solvent must have a high density and low solubility in water]. The extraction and auxiliary solvent mixture is immiscible with water and has a density higher than that of water, so the resulting fine droplets in the mixture containing the extracted analyte, self-sediment in a short time at the bottom of conical tube. The use of microcolumn and centrifugation are not required for separation of the extraction phase. The procedures presented in DLLME are not simple [22–27], so further research is necessary to complete experiments in this area. ILs have been successfully used as suitable alternative green extraction solvents to common toxic solvents [4]. However, most ILs are incompatible with GC analysis, commercially available ILs are too expensive and their relatively purities are below what would be typically specified for laboratory solvents. Hence, future activities should be oriented towards development of new extraction phases with low toxicity and good GC compatibility. Moreover, complete elimination of disperser solvents is highly desirable, since most disperser solvents are toxic and the use of organic disperser solvent usually decreases the performance of extraction of the analyte from the complex matrices. DLLME is time consuming when performed in manual mode, due to the centrifugation step. This time-consuming step can be avoided by ST-DLLME [34] and some other types of DLLME [22,46]. In spite of that, applications of DLLME in the analysis of pesticides remain few. The experimental parameters affecting uptake performance of DLLME are usually optimized by employing a step-by-step approach, in which each factor is varied sequentially. Recent years saw an upsurge of interest in automation of DLLME using different strategies [17]. However, the complexity of the procedures presented is far from the simplicity of DLLME, so further modifications and improvements should be made to make more progress in automating DLLME. The technique is unsuitable when the number of

influential factors is relatively large, and a step-by-step approach does not show the interaction among experimental parameters, so use of experimental designs is highly recommended to achieve the best extraction conditions quickly and in a relatively small number of experiments. DLLME will certainly be used for solid-state samples and connected to other extraction techniques. 6. Conclusion This review provides a snapshot of the field at this critical stage covering the recent advances in DLLME for pesticide analysis. We cover milestones, green aspects, advantages, and drawbacks of the well-known protocols of DLLME. We provide a brief overview of the principle of DLLME and manipulation and quantification methods in droplets including chromatographic and other techniques. DLLME is a newly developed sample-preparation technique. However, many improvements have been introduced since its innovation in 2006. The rapid development of sample preparation in the field of analytical chemistry in the past few years will continue in the foreseeable future, bringing solutions to many challenges in current bioanalytical, pesticides and drugs analysis. The widespread application of the DLLME technique is supported by the usefulness and the automation of the procedures. DLLME is a clear paradigm in this approach because most of the published articles have focused on its automation. Application of DLLME in analysis of pesticides has received great attention due to its low cost, high EFs, and high recoveries, and the short time it takes. Sample preparation involving DLLME for analysis of pesticides is ecofriendly and offers several advantages, such as ease of method development and high EFs from low volumes of water samples, which made it available to virtually all analytical laboratories. In the first years of development, DLLME techniques were used for sample pretreatment for analysis of a wide range of environmental water samples. Thus, DLLME techniques may also be utilized for green analytical chemistry, since it reduces consumption of hazardous chlorinated organic solvents. Additional advantages of the well-known protocols of the DLLME are low instrumental costs and easy operation. DLLME is a highly versatile sample-preparation method, because not only can it be used for practically all classes of analytes, but also it is compatible, directly or after solvent replacement, with most enrichment techniques (e.g. SPE, SBSE, SFE, and D-μ-SPE), in the extraction of analytes from various aqueous samples.

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[50]

[51]

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