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Developments of Dispersive Liquid-Liquid Microextraction Technique
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 37, Issue 2, February 2009 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2009, 37(2), 161–168.
Review
Developments of Dispersive Liquid-Liquid Microextraction Technique ZANG Xiao-Huan, WU Qiu-Hua, ZHANG Mei-Yue, XI Guo-Hong, WANG Zhi* Key Laboratory of Bioinorganic Chemistry, College of Science, Agricultural University of Hebei, Baoding 071001, China
Abstract: Dispersive liquid-liquid microextraction (DLLME) is a novel environmentally benign sample-preparation technique, possessing obvious advantages of simple operation with a high enrichment factor, low cost, and low consumption of organic solvent. Compared with those of single drop microextraction (SDME) and hollow fiber-based liquid-phase microextraction (HF-LPME), the extraction time of DLLME is greatly shortened. DLLME coupled with gas chromatography (GC), high performance liquid chromatography (HPLC), and atomic absorption spectrometry (AAS) have been widely applied to the analyses of environmental and food samples. The basic principles, parameters affecting the extraction efficiency, and the latest applications of DLLME are reviewed in the article. Key Words: Dispersive liquid-liquid microextraction; Sample pretreatment, Review
1
Introduction
Sample preparation is a crucial step for its whole analysis and is often a bottleneck to rapidly obtain an accurate and sensitive result in an analysis. Traditional methods for sample preparation including liquid-liquid extraction, soxhlet extraction, chromatography, distillation, and absorption[1], usually suffer from the disadvantages of time-consuming and tedium, large amounts of toxic organic solvent to be used, and difficulty in automation to some extent. Therefore, a lot of research efforts in separation science and related fields have been focused on the development of new sample preparation techniques, which are less time-consuming, more effective, and require smaller amounts of organic solvents[2–5]. In recent years, a lot of new sample preparation methods have been developed, such as solid-phase extraction (SPE)[6], molecular imprinting technique (MIT)[7], solid-phase microextraction (SPME)[8], single-drop microextraction (SDME)[9], and hollow fiber-based liquid-phase microextraction (HF-LPME). However, SPE is time-consuming and relatively expensive, sometimes shows a poor batch-to-batch reproducibility, and still needs a large amount of organic solvents. For MIT, the
recognition ability is greatly affected by solvent and the selectivity in aqueous solution is very poor. In the case of SPME, the fiber is quite expensive and fragile, with limited lifetime, and sometimes encounters sample carry-over problems. Although SPME coupled with GC is very effective for some applications, a special desorption apparatus is needed when it is used in coupling with HPLC. Single-drop microextraction often requires careful manual operation to prevent drop dislodgment and is instable especially when high-speed stirring is used. In addition, an extra filtration step is usually needed for the sample solutions with complex matrixes, and its sensitivity and the precision still need further improvements[10]. In 2006, Assadi and co-workers[11] developed a novel liquid–phase microextraction technique, named dispersive liquid–liquid microextraction (DLLME). This method is based on a ternary component solvent system in which the extraction solvent and disperser solvent are rapidly injected into the aqueous sample by syringe. The mixture is then gently shaken and a cloudy solution (water/disperser solvent/extraction solvent) was formed in the test tube. After centrifugation, the fine particles of extraction solvent were
Received 10 September 2008; accepted 24 October 2008 * Corresponding author. Email: [email protected]; Tel: +86-312-7521513; Fax: +86-312-7521513 This work was supported by the National Natural Science Foundation of Hebei province of China (No. B2008000210). Copyright © 2009, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(08)60082-1
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sedimented in the bottom of the conical test tube. The resultant sedimented phase is taken with a microsyringe and injected into GC for analysis. DLLME is a miniaturized LLE that uses microliter volumes of extraction solvent. The advantages of DLLME method are simplicity of operation, rapidity, low cost, high-recovery, high enrichment factor, and environmental benignity[11,12], with wide application prospects in trace analysis. The recent researches and developments of DLLME are reviewed in this article.
2
Principle of DLLME Fig. 1
Dispersive liquid–liquid microextraction is a miniaturized LLE using microliter volumes of extraction solvent, which is based on the equilibrium distribution process of the target analytes between sample solution and extraction solvent. Distribution coefficient (K) is defined as the ratio between the analyte concentration in extraction solvent and sample solution. Dispersive liquid-liquid microextraction is only applicable for the analytes with high or moderate lipophilic property (K > 500) and not fit to those neutral analytes with high hydrophilic property. As for the acidic or alkaline analytes, distribution coefficient could be increased by controlling the pH value of sample solution, making the analytes existing in nonionic state. The enrichment factor and extraction recovery are calculated as follows[11,12]: F = Csed/Co R = (Csed Vsed)/(Co Vaq) where, F, Csed and Co are the enrichment factor, the analyte concentration in the sediment, and the initial concentration of analyte in the aqueous sample, respectively; R, Vsed and Vaq are the extraction recovery, the volume of the sediment phase, and the volume of the aqueous sample, respectively. The extraction steps of DLLME are illustrated in Fig. 1. A certain volume of sample solution is placed in a 10-mL screw cap glass test tube with conic bottom (A), followed by the rapid injection of disperser solvent containing extraction solvent into the sample solution with a syringe or pipette. Then, the mixture was gently shaken; thus, a cloudy solution (water/disperser solvent/extraction solvent) is formed in the test tube (B). After that, the surface area between extraction solvent and aqueous phase (sample) is infinitely large, thereby, transition of analyte from aqueous phase (sample) to extraction phase is fast. Subsequently, equilibrium state is achieved quickly, resulting in a very short extraction time, which is the remarkable advantage of DLLME compared with those of other techniques. Finally, the dispersed fine particles of extraction phase are sedimented in the bottom of conical test tube through centrifugation (C). A certain volume of the sedimented phase is injected into chromatographic system using a microsyringe for further analysis (D).
3
Dispersive liquid-liquid microextraction procedure
Parameters affecting extraction efficiencies of DLLME
The extraction efficiency for the target analyte by DLLME is influenced by many factors, such as the kind of extraction and disperser solvent, and their volume, the extraction time, and salt addition. 3.1
Selection of extraction solvent
The selection of an appropriate extraction solvent is a major parameter for DLLME process. The extraction solvent should satisfy two conditions: one is the higher density of the extraction solvent than that of water, which makes it possible to separate extraction solvent from aqueous phase by centrifugation; the other is the extraction capability of extraction solvent for the compounds of interest, good chromatographic behavior, and low solubility of extraction solvent in water. Halogenated hydrocarbon, such as chlorobenzene, chloroform, carbon tetrachloride, and tetrachloroethylene (tetrachloroethane), are usually selected as extraction solvents because of their high density. 3.2
Selection of disperser solvent
Disperser solvent is soluble in extraction solvent and should be miscible in water, thus enabling the extraction solvent to be dispersed as fine particles in aqueous phase to form a cloudy solution (water/disperser solvent/extraction solvent). In such a case, the surface area between extraction solvent and aqueous phase (sample) can be infinitely large, thus to increase the extraction efficiency. The commonly used disperser solvents include methanol, ethanol, acetonitrile, acetone, and tetrahydrofuran. 3.3
Effect of extraction solvent volume
The extraction solvent volume has great effects on the enrichment factor. With the increase of the extraction solvent volume, the final organic phase obtained by centrifugation is
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increased, resulting in a decrease of the concentration of the target analyte in organic phase. Although the extraction recovery keeps almost constant, the enrichment factor will be decreased, leading to a decrease of the sensitivity of the determination for the target compounds. Therefore, the optimal extraction solvent volume should ensure both the high enrichment factors and the enough volume for the subsequent determination after centrifugation. In general, 5–100 ȝl of extraction solvent is selected. 3.4
Effect of disperser solvent volume
The disperser solvent volume directly affects the formation of the cloudy solution (water/disperser solvent/extraction solvent), the degree of the dispersion of the extraction solvent in aqueous phase, and subsequently, the extraction efficiency. Usually, 0.5–1.5 ml of disperser solvent is selected. 3.5
Effect of extraction time
In DLLME, extraction time is defined as the interval between injecting the mixture of disperser solvent and extraction solvent and centrifugation. It is revealed that extraction time has little effect on the extraction efficiency of DLLME. The reason for this is that the extraction solvent can be evenly dispersed after the formation of the cloudy solution, the transition of the analyte from aqueous phase (sample) to extraction phase can be very fast, and the equilibrium state can be subsequently achieved very quickly, resulting in a very short extraction time needed for equilibrium. Short extraction time is a remarkable advantage of the DLLME technique. 3.6
Effect of salt addition
The solubilities of the target analyte and organic extraction solvent in aqueous phase are usually decreased with the increase of ionic strength, which is favorable for reaching high recovery. However, at the same time, the obtained volume of organic phase is increased, resulting in a decrease of both the target analyte concentration and the enrichment factor.
4
Application of DLLME
As a novel sample preparation method, DLLME can be coupled with GC, HPLC, and AAS for application. It has been widely applied to the analyses of pesticide residues, heavy metals, and so on. The typical applications are shown in Table 1. 4.1
DLLME-GC
DLLME is very suitable to be coupled with GC since in most cases, the extraction solvent can be directly injected into GC for determination with a microsyringe, without additional
pretreatment. Therefore, DLLME-GC technique has achieved rapid development in a short time. Monitoring pollutants in water is one of the most important tasks for environmental analysis. DLLME-GC is easy to operate and is extremely applicable for water samples. Assadi, et al[11] first developed a novel method for the determination of polycyclic aromatic hydrocarbons (PAHs) in water samples (surface, river, and well waters) with DLLME coupled with gas chromatography-flame ionization detection (DLLME-GC-FID). First, 5.00 ml of double distilled water was placed in a 10-ml screw cap glass test tube with conic bottom and spiked at a level of 2 ȝg l–1 of PAHs. Then, 1.00 ml of acetone (as disperser solvent) containing 8.0 ȝl of C2Cl4 (as extraction solvent) was rapidly injected into a sample solution with a 1.00-ml syringe, and the mixture was gently shaken. A cloudy solution (water/acetone/tetrachloroethylene) was formed in the test tube (the cloudy state was stable for a long time). Then, the mixture was centrifuged for 1.5 min at 6000 rpm. Accordingly, the dispersed fine particles of extraction phase were sedimented in the bottom of the conical test tube, and 2.00 ȝl of the sedimented phase was removed by means of a 2.00-ȝl microsyringe and injected into GC for analysis. Under the optimum conditions, the obtained enrichment factor ranged from 603 to 1113. The linear range was 0.02–200 ȝg l–1, and the detection limit was 0.007–0.030 ȝg l–1 for most of the analytes. The relative standard deviation (RSDs) for 2 ȝg l–1 PAHs in water by virtue of internal standard was in a range of 1.4%–10.2% (n = 5). The recovery of PAHs from surface water at a spiked level of 5.0 ȝg l–1 was 82.0%–111.0%. In a further research[12], they developed another new method for the extraction of organophosphorus pesticides (OPPs) from water samples by DLLME-GC-FPD. Compared with SPME-GC-FPD, SPME-GC-NPD, SDME-GC-MS, and SDME-GC-FPD, DLLME–GC-FPD has the features of simplified device, simplicity of operation, low LOD, wide linear range, high enrichment factor, and very short extraction time of less than 3 min, which is far less than those of other methods (15–60 min). Assadi, et al. also applied DLLME to the analyses of trichloromethane[13], chlorobenzenes[14], polychlorinated biphenyls[15], etc. in environmental samples. Recently, we[16] have developed a novel method for the determination of pyrethroid pesticide residues in water samples by DLLME-GC-ECD. Under the optimum conditions⧎as high as a 708-fold to 1087-fold enrichment factor and the detection limit of 0.04–0.10 ȝg l–1 were achieved. This method has been successfully applied to the determination of pyrethroid pesticide residues in real water samples (tap water, well water, and river water) with recoveries falling in a range from 76.0% to 116.0%. Besides, DLLME has also been applied to the determination of phthalate esters[17], amitriptyline and nortriptyline[18], organophosphorus flame retardants[19], triazine herbicides[20], and amide herbicides[21], etc. in environmental water samples.
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Table 1 Application of dispersive liquid- liquid microextraction Matrix
Analytical method
Extraction solvent
Disperser solvent
EF
LOD (μg l–1)
Ref.
Water
GC-FID
Tetrachloroethylene
Acetone
603–1113
0.007–0.030
11
Water
GC-FPD
Chlorobenzene
Acetone
789–1070
3–20 ng l–1
12
Drinking water Water
GC-ECD GC-ECD
Carbon disulfide Chlorobenzene
Acetone Acetone
116–355 711–813
0.005–0.040 0.0005–0.05
13 14
Water
GC-ECD
Chlorobenzene
Acetone
383–540
0.001–0.002
15
Water Water
GC-ECD GC-MS
Chlorobenzene Chlorobenzene
Acetone Acetone
708–1087 681–889
16 17
Water
GC-FID
Carbon tetrachloride
Methanol
740–1000
0.10 – 0.04 0.002–0.008 0.005, 0.01 mg l–1
Water
GC-NPD
1,1,1-Trichloroethane
Acetone
190–830
0.01–0.080
19
Water Water
GC-MS GC-MS
Chlorobenzene Carbon tetrachloride
Acetone Acetone
151–722 437–460
0.021–0.12 0.02–0.04
20 21
Water
GC-MS
Chloroform
100
6.0–133 ng l– 1
22
Water
GC-FID
Chloroform
Methanol
225–257
0.02–0.18
23
Water
GC-ECD
Chlorobenzene
Acetone
0.010–2.0
24
Chlorophenols
Water
SPE-GC-ECD
Chlorobenzene
Acetone
0.0005–0.1
25
Selenium Butyl and phenyltin Compounds Anilines Organophosphorus Pesticides Volatile phenols Organosulfur pesticides Captan, Folpet and Captafol Methomyl Phthalate esters Polybrominated diphenyl ethers Clenbuterol Antioxidants
Water
GC-ECD
Chlorobenzene
Ethanol
287–906 4390– 17870 122
0.005
26
Analytes Polycyclic aromatic hydrocarbons Organophosphorus pesticides Trihalomethanes Chlorobenzenes Polychlorinated biphenyls Pyrethroid pesticide Phthalate esters Amitriptyline and nortriptyline organophosphorus flame retardants Triazine herbicide Amide herbicides Synthetic musk Fragrances, phthalate Esters and lindane Polycyclic aromatic Hydrocarbons Chlorophenols
Pyrethroid pesticide Methylparathion and Phoxim Atrazine Aromatic amines gold Selenium Cadmium Cadmium Lead Cobalt and nickel Copper
–1
18
Water
GC-FPD
Carbon tetrachloride
Ethanol
825–1036
0.2–1 ng l
27
Waste water Watermelon, cucumber Red wines Environmental and Beverage samples
GC-MS
Chlorobenzene
Acetone
212–645
28
GC-FPD
Chlorobenzene
Acetonitrile
41–50
29
GC-MS
Carbon tetrachloride
Acetone
0.04–0.09 0.010–0.190 μg kg–1 28–44
GC-FPD
Carbon tetrachloride
Methanol
176–946
0.21–3.05
31
Grape
GC-ECD
Chlorobenzene
Acetone
788–876
Water Water
HPLC-VWD HPLC-VWD
tetrachloroethane Carbon tetrachloride
Methanol Acetonitrile
70.7 44–196
Water
HPLC-VWD
Tetrachloroethane
Acetonitrile
268–305
water Waste water Environmental samples Environmental samples Environmental samples Water Water silicate ore sample Water Water Water Biological and water samples Environmental water and rice samples Water
HPLC HPLC-DAD
Tetrachloroethylene Carbon tetrachloride
Acetone Acetonitrile
175 168–220
HPLC
[C6MIM][PF6]
HPLC
[C6MIM][PF6]
HPLC
6.0–8.0 ȝg/kg 1.0 0.64–8 12.4–55.6 ng l–1 4.9 3–7
30
32 33 34 35 36 37
0.28–0.6
38
0.17, 0.29
39
[C6MIM][PF6]
0.601
40
HPLC
[BMIM][PF6]
0.45–2.6
41
GF AAS
Chlorobenzene
Acetone
388
0.005
42
GF AAS GF AAS GF AAS
Carbon tetrachloride Carbon tetrachloride Carbon tetrachloride
Ethanol Methanol Methanol
70 125 122
0.05 0.6 ng l–1 0.5 ng l–1
43 44 45
GF AAS
Carbon tetrachloride
Ethanol
78
39 ng l–1
46
50
–1
GF AAS
Carbon tetrachloride
Acetone
101, 200
FAAS
Chloroform
Methanol
42
21 ng l , 33 ng l–1 3.0
47 48
(To be continued)
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Table 1 (Continued) Analytes
Matrix
Lead Lead
Water Water
Cobalt
Water
Analytical method
Extraction solvent
Disperser solvent
EF
LOD (μg l–1)
Ref
FAAS ET AAS Spectrophotometric
Carbon tetrachloride Carbon tetrachloride
Methanol Acetone
450 150
0.5 0.02
49 50
Chloroform
Ethanol
125
0.5
51
0.25, 0.2
52
Palladium and cobalt
Water and synthetic sample
FO-LADS
1,2-Dichlorobenzene
Ethanol
162, 165
Samarium, europium, Gadolinium and Dysprosium
Water
ICP-OES
Chloroform
Methanol
78–103
The use of disperser solvent usually decreases the partition coefficient of analytes into the extractant solvent. Garcia-Jares et al[22] proposed a novel method based on ultrasound-assisted emulsification-microextraction (USAEME) and GC-MS for the analyses of synthetic musk fragrances, phthalate esters, and lindane in water samples, without the addition of an emulsifier. Aliquots of 10 ml of sample were, respectively, placed in 15-ml conical-bottom glass centrifuge tubes. Under the final optimized conditions, 100 ȝl of chloroform containing internal standard substance was added to each tube as extractant solvent. Extractions were performed at 40 kHz of ultrasound frequency and 100 W of power for 10 min so that oil-in-water (O/W) emulsions of chloroform (dispersed phase) in water (continuous phase) were formed. Emulsions were then disrupted by centrifugation at 5000 rpm for 3 min and the organic phases were sedimented at the bottom of each conical tube. The extracts thus obtained were stored at í20 ºC until analysis by GC-MS. An enrichment factor of 100 and a detection limit of 6–133 ng l–1 were finally achieved. Yamini et al[23] added 1.2 ml of methanol (as consolute solvent) containing 60 ȝl of chloroform (as extracting solvent) to a 2.5-ml of sample solution in a 11-ml screw cap test tube with conic bottom. Then, the mixture was gently shaken, and a homogeneous solution was obtained. After 0.6 g of NaCl was added to the solution and shaken, a cloudy mixture was formed in the test tube. The cloudy mixture was centrifuged for 3 min at 3500 rpm. Accordingly, the fine particles of the extraction phase were sedimented at the bottom of the conical test tube, and the sedimented phase was withdrawn and injected into a GC for analysis. This method was applied for the determination of PAHs in waste water samples. Under the optimum conditions, an enrichment factor of 225–257 and a detection limit of 0.02–0.18 ȝg l–1 were achieved. USAEME brings the unmixable aquatic phase and organic phase into a cloudy mixture by ultrasound rather than the addition of a disperser, resulting in a high distribution coefficient of the sample in extraction solvent, and therefore good extraction efficiency. In homogeneous DLLME, extraction solvent enters into aquatic phase to form a single phase (no phase interface exists between aquatic and organic phases). As a result, the contact area between the sample and the extraction solvent is
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increased to a maximum content, obtaining high extraction efficiency. Therefore, USAEME and homogeneous DLLME provide new approaches for the development of DLLME. For the strong polar and nonvolatile samples, which are unsuitable to be analyzed by GC, derivatization can be used to enhance their volatility and stability so as to make it possible to analyze them by GC. Assadi et al[24] simultaneously used DLLME and derivatization via GC-ECD to determine chlorophenols (CPs) in water samples, which greatly improved the extraction efficiency and sensitivity. In a 10-ml screw cap conical bottomed glass test tube were added 0.50 ml K2CO3 (5%, w/v) and 0.5 ml acetone (disperser solvent) containing 10.0 ȝl chlorobenzene (extraction solvent) and 50 ȝl anhydride acetic (derivatization reagent) to 5.00 ml of an aqueous solution. After shaking, a cloudy mixture was formed. In this step, chlorophenols were derivatized with acetic anhydride and extracted into the fine droplets of chlorobenzene in a few seconds. The determination of CPs in a water sample by DLLME-GC-ECD was compared with those by other methods such as SPE-GC-ECD, SPME-GCMS, and LPME-GC-MS, indicating that DLLME is a fast, reproducible, and simple technique, with low RSDs, wide linear range, and short extraction time (only several seconds). Assadi et al[25] also developed a preconcentration technique combining SPE with DLLME and derivatization for the determination of chlorophenols in water samples via GC-ECD. It provided an enrichment factor of as high as 4390–17870, and the detection limit ranged from 0.0005 to 0.1 ȝg l–1. They also performed DLLME-GC-FPD for the analysis of butyl and phenyltin compounds in water samples after derivatization with sodium tetraethylborate NaBEt4[27]. Chang et al[28] presented a one-step derivatization and extraction technique for the determination of anilines in river water by DLLME-GC-MS and derivatization of them with pentafluorobenzaldehyde (PFBAY). Application of DLLME coupled with derivatization provides a one-step derivatization and extraction technique, greatly simplifying the operation steps and shortening the analysis time. Dispersive liquid-liquid microextraction is more suitable for the treatment of the target compounds with simple matrix, resulting in its wide application in the analysis of water
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samples. By now, the pretreatment of samples with complex matrix by DLLME is still at the beginning stage of exploration. Zhao et al[29] developed a new method for the determination of OPPs in cucumber and watermelon by DLLME-GC-FPD, with a detection limit ranging from 0.010 to 0.190 ȝg kg–1 for the target pesticides. Fariña et al[30] proposed a new method for the analysis of volatile phenols in the aroma of red wine by combining DLLME with GC-MS. This method is now in commercial use. Xiong et al[31] developed a new method for the analysis of organosulfur pesticides (malathion, chlorpyrifos, buprofezin, triazophos, carbosulfan, and pyridaben) in environmental and beverage samples by GC-FPD via coupling DLLME with HF-LPME. In our previous work[32], a novel method was developed for the determination of chlorothalonil, captan, and folpet in grape samples by DLLME coupled with GC-ECD. Under the optimum conditions, the enrichment factor ranged between 788 and 876, and the detection limit was between 6.0 and 8.0 μg kg–1. Practical samples were successfully analyzed by the proposed method with satisfactory results. With the further development of this novel DLLME technique, the analysis of samples with complex matrix by DLLME will be more and more widely applied. 4.2
DLLME-HPLC
DLLME-GC is unsuitable for the analysis of thermo-labile compounds, surfactants, medicaments, and proteins, which are semivolatile or nonvolatile compounds. However, DLLMEHPLC can be a good alternative to the problems. Up to now, there have been two operation modes for DLLME-HPLC, one is the direct injection analysis for target compounds by HPLC after extraction by DLLME. Wei et al[33] developed a new method for the determination of methomyl in water samples by combining DLLME with HPLC-VWD, with an enrichment factor of 70.7 and a detection limit of 1.0 μg l–1, respectively. Comparison of DLLME with other methods such as SPE, SPME, and LPME for extraction and determination of NMCs from water samples indicates that the extraction time by DLLME is very short (approximately 2 min), the extraction equilibrium is attained very quickly, and it does not require any additional instrument. Therefore, it is very simple, rapid, easy to use, inexpensive, and environmentally benign. Phthalate esters[34] and polybrominated diphenyl ethers[35] were also determined by DLLME-HPLC. The other operation mode is to give some additional treatment of the target compounds after extraction by DLLME before injection into HPLC system for analysis. Melwanki et al[36] demonstrated a new sample pretreatment method by combining DLLME with in-syringe back extraction for the HPLC analysis of clenbuterol in river, lake, and stream water. The analytical compound was first extracted into organic extracting solvent by DLLME and then back extracted into a HPLC-compatible aqueous solution in a
syringe by repeated plunger movement. A 10 μl volume of 1% formic acid was drawn in a 100-μl syringe followed by 20 μl of the sedimented tetrachloroethylene (TCE) phase. The plunger was drawn back to the 100-μl line and mounted on a syringe pump (keeping the needle downwards). The syringe pump was preprogrammed to move the plunger to 60 μl “back” and “forth” at a speed of 200 μl min–1 in a continuous mode. Meanwhile, the organic and the aqueous acidic plugs also moved within the barrel of the syringe. After 5 min, the syringe pump was stopped, the TCE phase was discarded and the acidic aqueous phase was neutralized with 5 μl of 1% triethylamine. The total volume of 15 μl of the aqueous phase was injected into HPLC. As a result, a detection limit of 4.9 μg l–1, an enrichment factor of 175, and a relative recovery of 97% were obtained. Farajzadeh et al[37] presented a new method using DLLME and HPLC-DAD for the extraction and determination of Irganox 1010, Irganox 1076, and Irgafos 168 (antioxidants) in aqueous samples. The sedimented phase was completely transferred to another test tube with conical bottom with a 100-μl HPLC syringe, and after the evaporation of the solvent in a water bath, the residue was dissolved in 50 μl of HPLC grade methanol and injected into the separation system. Room temperature ionic liquids (ILs) are a class of liquids entirely comprised of ions, which usually exist as liquids well below room temperature. Typical formulations of ILs rely mostly on quaternary nitrogen cations such as alkylammonium, dialkylimidazolium, and alkylpyridinium, and anions such as Cl–, AlCl4–, PF6–, BF4-, N(CF3SO2)2–, CF3CO2– and CF3SO3–. Ionic liquids have attracted an increasing interest in the chemistry community as green alternatives to classical environmentally damaging media of volatile organic compounds (VOCs) because of their remarkable properties, such as a negligibly small vapor pressure, incombustibility, and high thermal stability. Therefore, it is also called a “Green Solvent”. Zhou et al[38] reported a novel, green, and environmental benign sample enrichment method termed temperature-controlled IL dispersive liquid phase microextraction. An IL of 1-hexyl-3methylimidazolium hexafluorophosphate [C6MIM][PF6] was used as the extraction solvent and pyrethroid pesticides as the model compounds. For this new extraction procedure, 45 μl of [C6MIM][PF6] was added into a 10-ml glass graduated conical tube, and ultra-pure water was added into it until the 10-ml mark. This solution was spiked at a concentration of 20 μg l–1 for the five target analytes. Then, the conical tubes were heated in a water bath with the temperature controlled at 70 ºC. The IL was dissolved completely and could mix with the solution entirely and the analytes migrated into the IL phase. The tube was thereafter cooled with ice water and the solution became turbid. Then, the solution was centrifuged for 20 min. The upper aqueous phase was removed with a syringe, the residue was dissolved in 200 μl of mobile phase, and 20 μL of
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the solution was injected into the HPLC system for analysis. Finally, the proposed method showed a good linear range of 1.5–100 μg l–1, a detection limit of 0.28–0.6 μg l–1, and a spiked recovery in a range of 76.7%–135.6%. The researchers also used the same method for the determination of organophosphorus pesticides[39] and atrazine[40] in environmental samples. Fan et al[41] demonstrated a new microextraction method termed IL based DLLME (IL-DLLME) via HPLC analysis of 2-methylaniline, 4-chloroaniline, 1-naphthylamine, and 4-aminobiphenyl in aqueous matrices with 1-butyl-3- methylimidazolium hexafluorophosphate ([Bmim][PF6]) as extraction solvent. Ionic liquids, as a kind of green solvents, can be specially designed for some use of special samples based on their own properties so as to increase the extraction efficiency of samples with different lipophilic property. Therefore, ILs as the extraction solvents for DLLME not only can enlarge the application range of DLLME but also provide a new approach for its development in future researches. 4.3
DLLME-AAS
DLLME can be coupled with atomic absorption spectrometry (AAS), which has been applied to the trace analysis of heavy metals in environmental water samples. There are two ways for the application of this method. One is to add chelating agent to the sample solution first, followed by the addition of appropriate extraction solvent and disperser solvent; the other is to add chelating agent, extraction solvent, and disperser solvent to the sample solution simultaneously. After shaking, a cloudy mixture is formed, and the metal ions react with chelating agent and are extracted into the fine droplets of extraction solvent. Then, the mixture was centrifugated to obtain extraction solvent for injection analysis. When DLLME is coupled with graphite furnace atomic absorption spectrometry (GFAAS), direct injection analysis can be performed just after the removal of the extraction solvent. Shamsipur et al[42] developed a new method based on highly efficient separation and preconcentration of gold by DLLME followed by its determination with GFAAS. It was successfully applied to the extraction and determination of gold in tap water and silicate ore samples with victoria blue B and 0.04% Pd(NO3)2 as chelating agent and chemical modifier, respectively. The analytical curve was linear in a concentration range of 0.03–0.5 μg l–1, and the detection limit was 0.005 μg l–1. Bidari[43] and Jahromi[44] reported a simple and powerful microextraction technique for the determination of selenium and cadmium in water samples with ammonium pyrrolidine dithiocarbamate (APDC) as chelating agent. The detection limit and enrichment factor of selenium were 0.05 ȝg l–1 and 70, and those of cadmium were 0.6 ng l–1 and 125, respectively. Moghimi et al[45] used the same method to
determine cadmium in natural waters using N,N’-bis(salicylidene) ethylenediamine instead of APDC as chelating agent. Similar results of an enrichment factor of 122 and a detection limit of 0.5 ng l–1 were obtained. Liang et al[46] successfully applied DLLME- GFAAS to determine the trace amounts of lead in human urine and water samples using 1-phenyl-3-methyl-4- benzoyl-5-pyrazolone (PMBP) as a chelating agent. The detection limit for lead was 39 ng l–1, and the enrichment factor reached 78. Jiang et al[47] developed a new method by combining DLPME with GFAAS for the determination of trace Co and Ni in environmental and food samples using 1-(2-pyridylazo)-2-naphthol (PAN) as chelating reagent. The enrichment factors were 101 and 200 and the detection limits were 21 and 33 ng l–1 for Co and Ni, respectively. When DLLME is coupled with flame atomic absorption spectrometry (FAAS), direct injection analysis can not be performed since the volume of extraction solvent is too little to be determined. Farajzadeh et al[48] proposed a new method for the quantitation of Cu2+ ions in different water samples by DLLME-FAAS using 8-hydroxy quinoline as a chelating agent. After the volume of the sedimented phase (chloroform) was removed with a 250-μl HPLC syringe, the sedimented phase was quantitatively transferred to another test tube and allowed to evaporate at room temperature. Finally, the residue was dissolved into 0.5 ml of 0.1 M nitric acid and the copper concentration was determined by flame atomic absorption spectrometry. The calibration graph was linear over a range of 50–2000 μg l–1, the detection limit was 3 μg l–1, and the enrichment factor was about 42. Naseri et al[49] coupled a specially designed microsample introduction system to FAAS through which a volume of 20 ȝl of the final organic extract was introduced into the flame in all the FAAS measurements. Dispersive liquid-liquid microextraction was combined with FAAS for the determination of lead in water samples with diethyldithiophosphoric acid (DDTP) as chelating agent. The obtained enrichment factor was 450, the calibration graph was linear in a range of 1–70 μg l–1 with a detection limit of 0.5 μg l–1, and the relative recoveries of lead in tap, well, river, and seawater samples ranged from 93.8% to 106.2%. They also used the same method for the detection of ultratrace amounts of lead in water samples by means of electrothermal atomic absorption spectrometry (ETAAS) instead of FAAS[50]. The obtained enrichment factor was 150, and the calibration graph was linear in a range of 0.05–1 ȝg l–1 with a detection limit of 0.02 ȝg l–1. Dispersive liquid-liquid microextractionatomic absorption spectrometry not only enlarges the application range of DLLME but also provides a new analytical method for the determination of metal ions. 4.4
DLLME coupled with other instruments
Dispersive liquid-liquid microextraction can also be
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
coupled with spectrophotometric instruments for the quantitative determination of metal ions. Gharehbaghi et al[51] used DLLME as a prior step for the determination of preconcentrate trace levels of cobalt in tap and river water samples by spectrophotometric method with PAN as chelating reagent. The enrichment factor and the detection limit were 150 and 0.5 ȝg l–1, respectively. Shokoufia et al[52] used fiber optic-linear array detection spectrophotometry (FO-LADS) in combination with DLLME for the simultaneous preconcentration and determination of palladium and cobalt in water and synthetic samples with PAN as chelating reagent. The calibration graphs were linear in a range of 2–100 ȝg l–1 and 1–70 ȝg l–1 with a detection limit of 0.25 ȝg l–1 and 0.2 ȝg l–1 for palladium and cobalt, respectively. The enhancement factors of 162 and 165 were obtained for palladium and cobalt. Shemirani et al[53] determined samarium, europium, gadolinium, and dysprosium by combining DLLME with inductively coupled plasma optical emission spectrometry (ICP-OES) using PAN as chelating reagent. The preconcentration factors of 80, 100, 103 and 78 were obtained for Sm, Eu, Gd and Dy, respectively.
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Dispersive liquid-liquid microextraction is a relatively new sample pretreatment method, combining sampling, extraction, and concentration all together. Compared with traditional extraction methods, DLLME has the advantages of simplicity of operation, rapidity, low-cost, high-recovery, high enrichment factor, and environmental benignity. It shows increased wide practical prospects in the field of trace analysis. It is predicted that DLLME will be developed rapidly in the following aspects: (1) to further develop applications in the analysis of samples with complex matrix. Since most of the target compounds detected by DLLME till now are relatively of simple matrix, it is important to make this method more applicable to samples with complex matrix; (2) to extend the selection range of extraction solvents for DLLME. At present, most of the reported extraction solvents are halogenated hydrocarbons. More extensive range of extraction solvents must lead to the more extensive range of applicable substrates preconcentrated by DLLME correspondingly, so as to make it have more practical prospects; (3) to investigate new techniques by combining DLLME with other more different instruments.
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Review
Developments of Dispersive Liquid-Liquid Microextraction Technique ZANG Xiao-Huan, WU Qiu-Hua, ZHANG Mei-Yue, XI Guo-Hong, WANG Zhi* Key Laboratory of Bioinorganic Chemistry, College of Science, Agricultural University of Hebei, Baoding 071001, China
Abstract: Dispersive liquid-liquid microextraction (DLLME) is a novel environmentally benign sample-preparation technique, possessing obvious advantages of simple operation with a high enrichment factor, low cost, and low consumption of organic solvent. Compared with those of single drop microextraction (SDME) and hollow fiber-based liquid-phase microextraction (HF-LPME), the extraction time of DLLME is greatly shortened. DLLME coupled with gas chromatography (GC), high performance liquid chromatography (HPLC), and atomic absorption spectrometry (AAS) have been widely applied to the analyses of environmental and food samples. The basic principles, parameters affecting the extraction efficiency, and the latest applications of DLLME are reviewed in the article. Key Words: Dispersive liquid-liquid microextraction; Sample pretreatment, Review
1
Introduction
Sample preparation is a crucial step for its whole analysis and is often a bottleneck to rapidly obtain an accurate and sensitive result in an analysis. Traditional methods for sample preparation including liquid-liquid extraction, soxhlet extraction, chromatography, distillation, and absorption[1], usually suffer from the disadvantages of time-consuming and tedium, large amounts of toxic organic solvent to be used, and difficulty in automation to some extent. Therefore, a lot of research efforts in separation science and related fields have been focused on the development of new sample preparation techniques, which are less time-consuming, more effective, and require smaller amounts of organic solvents[2–5]. In recent years, a lot of new sample preparation methods have been developed, such as solid-phase extraction (SPE)[6], molecular imprinting technique (MIT)[7], solid-phase microextraction (SPME)[8], single-drop microextraction (SDME)[9], and hollow fiber-based liquid-phase microextraction (HF-LPME). However, SPE is time-consuming and relatively expensive, sometimes shows a poor batch-to-batch reproducibility, and still needs a large amount of organic solvents. For MIT, the
recognition ability is greatly affected by solvent and the selectivity in aqueous solution is very poor. In the case of SPME, the fiber is quite expensive and fragile, with limited lifetime, and sometimes encounters sample carry-over problems. Although SPME coupled with GC is very effective for some applications, a special desorption apparatus is needed when it is used in coupling with HPLC. Single-drop microextraction often requires careful manual operation to prevent drop dislodgment and is instable especially when high-speed stirring is used. In addition, an extra filtration step is usually needed for the sample solutions with complex matrixes, and its sensitivity and the precision still need further improvements[10]. In 2006, Assadi and co-workers[11] developed a novel liquid–phase microextraction technique, named dispersive liquid–liquid microextraction (DLLME). This method is based on a ternary component solvent system in which the extraction solvent and disperser solvent are rapidly injected into the aqueous sample by syringe. The mixture is then gently shaken and a cloudy solution (water/disperser solvent/extraction solvent) was formed in the test tube. After centrifugation, the fine particles of extraction solvent were
Received 10 September 2008; accepted 24 October 2008 * Corresponding author. Email: [email protected]; Tel: +86-312-7521513; Fax: +86-312-7521513 This work was supported by the National Natural Science Foundation of Hebei province of China (No. B2008000210). Copyright © 2009, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(08)60082-1
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
sedimented in the bottom of the conical test tube. The resultant sedimented phase is taken with a microsyringe and injected into GC for analysis. DLLME is a miniaturized LLE that uses microliter volumes of extraction solvent. The advantages of DLLME method are simplicity of operation, rapidity, low cost, high-recovery, high enrichment factor, and environmental benignity[11,12], with wide application prospects in trace analysis. The recent researches and developments of DLLME are reviewed in this article.
2
Principle of DLLME Fig. 1
Dispersive liquid–liquid microextraction is a miniaturized LLE using microliter volumes of extraction solvent, which is based on the equilibrium distribution process of the target analytes between sample solution and extraction solvent. Distribution coefficient (K) is defined as the ratio between the analyte concentration in extraction solvent and sample solution. Dispersive liquid-liquid microextraction is only applicable for the analytes with high or moderate lipophilic property (K > 500) and not fit to those neutral analytes with high hydrophilic property. As for the acidic or alkaline analytes, distribution coefficient could be increased by controlling the pH value of sample solution, making the analytes existing in nonionic state. The enrichment factor and extraction recovery are calculated as follows[11,12]: F = Csed/Co R = (Csed Vsed)/(Co Vaq) where, F, Csed and Co are the enrichment factor, the analyte concentration in the sediment, and the initial concentration of analyte in the aqueous sample, respectively; R, Vsed and Vaq are the extraction recovery, the volume of the sediment phase, and the volume of the aqueous sample, respectively. The extraction steps of DLLME are illustrated in Fig. 1. A certain volume of sample solution is placed in a 10-mL screw cap glass test tube with conic bottom (A), followed by the rapid injection of disperser solvent containing extraction solvent into the sample solution with a syringe or pipette. Then, the mixture was gently shaken; thus, a cloudy solution (water/disperser solvent/extraction solvent) is formed in the test tube (B). After that, the surface area between extraction solvent and aqueous phase (sample) is infinitely large, thereby, transition of analyte from aqueous phase (sample) to extraction phase is fast. Subsequently, equilibrium state is achieved quickly, resulting in a very short extraction time, which is the remarkable advantage of DLLME compared with those of other techniques. Finally, the dispersed fine particles of extraction phase are sedimented in the bottom of conical test tube through centrifugation (C). A certain volume of the sedimented phase is injected into chromatographic system using a microsyringe for further analysis (D).
3
Dispersive liquid-liquid microextraction procedure
Parameters affecting extraction efficiencies of DLLME
The extraction efficiency for the target analyte by DLLME is influenced by many factors, such as the kind of extraction and disperser solvent, and their volume, the extraction time, and salt addition. 3.1
Selection of extraction solvent
The selection of an appropriate extraction solvent is a major parameter for DLLME process. The extraction solvent should satisfy two conditions: one is the higher density of the extraction solvent than that of water, which makes it possible to separate extraction solvent from aqueous phase by centrifugation; the other is the extraction capability of extraction solvent for the compounds of interest, good chromatographic behavior, and low solubility of extraction solvent in water. Halogenated hydrocarbon, such as chlorobenzene, chloroform, carbon tetrachloride, and tetrachloroethylene (tetrachloroethane), are usually selected as extraction solvents because of their high density. 3.2
Selection of disperser solvent
Disperser solvent is soluble in extraction solvent and should be miscible in water, thus enabling the extraction solvent to be dispersed as fine particles in aqueous phase to form a cloudy solution (water/disperser solvent/extraction solvent). In such a case, the surface area between extraction solvent and aqueous phase (sample) can be infinitely large, thus to increase the extraction efficiency. The commonly used disperser solvents include methanol, ethanol, acetonitrile, acetone, and tetrahydrofuran. 3.3
Effect of extraction solvent volume
The extraction solvent volume has great effects on the enrichment factor. With the increase of the extraction solvent volume, the final organic phase obtained by centrifugation is
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
increased, resulting in a decrease of the concentration of the target analyte in organic phase. Although the extraction recovery keeps almost constant, the enrichment factor will be decreased, leading to a decrease of the sensitivity of the determination for the target compounds. Therefore, the optimal extraction solvent volume should ensure both the high enrichment factors and the enough volume for the subsequent determination after centrifugation. In general, 5–100 ȝl of extraction solvent is selected. 3.4
Effect of disperser solvent volume
The disperser solvent volume directly affects the formation of the cloudy solution (water/disperser solvent/extraction solvent), the degree of the dispersion of the extraction solvent in aqueous phase, and subsequently, the extraction efficiency. Usually, 0.5–1.5 ml of disperser solvent is selected. 3.5
Effect of extraction time
In DLLME, extraction time is defined as the interval between injecting the mixture of disperser solvent and extraction solvent and centrifugation. It is revealed that extraction time has little effect on the extraction efficiency of DLLME. The reason for this is that the extraction solvent can be evenly dispersed after the formation of the cloudy solution, the transition of the analyte from aqueous phase (sample) to extraction phase can be very fast, and the equilibrium state can be subsequently achieved very quickly, resulting in a very short extraction time needed for equilibrium. Short extraction time is a remarkable advantage of the DLLME technique. 3.6
Effect of salt addition
The solubilities of the target analyte and organic extraction solvent in aqueous phase are usually decreased with the increase of ionic strength, which is favorable for reaching high recovery. However, at the same time, the obtained volume of organic phase is increased, resulting in a decrease of both the target analyte concentration and the enrichment factor.
4
Application of DLLME
As a novel sample preparation method, DLLME can be coupled with GC, HPLC, and AAS for application. It has been widely applied to the analyses of pesticide residues, heavy metals, and so on. The typical applications are shown in Table 1. 4.1
DLLME-GC
DLLME is very suitable to be coupled with GC since in most cases, the extraction solvent can be directly injected into GC for determination with a microsyringe, without additional
pretreatment. Therefore, DLLME-GC technique has achieved rapid development in a short time. Monitoring pollutants in water is one of the most important tasks for environmental analysis. DLLME-GC is easy to operate and is extremely applicable for water samples. Assadi, et al[11] first developed a novel method for the determination of polycyclic aromatic hydrocarbons (PAHs) in water samples (surface, river, and well waters) with DLLME coupled with gas chromatography-flame ionization detection (DLLME-GC-FID). First, 5.00 ml of double distilled water was placed in a 10-ml screw cap glass test tube with conic bottom and spiked at a level of 2 ȝg l–1 of PAHs. Then, 1.00 ml of acetone (as disperser solvent) containing 8.0 ȝl of C2Cl4 (as extraction solvent) was rapidly injected into a sample solution with a 1.00-ml syringe, and the mixture was gently shaken. A cloudy solution (water/acetone/tetrachloroethylene) was formed in the test tube (the cloudy state was stable for a long time). Then, the mixture was centrifuged for 1.5 min at 6000 rpm. Accordingly, the dispersed fine particles of extraction phase were sedimented in the bottom of the conical test tube, and 2.00 ȝl of the sedimented phase was removed by means of a 2.00-ȝl microsyringe and injected into GC for analysis. Under the optimum conditions, the obtained enrichment factor ranged from 603 to 1113. The linear range was 0.02–200 ȝg l–1, and the detection limit was 0.007–0.030 ȝg l–1 for most of the analytes. The relative standard deviation (RSDs) for 2 ȝg l–1 PAHs in water by virtue of internal standard was in a range of 1.4%–10.2% (n = 5). The recovery of PAHs from surface water at a spiked level of 5.0 ȝg l–1 was 82.0%–111.0%. In a further research[12], they developed another new method for the extraction of organophosphorus pesticides (OPPs) from water samples by DLLME-GC-FPD. Compared with SPME-GC-FPD, SPME-GC-NPD, SDME-GC-MS, and SDME-GC-FPD, DLLME–GC-FPD has the features of simplified device, simplicity of operation, low LOD, wide linear range, high enrichment factor, and very short extraction time of less than 3 min, which is far less than those of other methods (15–60 min). Assadi, et al. also applied DLLME to the analyses of trichloromethane[13], chlorobenzenes[14], polychlorinated biphenyls[15], etc. in environmental samples. Recently, we[16] have developed a novel method for the determination of pyrethroid pesticide residues in water samples by DLLME-GC-ECD. Under the optimum conditions⧎as high as a 708-fold to 1087-fold enrichment factor and the detection limit of 0.04–0.10 ȝg l–1 were achieved. This method has been successfully applied to the determination of pyrethroid pesticide residues in real water samples (tap water, well water, and river water) with recoveries falling in a range from 76.0% to 116.0%. Besides, DLLME has also been applied to the determination of phthalate esters[17], amitriptyline and nortriptyline[18], organophosphorus flame retardants[19], triazine herbicides[20], and amide herbicides[21], etc. in environmental water samples.
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
Table 1 Application of dispersive liquid- liquid microextraction Matrix
Analytical method
Extraction solvent
Disperser solvent
EF
LOD (μg l–1)
Ref.
Water
GC-FID
Tetrachloroethylene
Acetone
603–1113
0.007–0.030
11
Water
GC-FPD
Chlorobenzene
Acetone
789–1070
3–20 ng l–1
12
Drinking water Water
GC-ECD GC-ECD
Carbon disulfide Chlorobenzene
Acetone Acetone
116–355 711–813
0.005–0.040 0.0005–0.05
13 14
Water
GC-ECD
Chlorobenzene
Acetone
383–540
0.001–0.002
15
Water Water
GC-ECD GC-MS
Chlorobenzene Chlorobenzene
Acetone Acetone
708–1087 681–889
16 17
Water
GC-FID
Carbon tetrachloride
Methanol
740–1000
0.10 – 0.04 0.002–0.008 0.005, 0.01 mg l–1
Water
GC-NPD
1,1,1-Trichloroethane
Acetone
190–830
0.01–0.080
19
Water Water
GC-MS GC-MS
Chlorobenzene Carbon tetrachloride
Acetone Acetone
151–722 437–460
0.021–0.12 0.02–0.04
20 21
Water
GC-MS
Chloroform
100
6.0–133 ng l– 1
22
Water
GC-FID
Chloroform
Methanol
225–257
0.02–0.18
23
Water
GC-ECD
Chlorobenzene
Acetone
0.010–2.0
24
Chlorophenols
Water
SPE-GC-ECD
Chlorobenzene
Acetone
0.0005–0.1
25
Selenium Butyl and phenyltin Compounds Anilines Organophosphorus Pesticides Volatile phenols Organosulfur pesticides Captan, Folpet and Captafol Methomyl Phthalate esters Polybrominated diphenyl ethers Clenbuterol Antioxidants
Water
GC-ECD
Chlorobenzene
Ethanol
287–906 4390– 17870 122
0.005
26
Analytes Polycyclic aromatic hydrocarbons Organophosphorus pesticides Trihalomethanes Chlorobenzenes Polychlorinated biphenyls Pyrethroid pesticide Phthalate esters Amitriptyline and nortriptyline organophosphorus flame retardants Triazine herbicide Amide herbicides Synthetic musk Fragrances, phthalate Esters and lindane Polycyclic aromatic Hydrocarbons Chlorophenols
Pyrethroid pesticide Methylparathion and Phoxim Atrazine Aromatic amines gold Selenium Cadmium Cadmium Lead Cobalt and nickel Copper
–1
18
Water
GC-FPD
Carbon tetrachloride
Ethanol
825–1036
0.2–1 ng l
27
Waste water Watermelon, cucumber Red wines Environmental and Beverage samples
GC-MS
Chlorobenzene
Acetone
212–645
28
GC-FPD
Chlorobenzene
Acetonitrile
41–50
29
GC-MS
Carbon tetrachloride
Acetone
0.04–0.09 0.010–0.190 μg kg–1 28–44
GC-FPD
Carbon tetrachloride
Methanol
176–946
0.21–3.05
31
Grape
GC-ECD
Chlorobenzene
Acetone
788–876
Water Water
HPLC-VWD HPLC-VWD
tetrachloroethane Carbon tetrachloride
Methanol Acetonitrile
70.7 44–196
Water
HPLC-VWD
Tetrachloroethane
Acetonitrile
268–305
water Waste water Environmental samples Environmental samples Environmental samples Water Water silicate ore sample Water Water Water Biological and water samples Environmental water and rice samples Water
HPLC HPLC-DAD
Tetrachloroethylene Carbon tetrachloride
Acetone Acetonitrile
175 168–220
HPLC
[C6MIM][PF6]
HPLC
[C6MIM][PF6]
HPLC
6.0–8.0 ȝg/kg 1.0 0.64–8 12.4–55.6 ng l–1 4.9 3–7
30
32 33 34 35 36 37
0.28–0.6
38
0.17, 0.29
39
[C6MIM][PF6]
0.601
40
HPLC
[BMIM][PF6]
0.45–2.6
41
GF AAS
Chlorobenzene
Acetone
388
0.005
42
GF AAS GF AAS GF AAS
Carbon tetrachloride Carbon tetrachloride Carbon tetrachloride
Ethanol Methanol Methanol
70 125 122
0.05 0.6 ng l–1 0.5 ng l–1
43 44 45
GF AAS
Carbon tetrachloride
Ethanol
78
39 ng l–1
46
50
–1
GF AAS
Carbon tetrachloride
Acetone
101, 200
FAAS
Chloroform
Methanol
42
21 ng l , 33 ng l–1 3.0
47 48
(To be continued)
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
Table 1 (Continued) Analytes
Matrix
Lead Lead
Water Water
Cobalt
Water
Analytical method
Extraction solvent
Disperser solvent
EF
LOD (μg l–1)
Ref
FAAS ET AAS Spectrophotometric
Carbon tetrachloride Carbon tetrachloride
Methanol Acetone
450 150
0.5 0.02
49 50
Chloroform
Ethanol
125
0.5
51
0.25, 0.2
52
Palladium and cobalt
Water and synthetic sample
FO-LADS
1,2-Dichlorobenzene
Ethanol
162, 165
Samarium, europium, Gadolinium and Dysprosium
Water
ICP-OES
Chloroform
Methanol
78–103
The use of disperser solvent usually decreases the partition coefficient of analytes into the extractant solvent. Garcia-Jares et al[22] proposed a novel method based on ultrasound-assisted emulsification-microextraction (USAEME) and GC-MS for the analyses of synthetic musk fragrances, phthalate esters, and lindane in water samples, without the addition of an emulsifier. Aliquots of 10 ml of sample were, respectively, placed in 15-ml conical-bottom glass centrifuge tubes. Under the final optimized conditions, 100 ȝl of chloroform containing internal standard substance was added to each tube as extractant solvent. Extractions were performed at 40 kHz of ultrasound frequency and 100 W of power for 10 min so that oil-in-water (O/W) emulsions of chloroform (dispersed phase) in water (continuous phase) were formed. Emulsions were then disrupted by centrifugation at 5000 rpm for 3 min and the organic phases were sedimented at the bottom of each conical tube. The extracts thus obtained were stored at í20 ºC until analysis by GC-MS. An enrichment factor of 100 and a detection limit of 6–133 ng l–1 were finally achieved. Yamini et al[23] added 1.2 ml of methanol (as consolute solvent) containing 60 ȝl of chloroform (as extracting solvent) to a 2.5-ml of sample solution in a 11-ml screw cap test tube with conic bottom. Then, the mixture was gently shaken, and a homogeneous solution was obtained. After 0.6 g of NaCl was added to the solution and shaken, a cloudy mixture was formed in the test tube. The cloudy mixture was centrifuged for 3 min at 3500 rpm. Accordingly, the fine particles of the extraction phase were sedimented at the bottom of the conical test tube, and the sedimented phase was withdrawn and injected into a GC for analysis. This method was applied for the determination of PAHs in waste water samples. Under the optimum conditions, an enrichment factor of 225–257 and a detection limit of 0.02–0.18 ȝg l–1 were achieved. USAEME brings the unmixable aquatic phase and organic phase into a cloudy mixture by ultrasound rather than the addition of a disperser, resulting in a high distribution coefficient of the sample in extraction solvent, and therefore good extraction efficiency. In homogeneous DLLME, extraction solvent enters into aquatic phase to form a single phase (no phase interface exists between aquatic and organic phases). As a result, the contact area between the sample and the extraction solvent is
53
increased to a maximum content, obtaining high extraction efficiency. Therefore, USAEME and homogeneous DLLME provide new approaches for the development of DLLME. For the strong polar and nonvolatile samples, which are unsuitable to be analyzed by GC, derivatization can be used to enhance their volatility and stability so as to make it possible to analyze them by GC. Assadi et al[24] simultaneously used DLLME and derivatization via GC-ECD to determine chlorophenols (CPs) in water samples, which greatly improved the extraction efficiency and sensitivity. In a 10-ml screw cap conical bottomed glass test tube were added 0.50 ml K2CO3 (5%, w/v) and 0.5 ml acetone (disperser solvent) containing 10.0 ȝl chlorobenzene (extraction solvent) and 50 ȝl anhydride acetic (derivatization reagent) to 5.00 ml of an aqueous solution. After shaking, a cloudy mixture was formed. In this step, chlorophenols were derivatized with acetic anhydride and extracted into the fine droplets of chlorobenzene in a few seconds. The determination of CPs in a water sample by DLLME-GC-ECD was compared with those by other methods such as SPE-GC-ECD, SPME-GCMS, and LPME-GC-MS, indicating that DLLME is a fast, reproducible, and simple technique, with low RSDs, wide linear range, and short extraction time (only several seconds). Assadi et al[25] also developed a preconcentration technique combining SPE with DLLME and derivatization for the determination of chlorophenols in water samples via GC-ECD. It provided an enrichment factor of as high as 4390–17870, and the detection limit ranged from 0.0005 to 0.1 ȝg l–1. They also performed DLLME-GC-FPD for the analysis of butyl and phenyltin compounds in water samples after derivatization with sodium tetraethylborate NaBEt4[27]. Chang et al[28] presented a one-step derivatization and extraction technique for the determination of anilines in river water by DLLME-GC-MS and derivatization of them with pentafluorobenzaldehyde (PFBAY). Application of DLLME coupled with derivatization provides a one-step derivatization and extraction technique, greatly simplifying the operation steps and shortening the analysis time. Dispersive liquid-liquid microextraction is more suitable for the treatment of the target compounds with simple matrix, resulting in its wide application in the analysis of water
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
samples. By now, the pretreatment of samples with complex matrix by DLLME is still at the beginning stage of exploration. Zhao et al[29] developed a new method for the determination of OPPs in cucumber and watermelon by DLLME-GC-FPD, with a detection limit ranging from 0.010 to 0.190 ȝg kg–1 for the target pesticides. Fariña et al[30] proposed a new method for the analysis of volatile phenols in the aroma of red wine by combining DLLME with GC-MS. This method is now in commercial use. Xiong et al[31] developed a new method for the analysis of organosulfur pesticides (malathion, chlorpyrifos, buprofezin, triazophos, carbosulfan, and pyridaben) in environmental and beverage samples by GC-FPD via coupling DLLME with HF-LPME. In our previous work[32], a novel method was developed for the determination of chlorothalonil, captan, and folpet in grape samples by DLLME coupled with GC-ECD. Under the optimum conditions, the enrichment factor ranged between 788 and 876, and the detection limit was between 6.0 and 8.0 μg kg–1. Practical samples were successfully analyzed by the proposed method with satisfactory results. With the further development of this novel DLLME technique, the analysis of samples with complex matrix by DLLME will be more and more widely applied. 4.2
DLLME-HPLC
DLLME-GC is unsuitable for the analysis of thermo-labile compounds, surfactants, medicaments, and proteins, which are semivolatile or nonvolatile compounds. However, DLLMEHPLC can be a good alternative to the problems. Up to now, there have been two operation modes for DLLME-HPLC, one is the direct injection analysis for target compounds by HPLC after extraction by DLLME. Wei et al[33] developed a new method for the determination of methomyl in water samples by combining DLLME with HPLC-VWD, with an enrichment factor of 70.7 and a detection limit of 1.0 μg l–1, respectively. Comparison of DLLME with other methods such as SPE, SPME, and LPME for extraction and determination of NMCs from water samples indicates that the extraction time by DLLME is very short (approximately 2 min), the extraction equilibrium is attained very quickly, and it does not require any additional instrument. Therefore, it is very simple, rapid, easy to use, inexpensive, and environmentally benign. Phthalate esters[34] and polybrominated diphenyl ethers[35] were also determined by DLLME-HPLC. The other operation mode is to give some additional treatment of the target compounds after extraction by DLLME before injection into HPLC system for analysis. Melwanki et al[36] demonstrated a new sample pretreatment method by combining DLLME with in-syringe back extraction for the HPLC analysis of clenbuterol in river, lake, and stream water. The analytical compound was first extracted into organic extracting solvent by DLLME and then back extracted into a HPLC-compatible aqueous solution in a
syringe by repeated plunger movement. A 10 μl volume of 1% formic acid was drawn in a 100-μl syringe followed by 20 μl of the sedimented tetrachloroethylene (TCE) phase. The plunger was drawn back to the 100-μl line and mounted on a syringe pump (keeping the needle downwards). The syringe pump was preprogrammed to move the plunger to 60 μl “back” and “forth” at a speed of 200 μl min–1 in a continuous mode. Meanwhile, the organic and the aqueous acidic plugs also moved within the barrel of the syringe. After 5 min, the syringe pump was stopped, the TCE phase was discarded and the acidic aqueous phase was neutralized with 5 μl of 1% triethylamine. The total volume of 15 μl of the aqueous phase was injected into HPLC. As a result, a detection limit of 4.9 μg l–1, an enrichment factor of 175, and a relative recovery of 97% were obtained. Farajzadeh et al[37] presented a new method using DLLME and HPLC-DAD for the extraction and determination of Irganox 1010, Irganox 1076, and Irgafos 168 (antioxidants) in aqueous samples. The sedimented phase was completely transferred to another test tube with conical bottom with a 100-μl HPLC syringe, and after the evaporation of the solvent in a water bath, the residue was dissolved in 50 μl of HPLC grade methanol and injected into the separation system. Room temperature ionic liquids (ILs) are a class of liquids entirely comprised of ions, which usually exist as liquids well below room temperature. Typical formulations of ILs rely mostly on quaternary nitrogen cations such as alkylammonium, dialkylimidazolium, and alkylpyridinium, and anions such as Cl–, AlCl4–, PF6–, BF4-, N(CF3SO2)2–, CF3CO2– and CF3SO3–. Ionic liquids have attracted an increasing interest in the chemistry community as green alternatives to classical environmentally damaging media of volatile organic compounds (VOCs) because of their remarkable properties, such as a negligibly small vapor pressure, incombustibility, and high thermal stability. Therefore, it is also called a “Green Solvent”. Zhou et al[38] reported a novel, green, and environmental benign sample enrichment method termed temperature-controlled IL dispersive liquid phase microextraction. An IL of 1-hexyl-3methylimidazolium hexafluorophosphate [C6MIM][PF6] was used as the extraction solvent and pyrethroid pesticides as the model compounds. For this new extraction procedure, 45 μl of [C6MIM][PF6] was added into a 10-ml glass graduated conical tube, and ultra-pure water was added into it until the 10-ml mark. This solution was spiked at a concentration of 20 μg l–1 for the five target analytes. Then, the conical tubes were heated in a water bath with the temperature controlled at 70 ºC. The IL was dissolved completely and could mix with the solution entirely and the analytes migrated into the IL phase. The tube was thereafter cooled with ice water and the solution became turbid. Then, the solution was centrifuged for 20 min. The upper aqueous phase was removed with a syringe, the residue was dissolved in 200 μl of mobile phase, and 20 μL of
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
the solution was injected into the HPLC system for analysis. Finally, the proposed method showed a good linear range of 1.5–100 μg l–1, a detection limit of 0.28–0.6 μg l–1, and a spiked recovery in a range of 76.7%–135.6%. The researchers also used the same method for the determination of organophosphorus pesticides[39] and atrazine[40] in environmental samples. Fan et al[41] demonstrated a new microextraction method termed IL based DLLME (IL-DLLME) via HPLC analysis of 2-methylaniline, 4-chloroaniline, 1-naphthylamine, and 4-aminobiphenyl in aqueous matrices with 1-butyl-3- methylimidazolium hexafluorophosphate ([Bmim][PF6]) as extraction solvent. Ionic liquids, as a kind of green solvents, can be specially designed for some use of special samples based on their own properties so as to increase the extraction efficiency of samples with different lipophilic property. Therefore, ILs as the extraction solvents for DLLME not only can enlarge the application range of DLLME but also provide a new approach for its development in future researches. 4.3
DLLME-AAS
DLLME can be coupled with atomic absorption spectrometry (AAS), which has been applied to the trace analysis of heavy metals in environmental water samples. There are two ways for the application of this method. One is to add chelating agent to the sample solution first, followed by the addition of appropriate extraction solvent and disperser solvent; the other is to add chelating agent, extraction solvent, and disperser solvent to the sample solution simultaneously. After shaking, a cloudy mixture is formed, and the metal ions react with chelating agent and are extracted into the fine droplets of extraction solvent. Then, the mixture was centrifugated to obtain extraction solvent for injection analysis. When DLLME is coupled with graphite furnace atomic absorption spectrometry (GFAAS), direct injection analysis can be performed just after the removal of the extraction solvent. Shamsipur et al[42] developed a new method based on highly efficient separation and preconcentration of gold by DLLME followed by its determination with GFAAS. It was successfully applied to the extraction and determination of gold in tap water and silicate ore samples with victoria blue B and 0.04% Pd(NO3)2 as chelating agent and chemical modifier, respectively. The analytical curve was linear in a concentration range of 0.03–0.5 μg l–1, and the detection limit was 0.005 μg l–1. Bidari[43] and Jahromi[44] reported a simple and powerful microextraction technique for the determination of selenium and cadmium in water samples with ammonium pyrrolidine dithiocarbamate (APDC) as chelating agent. The detection limit and enrichment factor of selenium were 0.05 ȝg l–1 and 70, and those of cadmium were 0.6 ng l–1 and 125, respectively. Moghimi et al[45] used the same method to
determine cadmium in natural waters using N,N’-bis(salicylidene) ethylenediamine instead of APDC as chelating agent. Similar results of an enrichment factor of 122 and a detection limit of 0.5 ng l–1 were obtained. Liang et al[46] successfully applied DLLME- GFAAS to determine the trace amounts of lead in human urine and water samples using 1-phenyl-3-methyl-4- benzoyl-5-pyrazolone (PMBP) as a chelating agent. The detection limit for lead was 39 ng l–1, and the enrichment factor reached 78. Jiang et al[47] developed a new method by combining DLPME with GFAAS for the determination of trace Co and Ni in environmental and food samples using 1-(2-pyridylazo)-2-naphthol (PAN) as chelating reagent. The enrichment factors were 101 and 200 and the detection limits were 21 and 33 ng l–1 for Co and Ni, respectively. When DLLME is coupled with flame atomic absorption spectrometry (FAAS), direct injection analysis can not be performed since the volume of extraction solvent is too little to be determined. Farajzadeh et al[48] proposed a new method for the quantitation of Cu2+ ions in different water samples by DLLME-FAAS using 8-hydroxy quinoline as a chelating agent. After the volume of the sedimented phase (chloroform) was removed with a 250-μl HPLC syringe, the sedimented phase was quantitatively transferred to another test tube and allowed to evaporate at room temperature. Finally, the residue was dissolved into 0.5 ml of 0.1 M nitric acid and the copper concentration was determined by flame atomic absorption spectrometry. The calibration graph was linear over a range of 50–2000 μg l–1, the detection limit was 3 μg l–1, and the enrichment factor was about 42. Naseri et al[49] coupled a specially designed microsample introduction system to FAAS through which a volume of 20 ȝl of the final organic extract was introduced into the flame in all the FAAS measurements. Dispersive liquid-liquid microextraction was combined with FAAS for the determination of lead in water samples with diethyldithiophosphoric acid (DDTP) as chelating agent. The obtained enrichment factor was 450, the calibration graph was linear in a range of 1–70 μg l–1 with a detection limit of 0.5 μg l–1, and the relative recoveries of lead in tap, well, river, and seawater samples ranged from 93.8% to 106.2%. They also used the same method for the detection of ultratrace amounts of lead in water samples by means of electrothermal atomic absorption spectrometry (ETAAS) instead of FAAS[50]. The obtained enrichment factor was 150, and the calibration graph was linear in a range of 0.05–1 ȝg l–1 with a detection limit of 0.02 ȝg l–1. Dispersive liquid-liquid microextractionatomic absorption spectrometry not only enlarges the application range of DLLME but also provides a new analytical method for the determination of metal ions. 4.4
DLLME coupled with other instruments
Dispersive liquid-liquid microextraction can also be
ZANG Xiao-Huan et al. / Chinese Journal of Analytical Chemistry, 2009, 37(2): 161–168
coupled with spectrophotometric instruments for the quantitative determination of metal ions. Gharehbaghi et al[51] used DLLME as a prior step for the determination of preconcentrate trace levels of cobalt in tap and river water samples by spectrophotometric method with PAN as chelating reagent. The enrichment factor and the detection limit were 150 and 0.5 ȝg l–1, respectively. Shokoufia et al[52] used fiber optic-linear array detection spectrophotometry (FO-LADS) in combination with DLLME for the simultaneous preconcentration and determination of palladium and cobalt in water and synthetic samples with PAN as chelating reagent. The calibration graphs were linear in a range of 2–100 ȝg l–1 and 1–70 ȝg l–1 with a detection limit of 0.25 ȝg l–1 and 0.2 ȝg l–1 for palladium and cobalt, respectively. The enhancement factors of 162 and 165 were obtained for palladium and cobalt. Shemirani et al[53] determined samarium, europium, gadolinium, and dysprosium by combining DLLME with inductively coupled plasma optical emission spectrometry (ICP-OES) using PAN as chelating reagent. The preconcentration factors of 80, 100, 103 and 78 were obtained for Sm, Eu, Gd and Dy, respectively.
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Dispersive liquid-liquid microextraction is a relatively new sample pretreatment method, combining sampling, extraction, and concentration all together. Compared with traditional extraction methods, DLLME has the advantages of simplicity of operation, rapidity, low-cost, high-recovery, high enrichment factor, and environmental benignity. It shows increased wide practical prospects in the field of trace analysis. It is predicted that DLLME will be developed rapidly in the following aspects: (1) to further develop applications in the analysis of samples with complex matrix. Since most of the target compounds detected by DLLME till now are relatively of simple matrix, it is important to make this method more applicable to samples with complex matrix; (2) to extend the selection range of extraction solvents for DLLME. At present, most of the reported extraction solvents are halogenated hydrocarbons. More extensive range of extraction solvents must lead to the more extensive range of applicable substrates preconcentrated by DLLME correspondingly, so as to make it have more practical prospects; (3) to investigate new techniques by combining DLLME with other more different instruments.
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