Journal of Chromatographic Science, 2016, Vol. 54, No. 6, 1061–1067
doi: 10.1093/chromsci/bmw020
Advance Access Publication Date: 3 March 2016
Article
Article
Homogeneous Liquid–Liquid Microextraction for
Determination of Organophosphorus Pesticides
in Environmental Water Samples Prior to Gas
Chromatography-Flame Photometric Detection
Sana Berijani1,*, Mirhanif Sadigh2, and Elham Pournamdari3
1
Department of Applied Chemistry, Faculty of Science, Islamic Azad University, South Tehran Branch, Tehran, Iran,
Department of Civil Engineering, University of Science and Culture, Tehran, Iran, and 3Department of Chemistry,
College of Science, Islamshahr Branch, Islamic Azad University, Islamshahr, Iran
2
*Author to whom correspondence should be addressed. Email: [email protected]
Received 14 July 2015; Revised 31 July 2015
Abstract
In this study, homogeneous liquid–liquid microextraction (HLLME) was developed for preconcentration
and extraction of 15 organophosphorus pesticides (OPPs) from water samples coupling with
gas chromatography followed by a flame photometric detector (HLLME-GC-FPD). In this method,
OPPs were extracted by the homogeneous phase in a ternary solvent system (water/acetic acid/
chloroform). The homogeneous solution was excluded by the addition of sodium hydroxide as a
phase separator reagent and a cloudy solution was formed. After centrifugation (3 min at 5,000 rpm),
the fine particles of extraction solvent (chloroform) were sedimented at the bottom of the conical test
tube (10.0 ± 0.5 µL). Furthermore, 0.5 µL of the sedimented phase was injected into the GC for separation and determination of OPPs. Optimal results were obtained under the following conditions: volume
of the extracting solvent (chloroform), 53 µL; volume of the consolute solvent (acetic acid), 0.76 mL and
concentration of sodium hydroxide, 40% (w/v). Under the optimum conditions, the enrichment factors
of (260–665), the extraction percent of 75.8–104%, the dynamic linear range of 0.03–300 µg L−1 and the
limits of detection of 0.004–0.03 µg L−1 were obtained for the OPPs. This method was successfully
applied for the extraction and determination of the OPPs in environmental water samples.
Introduction
Organophosphorus pesticides (OPPs) represent one of the most important classes of cholinesterase inhibitors (1). They are very toxic
when absorbed by human organisms because of acetyl-cholinesterase
deactivation. In addition, OPPs are known to reduce the activity of
neurotransmitters and hence to cause irreversible effects on the
nervous system (2). They are also important causes of morbidity and
mortality following intentional self-poisoning or in the case of occupational or environmental exposure (3). Determination of OPPs in water is
usually carried out by methods involving gas chromatography (GC), gas
chromatography–mass spectrometry (GC–MS) or high-performance
liquid chromatography (HPLC) (4–9). Monitoring environmental pollutants at an ultra-trace level needs an effective sample preconcentration
step (10) and due to the low OPP concentration and complex matrices,
the sample is prepared by liquid–liquid extraction (LLE) and also by
solid-phase extraction (SPE) (11, 12). SPE is a solvent-free process, developed by Anrthur and Pawliszyn (13), that features simultaneous extraction and preconcentration of analytes directly from an aqueous and
gas sample or from the headspace of aqueous and solid samples (14, 15).
This technique is fast, portable, easy to use and is applied for determination of OPPs in water samples (6, 16, 17). Liquid-phase microextraction
(LPME) and also single-drop microextraction (SDME) were reported in
1997 (18, 19). SDME was used for determination of OPPs in water samples (20, 21). Cloud point extraction (CPE), a kind of LLE technique,
has been used in different fields of analytical chemistry, mainly those
focusing on separation and preconcentration based on cloud point
© The Author 2016. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
1061
1062
procedures (22–24). This technique has also been used for extraction of
OPPs (25, 26). Dispersive liquid–liquid microextraction (DLLME) was
developed and reported as a simple and rapid technique for determination of different compounds in water and was also performed for determination of OPPs in water (27–31). Ultrasound-assisted emulsification
microextraction is another sample pretreatment method, which has been
introduced by Regueiro in 2008, and has been applied for determination
of different pollutants (32–35). The homogeneous liquid–liquid microextraction (HLLME) method, which extracts the desired analyte existing
in the homogeneous aqueous solution into the water-immiscible sediment phase, is based on the phenomenon of phase separation (36). In
this method, the initial condition is a homogeneous solution and there
is no interface between the water phase and the extraction solvent phase.
Therefore, it has the advantage of extremely fast extraction speed due to
the absence of obstacles from the surface contact between the aqueous
phase and the organic phase during the extraction procedure. This method was mainly studied as a high-powered preconcentration method
for the separation of the desired component or instrumental analysis
(37–41). The ternary component solvent system and the perfluorinated
surfactant system are the two usual modes of homogeneous LLE (42).
The goal of this study was to assess the technique suitability for the detection of 15 OPPs in the water samples. The analytes were monitored
by gas chromatography combined with flame photometric detector
(GC-FPD). Finally, this recommended method was employed to investigate the levels of the target species in environmental water samples.
Experimental
Instrumentation and reagents
All OPPs (dichlorovos, mevinphose, phorate, diazinon, disolfotane,
methylparathion, sumithion, cloropyrifos, malathion, fenthion, profenphose, ethion, phosalone, azinphose-methyl, co-ral) were purchased from Poly Science Company. Chloroform, acetic acid,
triphenyl phosphate (as internal standard) and sodium hydroxide
were obtained from Merck (Germany). Bidistilled water was used
for preparation of aqueous solution. The proper amount of each
OPP was dissolved in 10.0 mL methanol to obtain a standard solution
with a concentration of 1,000 mg L−1. A fresh 2.00 µg mL−1 standard
solution containing 15 OPPs was prepared in methanol every week
and stored at 4°C. Farm water samples, used for development of the
method, were collected in glass bottles from Tehran (Iran), and stored
at 4°C. Water samples were filtrated through a 0.45-µm PTFE syringe
filter (Osmonics, Inc).
A gas chromatograph (Shimadzu GC 2010) with a split/splitless injector system, and a flame photometric detector was used for separation and determination of OPPs. Ultrapure helium (99.9999%; Air
Products, UK) passes through a molecular sieve trap and oxygen
trap (Crs, USA) was used as the carrier gas at a constant linear velocity
of 35 cm s−1. The injection port was held at 250°C and used in the
splitless mode with a splitless time of 0.5 min. Separation was carried
out on a ZB-35, 30 m × 0.25 mm capillary column with a 0.15-µm
stationary film thickness, 65% methyl–35% diphenylpolysiloxane copolymer column (Phenomenex, USA). The oven temperature was programed as follows: initial 100°C, from 100°C (held 2 min) to 150°C at
a rate of 25°C/min; from 150 to 175°C at a rate of 5°C/min; from
175 to 195°C at a rate of 2°C/min and from 195 to 275°C at a rate
of 10°C/min (held 5 min). The total time for one GC run was
32 min. The FPD temperature was maintained at 300°C, and hydrogen gas was generated with a hydrogen generator (OPGU–2200s,
Shimadzu) for FPD at a flow rate of 80 mL min−1. The flow of air
Berijani et al.
(99.999, Air Products, UK) for FPD was 120 mL min−1 and the centurion scientific Ltd (model 2010D, UK) was used for centrifuging.
Methods
About 7 mL of bidistilled water was placed in a 10-mL screw cap glass
test tube with conical bottom and spiked at a level of 2 µg L−1 of OPPs
and triphenyl phosphate (as internal standard). Then, 0.76 mL of acetic acid (as consolute solvent) and 53 µL chloroform (as extracting solvent) were injected into the sample solution by using a 1-mL syringe
(gastight, Hamilton). A homogeneous solution (water/acetic acid/
chloroform) was prepared under this condition. Then, to separate
the analytes from this homogeneous solution, 1 mL of sodium hydroxide 40 (w/v %) as phase separator reagent was added to the solution. In this step, a cloudy solution, which contains the OPPs, was
produced in the test tube. The cloudy mixture was centrifuged for
3 min at 5,000 rpm. After this process, the fine particles of chloroform
were sedimented at the bottom of the conical test tube (10 ± 0.5).
Then, 0.5 µL of the sediment phase were removed by a 1-µL micro syringe (zero dead volume, cone tip needle, SGE) and injected into GC.
Figure 1 shows the scheme of the performed procedure.
Results
In ternary solvent system (water/acetic acid/chloroform), acetic acid
molecules can solvate chloroform molecules and a homogeneous solution will be formed. When the solvation effect of the acetic acid is excluded by addition of sodium hydroxide, acetic acid converts to
acetate ions due to the acid–base reaction of acetic acid and sodium
hydroxide, so the water-immiscible sedimented phase, consisting of
hydrophobic solvent, is separated from the aqueous solution. To obtain a good recovery, the effect of some parameters was examined and
optimum conditions were selected. To study the explained parameters,
extraction recovery and enrichment factor have been used (Equations
(1) and (2)).
R% ¼
Csed × Vsed
× 100;
C0 × Vaq
ð1Þ
Csed
;
C0
ð2Þ
EF ¼
where R% is the extraction recovery, EF defines as enrichment factor,
Csed and C0 refer to the concentration of analytes in the sediment
phase and the initial aqueous phase, Vsed and Vaq are the volume of
the sediment phase and the aqueous phase, respectively. All the mentioned parameters are known except Csed. Calculation of Csed was
done by direct injection of OPPs standard solutions with different concentrations in the range of 0.5–2.5 mg L−1.
Discussion
Selection of volume of the extracting solvent
In this ternary solvent system, chloroform acts as an extracting solvent.
To examine the effect of the volume of chloroform, solutions containing different volumes of chloroform were examined with the same
HLLME procedure. Hence, 0.76 mL acetic acid containing 45, 49,
53, 57 and 60 µL chloroform were tested. Figures 2, 3 and 4 show
the curves of volume of sediment phase, extraction recovery and enrichment factor versus the volume of chloroform. As shown in Figure 2, by increasing the volume of chloroform from 45 to 60 µL, the
volume of the sediment phase increases from 4 to 18 µL. Figure 3 indicates that by increasing the volume of chloroform, the extraction recovery increases up to 53 and it will remain constant from 53 to 60 µL
Determination of Organophosphorus Pesticides in Environmental Water Samples
1063
Figure 1. Scheme of HLLME in this research.
Figure 2. Effect of the volume of chloroform on the volume of sediment phase.
Figure 4. Effect of the volume of chloroform on the enrichment factor of OPPs.
of chloroform. Figure 4 shows that increasing the volume of the chloroform and sediment phase leads to decreasing of enrichment factors.
It should be noticed that by using the volumes <40 µL of chloroform,
no sediment phase is formed and the volumes >60 µL cause no homogeneous solution. Therefore, 53 µL was selected as the optimum volume of chloroform.
Selection of phase separator reagent concentration
Figure 3. Effect of the volume of chloroform on the recovery of OPPs.
In HLLME after formation of homogeneous solution according to the
components of the solution, a reagent is spiked in order to separate the
organic and aqueous solutions. In the selected system, homogeneous
solution of chloroform in water in the presence of acetic acid was created in which the chloroform molecules are solvated by acetic acid
molecules and dissolved in water. Sodium hydroxide acts as a reagent,
which excludes the homogeneous solution and makes two phases.
1064
Sodium hydroxide enters an acid–base reaction with acetic acid, and
sodium acetate is formed in the solution. In this way, chloroform
which contains the analytes separates from aqueous solution and
the homogeneous solution converts to a cloudy solution. After centrifugation, a sediment phase was formed and 0.5 µL of this phase was
Berijani et al.
used for analysis. Different concentrations of sodium hydroxide 25,
30, 35 and 40 (% w/v) were prepared and tested in the experiment.
The addition of ionic strength promotes the transport of analytes
into the organic phase, so the extraction efficiency increases while increasing the NaOH up to 40% (w/v). However, an increase in the sediment phase volume and dilution effect of chloroform leads to a
decrease in the preconcentration of OPPs (Figs 5–7). Finally, sodium
hydroxide 40 (%w/v) was selected for the analysis. It is recognizable
that the use of higher concentration of sodium hydroxide was not possible because of the heat produced in acid–base reaction that may
evaporate the chloroform, and also the alkaline medium may destroy
some of the OPPs.
Effect of extraction time
Extraction time is one of the major parameters affecting the extraction efficiency. In HLLME, the profile of extraction time was studied
by monitoring the variation of the relative peak area of the analyte
with the time of cloudy state before starting centrifugation. The
effect of time was examined in the range of 0–60 min under optimum extraction conditions (Figure 8). According to the curves,
Figure 5. Effect of sodium hydroxide concentration on the volume of sediment
phase.
Figure 6. Effect of sodium hydroxide concentration on the recovery of OPPs.
Figure 8. Effect of extraction time on the peak area of OPPs.
Figure 7. Effect of sodium hydroxide concentration on the enrichment factor of
OPPs.
1065
Determination of Organophosphorus Pesticides in Environmental Water Samples
Table I. Quantitative results of HLLME-GC-FPD of OPPs from the water sample
OPPs
RSD %a (n = 5)
RSD %b (n = 5)
EF
LDR (µg L−1)
r 2c
r 2d
LOD (µg L−1)
Dichlorovos
Mevinphose
Phorate
Diazinon
Disolfotane
Methylparathion
Sumithion
Chloropyrifose
Malathion
Fenthion
Profenphose
Ethion
Phosalone
Azinphose methyl
Co-Ral
1.6
3.0
4.0
1.9
5.4
5.7
3.6
1.8
6.3
3.0
3.4
4.4
4.2
8.5
1.1
2.4
6.3
7.1
5.2
8.8
8.2
6.6
5.2
7.4
5.6
4.6
5.6
3.9
10.8
3.8
317
274
327
325
260
665
369
344
455
324
658
315
478
562
394
0.05–300
0.05–300
0.02–300
0.02–300
0.03–300
0.03–300
0.03–300
0.03–300
0.03–300
0.03–300
0.03–300
0.02–300
0.03–300
0.05–300
0.1–300
0.998
0.998
0.999
0. 999
0.998
0.998
0.998
0. 991
0.998
0.999
0.999
0.998
0.999
0. 996
0.998
0.996
0.994
0.994
0.998
0.996
0.995
0.997
0.994
0.993
0.993
0.992
0.998
0.993
0.994
0.993
0.02
0.02
0.005
0.005
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.005
0.01
0.02
0.04
EF, enrichment factor; LOD, limit of detection for a S/N = 3.
a
RSD% by using the internal standard at a concentration of 2 µg L−1 of each OPP.
b
RSD% without using the internal standard.
c
Calculated by using the internal standard.
d
Without using the internal standard.
Table II. Comparison of HLLME with DLLME, SDME and SPME for determination of OPPs in water
Methods
LOD (µg L−1)
LDR (µg L−1)
RSD (%)
Sample volume (mL)
Extraction time (min)
References
SPME-GC-FPD
SPME-GC-NPD
SDME-GC-MS
SDME-GC-FPD
DLLME-GC-FPD
HLLME-GC-FPD
0.03–0.4
0.02–0.04
0.01–0.07
0.002–0.02
0.003–0.02
0.004–0.03
1–50
0.1–10
0.5–100
0.01–100
0.01–100
0.03–300
5–8
7–17
8.5–15
7.9–13
4.6–6.5
1.1–7.5
3
3
5
5
5
7
30
60
15
40
3
5
7
9
20
8
29
Represented method
Table III. Results of real sample analysis
OPPs
Concentration in real sample (SD)a
(µg L−1)
Added concentration
(µg L−1)
Founded concentration (SD)
(µg L−1)
Relative recovery
(%)
Dichlorovos
Mevinphose
Phorate
Diazinon
Disolfotane
Methylparathion
Sumithion
Chloropyrifose
Malathion
Fenthion
Profenphose
Ethion
Phosalone
Azinphose-methyl
Co-Ral
ndb
nd
nd
0.113 ± 0.005
nd
nd
nd
nd
nd
nd
nd
0.042 ± 0.007
nd
nd
nd
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.50
0.379 ± 0.010
0.503 ± 0.078
0.486 ± 0.002
0.563 ± 0.018
0.519 ± 0.003
0.465 ± 0.013
0.470 ± 0.009
0.524 ± 0.011
0.493 ± 0.002
0.490 ± 0.011
0.388 ± 0.009
0.563 ± 0.004
0.500 ± 0.045
0.515 ± 0.032
0.516 ± 0.046
75.8 ± 2.0
100.6 ± 15.6
97.2 ± 0.4
90.0 ± 2.8
103.8 ± 0.6
93.0 ± 2.6
94.0 ± 1.8
104.0 ± 2.18
98.6 ± 0.4
98.0 ± 2.2
77.6 ± 1.8
104.2 ± 0.7
100.0 ± 9.0
103.0 ± 6.4
103.2 ± 9.2
SD, standard deviation; nd, not detected.
time has no effect on extraction efficiency. It is revealed that after
formation of cloudy solution, the surface area between the extraction solvent and the aqueous phase is infinitely large. Thereby,
transition of analytes from the aqueous phase (sample) to the
extraction solvent is fast. Subsequently, equilibrium state is achieved
quickly because of the short time of the extraction. In this extraction
method, the only time-consuming step is centrifugation, which is
∼3 min.
1066
Berijani et al.
Figure 9. Chromatogram of farm water (І) and spiked farm water at a concentration of 0.5 µg L−1 of each OPP (B) obtained using HLLME combined with GC-FPD. Peak
identification: (1) dichlorovos, (2) mevinphose, (3) phorate, (4) diazinon, (5) disolfotane, (6) methylparathion, (7) sumithion, (8) chloropyrifose, (9) malathion, (10)
fenthion, (11) profenphose, (12) ethion, (13) phosalone, (14) azinphose-methyl and (15) Co-Ral. (IS): Triphenyl phosphate.
Quantitative analysis
The calibration curves shown in Table I were obtained under
optimized conditions. Linearity was observed over the range of
0.03–300 µg L−1 (four order magnitude) for most of analytes. The coefficient of correlation (r 2) ranged from 0.991 to 0.998 with internal
standard and from 0.990 to 0.998 without internal standard. The enrichment factors of this method were from 260 to 665. The reproducibility in peak responses was investigated on five replicate experiments
under the optimized conditions. The relative standard deviations (RSD
%) of OPPs were from 1.1 to 8.5% with an internal standard and 2.4
to 13.8% without an internal standard. The limit of detection (LOD),
based on a signal-to-noise ratio (S/N) of 3, ranged from 0.0075 to
0.04 µg L−1. The represented method is compared with other separation and determination techniques (Table II).
Environmental water analysis
To evaluate the accuracy and also applicability of the proposed method (HLLME), for real water samples, the extraction and determination of the OPPs in environmental water samples were performed.
Two different farm waters were collected from Tehran (Iran) and
were analyzed by the HLLME method combined with GC-FPD
(Table III). Water samples were filtered and restored in a freezer for
being prepared for analysis. In the farm water (І), diazinon and ethion
were detected and they were confirmed by spiking OPPs into the farm
water. Figure 8 shows the chromatograms obtained for farm water (І).
The concentration of diazinon and ethion in the farm water
were determined to be 0.113 ± 0.005 and 0.042 ± 0.007 µg L−1, respectively. The relative recoveries for this real sample were 75–104. In the
farm water (ІІ), the results show the presence of mevinphose
(0.083 ± 0.004), diazinon (2.28 ± 0.03) and ethion (0.0050 ± 0.001).
This sample is spiked by OPPs, and the relative recoveries were in
the range of 96–125. These results demonstrate that the farm water
matrices, in our present context, had little effect on HLLME (Figure 9).
Comparison with other similar methods
Table II indicates the LOD, linear dynamic range (LDR), RSD, extraction time and sample volume in the SPME, SDME, DLLME and
HLLME (represented method) for extraction and determination of
OPPs from the water sample. The comparison of the results shows
that the extraction time in HLLME is very short, ∼5 min, and also
extraction equilibrium is achieved very quickly. Otherwise, the extraction time for SPME and SDME ranges from 15 to 60 min. The RSDs
are low (1.1–7.5%) for HLLME, which are probably because of quick
achievement of equilibrium (Figure 8). In comparison with other
extraction methods, HLLME has low LODs (0.004–0.03 µg L−1)
and wide linear range (0.03–300 µg L−1) and can be applied for
determination of OPPs successfully with satisfactory results as other
extraction and preconcentration techniques.
Conclusion
This article has outlined the successful development and application
of a method based on the HLLME technique combined with the
GC-FPD for the analysis of OPPs in water samples. The designed
method is concluded to be precise, reproducible and linear over a
broad range with sufficient selectivity. This analytical technique offers
numerous advantages such as simplicity, low cost, ease of operation
and high enrichment factors. In addition, the technique requires
only a small volume of organic solvent, being therefore an environmentally friendly approach. The results show that performance of
this procedure in the extraction of OPPs from different water samples
with various matrices was excellent. Subsequently, this method can be
used routinely for screening purposes.
References
1. Buonasera, K., D’Orazio, G., Fanali, S., Dugo, P., Mondello, L.; Separation
of organophosphorus pesticides by using nano-liquid chromatography;
Journal of Chromatography A, (2009); 1216: 3970–3976.
2. Jia, G., Lv, C., Zhu, W., Qiu, J., Wang, X., Zhou, Z.; Applicability of cloud
point extraction coupled with microwave assisted back-extraction to the
determination of organophosphorus pesticides in human urine by gas chromatography with flame photometry detection; Journal of Hazardous
Materials, (2008); 159: 300–305.
3. Salm, P., Taylor, P.J., Roberts, D., Silvia, J.; Liquid chromatography–
tandem mass spectrometry method for the simultaneous quantitative
determination of the organophosphorus pesticides dimethoate, fenthion,
diazinon and, chlorpyrifose in human blood; Journal of Chromatography
B, (2009); 877: 568–574.
Determination of Organophosphorus Pesticides in Environmental Water Samples
4. Musshoff, F., Junker, H., Madea, B.; Simple determination of 22
organophosphorous pesticides in human blood using headspace solid-phase
microextraction and gas chromatography with mass spectrometric detection; Journal of Chromatographic Science, (2002); 40: 29–39.
5. Ito, M.; Organophosphorous pesticides: organic and biological chemistry.
CRC Press, Inc., Cleveland, OH, (1974), 386 pp, Journal of Chromatographic Science, (1975); 13(5): 20A.
6. Eisert, R., Levsen, K., Wunsch, G.; Element-selective detection of pesticides
by gas chromatography–atomic emission detection and solid-phase extraction; Journal of Chromatography A, (1994); 683: 175–183.
7. Su, P.G., Huang, S.D.; Determination of organophosphorus pesticides in
water by solid-phase microextraction; Talanta, (1999); 49, 393–402.
8. Camel, V., Caude, M., Tambuté, A.; SFE of an organophosphorous compound from soils with capillary GC analysis; Journal of Chromatographic
Science, (1995); 33: 123–132.
9. Beltran, J., Lopez, F.J., Cepria, O., Hernandez, F.; Solid-phase microextraction for quantitative analysis of organophosphorus pesticides in environmental water samples; Journal of Chromatography A, (1998); 808:
257–263.
10. Li, Z., Chang, X., Hu, Z., Huang, X., Zou, X., Wu, Q., et al. Zinconmodified activated carbon for solid-phase extraction and preconcentration
of trace lead and chromium from environmental samples; Journal of
Hazardous Materials, (2009); 166: 133–137.
11. Eisert, R., Levesen, K.; Determination of pesticides in aqueous samples
by solid-phase extraction in-line coupled to gas chromatography–mass
spectrometry; Journal of American Society Mass Spectrometry, (1995); 6:
1119–1130.
12. Eisert, R., Levesen, K.; Determination of organophosphorus, triazine
and 2,6-dinitroaniline pesticides in aqueous samples via solid-phase extraction (SPME) and gas chromatography with nitrogen-phosphorus detection;;
Fresenius’ Journal of Analytical Chemistry, (1995); 351: 555–562.
13. Arthur, C.L., Pawliszyn, J.; Solid phase extraction with thermal desorption
using fused silica optical fibers; Analytical Chemistry, (1990); 62:
2145–2148.
14. Djozan, D.J., Assadi, Y.; Modified pencil lead as a new fiber for solid-phase
extraction; Chromatographia, (2004); 60: 313–317.
15. Djozan, D.J., Assadi, Y., Karim-Nezhad, G.; Modified copper wire as
solid-phase extraction fiber, selective extraction of some amines;
Chromatographia, (2002); 56: 611–616.
16. Magdic, S., Bayd-Boland, A., Jimo, K., Pawliszyn, J.; Analysis of organophosphorus insecticides from environmental samples using solid-phase
extraction; Journal of Chromatography A, (1996); 736: 219–228.
17. Choudhury, T.K., Gerhordt, O., Mawhinney, T.P.; Solid-phase extraction
of nitrogen- and phosphorus-containing pesticides from water and gas chromatographic analysis; Environmental Science and Technology, (1996); 30:
3259–3265.
18. He, Y., Lee, H.K.; Liquid-phase extraction in a single drop of organic solvent by using a conventional microsyringe; Analytical Chemistry, (1997);
69: 4634–4640.
19. Wang, Y., Kwok, Y.C., He, Y., Lee, H.K.; Application of dynamic
liquid-phase extraction to the analysis of chlorobenzenes in water by using
conventional microsyringe; Analytical Chemistry, (1998); 70: 4610–4614.
20. Ahmadi, F., Assadi, Y., Milani Hosseini, M.R., Rezaee, M.; Determination of
organophosphorus pesticides in water samples by single drop extraction and gas
chromatography–flame photometric detector; Journal of Chromatography A,
(2006); 1101: 307–312.
21. Lambropoulon, D.A., Psillakis, E., Albanis, T.A., Kalogerakis, N.; Singledrop extraction for the analysis of organophosphorous insecticides in
water; Analytica Chimica Acta, (2004); 516: 205–211.
22. Filik, H., Cengel, T., Apak, R.; Selective cloud point extraction and graphite
furnace atomic absorption spectrometric determination of molybdenum (VI)
ion in seawater samples; Journal of Hazardous Materials, (2009); 169: 766.
23. Stalikas, C.D.; Micelle-mediated extraction as a tool for separation and preconcentration in metal analysis; Trends in Analytical Chemistry, (2002); 21:
343–355.
24. Paleologos, E.K., Giokas, D.L., Karayannis, M.I.; Micelle-mediated separation and cloud-point extraction; Trends in Analytical Chemistry, (2005);
24: 426–436.
1067
25. Sanz, C.P., Halko, R., Ferrera, Z.S.; Micellar extraction of organophosphorus pesticides and their determination by liquid chromatography; Analytica
Chimica Acta, (2004); 524: 265–270.
26. Carabias-Martinez, R., Rodriguez-Gonzalo, E., Moreno-Cordero, B.,
Perez-Pavon, J.L., Garcia-Pinto, C.C., Fernandez-Laespada, E.; Surfactant
cloud point extraction and preconcentration of organic compounds prior to
chromatography and capillary electrophoresis; Journal of Chromatography
A, (2000); 902: 251–265.
27. Rezaee, M., Assadi, Y., Milani Hosseini, M.R., Aghaee, E., Ahmadi, F.,
Berijani, S.; Determination of organic compounds in water using dispersive
liquid liquid microextraction; Journal of Chromatography A, (2006); 1116:
1–9.
28. Berijani, S., Assadi, Y., Anbia, M., Milani Hosseini, M.R., Aghaee, E.; Dispersive liquid liquid microextraction combined with gas chromatography–
flame photometric detection very simple, rapid and sensitive method for the
determination of organophosphorus pesticides in water; Journal of
Chromatography A, (2006); 1123: 1–9.
29. Mashayekhi, H.A., Abroomand-Azar, P., Saber-Tehrani, M., Waqif-Husain, S.;
Rapid and sensitive determination of biphenyl and biphenyl oxide in water
samples using dispersive liquid–liquid microextraction followed by gas chromatography; International Journal of Environmental Analytical Chemistry,
(2011); 91: 516–524.
30. Ma, J., Xiao, R., Li, J., Zhao, X., Shi, B., Li, S.; Determination of organophosphorus pesticides in underground water by SPE–GC–MS; Journal of
Chromatographic Science, (2009); 47: 110–115.
31. Yang, Z., Liu, Y., Liu, D., Zhou, Z.; Determination of organophosphorus
pesticides in soil by dispersive liquid–liquid microextraction and gas chromatography; Journal of Chromatographic Science, (2012); 50: 15–20.
32. Regueiro, J., Liompart, M., Garcia-Jares, C., Garcia-Monteagudo, J.C.,
Cela, R.; Ultrasound-assisted emulsification–microextraction of emergent
contaminants and pesticides in environmental waters; Journal of
Chromatography A, (2008); 1190: 27–38.
33. Chang, Q., Zhang, J., Du, X., MA, J., Li, J.; Ultrasound-assisted emulsification solidified floating organic drop microextraction for the determination
of trace amounts of copper in water samples; Frontiers of Environmental
Science & Engineering in China, (2010); 4: 187–195.
34. Wu, C., Liu, N., Wu, Q., Wang, C., Wang, Z.; Application of ultrasoundassisted surfactant-enhanced emulsification microextraction for the determination of some organophosphorus pesticides in water samples; Analytica
Chimica Acta, (2010); 679: 56–62.
35. Ma, J.J., Du, X., Zhang, J.W., Li, J.C., Wang, L.Z.; Ultrasound-assisted
emulsification–microextraction combined with flame atomic absorption
spectrometry for determination of trace cadmium in water samples;
Talanta, (2009); 80: 980–984.
36. Igarashi, S., Takahashi, A., Ueki, Y., Yamaguchi, H.; Homogeneous liquid–
liquid extraction followed by X-ray fluorescence spectrometry of a micro
droplet on filter-paper for the simultaneous determination of small amounts
of metals; Analyst, (2000); 125: 797–798.
37. Takagai, Y., Igarashi, S.; Homogeneous liquid–liquid extraction as a simple
and powerful preconcentration method for capillary gas chromatography
and capillary electrophoresis; American Laboratory News, (2002); 34:
29–30.
38. Sudo, T., Igarashi, S.; Homogeneous liquid–liquid extraction method for
spectrofluorimetric determination of chlorophyll a; Talanta, (1996); 43:
233–237.
39. Li, X., Gan, P., Peng, R., Huang, C., Yu, H.; Determination of 23 organophosphorous pesticides in surface water using SPME followed by GC–MS;
Journal of Chromatographic Science, (2010); 48: 183–187.
40. Farajzadeh, M.A., Bahram, M., Zorita, S., Ghorbani Mehr, B.;
Optimization and application of homogeneous liquid–liquid extraction in
preconcentration of copper (II) in a tyernary solvent system; Journal of
Hazardous Materials, (2009); 161: 1535–1543.
41. Takahashi, A., Ueki, Y., Igarashi, S.; Homogeneous liquid–liquid extraction
of uranium (VI) from acetate aqueous solution; Analytica Chimica Acta,
(1999); 387: 71–75.
42. Ebrahimzadeh, H., Yamini, Y., Kamarei, F., Shariati, S.; Homogeneous
liquid–liquid extraction of trace amounts of mononitrotoluenes from
waste water samples; Analytica Chimica Acta, (2007); 594: 90–100.