Ultrasound‐Assisted Emulsification Microextraction

Surfactant‐Based Materials*

4.3 Surfactant‐Based Liquid‐Phase Extraction

4.3.3 Ultrasound‐Assisted Emulsification Microextraction

A novel ultrasound‐assisted surfactant‐enhanced emulsification microextraction (UASEME) was first introduced in 2010 by Wu et al. [46]. The method was initially applied for the determination of carbamate pesticides in water samples. The surfactant could serve as an emulsifier to enhance the dispersion of extractant into the aqueous sample phase by accelerating the emulsification of the water‐immiscible extraction sol- vent into aqueous sample solution under ultrasound radiation, which is favorable for the mass transfer of the analytes from the aqueous phase to the organic phase. In this work, Tween 20 was used as emulsifier, and chlorobenzene and chloroform were used as dual extraction solvent. Compared with conventional DLLME, UASEME provided higher extraction efficiency and there is no need for the addition of an organic disperser.

In addition, UASEME required a shorter extraction time than the extraction process without ultrasound application.

Ultrasonic energy provides various physical and chemical phenomena including agita- tion, vibration, pressure, shock waves, shear forces, microjets, compression and rarefac- tion, acoustic streaming, cavitation, and also radical formation [53]. In extraction process, the major effect of sonication is acoustic cavitation, in which very fine emul- sions of immiscible liquids can be formed. The ultrasonic energy has been employed to assess the dispersion of the extractant phase in an aqueous sample solution. Therefore, the mass transfer of the analytes is accelerated resulting in enhanced extraction efficiency.

The basis of the ultrasound‐assisted extraction process is emulsification. An emul- sion is a non‐homogenous system of two immiscible liquids, where one (the dispersed phase) is well dispersed in the other (the continuous phase), forming droplets that do not exceed 0.1 μm in diameter. Even though an emulsion is a non‐homogenous system, the emulsification process leads to a homogenous structure. Ultrasonic energy cata- lyzes dispersion of one phase in the other to create a very stable blend by breaking large droplets of the dispersed phase into smaller particles (due to the shearing forces, which decrease the liquid viscosity that causes friction and absorbs large amounts of the applied ultrasonic energy). Since that time, research has been carried out to describe in detail all the processes that occur during ultrasound‐assisted emulsification in order to develop optimum emulsification conditions for the best possible mass transfer and, consequently, higher enrichment factors for applying the emulsifier at the analytical scale. The ultrasound extraction time, generally defined as the interval of time between the addition of the extraction solvent (the start of the ultrasonication) to the sample and the end of the ultrasonication, affects both emulsification and mass transfer process, thus influencing the extraction recovery of the analytes [54]. The experimental scheme of the UASEME procedure is shown in Figure 4.4. Applications of this technique for various types of target analytes are summarized in Table 4.3.

The development of UASEME procedures for extraction of different pesticide resi- dues has been documented. For the extraction of organophosphorus pesticides in water samples, Triton X‐100 and chlorobenzene were used as emulsifier and extraction sol- vent, respectively [55]. During the sonication, the solution became turbid because of the dispersion of very fine chlorobenzene droplets into the aqueous sample. The ana- lytes in the sample were extracted into the fine droplets of the extractant in this step.

After disrupting the emulsion by centrifugation, the organic phase was sedimented at the bottom of the tube and was analyzed by HPLC. The established method has been successfully applied for the determination of organophosphorus pesticides in real water samples. For application in the determination of pesticide residues in wine samples, UASEME was developed for carbamates prior to analysis by HPLC‐MS/MS [56]. Triton X‐114 and 2‐butanone were used as emulsifier and extraction solvent, respectively.

Extraction solvent + Surfactant

Ultrasonication Centrifugation

Withdrawing

OR Analysis

or Vortex mixing

Aqueous sample Cloudy state Phase separation Figure 4.4 Schematic diagram of UASEME (or VSLLME) procedure.

Table 4.3 Selected applications of ultrasound‐assisted surfactant‐based microextraction (UASEME).

Analytes (Reference) Sample matrix Extraction conditions/Analytical technique Analytical performance Carbamates (metolcarb, carbofuran, carbaryl,

pirimicarb, isoprocarb, diethofencarb) [46] Water UASEME/HPLC‐DAD Sample: 5.00 ml

Extractant: 150  μ l CHCl 3 –C 6 H 5 Cl (1  : 1, v/v) Emulsifier: 30  μ l Tween 20 (1.0 × 10 ⁻² mol l ⁻¹ ) Ultrasonication: 3 min (25 ± 2 °C) Centrifugation: 5 min (3500 rpm)

Linear range: 0.3–200 ng ml ⁻¹ RSDs: 3.2–4.8% ( n  = 5) EFs: 170–246 LODs: 0.1–0.3 ng ml ⁻¹ Recovery: 81.0–97.5%

Organophosphorus (isocarbophos, phosmet, parathion, parathion‐methyl, fenitrothion, fonofos, phoxin) [55]

Water UASEME/HPLC‐DAD

Sample: 5.00 ml Extractant: 150  μ l C 6 H 5 Cl

Emulsifier: 100  μ l Triton X‐100 (1.0 × 10 ⁻² mol l ⁻¹ ) Ultrasonication: 3 min (59 kHz, 200 W, 23 °C) Centrifugation: 5 min (3500 rpm)

Linear range: 1–200 ng ml ⁻¹ RSDs: 3–6% ( n  = 5) EFs: 210–242 LODs: 0.1–0.3 ng ml ⁻¹

Recovery: 83–106% (RSDs 3.3–5.6%)

Carbamates (25 carbamates) [56] Wine UASEME/UHPLC–MS/MS

Sample: 1 ml (diluted with 3 ml 30% w/v MgSO 4 ) Extractant: 1150  μ l 2‐butanone

Emulsifier: 333  μ l Triton X‐114 (1.8 mmol l ⁻¹ ) Ultrasonication: 5 min (50 kHz, 200 W, 25 °C) Centrifugation: 5 min (2000 rpm)

Linear range: 1–100  μ g l ⁻¹ RSDs: 6%

LODs: 0.04–0.31  μ g l ⁻¹ Recovery: 74–102%

Fungicides (diethofencarb, pyrimethanil) [57] Water, fruit

juices UASEME/HPLC‐DAD/ESI‐MS

Sample: 5.00 ml Extractant: 20  μ l CCl 4 Emulsifier: 0.05 mg Tween 80

Ultrasonication: 3 min (40 kHz, 100 W, 25 ± 2 °C) Centrifugation: 2 min (3500 rpm)

Linear range: 0.05–2000  μ g l ⁻¹ RSDs: <8.0% ( n  = 6)

EFs: 265 (diethofencarb), 253 (pyrimethanil) LODs: 0.01  μ g l ⁻¹

Recovery: 88–114% (water), 86–117% (fruit juices)

(Continued)

Analytes (Reference) Sample matrix Extraction conditions/Analytical technique Analytical performance Phthalate esters (dimethyl phthalate, diisooctyl

phthalate, diethyl phthalate, dibutyl phthalate, diethyl phthalate) [58]

Beverages UASEME/GC‐FID Sample: 8.00 ml Extractant: 40  μ l CCl 4

Emulsifier: 2.0 mmol l ⁻¹ Triton X‐100 Salt: 6% NaCl

Ultrasonication: 2 min (40 kHz, 25 ± 2 °C) Centrifugation: 5 min (4000 rpm)

Linear range: 0.86–251.05  μ gl l ⁻¹ RSDs: ≤5.46% (intra‐day), 5.81% (inter‐day) EFs: 230–288

LODs: 0.41–0.79  μ g l ⁻¹

Recovery: 89.3–100.0% (RSDs ≤ 8.13%)

Estrogens (17 β ‐estradiol, estrone, diethylstilbestrol)

[59] Water UASEME/HPLC‐DAD

Sample: 10 ml Extractant: 50  μ l CCl 4

Emulsifier: 0.3 ml Triton X‐100 (0.8 × 10 ⁻⁵ mol l ⁻¹ ) Ultrasonication: 5 min (35 kHz, 25 °C) Centrifugation: 5 min (4000 rpm)

Linear range: 10–1000 ng ml ⁻¹ RSDs: 0.85–1.28% ( n  = 5) EFs: 85.29–173.45 LODs: 0.100–0.200 ng ml ⁻¹ Recovery: ≥89.82%

Benzimidazole anthelmintics (thiabendazole, oxfendazole, mebendazole, albendazole, fenbendazole) [60]

Milk UASEME/HPLC‐PDA

Sample: 5.0 ml

Extractant: 100  μ l dichloromethane Emulsifier: 0.5% w/v Triton X‐114 Salt: 6% w/v sodium acetate

Ultrasonication: 4 min (40 kHz, 500 W, 25 °C) Centrifugation: 10 min (3500 rpm)

Linear range: 10–150  μ g l ⁻¹ RSDs: <0.8% (intra‐day, n  = 5), <9.2%

(inter‐day, n  = 4 × 3) EFs: 46–60 LODs: 1.8–3.6  μ g l ⁻¹ Recovery: 72.5–113.5%

Ketoconazole, econazole nitrate [61] Human blood UESA‐DLLME/HPLC‐DAD Sample: 5 ml (adjusted pH to 6 using NaHPO 4 ‐NaOH)

Extractant: 40  μ l chloroform Disperser: 0.068 mg ml ⁻¹ CTAB Ultrasonication: 2 min (25 ± 2 °C) Centrifugation: 3 min (1500 g)

Linear range: 4–5000  μ g l ⁻¹ (ketoconazole), 8–5000  μ g l ⁻¹ (econazole nitrate) RSDs: 5.2–7.8% ( n = 5)

EFs: 129 (ketoconazole), 140 (econazole nitrate)

LODs: 1.1  μ g l ⁻¹ (ketoconazole), 2.3  μ g l ⁻¹ (econazole nitrate)

(Continued) Analytes (Reference) Sample matrix Extraction conditions/Analytical technique Analytical performance

Cobalt [62] Food, water UASEME/GFAAS

Sample: 5.0 ml + 0.8 mg ml ⁻¹ DDTC (adjusted to pH 7.0 with 0.1 mol l ⁻¹ NH 4 Ac‐NH 3 buffer solution)

Extractant: 50  μ l CHCl 3 Emulsifier: 20  μ l SDS (0.5 mg ml ⁻¹ ) Ultrasonication: 1 min (60 kHz, 25 °C) Centrifugation: 5 min (3000 rpm)

Linear range: 0.1–5 ng ml ⁻¹ RSD: 4.3% ( n  = 7) EF: 58 LOD: 15.6 ng l ⁻¹ Recovery: 94–106%

PAHs (naphthalene, acenaphthene, fluorene, phenanthrene, anthracene, pyrene, benzo[ a ] anthracene, benzo[ b ]fluoranthene, benzo[ k ] fluoranthene, benzo[ a ]pyrene, dibenzo[ a,h ] anthracene) [63]

Water UASEME/HPLC‐FLD

Sample: 5.00 ml

Extractant: 20  μ l cyclohexane Emulsifier: 10  μ l Tween 80 (0.5 g L ⁻¹ ) Salt: 6% w/v NaCl

Ultrasonication: 1 min (40 kHz, 0.138 kW, 25 ± 2 °C)

Centrifugation: 3 min (3500 rpm)

Linear range: 2–2000 ng l ⁻¹ RSDs: <10.8% ( n  = 6) EFs: 90–247 LODs: 0.6–62.5 ng l ⁻¹ Recovery: 73–120%

Benzimidazole anthelmintics (oxfendazole, albendazole, mebendazole, flubendazole, fenbendazole, niclosamide [64]

Milk UASEME/HPLC‐UV

Sample: 10.00 ml Extractant: 100  μ l 1‐octanol

Emulsifier: 100 μ l Tergitol TMN‐6 (25% w/v) Salt: 0.3 g CH 3 COONa

Ultrasonication: 4 min Centrifugation: 10 min (3500 rpm)

Linear range: 0.5–5000  μ g l ⁻¹ RSD: <15% ( n  = 3×3) inter‐day EF: 89

LOD: 0.50–6.00  μ g l ⁻¹ Recovery: 79.3–118.0%

Analytes (Reference) Sample matrix Extraction conditions/Analytical technique Analytical performance SUHs (metsulfuron‐methyl, chlorsulfuron,

bensulfuron‐methyl) [65] Water, soil UASEME/HPLC‐UV

Sample: 10 ml Extractant: 35 μl 1‐octanol Emulsifier: 3% w/v Aliquat‐336 Ultrasonication: 1 min (40 kHz, 0.138 kW, 25 ± 2 °C)

Centrifugation: 5 min (440 RCF)

Linear range: 1–100 μg l⁻¹ RSDs: 4.7–6.1%

EFs: 103–153 LOD: 0.5 μg l⁻¹

Recovery: 89.8–110.1% (water), 88.4–94.5%

(soil) Preservatives (benzoic acid, methylparaben,

ethylparaben, propylparaben, butylparaben) [66] Water, beverages, personal care products

UASEME/HPLC‐UV Sample: 10.00 ml Extractant: 125 μl 1‐octanol Emulsifier: 0.05 mmol l⁻¹ Tween 20 Salt: 0.5% NaCl

Ultrasonication: 6 min (35 kHz, 160/640 W) Centrifugation: 10 min (3500 rpm)

Linear range: 0.5–7000 μg l⁻¹ RSD: <7% (n = 3 × 3) inter‐day EF: 15–184

LOD: 0.03–10.00 μg l⁻¹ Recovery: 70.0–138.1%

Fungicides (pyrimethanil, fludioxonil, procymidone, cyprodinil, kresoxim‐methyl, pyraclostrobin) [67]

Juice, red wine UASEME‐SFO/HPLC‐DAD Sample: 8.0 ml

Extractant: 30 μl 1‐dodecanol Emulsifier: 24 μl Tween 80 (10 mmol l⁻¹) Ultrasonication: 1 min (40 kHz, 100 W, 25 ± 2 °C) Centrifugation: 3 min (3800 rpm)

Ice bath: 1 min

Linear range: 5–1000 μg l⁻¹ RSDs: <5.8% (n = 5) LODs: 0.4–1.4 μg l⁻¹

Recovery: 79.5–113.4% (RSDs 0.4–12.3%) Table 4.3 (Continued)

11/19/2018 9:10:37 PM

Analytes (Reference) Sample matrix Extraction conditions/Analytical technique Analytical performance Strobilurin fungicides (kresoxim‐methyl,

picoxystrobin, pyraclostrobin, trifloxystrobin) [68] Fruit juices UASEME‐SFOD/HPLC‐VWD Sample: 5.00 ml

Extractant: 30 μl 1‐undecanol Emulsifier: 15 μl Tween 80 (5.0 mg ml⁻¹) Salt: 1% w/v NaCl

Ultrasonication: 1 min (60 kHz, 30 °C) Centrifugation: 5 min (3000 rpm) Ice bath: 10 min

Linear range: 5–10 000 ng ml⁻¹ RSDs: 3.2–4.9% (n = 7) EFs: 95–105 LODs: 2–4 ng ml⁻¹

Recovery: 82.6–97.5% (RSDs 3.0–6.2%)

Nitrazepam, midazolam [69] Human serum UA‐SEME‐SFOD/HPLC‐UV

Sample: 10.0 ml

Extractant: 29.1 μl 1‐undecanol

Emulsifier: 10.0 μl SDS (25.0 μg ml⁻¹) and Tween 80 (25.0 μg ml⁻¹)

Salt: 1.36% NaCl

Ultrasonication: 20 min (room temperature) Centrifugation: 5.0 min (4000 rpm) Ice bath: 5.0 min

Linear range: 0.5–6.5 × 10³ ng ml⁻¹ (nitrazepam), 0.8 – 5.5 × 10³ ng ml⁻¹ (midazolam)

RSDs: 2.06–6.5% (intra‐day), 1.3–3.5%

(inter‐day)

LODs: 0.017–0.086 ng ml⁻¹ Recovery: 91.0–108.0% (RSDs 0.8–3.6%)

Flavoring compounds (para‐anisaldehyde,

trans‐anethole, estragole) [70] Plant extracts,

urine IL‐UA‐SE‐ME/HPLC‐UV

Sample: 10 ml

Extractant: 90 μl [C6MIM][PF6]

Disperser: 5 mg N‐dodecylbenzenesulfonic acid sodium salt

Ultrasonication: 5 min (25 ± 2 °C) Centrifugation: 5 min (5000 rpm)

Linear range: 0.04–90 μg ml⁻¹ RSDs: 3.1–5.3%

EFs: 118–127 LODs: 16–22 ng ml⁻¹ Recovery: 94.3–101.1%

(Continued)

0004177225.INDD 12711/19/2018 9:10:37 PM

Analytes (Reference) Sample matrix Extraction conditions/Analytical technique Analytical performance Antidepressant (doxepin), antipsychotic

(perphenazine) drugs [71] Urine IL‐SE‐UA‐ME/HPLC‐UV

Sample: 6 ml

Extractant: 50 μl [C6MIM][PF6] Emulsifier: 4 mg SDS

Ultrasonication: 10 min (60 kHz, 130 W, 25 ± 2 °C) Centrifugation: 5 min (5000 rpm)

Linear range: 0.3–1000 μg l⁻¹ (doxepin), 5–1000 μg l⁻¹ (perphenazine) RSDs: <3.5%

EFs: 180 (doxepin), 220 (perphenazine) LODs: 0.1 μg l⁻¹ (doxepin), 1 μg l⁻¹ (perphenazine)

Recovery: 89–98%

Fungicides (pyrimethanil, fludioxonil,

cyprodinil) [72] Juice IL‐MSUASEME/HPLC‐UV

Sample: 5 ml (adjusted to pH 5 with 1 ml⁻¹ HCl) Extractant: 30 μl [HMIMN][TF2] Emulsifier: 50 μl NP‐10 (5 mmol l⁻¹) Salt: 1% w/v NaCl

Shaking: 15 s up and down (30 times) Ultrasonication: 1 min

Centrifugation: 5 min (3800 rpm)

Linear range: 5–200 μg l⁻¹ (cyprodinil, pyrimethanil), 10–400 μg l⁻¹ (fludioxonil) RSDs: <9.4% (n = 5)

LODs: 0.4–1.6 μg l⁻¹

Recovery: 61.4–86.0% (RSDs 1.8–9.7%)

Acetonin [73] Butter USA‐RM‐DLLME/HPLC‐UV

Sample: 2 g Diluent: 2.0 ml hexane Extractant: 400 μl distilled water Emulsifier: Triton X‐100 (1.25% w/v) Ultrasonication: 4 min (40 kHz, 130 W) Centrifugation: 8.5 min (3100g)

Linear range: 0.6–200 mg l⁻¹ RSD: <5% (n = 5) LOD: 0.2 mg l⁻¹ Recovery: 93.9–107.8%

Table 4.3 (Continued)

11/19/2018 9:10:37 PM

Analytes (Reference) Sample matrix Extraction conditions/Analytical technique Analytical performance Aromatic amines (2,4,5‐trimethylanilibe,

4‐chloro‐o‐toluidine, 4‐aminoazobenzene, 3,3′‐dimethyl‐4,4′‐diaminobiphenylmethane, 3,3′‐dimethylbenzidine, 3,3′‐dichlorobenzidine, 4,4′‐methylene‐bis(2‐chloroaniline), 3,3′‐

dimethoxybenzidine) [74]

Water SA‐USAEME/GC–MS

Sample: 5 ml

Extractant: 150 μl dichloroethane Emulsifier: 100 μl polymer surfactant sodium alginate (0.20 g l⁻¹)

Salt: 3% NaCl

Ultrasonication: 1 min (40 kHz, 120 W) Centrifugation: 3 min (4000 rpm)

Linear range: 0.05–200 μg l⁻¹ RSDs: <10.3% (intra‐day, n = 5), <11.9%

(inter‐day) LODs: 0.08–0.3 μg l⁻¹ Recovery: 74.2–115.4%

Chromium [75] Water, air,

biological samples

UEAASLLM‐SFO/ETAAS Sample: 10 ml

Extractant: 30 μl 1‐undecanol Emulsifier: 1 ml SDS (10⁻³ mol l⁻¹) Ultrasonication: 30 s Centrifugation: 3 min (500 rpm) Ice bath: 2 min

Linear range: 0.01–0.3 μg l⁻¹ RSD: 3.65% (n = 5) EF: 174 LOD: 0.003 μg l⁻¹ Recovery: 97.4%

UASEME: Ultrasound‐assisted surfactant‐enhanced emulsification microextraction; DAD: diode array detector; UHPLC: ultra‐high performance liquid chromatography; ESI‐MS:

electrospray ionization‐mass spectrometry; GC‐FID: gas chromatography‐flame ionization detector; PDA: photodiode array detector; UESA‐DLLME: ultrasound‐enhanced sur- factant‐assisted dispersive liquid–liquid microextraction; CTAB: cetyltrimethylammonium bromide; GFAAS: graphite furnace atomic absorption spectrometry; DDTC: diethylam- monium‐N,N′‐diethyldithiocarbamate; PAHs: polyaromatic hydrocarbons; FLD: fluorescence detector; SUHs: sulfonylurea herbicides; UASEME‐SFO or UASEME‐SFOD:

ultrasound‐assisted surfactant‐enhanced emulsification microextraction based on the solidification of a floating organic droplet; VWD: variable wavelength detector; IL‐UA‐SE‐ME or IL‐SE‐UA‐ME: ionic liquid‐based ultrasound‐assisted surfactant‐emulsified microextraction; IL‐MSUASEME: ionic‐liquid‐based, manual shaking‐ and ultrasound‐assisted, sur- factant‐enhanced emulsification microextraction; USA‐RM‐DLLME: ultrasound‐assisted reverse micelles dispersive liquid–liquid microextraction; SA‐USAEME: ultrasound‐assisted polymer surfactant‐enhanced emulsification microextraction; UEAASLLM‐SFO: ultrasound enhanced air‐assisted surfactant liquid–liquid microextraction based on the solidifica- tion of an organic droplet; ETAAS: electrothermal atomic absorption spectrometry.

0004177225.INDD 12911/19/2018 9:10:37 PM

The UASEME was optimized by means of an experimental design. Ultrasonication was applied to shorten the extraction time and enhance the emulsification. The proposed method is adequate for the determination of very low levels of these contaminations in a complex matrix with good recoveries. For UASEME of diethofencarb and pyrimetha- nil fungicides in water and fruit juice samples, Tween 80 and carbon tetrachloride were used as emulsifier and extractant, respectively [57]. Ultrasound irradiation was applied for 3 min. The surfactant could serve as an emulsifier to enhance the dispersion of the water‐immiscible phase into the aqueous phase and accelerate the formation of fine droplets of the extraction solvent in an aqueous sample solution without using an organic dispersive solvent, while also decreasing the extraction time. The developed UASEME method was coupled to a HPLC‐DAD/ESI‐MS instrument and provided low LODs, good repeatability, high EFs, and good recoveries in a short analysis time.

The rapid screening of phthalate esters from beverages was proposed using UASEME coupled with the GC‐FID technique [58]. The non‐ionic surfactant Triton X‐100 was added as emulsifier, without using any organic disperser. The non‐ionic surfactant was selected because it had a larger solubilization capacity for the analytes than the ionic surfactants, and non‐polar analytes were easily emulsified by non‐ionic surfactant.

Formation of fine droplets of extraction solvent (carbon tetrachloride in this work) was accelerated under ultrasound radiation. The proposed method proved to be a simple, rapid, and efficient method for isolation and determination of trace levels of phthalate esters in beverages. The UASEME method using carbon tetrachloride and Triton X‐100 as extractant and emulsifier, respectively, was also proposed for the determination of estrogens [59]. Triton X‐100 is a typical non‐ionic surfactant containing an average of 9.5 oxyethylene units per molecule and is widely applied in the UASEME process.

Extraction was performed under ultrasonication to assist emulsification. The emulsion was then detached by centrifugation for phase separation. Determination of estrogens was carried out by HPLC. The method is simple and reliable for the determination of estrogen residues in water.

For application in food samples, UASEME was developed for the preconcentration of benzimidazole anthelmintics in milk prior to HPLC‐PDA analysis [60]. Good recover- ies were obtained using dichloromethane and Triton X‐114 as extractant and emulsifier, respectively. Dichloromethane in the presence of Triton X‐114 has a more suitable polarity and is more favorable for extraction of the polar target compounds than other extraction media. Extraction performance also increased with increasing sonication time. The use of surfactant and less consumption of organic extractant are in good accordance with green analytical methods.

Application of the method has been extended to a complex matrix sample by deter- mination of ketoconazole and econazole nitrate in human blood [61]. In this method, the cationic surfactant CTAB was used as dispersant, while chloroform was used to extract the analytes under ultrasonication. After centrifugation, the sedimented chloro- form phase was collected for HPLC analysis. Because the emulsification and the equi- librium of mass transfer were easily achieved under combination of ultrasound radiation and surfactant, the temperature has no significant effect on the extraction efficiency.

This method offered good analytical performance, proving to be a useful tool for rapid analysis of two azoles in clinical pharmaceutical analysis.

For application in metal analysis, UASEME was developed for preconcentration of Co(II) in food and water samples prior to its determination by GFAAS [62]. Chloroform

Surfactant-Based Materials 131

and SDS were used to extract the complex of Co and DDTC from the studied samples.

Ultrasound was applied to assist emulsification. After centrifugation, the sediment phase was analyzed. The proposed method possesses a low detection limit and high enrichment factor, making it suitable for the determination of trace amounts of Co(II) in various samples.

The commonly used extraction solvents in typical UASEME are halogenated solvents, which are potentially hazardous to the handlers and are not compatible with the instru- mental techniques, e.g. the mobile phase of HPLC. Therefore, less toxic and low‐density organic solvents have been introduced as alternative solvents. UASEME using low‐

density extraction solvent, cyclohexane, and Tween 80 as emulsifier was developed for analysis of polycyclic aromatic hydrocarbons at trace levels [63]. After applying ultra- sonication, the tube was turned upside down and centrifuged. The finely dispersed droplets of cyclohexane were collected at the bottom of the tube and injected into HPLC for analysis. For extraction of benzimidazole anthelmintics, 1‐octanol, and Tergitol® TMN‐6 non‐ionic surfactant were used as extractant and emulsifier, respec- tively [64]. Formation of a cloudy solution was accelerated using ultrasonic energy.

Extraction could be easily performed using simple glassware without any further modi- fication. After centrifugation, the octanol‐rich phase containing analytes was observed on the top of the extraction tube and was then analyzed by HPLC. Application of UASEME using low‐density extraction solvent, 1‐octanol, for sulfonylurea herbicides was reported in 2015 [65]. In this method, aliquat‐336 was added as emulsifier under ultrasonication, the analytes were extracted into an extraction phase, and dispersed in an aqueous solution. After that, the organic phase on the top of the solution was with- drawn into a syringe for injection into a HPLC instrument. The method was sufficient, fast, and inexpensive for determination of sulfonylurea herbicides in environmental aqueous and soil samples. Later, 1‐octanol was also demonstrated for extraction of pre- servatives [66]. Tween 20 non‐ionic surfactant was used as emulsifier with the aid of ultrasound radiation to enhance the extraction efficiency. The method offers good pos- siblities for application in various sample matrices.

The cationic micellar precipitation (CMP) procedure has been proposed for the determination of benzimidazoles [76]. CTAB was used as dispersant or precipitation solvent to accelerate the formation of fine droplets of the extraction solvent (1‐octanol) in an aqueous sample solution, which enhances the mass transfer of the analytes from the aqueous phase to the organic phase. The extraction was performed at ambient tem- perature in the absence of any organic dispersive solvent and showed reliability with an analytical detection range well‐suited for application in milk samples.

The low‐toxicity solvent 1‐dodecanol was employed as an extractant in the develop- ment of ultrasound‐assisted surfactant‐enhanced emulsification microextraction technique based on the solidification of a floating organic droplet (UASEME‐SFO) for determination of fungicide residues [67]. For its low density and proper melting point near room temperature, the extractant droplet was collected easily by solidifying it at a low temperature. Tween 80 was applied as emulsifier to enhance dispersion of the fine droplet water‐immiscible extraction solvent into an aqueous phase under ultra- sound irradiation. The technique is more environmentally friendly than the conven- tional DLLME‐SFO method in which the organic disperser is absent. The negative impact of surfactant used in this method could be ignored owing to its lower concen- tration. The use of UASEME‐SFO was also proposed for determination of strobilurin

fungicides in fruit juice samples [68]. The low‐density extractant 1‐undecanol was used in combination with Tween 80 as emulsifier. The target analytes were extracted under ultrasonication.

Recently, UASEME‐SFO was proposed for preconcentration and determination of nitrazepam and midazolam drugs [69]. A Box–Behnken design was used to optimize the experimental parameters. 1‐Undecanol was used as extractant, while SDS and Tween 80 were selected as emulsifier in an extraction time of 20 min under ultrasound conditions. The developed methodology was successfully applied for the determination of target analytes in several human serum samples.

An ionic liquid (IL) has been used as extraction solvent in combination with sur- factant in microextraction techniques. For extraction of flavoring compounds in plant extracts and urine samples [70], [C6MIM][PF6] was injected into the mixture of sam- ple and surfactant (as disperser) solution. N‐Dodecylbenzenesulfonic acid sodium salt was applied as disperser. Extraction took place with ultrasonication before apply- ing centrifugation for phase separation. The IL phase stuck to the parapet of the tube and was analyzed by HPLC. The method was simple, sensitive, and more effective, and did not need to use hazardous extraction and disperser solvents. For the determi- nation of antidepressant and antipsychotic drugs in urine samples [71], 1‐hexyl‐3‐

methylimidazolium hexafluorophosphate ([C6MIM][PF6]) and SDS were applied as extraction media. The lipophilic tails of SDS have tendency for sorption and trapping analytes via mechanisms such as electrostatic attraction and hydrogen bonding between surfactant, IL, and analytes. The extraction was accelerated by ultrasound radiation. Several experimental parameters were optimized using a fractional facto- rial design. The developed method provided better dynamic ranges and detection limits compared to other previously reported methods. The IL was also applied in a new extraction procedure, based on manual‐shaking and UASEME, for extraction of fungicide residues in juice samples [72]. The IL 1‐ethyl‐3‐methylimidazolium bis[(trifluoromethyl)sulfonyl]imide was used as extractant instead of a volatile organic solvent. The surfactant NP‐10 was used as emulsifier. Manual shaking for 10 s was performed before ultrasound to preliminarily mix the extraction solvent and the aqueous sample, and the time taken to disperse the IL into aqueous solution could be shortened by ultrasound irradiation.

Ultrasound radiation was applied in a microextraction method based on reverse micelle DLLME for determination of acetonin in butter prior to HPLC analysis [73].

Hexane and Triton X‐100 were used to dilute and homogenize the butter samples, respectively. Extraction was accelerated under ultrasonication after adding distilled water. Optimization was performed using the Box–Behnken design combined with desirability function. The use of water and surfactant as extraction media lead to elimi- nation of a relatively large amount of chlorinated and organic dispersive solvent widely used in conventional DLLME.

A polymer surfactant has been introduced as emulsifier in a UASEME method for the determination of aromatic amines in water samples [74]. Sodium alginate, polymer sur- factant, was added as emulsifier using dichloroethane as extractant. Extraction was performed in an ultrasound water bath. After centrifugation, the sediment phase was analyzed by GC‐MS. The water‐soluble polymer surfactant in this method could solve the problems of potential pollution and decrease the GC limitation since the polymer surfactant is natural and is insoluble in extractive solvent.