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Pressurized hot water extraction (PHWE)

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 Journal of Chromatography A, 1217 (2010) 2484–2494

Contents lists available at ScienceDirect

Journal of Chromatography A
journal homepage: www.elsevier.com/locate/chroma

Review

Pressurized hot water extraction (PHWE)

Chin Chye Teo a,b , Swee Ngin Tan a , Jean Wan Hong Yong a , Choy Sin Hew b , Eng Shi Ong c,∗
a
Natural Sciences and Science Education Academic Group, Nanyang Technological University, 1 Nanyang Walk, Singapore 637616, Singapore
b
Institute of Advanced Studies, Nanyang Technological University, Nanyang Executive Centre #02-18, 60 Nanyang View, Singapore 639673, Singapore
c
Department of Epidemiology and Public Health, Yong Loo Ling School of Medicine, National University of Singapore, 8 Medical Drive, Singapore 117597, Singapore

a r t i c l e i n f o a b s t r a c t

Article history: Pressurized hot water extraction (PHWE) has become a popular green extraction method for different
Available online 4 January 2010 classes of compounds present in numerous kinds of matrices such as environmental, food and botanical
samples. PHWE is also used in sample preparation to extract organic contaminants from foodstuff for
Keywords: food safety analysis and soils/sediments for environmental monitoring purposes. The main parameters
Pressurized hot water extraction which influence its extraction efficiency are namely the temperature, extraction time, flow rates and
Temperature
addition of modifiers/additives. Among these different parameters studied, temperature is described as
Green extraction
the most important one. It is reported that the extraction of certain compounds is rather dependent
Scaled-up
Pilot-scale
on pressurized water with different applied temperature. Thus, the stability and reduced solubilities of
Metabolites certain compounds at elevated temperatures are highlighted in this review. With some modifications,
a scaled-up PHWE could extract a higher amount of desirable compounds from solid and powdered
samples such as plant and food materials. The PHWE extracts from plants are rich in chemical compounds
or metabolites which can be a potential lead for drug discovery or development of disease-resistant food
crops.
© 2009 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2485
2. Fundamental principles of PHWE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2485
2.1. Changes in physicochemical properties of water . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2485
2.2. Extraction mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2485
2.3. Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2487
3. Parameters affecting the extraction process in PHWE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2487
3.1. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2487
3.2. Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2488
3.3. Dynamic or static extraction mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2488
3.4. Modifiers and additives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2488
4. Applications of PHWE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2488
4.1. Extraction of bioactive and nutritional compounds from plant and food materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2488
4.2. Removal of organic contaminants in foodstuff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2488
4.3. Environmental samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2492
4.4. Pesticides and herbicides in soil and sediments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2492
5. Future outlooks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2492
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2492
Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2492
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2493

∗ Corresponding author. Tel.: +65 6516 4996; fax: +65 6779 1489.
E-mail addresses: ephoes@nus.edu.sg, cofoes@nus.edu.sg (E.S. Ong).

0021-9673/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.chroma.2009.12.050
 C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494 2485

1. Introduction to remove contaminants from soil samples. At the same time, lim-
itations of PHWE are discussed in this review as well.
In the analysis of solid samples, the method of extraction is
regarded as a crucial step in the sample preparation. Classical sam-
2. Fundamental principles of PHWE
ple preparation techniques that rely on extraction with solvents
such as liquid–liquid extraction (LLE), sonication, Soxhlet extrac-
2.1. Changes in physicochemical properties of water
tion and other methods have been used. However, these traditional
methods may often be time consuming with low extraction effi-
Water is a highly polar solvent with a high dielectric constant
ciency and also require large volume of non-environmental friendly
(ε) at room temperature and atmospheric pressure due to the pres-
organic solvents. In recent years, there is steady progress in extrac-
ence of extensive hydrogen-bonded structure. Hence, traditionally
tion technology with the development of new and simpler sample
water is not considered as a suitable extraction fluid for non-polar
preparation methods such as supercritical fluid extraction (SFE),
or organic compounds at room temperature. When the tempera-
microwave-assisted extraction (MAE), pressurized liquid extrac-
ture of water is raised, there is a steady decrease in its permittivity,
tion (PLE) and pressurized hot water extraction (PHWE) which have
viscosity and surface tension but an increase in its diffusivity char-
been described in several earlier reviews [1–20].
acteristics. With enough pressure to maintain water in the liquid
To reduce the usage of organic solvents, PHWE is a feasible
phase at elevated temperature, the initial value of the dielectric
green solvent extraction method as it utilizes pressurized water
constant of 80 at 25 ◦ C decreases to 27 at 250 ◦ C and 50 bar, which
at elevated temperature and controlled pressure conditions. Var-
falls between those of methanol (ε = 33) and ethanol (ε = 24) at
ious reports have shown that at certain temperature and applied
25 ◦ C. Under these conditions, water behaves like certain organic
pressure, the polarity of water can be varied close to those of alco-
solvents which can dissolve a wide range of medium and low polar-
hols. Thus, it can dissolve a wide range of medium and low polarity
ity analytes [21–36].
analytes [21–36]. The major advantage of PHWE is the reduction
in the consumption of organic solvents. Moreover, water is easily
available, non-toxic and can be recycled or disposed with mini- 2.2. Extraction mechanism
mal environmental problems. Hence, PHWE has steadily become
an efficient and low cost method of extraction for less-polar organic The extraction mechanism in PHWE is proposed to involve four
components from environmental soil, sediments and plant mate- sequential steps which take place in the extraction cell filled with
rials [15,16,23–38]. sample materials and a high portion of sands. The first step is the
The application of pressurized water as an extraction fluid desorption of solutes from the various active sites in the sample
at elevated temperatures was first reported in the pioneering matrix under the pressurized and elevated temperature conditions.
work of Hawthorne and co-workers for extraction of some polar The second step may involve the diffusion of extraction fluid into
and non-polar analytes from soil samples in 1994 [39]. Their the matrix. Next, depending on the sample matrix, the solutes may
works have since changed the perception that highly polar water partition themselves from the sample matrix into the extraction
could be transformed into a suitable extraction solvent for organic fluid and finally be chromatographically eluted out of the extrac-
compounds under certain high temperatures and controlled pres- tion cell to the collection vial [15,18,39]. An earlier theoretical study
surized conditions. The term “pressurized hot water” is used to has suggested that the extraction mechanism in PHWE could fit in
denote the region of condensed phase of water between the tem- a thermodynamic model [49]. In this model, the extraction of any
perature range from 100 ◦ C (boiling point of water) to 374 ◦ C compound from a solid matrix requires two steps: (1) the com-
(critical point of water). Other common terms such as “superheated pound must be desorbed from its original binding sites in (or on)
water”, “near critical water”, “subcritical water”, “high tempera- the sample matrix (generally modeled by rate processes such as
ture extraction” and “extraction using hot compressed water” have diffusion) and (2) the compound must be eluted from the sample
also been used. In the case of PHWE, the density of water remains in a manner analogous to frontal elution chromatography (con-
almost constant over this range of temperature so that the pres- trolled by the thermodynamic partitioning coefficient, KD ). Hence, a
sure effect on the properties of water is minimal [40]. During the model based solely on the thermodynamic partitioning coefficient
extraction, moderate pressures are needed to keep a condensed KD , which assumes that analyte desorption from the matrix is rapid
phase of water such as 15 bar at 200 ◦ C and 85 bar at 300 ◦ C. If the compared to elution that is used to describe the extraction profiles
pressure decreases below the boiling point at any pressure, super- obtained with PHWE.
heated steam will be formed. The basic principle of PHWE and its The enhancement on the extraction efficiency of PHWE can be
feasibility as a green solvent extraction method to extract organic attributed to: (1) an improvement in the solubility and mass trans-
and non-polar compounds from numerous kinds of matrices have fer effects and (2) an increased disruption of surface equilibria
been described in earlier reviews [15–20,41–48]. [18]. With the modification of the properties of water at elevated
In this review, the fundamental principles of the PHWE are temperatures, the capacity of the fluid to solubilize analytes is
briefly explained. The main parameters affecting its extraction increased. There is reduced viscosity but improved diffusivity of
efficiency, namely the temperature, pressure, static or dynamic water to allow better penetration through the matrix particles. If
operation mode in terms of extraction time/flow rate and also fresh water is continuously introduced during a dynamic extraction
modifiers/additives are covered. The current review will evaluate in PHWE, it improves the mass transfer and hence, increases extrac-
the extraction efficiencies of PHWE applied on different com- tion rate. Both the high temperatures and pressures could disrupt
pounds from a variety of sample matrices like environmental the surface equilibria. The increased temperature can overcome the
soils/sediments, plant and food samples. The PHWE is also used solute–matrix interaction caused by van der Waals forces, hydro-
in sample preparation to extract organic contaminants from food- gen bonding, dipole attraction of the solutes molecules and active
stuff for food safety analysis and soils/sediments for environmental sites in the matrix. Thus, the thermal energy supplied can disrupt
monitoring purposes. It is noted that there is a steady growing trend cohesive (solute–solute) and adhesive (solute–matrix) interaction
to use PHWE to extract bioactive and nutritional compounds from by decreasing the activation energy required for desorption pro-
plant and food materials (Table 1). Finally, the scaled-up use of cess. The transfer of the analytes from matrix to pressurized hot
PHWE as a green solvent extraction method for industrial applica- water is achieved by the diffusion and convection processes [16].
tion is discussed with reference to a successful pilot-scale project However, thermally labile compounds are degraded at elevated
 2486
Table 1
Analyses of plants and food materials based on PHWE.

Analyte(s) Matrix Temperature (◦ C) Pressure Mode Flow-rate Extraction Reference method(s) Sample Analysis method Reference
(ml/min) time (min) pre-treatment
Plants
Stevioside, rebaudioside A Stevia rebaudiana 100 11–13 bar Dynamic 1.5 15 Reflux Nil HPLC [22]
Gastrodin, Vanillyl alcohol Gastrodia elata 100 8–10 bar Dynamic 1.5 20 Reflux Nil HPLC [26]
Phenolic compounds Momordica charantia 150–200 10 MPa Dynamic 2.0 320 Soxhlet extraction Nil Anti-oxidant study [59]
Tanshinone I and IIA Salvia miltiorrhiza 95–140 10–20 bar Dynamic 1.0 20, 40 Soxhlet extraction SPE HPLC, LC–MS [53]
Essential oil Fructus amomi 150 50 bar Dynamic 1.0 5 Recovery and repeatability SPME GC–MS [76]
Essential oil Acorus tatarinowii 150 50 bar Dynamic 1.0 5 Steam distillation SPME GC–MS [77]
Essential oil Fructus amomi 160 60 bar Dynamic 1.0 5 Steam distillation LPME GC–MS [78]
Borneol, terpinen-4-ol, carvacrol Origanum anites 100, 125, 150, 175 60 bar Dynamic 2.0 30 Steam distillation, Soxhlet SPE GC × GC/TOF–MS [79]
extraction
Essential oils Origanum micrathum 100, 125, 150, 175 40–80 bar Dynamic 1.0–3.0 30 Nil SPE GC × GC/TOF–MS [80]
Pulegone, terpinen-4-ol, Ziziphora taurica 150 60 bar Dynamic 2.0 30 Steam distillation, direct SPE GC × GC/TOF–MS [81]
trans-carveol, verbenone thermal desorption
Glycyrrhizin Glycyrrhiza glabra 30–120 5 atm Static Nil 60–120 Nil Nil UV [85]

C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494

Anthocyanins Brassica oleracea 80–120 50 bar Static Nil 11 Nil Nil HPLC [86]
Anthraquinones Morinda citrifolia 80, 120 4 MPa Dynamic 4.0 120 Nil Nil HPLC [87]
Gallic acid, ellagic acid, corilagin Terminalia chebula Retz 120–200 4 MPa Dynamic 2.0–4.0 150 Soxhlet extraction, Hot water Nil Anti-oxidant study [88]
extraction in stirred vessel
Saponins, cyclopeptides Vaccaria segetalis Garcke, 160 750 psi Dynamic 0.5, 1.0, 2.0, 4.0, 80 Ultrasonication extraction Nil HPLC [89]
Saponaria vaccaria 8.0
Terpenes (␣-pinene, limonene, Basil and oregano leaves 100, 150, 200, 250 Nil Static Nil 30, 300 Sonication extraction Nil GC–FID [91]
camphor, citronellol, carvacrol)
Volatile oil Cuminum cyminum L. 100–175 20 bar Dynamic 2.0,4.0 Nil Hydrodistillation, Soxhlet Nil GC–FID, GC–MS [92]
extraction
Lignans Linum usitatissimum 140 5.2 MPa Dynamic 0.5 400 Nil Nil HPLC [97]
Rosmarinic acid, carnosic acid Rosmarinus officinalis 60–100 1500 psi Static Nil 25 Nil Nil CE–ESI-MS [98]
Anti-oxidants Spirulina platensis 60, 115, 170 1500 psi Static Nil 3, 9, 15 Nil Nil Anti-oxidant study [100]
Anti-oxidants Spirulina platensis 115, 170 1500 psi Static Nil 9, 15 Nil Nil MEKC–DAD [101]
Cedarwood oil Juniperus virginianna 50, 100, 150, 200 500,750,1500,
Static Nil 15 Liquid and SFE LLE SFC [103]
3000 psi
1,1-Diphenyl-2-picrylhydrazyl Diascorea alata 100 1.34 MPa Dynamic 10.0 <180 Nil Nil HPLC [104]
Anthraquinones Morinda citrifolia 100, 170, 220 7 MPa Dynamic 2.0, 4.0, 6.0 18 Organic solvent extraction Nil UV [106]
Shikimic acid Chinese star anise (Illicium 30–200 5–15 MPa Dynamic 5.0–15.0 g/min 10 Nil Nil HPLC [107]
verum Hook. f.)
Food
Total sugars, proteins Defatted rice bran 200 Nil Static Nil 5 Nil Nil Anti-oxidant study [69]
Isoflavones Soybeans 100 1000 psi Static Nil Nil Vortexing, shaking, stirring, Nil HPLC [90]
sonication and Soxhlet
Lignans, proteins and carbohydrates Defatted flaxseed meal 130,160, 190 750 psi Dynamic 1.0 400 Nil Nil HPLC [93]
Flavonoids Knotwood of aspen 150 220 bar Static Nil 35 Soxhlet, ultrasonic extraction Nil GC–FID, GC–MS, [94]
and reflux in methanol HPLC–UV, HPLC–MS
Catechins, Proanthocyanidins Grape seed 50, 100, 150 1500 psi Static Nil 30 Extraction with 75% methanol Nil HPLC [95]
Capsaicin, dihydrocapsaicin Peppers 50–200 100 atm Static Nil Nil Nil Nil LC–MS [96]
Anthocyanins, phenolics Dried red grape skin 100–160 Nil Static Nil 40 s Conventional hot water, Nil HPLC [99]
aqueous 60% methanol
extraction
Catechin, epicatechin Tea leaves, grape seeds 100–200 1500 psi Static Nil 5, 10 Ultrasound-assisted extraction Nil HPLC [102]
Isoflavones Defatted soybean flakes 110 641 psig Static Nil 2.3 h Soxhlet extraction SPE HPLC [105]
Total phenolic content Citrus pomaces 25–250 0.1–5.0 MPa Static Nil 10, 30, 60 Nil Whatmann Anti-oxidant study [108]
No. 1 filter
paper
 C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494 2487

time, flow rates and modifiers/additives. The geometry of the

extraction cell and flow direction have little effect on the recov-
ery of the analytes from sample materials [58]. For the validation
of analytical methods, the extraction efficiency of PHWE is often
compared with other reference methods such as heating under
reflux, Soxhlet extraction, sonication and others which rely on
pure or aqueous mixture of organic solvents [3,22,24–27,59–62].
In botanicals, the bioactive or marker compounds are present
naturally and significant analyte–matrix interaction will be
present. Hence, spiking of the target compounds into the plant
matrix will not mimic the real environment of the matrix
[22,26,51–53,62–64].

3.1. Temperature

Temperature is the main factor which could affect the extrac-

tion efficiency and selectivity in PHWE. It could influence the
Fig. 1. A schematic diagram of a laboratory self-assembled PHWE system [54]. physicochemical properties of water and also subject thermally
labile analytes to their decomposition or hydrolytic attack. In
PHWE, the applied extraction temperature is usually above the
temperatures. Sufficient pressure is required to be exerted on water
normal boiling point of the fluid used. The physical advantages
when temperature above its boiling point is used. The presence of
such as high diffusion, low viscosity and low surface tension
pressure could facilitate extraction from samples where analytes
are achieved at elevated temperature condition. The increased
are trapped in the matrix pores. This pressure forces the water into
vapour pressures and rapid thermal desorption of target com-
areas of the matrices which are not normally covered if water at
pounds from matrices could enhance the extraction efficiency of
atmospheric pressure is used [50].
PHWE [16]. The high temperatures have also changed the proper-
ties of water and thus making the polarity of water closer to those
2.3. Instrumentation of non-polar compounds. This will enhance the solubility of less-
polar compounds in water for extraction from different matrices
The basic experimental set-up for PHWE is similar to that used [22,29,54,56,59,62,65–71]. However, degradation of compounds
in Accelerated Solvent Extraction (ASE) as reported in an earlier and the intensity of reactions such as hydrolysis and oxidation can
review [16]. PHWE can extract wet samples directly as compared occur with increased temperature. The amount of PAHs extracted
to SFE with carbon dioxide (CO2 ) as an extraction fluid. It is eas- from sediments did not increase beyond 250 ◦ C but there was
ier to operate with water than liquefied CO2 . In a laboratory a degradation phenomenon at higher temperature [28,72]. For
self-assembled system, degassing of water is needed to prevent non-polar organic pollutants, pressurized water at temperature
potential oxidative corrosion of extraction cell and delivery lines higher than 300 ◦ C could enhance the solubility and extraction
due to the presence of dissolved oxygen in water. Most published efficiencies of these compounds by PHWE due to its decreasing
studies have described a similar construction of the PHWE sys- dielectric constant [39,73,74]. Thus, a systematic optimization on
tem [21,16,19,20,51–53]. Typically, it consists of a water supply, temperature has to be carried out for the respective classes of
a pump for transporting the solvent, a heater for heating solvent, compounds.
a pressure vessel where the extraction occurs, a means to control Herbicides such as phenoxy acids were found to degrade at
the pressure in the system and a collection vessel for the extract. relatively low temperature above 120 ◦ C [75]. Thus, PHWE has
Thus, a general instrumentation set-up can be described by the fol- an important impact on the extraction efficiencies of compounds
lowing laboratory assembled system in Fig. 1 adapted from Ong et present in plants [22,24,26,59–62,76–81]. The bioactive and marker
al. [54]. The set-up consists of a stainless steel preheating coil to compounds in plants may be non-polar or polar and can also
ensure that water is at its operating temperature before entering be thermally labile and/or prone to hydrolytic attack. To extract
into the stainless extraction cell. A pump is used and the extrac- non-polar compounds from plant matrix, an increase in applied
tion is carried out at an elevated temperature maintained by a gas temperature up to 200 ◦ C (Table 1) may be needed. However, the
chromatography oven. The extraction processes is operated in pres- degradation of target compounds at higher applied temperature
sures of between 10 and 60 bars. The outlet flow is controlled by beyond 250 ◦ C was observed in the extraction of marker com-
a miniature back pressure regulator to generate the back pressure. pounds such as stevioside, rebaudioside A, berberine, aristolochic
PHWE using commercially available system like the Dionex ASE acids, baiclein, glycyrrhizin, tanshinone I and IIA and others in
200 has been reported [55,56]. For certain set-ups, a second pump medicinal plants [22,26,59–62,51–53,76–82].
is used to deliver chloroform/dichloromethane into a fused silica- It was noted that the behaviour of marker compounds from
lined tee placed in the oven between extraction cell and collection Radix Codonopsis pilosula by PLE with methanol and PHWE were
valve to prevent deposition of analytes when water cools during rather different under elevated temperature conditions [52]. It was
collection. Depending on its applications, a cooling trap may be proposed that the presence of dissolved gases such as oxygen and
used to cool fluid coming out of the extraction cell to room tem- nitrogen in the compressed dense water at higher temperature
perature [18]. With quality data and determination of scaled-up might reduce the solubility of certain target compounds. At the
factors, the design of pilot or industrial PHWE equipment has been same time, PLE with methanol showed better recovery of stevioside
reported [57]. from Stevia rebaudiana leaves than PHWE within the temperature
range of 110–160 ◦ C [83]. Another observation showed that under
3. Parameters affecting the extraction process in PHWE increasing temperature and extraction time conditions, PHWE
enhanced the recovery of flavonoids but had a lower recovery of
The main parameters that influence the selectivity and extrac- carotenoids from yellow Thai silk waste compared to PLE with
tion efficiency of PHWE include temperature, pressure, extraction ethanol at 50–79 ◦ C [84].
2488 C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494

3.2. Pressure and fragrances, and also for their bioactive compounds (Table 1).
In addition, methods using PHWE have also used in the extrac-
The effect of adjusting pressure could change the phases of tion of organic contaminants from foodstuff for the food safety
water. Moderate pressures such as 15 bar at 200 ◦ C and 85 bar at analysis (Table 2). The changes in the physicochemical properties
300 ◦ C are required to maintain the liquid phase of water. Pres- of water have enabled the extraction of non-polar organics such
sure is usually varied from 10 to 80 bars to maintain water in its as polycyclic aromatic hydrocarbons (PAHs) and polychlorinated
liquid phase at extraction temperature and often has little effect biphenyls (PCBs) from environmental soil and sediment samples
on the extraction efficiency of PHWE. The recovery of organic pol- (Table 3). Due to its green nature and feasibility to extract a wide
lutants from solid environmental samples was suggested to have range of compounds under certain extraction conditions, PHWE is
little dependence on pressure [39,74]. Similarly, varying pressure noted to aid in bioremediation processes by recovering pesticides
did not improve the recovery of essential oils from medicinal plants and herbicides from soils/sediments (Table 4).
and ginsenosides from American ginseng [77,78].
4.1. Extraction of bioactive and nutritional compounds from
3.3. Dynamic or static extraction mode plant and food materials

PHWE can be performed in either static or dynamic mode. In recent years, PHWE has gradually become a useful option for
In the dynamic extraction mode, both extraction time and flow the isolation of bioactive and nutritional compounds from plants
rate are important parameters for the optimization of PHWE. The and food materials. PHWE is a direct method to recover analytes
extraction time strongly depends on the extraction temperature, without the need to cleanup. This will reduce costs, as the ana-
nature of matrix and analytes. Using PHWE, it was observed that lytes extracted are safe for further testing, processing and human
an extraction time of 20 min at 100 ◦ C gave higher yields of ste- consumption. The nature of these materials is soft and thus can
vioside and rebaudioside A from Stevia rebaudiana as compared to be easily reduced into smaller sizes to improve the extraction
heating under reflux for 60 min [22]. Dynamic PHWE enhances the efficiency. As seen in Table 1, the studies on the plant materials
extraction compared to boiling in a flask (static type pf extrac- concentrate on the extraction of bioactive compounds and also
tion). In PHWE, the water is forced through a narrow sample volatile essential oils at optimized extraction conditions. The usage
cell at high pressure which generally enhances the extraction. of pure water mimics the tradition herbal preparations which usu-
However, prolonged heating may result in compound degrada- ally involve sequential steps with boiling in water. The extraction
tion and thus optimization of extraction time is very important efficiencies of the marker compounds from Gastrodia elata and Ste-
[22,26,59–62,51–53,76–82]. Using the dynamic extraction mode, via rebaudiana using PHWE were found comparable or higher than
the equilibrium is displaced to completion as fresh solvent is con- heating under reflux using water [22,26]. The chromatograms in
tinuously pumped through the sample. Thus, it requires more Fig. 2 showed that both marker compounds GA and VA present
volume of fluid compared to the static mode. A flow rate of 1 in Gastrodia elata could be extracted by PHWE as an alternative
or 1.5 ml/min is usually used in the dynamic extraction mode extraction method to the traditional heating under reflux [26].
(Tables 1–4). However, a higher flow rate will generally improve Table 1 demonstrated the feasibility of PHWE for the extraction
extraction efficiencies of highly concentrated samples because the of volatile components from botanicals at optimized conditions.
total volume of water is increased and also its enhancement in The extraction of volatile essential oil from Cuminum cyminum
physical mass transfer of analytes from matrix [15,39]. Hence, the L. at a higher temperature of 150 ◦ C by PHWE gave comparable
extraction time or flow rate in PHWE need to be determined during yields with reference to Soxhlet extraction and steam distillation
validation process. (hydrodistillation) [92]. Comparable results were also reported for
In the static extraction mode, its extraction efficiency strongly PHWE, hydrodistillation and Soxhlet in the extractions of Borneol
depends on the partition-equilibrium constant and solubility of [79] and Pulegone [81] in plant materials. Thus, PHWE is offered
compounds at elevated temperatures. Thus, highly concentrated as a fast, clean and high efficiency extraction method for volatile
samples or low solubility analytes may lead to incomplete extrac- components present in plants.
tion due to limited volume of water used. PHWE is also a common method to extract compounds from
food materials (Table 1) [69,90,93–96,99,102,105]. The stability of
3.4. Modifiers and additives these compounds at elevated temperature and their extraction effi-
ciencies compared with other methods of extraction were studied.
The addition of some organic, inorganic modifiers and addi- The total sugar present in defatted rice bean was determined to
tives may enhance the solubility of analytes in water and increase be the highest using PHWE at 200 ◦ C [69]. The extraction of cat-
the interactions of target analytes with water. They can also alter echins and proanthocyanidins from dried grape seeds was found
the physicochemical properties of water at elevated temperature. to be comparable to conventional extraction with 75% methanol
It was reported that a higher amount of natural sweetener from [95]. Using PHWE, five different capsaicinoids (nordihydrocap-
licorice (Glycyrrhiza glabra) roots could be achieved by PHWE with saicin, capsaicin, dihydrocapsaicin, an isomer of dihydrocapsaicin,
dissolved ammonia (0.01%, w/v) [85]. PHWE with extraction fluids and homodihydrocapsaicin) present in peppers were successfully
containing 5% ethanol was also reported to enhance the extraction isolated at 200 ◦ C and quantified by HPLC before they the extraction
of anthocyanins in red cabbage [86]. The degradation of compounds yield decreased at higher applied temperatures [96]. The feasibil-
could be reduced by micelle-mediated extraction (MMPHWE) with ity of PHWE as a green method to extract natural compounds from
Triton X-100 compared with PHWE without the use of surfactant food materials was also validated with reference to other methods
[87]. such as Soxhlet extraction, ultrasonic extraction and heating under
reflux with pure or aqueous mixture of alcohols (Table 1).
4. Applications of PHWE
4.2. Removal of organic contaminants in foodstuff
PHWE has been mainly applied on solids and powdered samples
because these matrices are more compatible with a flow extrac- The analysis of chemical contaminants in food has grown con-
tion system. Methods using PHWE have been applied successfully siderably in recent years. These chemical contaminants can be
on food and plant materials for the extraction of their flavours broadly classified into 4 main categories: (1) pesticides, (2) vet-
Table 2
Analyses of organic contaminants in foodstuff.

Analyte(s) Matrix Temperature (◦ C) Pressure Mode Flow-rate Extraction Reference Sample Analysis Reference
(ml/min) time (min) method(s) pre-treatment method

Sulfonamide (SAs) Cattle and fish 80 Nil Dynamic 1.0 4 Nil Cellulose filter LC–MS [110]
muscle tissue
Carbamates Bovine milk 90 Nil Dynamic 1.0 5 Nil Cellulose filter LC–MS [111]
(carbamate

C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494

insecticides)
Sulfonamides (SAs) Pork meat 160 1500 psi Static Nil 5 Nil Oasis HLB cartridge CE–MS [112]
Antibiotics Cattle and pig 70 1500 psi Static Nil 10 Nil Nil LC–MS [61]
meat samples
Herbicides Wheat flours and 120 100 atm Static Nil 15 Nil Filter LC–MS-MS [113]
(Chlormequat flour-based baby
and mepiquat) foods
Cholesterol Solid food 135 10 bar Dynamic 3.0 5 Nil C18 cartridge UV–vis [114]
Fluorescent Paper used for 250 100 atm Static 0.5 201 Dynamic Nil HPLC [115]
whitening agents wrapping food followed by Sonication-assisted
(FWAs) and azo Dynamic extraction, Soxhlet
dyes (AZOs) extraction
Insecticides: Seed-pellet
(i) Carbofuran (i) 150 Nil Static Nil 30 Nil SPE HPLC [116]
(ii) Imidacloprid (ii) 100–150
Pesticides Skin of grapes 120 Nil Dynamic 1.0 40 Nil Microporous GC–MS [73]
membrane
liquid–liquid
extraction
(MMLLE)
Pesticides Beef kidneys 100 50 atm Static (30% Nil 10 per Nil SPME GC–MS [117]
(Atrazine) Ethanol) cycle
Pesticides Water 50–125 50 atm Static Nil Nil Nil Nil On-line LC [118]
(Atrazine) (Modified
with
ethanol
and urea)

2489
 2490
Table 3
Analyses of environmental samples based on PHWE.

Analyte(s) Matrix Temperature (◦ C) Pressure Mode Flow-rate Extraction Reference Sample pre-treatment Analysis method Reference
(ml/min) time (min) method(s)

PAHs Environmental solids 50–400 5–600 bar Static Nil Nil Nil Nil GC–FID, GC–MS [39]
TNT (2,4,6-trinitrotoluene) Soils 150, 175, 200, 225 Nil Static Nil Nil Nil Nil HPLC [27]
PAHs Sediments 150 2000 psi Static Nil 5 Nil SPME GC–MS [28]
PAHs Sediments 100, 150 15 MPa Static Nil 10 Soxhlet Nil GC–MS [71]
extrac-
tion,
MAE
PAHs Soils 250 17.2 MPa Dynamic 0.20 Nil Soxhlet Nil UV [72]
extrac-
tion
Heterocyclic analogs of anthracene, Solids 313 K 5 MPa Dynamic 0.017 g/s Nil Nil Nil Nil [120]
phenanthrene and fluorene
Phenanthrene, PAHs Environmental solids 100–350 Nil Static Nil 30 Nil Nil GC–MS [121]
PAHs Environmental solids 313–498 K 0.1 MPa Static Nil Nil Nil Nil GC–MS [122]
PAHs Soil 250 1000 psi Dynamic 265.0, 1, 2 h Nil Nil GC–FID, GC–MS [123]

C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494

132.5,
121.24,
242.13,
253.57 ml/h
PAHs Environmental solids 100–185 100–160 bar Static Nil 8 Nil Nil GC–MS [124]
Organics Sediments 120, 200 Nil Static Nil 3 Nil SPE GC–MS [125]
Semivolatile organics Sediments 120, 200 Nil Static Nil 3 Nil SPE GC–MS [126]
Acenaphthene, anthracene, and pyrene Environmental solids 300 5 MPa Dynamic 0.10 Nil Nil Nil GC–MS [127]
Linear alkylbenzene sulfonates Sediments 100 20–30 bar Dynamic 2.0 70 Soxhlet Nil HPLC–UV [128]
extrac-
tion
PAHs Sediments 22, 100, 200 Nil Static Nil 30, 60, 90 Nil Nil GC–MS [129]
Dioxins, PCBs Soil 25–350 0.2–25 MPa Static Nil Nil Nil Nil GC–MS [130]
Phenolic compounds (phenol, Sands 50, 100, 200, 300 Nil Static Nil 20 Nil Nil CZE, GC–MS [131]
3-methylphenol,
4-chloro-3-methylphenol and
3,4-dichlorophenol)
PAHs Sea sand and soil 300 290 bar Dynamic 1.0 20 Soxhlet Nil GC–MS [135]
extrac-
tion
Brominated flame retardants (BFRs) Sediments 250–350 118 bar Dynamic 1.0 40 Soxhlet Solid-phase trap GC–MS [136]
extrac-
tion
Brominated flame retardants (BFRs) Sediments 325 118 bar Dynamic 1.0 40 Nil Phase-phase trap LC–GC–FID [137]
Polychlorinated dibenzofurans and Industrial oil and 200–400 10–250 atm Dynamic 1.0 30 Soxhlet Nil GC–MS [138]
naphthalenes seasand extrac-
tion
PAHs Airborne particulate 250 5.5 MPa Dynamic 2.0 18 Nil Poly(styrene divinyl- GC–FID, GC–MS [139]
matter samples benzene)membrane
Organic liquid products Fossil fuels 300–400 Nil Static Nil Nil Nil Nil CHN analyzer [140]
Oxygenated materials Humic soils 150 and 250 3–120 atm Dynamic 0.5–1.0 0.5, 3, 10 h Nil Nil GC–MS HPLC [141]
Surfactants and some of their metabolites Sewage sludge 150–230 100 bar Static/Dynamic 1.0 1017 Soxhlet SPE LC–MS [142]
extrac-
tion
Reductive dechlorination of PCBs Oils 150–300 10 MPa Static Nil Nil Nil Nil GC–ECD/FID [143]
Amino acids Soil 150–250 17.2 MPa Static Nil 10 Nil Nil GC–MS [144]
Organophosphate triesters (OPs) Sediments 90 1500 psi Static Nil 5 Nil Oasis HLB cartridge GC–MS [145]
 C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494 2491

Reference

[146]
[148]

[149]

[151]

[152]
[153]
[150]

[154]

[155]

[147]
[63]
On-line coupled to
Analysis method

HPLC–UV
GC–FID
GC–MS

GC–MS

GC–MS
LC–MS

HPLC
HPLC

HPLC
UV

GC
pre-treatment

Liquid–liquid
C-18 trap

C-18 trap
Sample

SPE
Nil
Nil

Nil
Nil

Nil

Nil

Nil
Soil desorption
method(s)

extraction

extraction

extraction

extraction
Reference

method
Organic
Soxhlet

Soxhlet
Stir bar
Nil

Nil
Nil

Nil
Nil

Nil
Extraction

270, 360
90, 180,
(min)
time

Nil

Nil
25
20
10

30

30
60

90
8

Fig. 2. Chromatograms obtained for GA and VA in Gastrodia elata by (A) PHWE at

Flow-rate

Varies for

100 ◦ C and (B) heating under reflux with 60 ml pure water for 60 min. HPLC con-
1.6 g/min
(ml/min)

0.4–3.5

1.0–2.0

1.0 and

ditions: 0.1% formic acid in water (solvent A) and 0.1% formic acid in methanol
study

(solvent B) as mobile phase. At initial condition, gradient of pump B was set at 10%
2.5
1.0

1.0

0.5

1.0

2.0
Nil

and increased to 100% in 25 min and then returned to initial condition for 10 min.
UV detection was at 270 nm. Oven temperature was at 40 ◦ C and flow rate was set
at 0.7 ml/min [26].
Static–dynamic
Dynamic and
Dynamic

Dynamic
Dynamic

Dynamic

Dynamic

Dynamic

Dynamic

Dynamic

Dynamic

erinary drugs, (3) persistent environmental chemicals and (4)

Mode

Static

naturally occurring toxicants as described in a review [109]. The

extraction of pollutants in food is usually associated with long
extraction and cleanup procedures based on the use of Soxhlet
65–500 atm

75–300 atm

method and/or saponification [41,45]. These procedures are labori-

Pressure

5.0 MPa

120 kPa
100 bar

<20 bar

ous and time consuming and usually employ large volumes of toxic
50 bar

8 MPa
Nil

Nil

Nil

organic solvents. From Table 2, the applications of this green PHWE

method can aid to extract various chemical constituents present in
Analyses of pesticides and herbicides in soils and sediments based on PHWE.

the food matrices.

Temperature (◦ C)

In Table 2, sulfonamides (SAs) are bacteriostatic compounds

90–130 pH 7.5

routinely used in veterinary medicine to treat a variety of bac-

(phosphate
buffered)
120–200

170–250

terial and protozon infections in poultry. However, they can be

260, 280
50–150

50–150
50–200

50–300

carcinogenic and thus pose human health risk. The recovery of

130

120

105

SAs from meat samples was successfully achieved by PHWE using

dynamic and static modes [110,112]. Water was the extraction fluid
of choice because of its low affinity towards fats and the polar
Soil and sediments

Soil and sediments

Marine sediments

character of the analytes. In the static mode, a higher tempera-

Spiked compost

ture of 160 ◦ C was needed for the recovery of SAs compared to the
dynamic mode with 4 ml of water at 1.0 ml/min passed through
samples
Matrix

the extraction cell heated at 80 ◦ C [110]. There were also the pesti-
Soils

Soils

Soils

Soils

Soils
Soil

Soil

cides and herbicides residues on food and animal feeds due to pest
and fungal control (Table 2). These harmful chemicals can enter
the human system through direct consumption of contaminated
cloransalum-methyl

food or through milk, meat and other products obtained from ani-
Chlorophenoxy acid

Naphthalene (PAH)
Triazine herbicides

mals that feed on contaminated feed and fodder [109,111]. PHWE

Organochlorine

Pesticides and

has proved to be feasible to extract these harmful chemicals from

herbicides

herbicides
pesticides

Tricyclazole

the skin of grapes at 120 ◦ C at a flow rate of 1.0 ml/min for 40 min
Analyte(s)

Pesticides

Pesticides

Pesticides

Pesticides
Herbicide

[73], carbamate insecticides in bovine milk at 90 ◦ C at a flow rate of

Table 4

1.0 ml/min for 5 min [111] and herbicides in wheat flours and their
products under static extraction for 15 min at 120 ◦ C [113]. Hence,
2492 C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494

PHWE has gradually increased its role in the sample preparation and herbicides from environmental soil and sediment samples to
for food safety analysis. allow for remediation [15,16,146–156]. As seen in Table 4, PHWE is
a suitable method for the recovery of pesticides/herbicides because
4.3. Environmental samples of its compatibility with the solid samples. PHWE could recover
pesticides from the environmental samples at a temperature less
The analysis of environmental samples is always challenging than 300 ◦ C (Table 4). The optimization of extraction temperature
due to the diversity and complexity of sample matrices with a vari- was required to improve the selectivity of the PHWE to extract dif-
ety of trace level organics [119]. The earlier successful application of ferent classes of compounds from the environmental specimens.
PHWE by Hawthorne and co-workers has demonstrated the feasi- It was found that pesticides such as malathion, heptachlor, aldrin,
bility of using polar solvent such as water to extract some PAHs dieldrin, butachlor, metalaxyl and propiconazole were extracted at
from soil under appropriate controlled experimental conditions 160 ◦ C while chlordane and thiobencarb were recovered at 120 ◦ C
[39]. A comparison on the recoveries of PAHs by the conventional and 180 ◦ C respectively from sand [146]. A combination of stir bar
Soxhlet extraction, PHWE, SFE and PFE methods showed that the sorptive extraction (SBSE) with PHWE was shown as a compat-
qualities of the extracts were rather different. The colour of the ible alternative to recover organochlorine pesticides (OCPs) and
PHWE extracts was lighter than the extracts obtained from the chlorobenzenes in soils [148]. An on-line method of extraction
other methods. This observation was due to n-alkanes which were using PHWE was developed for the analysis of five triazine her-
more readily extracted by other methods as compared to PHWE bicides from a spiked complex compost matrix with inclusion of
[39]. The solubility behaviour of three PAHs, namely the acenaph- cleanup steps [150]. Hence, PHWE can also aid in the soil/sediment
thene, anthracene, and pyrene, in superheated water was studied at remediation effort for environmental monitoring and safety.
temperatures from 50 to 300 ◦ C to understand the mechanisms of
extraction in PHWE [127]. As seen in Table 3, PHWE could recover 5. Future outlooks
PAHs from the environmental samples at a temperature less than
250 ◦ C. The extraction yields of PAHs were also found to be com- Other than the wide ranging analytical applications of PHWE
parable to the other reference methods such as Soxhlet extraction illustrated in Tables 1–4, it is noted that the likely future trend
[71,72,128]. The kinetic removal of PAHs from soils was also studied for this technology is towards scaled-up operation so as to extract
by PHWE designed for semi-continuous experiments with resi- large volume of samples. The design of industrial-scale equipment
dence times of 1 and 2 h at 250 ◦ C [123]. A review on the usage of is usually preceded by laboratory (bench) and pilot-scale systems
high temperature pressurized water (both in sub- and supercritical after obtaining sufficient preliminary data and the process is simi-
conditions) in the presence of oxidants such as hydrogen perox- lar to existing one [156]. The key parameters such as temperature,
ide, oxygen, persulfate was reported for the extraction, destruction pressure, flow rate or pH are usually fixed to achieve desirable
and oxidation of PAHs from soil samples [132,133]. The subcritical extraction efficiency or rate [157]. The feasibility of PHWE as a
fluid extraction method was also highlighted as one of the reme- green solvent extraction method for industrial applications has
diation technologies specifically for PAH-contaminated soils in a been established in a pilot-scale project to recover compounds from
recent review [134]. highly contaminated soils. The capacity of a laboratory unit was
Apart from its wide applicability to recover the PAHs, PHWE scaled-up by a factor of 1000 to handle an increased amount of soil
is also shown as feasible option for other classes of compounds processed from 8 g to 8 kg [57]. With some modifications, PHWE
such as nitrogen-based pollutants [27], dioxins [131], brominated could be scaled-up to extract high volume of desirable compounds
based compounds [136,137], chlorinated organic pollutants [138], from other solid and powdered samples such as plant and food
organic liquid products [139] and surfactants [142] present in envi- materials. The botanical extracts are rich in chemical compounds
ronmental samples (Table 3). At 200–400 ◦ C, PHWE can extract or metabolites which can be a potential lead for drug discovery or
these analytes which are usually bound tightly to the sample matrix development of disease-resistant food crops. The scaled-up PHWE
in either dynamic or static mode (Table 3). There was also an could remove more organic contaminants from larger amount of
attempt to optimize PHWE to extract alanine, aspartic acid, glu- foodstuff samples to increase the productivity in food safety analy-
tamic acid, glycine, serine and valine in soil samples over the sis. This potential application could also treat more environmental
temperature range of 30–325 ◦ C at pressures of 17.2 or 20.0 MPa soil and sediment samples for remediation purposes.
[144]. None of the amino acids was extracted at 30 ◦ C (at 17.2 MPa)
as they might be too strongly bounded by the soil matrix to be
extracted at such a low temperature. The extraction efficiencies of 6. Conclusions
glycine, alanine, and valine were increased with increasing extrac-
tion temperatures from 150 to 250 ◦ C (at 17.2 MPa). The increased Despite the certain limitations discussed as compared to certain
solubility of these acids at higher temperatures could be due to classical method of extraction, PHWE is a feasible green extrac-
the decreasing dielectric constant of water. However, amino acids tion method to be exploited in the future technologies for more
were not detected in extracts collected at 325 ◦ C (at 20.0 MPa) due anlaytes to be used on a bigger scale. This simple technique uti-
to amino acid decomposition at this temperature. lizes the cheap and non-toxic water as an extraction fluid which
is environmentally friendly with little disposal issue. Under opti-
4.4. Pesticides and herbicides in soil and sediments mized conditions, PHWE could be a suitable technique for scale
up to handle larger sample sizes for industrial applications. Other
Agricultural consumption of chemicals, in the form of pesticides potential applications include the coupling of PHWE with chemical
and herbicides for pest and fungal control, has been viewed as a fingerprints and pattern recognition tools to aid in quality control
source of potential adverse environmental impact. The recovery of medicinal plants, improve the nutritional value of food crops or
of these chemicals from the soils/sediments is usually achieved produce a potential lead for drug discovery purposes.
with organic solvents such as acetone, ethyl acetate, or methanol.
These procedures are labour intensive, of low extraction efficiency Acknowledgement
and also involve high consumption of hazardous organic solvents.
Under certain optimized conditions, PHWE was proposed as a fea- The authors would like to acknowledge the financial support
sible alternative method to extract different classes of pesticides from Nanyang Technological University (NTU), Singapore.
 C.C. Teo et al. / J. Chromatogr. A 1217 (2010) 2484–2494 2493

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