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1258 TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 95, No. 5, 2012
SPECIAL GUEST EDITOR SECTION
Review of Sample Preparation Techniques for the Analysis of
Pesticide Residues in Soil
Jost L. TADEO, ROSA ANA PEREZ, BEATRIZ ALBERO, ANA I. GARCIA-VALCARCEL, and CONSUELO SANCHEZ-BRUNETE
Instituto Nacional de InvestigaciOn y Tecnologia Agraria y Alimentaria (INIA), Departamento de Medio Ambiente, Ctra de la
Coruna Km 7, 28040 Madrid, Spain
This paper reviews the sample preparation
techniques used for the analysis of pesticides
in soil. The present status and recent advances
made during the last 5 years in these methods are
discussed. The analysis of pesticide residues in
soil requires the extraction of analytes from this
matrix, followed by a cleanup procedure, when
necessary, prior to their instrumental determination.
The optimization of sample preparation is a very
important part of the method development that can
reduce the analysis time, the amount of solvent,
and the size of samples. This review considers all
aspects of sample preparation, including extraction
and cleanup. Classical extraction techniques,
such as shaking, Soxhlet, and ultrasonic-assisted
extraction, and modern techniques like pressurized
liquid extraction, microwave-assisted extraction,
solid-phase microextraction and QuEChERS (Quick,
Easy, Cheap, Effective, Rugged, and Safe) are
reviewed. The different cleanup strategies applied for
the purification of soil extracts are also discussed.
In addition, the application of these techniques to
environmental studies is considered.
p
esticides play an important role in agriculture, but they
must be used efficiently in order to be both economically
viable and environmentally sustainable. Many pesticides
have been classified as persistent organic pollutants and highly
toxic; hence, they have been banned by regulatory organizations
and replaced by more environmentally friendly products. The
legislation of many countries for environmental protection
from pesticide contamination makes necessary the development
of analytical methods suitable for detecting pesticides at low
concentration levels in the environment; soil is an important
matrix where pesticides are often directly applied or found after
their application to the herial part of plants.
Sample preparation is a very important part of the analytical
method. The development of an appropriate sample preparation
procedure includes a number of steps, such as extraction and
cleanup, to obtain a final extract concentrate of target analytes
as free as possible of matrix compounds. Due to the low levels
of pesticides that may be found in soil, an enrichment of the
Guest edited as a special report on "Methods of Pesticide Residue
Analysis" by Tomasz Tuzimski.
Corresponding author's e-mail: [email protected]
DOI: 10.5740/jaoacint.SGE_Tadeo
analyte concentration must be achieved before its instrumental
determination.
The selective extraction of pesticides from soil is based on
differences in their chemical and physical properties. These
include solubility, polarity, MW, and volatility. A more selective
extraction technique may eliminate or reduce the cleanup
required.
Classical methods for the determination of trace pesticides
in soil usually involve a large amount of sample and require
much manual handling of the extracts. These methods are
tedious and time-consuming and require large amounts of
solvents. Ultrasonic-assisted extraction (UAE) is a powerful
tool used to accelerate the analytical process in soil. This
technique is expeditious, inexpensive, and an alternative to
other conventional extraction methods. Moreover, several
novel extraction techniques have been developed in recent
years in an attempt to overcome the main limitations of
conventional methods. In general, these techniques allow a
reduction of organic solvent consumption and an increase of
sample throughput, or they are solventless, such as solid-phase
microextraction (SPME). Among those techniques, enhanced
extraction efficiency can be achieved with microwave energy by
doing a microwave-assisted extraction (MAE), or with solvents
at high pressure and temperature by means of pressurized liquid
extraction (PLE). In addition, QuEChERS (Quick, Easy, Cheap,
Effective, Rugged, and Safe) is a novel sample preparation
methodology that involves an initial extraction with acetonitrile
followed by an extraction/partitioning step after the addition of
a salt mixture.
Figure 1 shows the relative percentages of published articles
on the different techniques used for the extraction of pesticides
from soil, found in the available scientific literature during
2007-2010. PLE is the modern extraction technique most often
applied, followed by SPME. Among the classical techniques,
shaking continues to be a widely used procedure, and UAE is
still a popular technique.
Different aspects of the analysis of pesticides in environmental
samples have been previously dealt with by our group (1). The
aim of this paper is to review the main extraction and cleanup
procedures applied to the analytical determination of pesticides
in soil samples in the review period.
Classical Extraction Methods
Solid—liquid extraction (SLE) is the procedure most widely
used in the analysis of pesticides in soil; it is based on the
contact of a sample with an appropriate solvent. SLE includes
three widely used techniques: shaking, Soxhlet, and UAE.
TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 95, NO. 5, 2012 1259
consumption of solvents (to a total volume of 10-30 mL) have
been reported in recent years (6).
Extraction techniques
Soxhlet
MAE
QuEChERS
UAE
SPME
Shaking
PLE
4%
Soxhlet Extraction
11%
12%
16%
18%
19%
20%
Percentage
Figure 1. Publications on extraction techniques for
the analysis of pesticides in soil during 2007-2010.
Total number of articles: 74.
Shaking
This extraction technique involves shaking (manually or
mechanically) the soil in presence of an appropriate solvent
for a certain period of time. Several types of equipment, such
as orbital, vortex, and stirring shakers, have been used, among
others, for mechanical extraction. The most commonly used
solvents are acetone, methanol, and acetonitrile, due to their
miscibility with water, although immiscible solvents, such as
dichloromethane or hexane, can also be used. The published
shaking procedures for extraction pesticides from soil are
summarized in Table 1.
Polar herbicides like phenoxyacids (2, 3) and benzonitriles (4),
which are also acidic pesticides, are usually extracted from
soil with organic solvent-water mixtures at acidic pH, with a
solvent of medium polarity or with an alkaline solution, using
manual or mechanical shaking. The solvents most often used
were ethyl acetate and methanol. For less-polar pesticides such
as chloroacetamides (7), triazines and their metabolites (8, 9),
and acetamides (10), organic solvents like methanol and
acetonitrile, alone or in mixtures with water, were commonly
used. Typical characteristics of carbamate pesticides are their
high polarity and solubility in water. Therefore, extraction of
these compounds in soil has been carried out by shaking with
methanol or acetonitrile (5). Chlorothalonil, a tow polarity
fungicide, and its more polar degradation products were
extracted from soil with a dichloromethane—hexane solution
using a rotary shaker for 2 h (11).
Reliable multiresidue methods for the analysis of different
chemical families within a wide range of polarity are needed
for monitoring programs of pesticide residues in soils. For this
purpose, soil was shaken with acetone alone or in a mixture
with water (12-14, 17), with methanol—ethyl acetate (15), or
with acetonitrile (16). The addition of water has been reported
to increase the recoveries of polar pesticides, like some
organophosphates, as well as nonpolar pesticides, such as
pyrethroids, because it favors the mass transfer from soil to the
organic solvent phase.
This technique of sample preparation is simple, but it
is time-consuming and usually involves a great amount
of glassware and large volumes of solvents harmful to the
environment. In order to minimize these drawbacks, a decrease
of the extraction time (to approximately 15 min) and the
Soxhlet extraction is a general and well-established technique
used for the isolation of nonpolar and semipolar pesticides from
soil. The sample is placed in an apparatus (Soxhlet extractor),
and the extraction of pesticides is achieved by means of a hot
condensate of an organic solvent that is continuously refluxed
through the sample distilling in a closed system. The papers
published during the review period on the Soxhlet extraction
of pesticides from soil are summarized in Table 1. Soxhlet
extraction has been used for the isolation of organochlorine
pesticides using dichloromethane (18, 19) or a mixture of
hexane—acetone (20) as the extraction solvent.
The main advantages of this methodology are that the extract
obtained does not need to be filtered or centrifuged and that a
large amount of sample can be used. On the other hand, the high
volume of solvent required and the long duration of the process
are the drawbacks of this procedure. This technique, although
exhaustive, is not selective, and a cleanup is often necessary.
After the extraction step, rotary evaporation-concentration of
the extracts before cleanup and analysis is necessary due to
the large volumes of organic solvents used, which may lead to
losses of the most volatile compounds. In some cases, Soxhlet is
a more suitable procedure when shaking is not effective enough
to extract pesticides strongly bound to soil and an increase in
temperature is required. However, due to the high temperatures
involved in Soxhlet extraction, degradation of thermally labile
compounds may occur. Other disadvantages are extraction
times of about 20 h and organic solvent volumes in the range
of 50 to 200 mL.
Recent improvements of this technique, namely S oxtec®
System HT, Soxwave-100, and focused microwaveassisted Soxhlet extraction, have been proposed, aimed at
overcoming most of the shortcomings of conventional Soxhlet.
Automated Soxhlet extraction may reduce extraction times
significantly and perform boiling, rinsing, and solvent recovery
automatically (21).
Ultrasonic-Assisted Extraction (UAE)
UAE is a conventional technique based on the extraction of
soil samples with an appropriate organic solvent by applying
ultrasound radiation in a water bath or with other devices, such
as probes, sonoreactors, or microplate horns. The mechanical
effect of ultrasound induces a greater penetration of solvent into
soil and improves mass transfer, leading to an enhancement of
analyte extraction efficiency. The most available and cheapest
source of ultrasound irradiation is the ultrasonic bath, but at
present a more efficient system using a cylindrical, powerful
probe for the sonication of samples has been developed (22).
To maximize extraction, it is necessary to optimize different
factors, such as the type of solvent and irradiation conditions
(temperature and amplitude of sonication). Other parameters
that influence extraction efficiency are sonication time, sample
particle size, sample amount, and the ultrasound device used
(bath or probe). Some examples of analysis of pesticides in soil
using UAE are summarized in Table 2.
Organochlorine pesticides are lipophilic compounds and tend
1260
TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 95, No. 5, 2012
Table 1. Shaking and Soxhlet techniques for extraction of pesticides from soil
Analyte (number)
Type
Sample size, g
Time, min
Solvent, mL a
Ref.
2
Shaking
MCPA
Manual
20
2
DCM, 50
Chlorophenoxy acids (2)
Manual
10
2
NaOH—EtAc, 40
3
Mechanical
10
45
Acetone, 50
4
Carbamates/carbamoyloxime (10)
Mechanical
5
120
MCAAB, 20
5
Carbaryl, triazophos
Mechanical
1
30
Me0H,10
6
Chloroacetamides (2)
Mechanical
20
90
Me0H—water (1 + 1, v/v), 80
7
Vortex
100
30
Me0H—ACN (1 + 1, v/v), 100
8
Bromoxynil
Triazines (3)
Atrazine and metabolites
Mechanical
10
5
Me0H—water (1 + 1, v/v), 30
9
Chlorophacinone
Mechanical
20
60
Me0H—ammonium bicarbonate 0.01 M, 100
10
Chlorothalonil
Mechanical
8-10
120
DCM—hexane (1 + 1, v/v), 40
11
Multiclass (37)
Mechanical
8
30
Acetone—HAc 1%, 30
12
Multiclass (32)
Mechanical
25
240
Acetone—EtAc—water (2 + 2 + 1, v/v/v), 50
13
Multiclass (62)
Mechanical
50
60
Acetone—ammonium chloride, 130
14
Multiclass (9)
Mechanical
10
240
Me0H or Me0H—EtAc (70 + 30, v/v), 20
15
Multiclass (6)
Mechanical
10
60
ACN, 50
16
Multiclass (3)
Mechanical
4
30
Acetone, 10
17
20
24 h
DCM, 100
18, 19
10
16 h
Acetone—hexane (1 + 1, v/v), 250
20
Soxhlet
Organochlorine (8, 13)
Organochlorine (20)
a
Automatic
DCM = Dichloromethane; EtAc = ethyl acetate; MCAAB = monochloroacetic buffer; HAc = acetic acid; ACN = acetonitrile; Me0H = methanol.
to remain adsorbed onto the surface of organic matter present
in soil. Methanol (24) or mixtures of n-hexane with a more
polar organic solvent such as acetone (25, 26) have been used
in the extraction of these analytes from soil. Organophosphorus
pesticides are polar compounds soluble in water that have been
extracted from soil by sonication with methanol or a mixture
of solvents (28). For nitrogen-containing pesticides, such
as triazines (33) and pyrimidines (34), organic solvents like
methanol, alone or in mixtures with water, have been commonly
used.
When pesticide residues belonging to different chemical
classes with a wide polarity range have to be determined
simultaneously, the solvent selection is critical and the
extraction is often performed with a semipolar solvent such as
ethyl acetate or acetonitrile (35). A fast UAE procedure, using
acetonitrile–water (1 + 1, v/v) as the extraction solvent, was
used for the simultaneous determination of 54 pesticides in
soils (22).
Although traditional extraction procedures, as described
above, allow the isolation of the pesticide analytes adequately,
the trend is to develop analytical methods in which the sample
preparation is less time-consuming and requires lower solvent
consumption (27). A miniaturized technique for reducing both
the amount of sample and the volume of organic solvent, named
sonication-assisted extraction in small columns (SAESC), has
been developed in our laboratory (35). In this method, the soil
sample located in a small column is placed in an ultrasonic
water bath wherein the pesticides are extracted with a low
solvent volume assisted by sonication. Analyses of fungicides,
insecticides, and herbicides in soil samples have been reported
using SAESC with three different solvents: ethyl acetate,
methanol, and acetone. This methodology requires lower
solvent volumes and shorter extraction times, thus reducing the
costs and toxic wastes generated, and uses low-cost equipment,
making SAESC a very attractive technique for extraction of
pesticides in soils. Figure 2 shows a schematic diagram of the
extraction of soil samples using SAESC.
The application of UAE to the extraction of pesticide
residues from soil is generally carried out in an ultrasonic
bath (24, 27, 28, 35); however, an analytical method applying
sonoreactors for the determination in soil of Cl-containing
herbicides has been lately reported by Ueno et al. (36).
UAE has been recently carried out using a dynamic extraction
setup (DUAE) with a system that continuously supplies fresh
extraction solvent to the extraction cell placed in an ultrasonic
water bath or in a water bath equipped with an ultrasonic probe.
The DUAE technique not only reduces extraction time and
solvent consumption, but also allows the possibility of coupling
to several techniques, making possible, in this way, the online
coupling of DUAE to instrumental analysis (37).
An interesting approach in sample preparation is to couple
UAE with other extraction techniques, such as stir bar sorptive
extraction (SBSE; 38), to make the most of both procedures
in order to achieve good extraction yields, with lower solvent
consumption, and be cost-effective (29, 39).
Modern Extraction Methods
MAE. MAE, also known as microwave-assisted solvent
extraction, uses microwave energy for the extraction of
—
TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 95, No. 5, 2012
Table 2.
1261
UAE of pesticides from soil
Analyte (number)
Type'
Chlorothalonil
Organochlorine (17)
Sample size, g
10
UB
Organochlorine (21)
Time, min
30
x
2
Solvent, mL b
Ref
Acetone, 50
23
1
30
Me0H, 15
24
5
4x3
Hexane—acetone (1 + 1, v/v), 90
25
2
30 a 2
Hexane—acetone (1 + 1, v/v), 30
26
Organochlorine (18)
UB
0.5
5x3
Acetone—petroleum ether (1 + 1, v/v), 15
27
Organophosphorus (7) + buprofezin
UB
1.5
15
Me0H—ACN (1 + 1, v/v), 10
28
1
60
ACN, 20
29
Permethrin
0.5-5
15 x 2
DCM, 20
30
Triazoles (3)
20
30
ACN, 50
31
Enestroburin
20
30
ACN, 70
32
Sulfonylureas (4)
20
30
Me0H—water (1 + 1, v/v), 60
33
10
20
Me0H—water (2 + 1, v/v), 18 + 0.1 M HCI, 2
34
UB
5
15 x 2
EtAc, 10
35
UP
10
15
ACN—water (1 + 1, v/v), 20
22
Metribuzin, quinalofop-ethyl
Triazolopyrimidine herbicides (4)
UB
Multiclass (50)
Multiclass (54)
a
UB = Ultrasonic bath; UP = ultrasonic probe.
ACN = Acetonitrile; Me0H = methanol; DCM = dichloromethane; EtAc = ethyl acetate.
analytes from a sample. MAE is a modern extraction technique
that provides a significant reduction in solvent consumption
requires lower amount of sample, and shortens extraction times
compared to traditional extraction procedures. Nevertheless,
filtration and cleanup of the extracts from soil samples are
usually required prior to chromatographic analysis. In MAE,
the microware energy causes molecular motions that heat
the extractive solvent and promote extraction of the analytes
from the matrix into the solvent. Because microwaves are
electromagnetic waves, this energy is only absorbed by
molecules with a high dielectric constant. For this reason, to
carry out the extraction of analytes, the solvent or mixture
of solvents must be polar or include some proportion of a
polar solvent (as methanol, ethanol, water, or acetone). Thus,
although hexane is not potentially a good solvent for MAE, it
has been described as adequate for the extraction of different
types of pesticides from soil (40-42) when it was used together
with a polar solvent. Two types of MAE have been developed:
pressurized and focused MAE, depending on the microwave
energy application to the samples using closed vessels (with
controlled pressure and temperature) or open vessels (under
atmospheric pressure), respectively. Pressurized MAE is the
technique used in the extraction of pesticides from soil (Table 3)
that allows extracting multiple samples simultaneously. The
main parameters that, affect pressurized MAE of pesticides
from soil are the extraction solution, temperature, extraction
time, and microwave power. Usually, 5 g of dried and sieved
soil and 20-25 mL of solvent are used in the extraction process.
Nevertheless, Fuentes et al. (41) and Morozova et al. (43) used
lower volumes of solvent to carry out pesticide extractions from
1 and 2.5 g of soil, respectively.
Fuentes et al. (41) and Hernandez-Soriano et al. (44) used the
surface response approach to improve the recoveries obtained
when MAE was used for the determination of organophosphorus
and pyrethroid pesticides in soil samples. In these works, the
surface methodology was applied to find the optimum values
of the parameters involved in the extraction of analytes, and the
proposed methods were applied to the analysis of real samples.
This methodological approach (41) was recently applied by
Rodriguez-Liebana et al. (48) to evaluate the effect of the use of
wastewater, or dissolved organic carbon and some of the salts
present in wastewater, on pesticide sorption onto soil.
PLE. PLE, also known as accelerated solvent extraction,
is a technique introduced in 1995 (49, 50) that, like MAE, is
less time-consuming and provides a lower solvent consumption
than traditional extraction procedures, such as shaking or
Soxhlet extraction. In PLE, solvent is pumped into an extraction
cell containing the sample, where it is subjected to elevated
temperature and pressure. The sample cell sizes can vary from
1 to 120 mL, although it depends on the PLE system used. After
extraction, the extract is automatically transferred from the cell
to a collector vial.
Each extractive cycle involves three steps: heating, hold,
and discharge. During the heat up period, the pressure inside
the extraction cells slowly increases to the set value of the
extraction method. The extraction conditions remain constant
during the hold period; finally there is pressure compensation
and extracts are collected in the vials in the discharge step. The
whole process is automated; each step can be programmed to
allow sequential or simultaneous extractions of several samples,
depending on the PLE instrument.
—
5 ml AcEt
5 g soil
2 Filter
papers
2g NauSOc anh.
Extraction
Vacuum
manifold
Filtration
Figure 2. Schematic diagram of SAESC procedure
(from ref. 88).
1262
TADEO ET AL.: JOURNAL
Table 3.
OF AOAC INTERNATIONAL VOL. 95, No. 5, 2012
MAE of pesticides from soil
Sample
size, g
Solvent'
5
25 mL n-hexane—acetone (1 + 1, v/v)
1200
110°C/10 min ramp to
110°C, 10 min hold time
40
1-2 mL water—MeOH (1 + 1, v/v) and 0.02 M KH 2 PO 4
homogenization 5 mL n-hexane
500
10 min
41
5
20 mL hexane—acetone (1 + 1 , v/v)
1500
100°C/10 min ramp to
100°C, 10 min hold time/
magnetic stirring
42
2.5
5 or 10 mL EtOH—water (40%)
1200
15 min
43
Pyrethroid and organophosphorus (5)
5
25 mL EtAc
110°C/10 min ramp
to 110°C, 5 min hold
time/1 mL extra water/
highest speed stirring
44
Carbamate and urea pesticides (8)
5
20 mL ACN
1500
70°C/10 min ramp to 70°C,
10 min hold time/
medium speed stirring
45
200 pL Alkyl benzene sulfonate (10 mg/mL in acetone),
200 pL benzalkonium chloride (3 mg/L in acetone),
8 mL HNO 3 (65%), 2 mL HCI (37%), and 2 mL HF (48%)
20 mL 4% Boric acid aqueous solution
800
200°C/30 min ramp to
200°C, 20 min hold time
180°C/15 min ramp to
180°C, 15 min hold time
46
50 mL Me0H—water (50 + 50, v/v)
1000
100°C/10 min
47
Analyte (number)
Organochlorine (10 )
Organophosphorus (6)
Chlorfenvinphos
Chlorophenoxy acids (2)
Quaternary ammonium herbicides (3)
Chloroacetanilide pesticide (2)
and their acidic metabolites (6)
a
10
Microwave Temperature, °C/time, min/
power, W
other conditions
Ref.
ACN = Acetonitrile; EtOH = ethanol; Me0H = methanol; DCM = dichloromethane; EtAc = ethyl acetate.
The main parameters involved in PLE procedures are
temperature, pressure, extraction solvent, number of cycles,
and hold time. There is another parameter, the flush volume,
that is important to avoid carryover of extract residues to a
subsequent run. Temperature is the parameter that has the
highest impact on the speed of the extraction and the recovery
of analytes. Nevertheless, it is necessary to take into account
that heat-sensitive compounds could be degraded during the
extractive process. For this reason, it is generally preferable
to work above, but close to, the boiling point of the solvent.
The elevated pressures used in PLE increase the penetration
of organic solvents into the matrix as a consequence of the
decrease in viscosity and surface tension, and the ability to
overcome strong solvent—matrix interactions (51). A pressure
of about 10 MPa (1500 psi) is usually used for a very broad
range of applications. As in other extractive techniques, the
extracting solvent has a major impact in the extraction, being
a parameter to be optimized in the PLE process. Various cycles
are necessary when the extraction efficiency is limited by the
saturation of the solvent with the analyte. On the other hand,
when the extraction efficiency is limited by the time required for
the solvent to penetrate the matrix and to dissolve the analyte,
it is more efficient to increase the hold time. Additionally, it
is necessary to carry out an adequate sample preparation, and
it is usually recommended to disperse the sample with inert
materials to avoid aggregation of sample particles. The PLE
conditions reported for the extraction of pesticides from soil
during recent years are shown in Table 4.
In the period reviewed, organochlorine pesticides
have been the main pesticide group analyzed in soil by
PLE (40, 52-56). In these works, different mixtures containing
10 to 17 organochlorine pesticides were studied. The sample
size varied from 1 to 10 g; the dispersing agent was usually
diatomaceous earth. Vega Moreno et al. (56) used aluminum
oxide as dispersing agent, but it was not reported why this agent
was selected nor its quantity, whereas other parameters like the
effects of different solvent mixtures, the extraction temperatures
and times, and the flush volume were evaluated in the study.
Several solvent mixtures, such as hexane—acetone (1 +1, v/v or
3+1, v/v; 52, 53, 56), ethyl acetate—n-heptane (1+1, v/v; 54),
and acetone—n-heptane (1 +1, v/v; 54, 55), have been reported
as adequate for the extraction of organochlorine pesticides
from soil by PLE. These extractions are usually carried out at
100°C, at about 10 MPa, and with three cycles of 5 min of static
extraction. However, Vega Moreno et al. (56) have reported
organochlorine pesticide recoveries from 64 to 103%, with a
temperature of 50°C, 5 min of static extraction, and only one
cycle. The availability of different extraction cell sizes makes
it possible to carry out an in-cell cleanup simultaneously with
the extractive process. Thus, the use of 10 g of Florisil topped
with 2 g of sodium sulfate and the soil sample homogenized
with diatomaceous earth have been described as adequate to
perform a simultaneous extraction and cleanup for multiresidue
analysis of organochlorine pesticides in soil with good
recoveries (54). This method was successfully applied to the
assessment of organochlorine pesticide pollution in Upper
Awash Ethiopian state farm soils (55). Other pesticide groups
that have been extracted from soils by PLE are pyrethroids (61)
and triazines (62, 63), using quite different sample quantity and
conditions (Table 4).
Some works have focused on development of PLE methods
for the multiresidue analysis of widely used pesticides (5760) instead of a group of pesticides in the same family (61).
In these methods, the procedures are quite different. Thus,
sample size was between 1 and 100 g of soil and the dispersing
agent was Florisil, silica gel, hydromatrix, or another agent. In
TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 95, NO. 5, 2012
Table 4.
1263
PLE of pesticides from soil
Analyte (number)
Sample
size, g
Dispersion agent
(amount, g)
Other agent in cell
Solvent°
Temperature, °C/P, psi b /t, min/flush
volume, %
Cycles
Ref.
3
52
Organochlorine
(11)
1
Diatomaceous
earth (0.25)
Hexane-acetone
(3 + 1 , v/v)
100°C; 2000 psi, 5 min heat time, 5 min
static extraction, flush volume 60%,
100 s purge time
Organochlorine
(17)
1
Diatomaceous
earth (0.25)
Hexane-acetone
(1 + 1 , v/v)
100°C; 1500 psi, 5 min static extraction
Organochlorine
(13)
4
Diatomaceous
earth (1)
EtAc-n-heptane
(1 + 1, v/v)
100°C; 1494 psi, 5-8 min heat time,
flush volume 50%, 60 s purge time
3
54
55
Acetone-DCM
(1 + 1, v/v)
140°C, 1500 psi, 6 min heat time, 5 min
static extraction, purge using N2 at
1500 psi
1
40
Hexane-acetone
(1 + 1 , v/v)
50°C; 1500 psi, 5 min heat time, 5 min
static extraction, flush volume 60%,
300 s purge time
1
56
Acetone-DCM
(1 + 1, v/v)
130°C; 1500 psi, 5 min static extraction
time, flush volume 60%,
60 s purge time
2
57
Water-ACN
(1 + 2, v/v)
140°C; 1595 psi, 20 min
3
58
Acetone
100°C; 1450 psi, 3 min static extraction,
flush volume 60%, 120 s purge time
3
59
Organochlorine
(10)
10
Organochlorine
(13)
5
Aluminum oxide
Multiclass (30)
1
Florisil (5)
Multiclass (24)
5
Silica gel (1)
Multiclass (7)
100
10 g Florisil topped
by 2 g sodium
sulfate anhydrous
2 g Florisil
53
Acetone-n-heptane
(1 + 1, v/v)
Multiclass (122)
6
Hydromatrix (7.5)
EtAc-MeOH
(3 + 1, v/v)
85°C; 1500 psi, 2 min heat time, 5 min
static extraction, flush volume 60%,
60 s purge time
2
60
Pyrethroids (12)
10
Diatomaceous
earth (2)
Acetone-DCM
(1 + 1, v/v)
100°C; 1500 psi, 10 min static
extraction
3
61
Triazines (11)
2
Hydromatrix
Acetone-MeOH
(50 + 50, v/v)
65°C; 1500 psi, 5 min heat time, 5 min
static time, 3 min extraction time, flush
volume 60%, 60 s purge time
3
62
Atrazine and
2 metabolites
15 or 60
Acetone
60°C, 1494 psi, 5 min heat time, 5 min
static extraction, flush volume 90%,
120 s purge time
3
63
Chlormequat
30
Me0H-water
(35 + 65, v/v)
130°C; 1500 psi, 5 min static
extraction time
2
64
Trifloxystrobin
10
Me0H-water
(50 + 50, v/v)
40°C, 1485 psi, 5 min heat time, 5 min
static extraction, flush volume 60%,
60 s purge time (150 psi)
1
65
Diatomaceous earth
a
ACN = Acetonitrile; Me0H = methanol; DCM = dichloromethane; EtAc = ethyl acetate.
b
Pressure units commonly used in PLE, 1500 psi = 10 MPa.
general, PLE methods for the extraction of a reduced number
of pesticides used a large amount of sample ( 10 g), and the
quantity of the dispersion agent was low (61), none (63, 64), or
not specified (65).
Hildebrandt et al. (57) developed a PLE method for the
analysis of 30 widely used pesticides and various transformation
products and, after evaluation of the extraction solvent, the
sorbents used for in-cell cleanup, and the effects of extraction
temperature and pressure, the recoveries obtained ranged from
48 to 144%.
Schreck et al. (59) applied PLE to the extraction of seven
pesticides from different chemical families. In this work, a large
quantity of sample (100 g)—without homogenization with any
sorbent—was used, and the extraction solvent, temperature, and
number of cycles were evaluated in order to achieve efficient
extractions. Recoveries higher than 93% were obtained for all
compounds after three extractive cycles.
Recently, Martinez Vidal et al. (60) developed a multiresidue
method for the analysis of 94 nonpolar pesticides and 28 polar
pesticides in soil. Several parameters, such as extraction solvent,
number of extraction cycles, preheating time, and extraction
temperature, were evaluated. The best results, both for nonpolar
and polar pesticides, were obtained using ethyl acetate–methanol
(3 + 1, v/v) as the solvent. Because lower recoveries of nonpolar
pesticides were obtained at high temperature, the extraction
temperature and preheating time were finally set at 85°C and
2 min, respectively. The average extraction recoveries reported
were in the range of 71 to 108%, and the method had a sensitivity
good enough to carry out analysis at concentrations lower than
the levels set by legislation (66).
QuEChERS.—In 2003, Anastassiades et al. (67) developed
the procedure called QuEChERS to extract pesticides in fresh
fruits and vegetables, matrixes with high water content. This
procedure involves the extraction of 10 g sample with 10 mL
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TADEO ET AL.: JOURNAL
OF AOAC INTERNATIONAL VOL. 95, No. 5, 2012
Table 5. QuEChERS extraction of pesticides from soil
Sample
size, g
Water, mL
Acetonitrile, mL
Salts, buffer
Ref.
Ecological insecticides: azadyrachtin,
spinosad, rotenone
5
5
10 + HAc, 0.1 8
4 g MgSO 4 , 1 g NaCI,
0.5 g Disodium citrate sesquihydrate
1 g Trisodium citrate dihydrate
69
Metaflumizone
10
5
10
4 g MgSO 4 , 1 g NaCI
70
3
Analyte (number)
Procymidone
10
10
2 g NaCI
71
Oxadiargyl
10
15
4 g MgSO 4 , 1 g NaCI
72
Organophosphorus (10) + buprofezin
10
20
4 g MgSO 4 , 1 g NaCI,
0.5 g Disodium citrate sesquihydrate
73
Organochlorine (19)
5
10 or 1 M Na 2 -EDTA, 10
9.9 + HAc, 0.1
4 g MgSO 4
1.7 g Sodium acetate trihydrate
74
Carbamates
5
3
5
2 g MgSO 4 , 0.5 g NaCI
75
Phenolic compounds
10
5
9.9 + HAc, 0.1
6 g MgSO 4 , 4 g NaCI
1.7 g Sodium acetate trihydrate
76
Multiclass (24)
10
20
4 g MgSO 4 , 1 g NaCI,
1 g Sodium citrate dihydrate
0.5 g Disodium citrate sesquihydrate
58
Multiclass (38)
10
20
8 g MgSO 4 , 2 g NaCI,
1 g Disodium citrate sesquihydrate
2 g Trisodium citrate dihydrate
77
4
a HAc = Acetic acid.
acetonitrile, followed by liquid partitioning using 4 g MgSO 4
and 1 g NaC1 to remove water, and a dispersive solid-phase
extraction (d-SPE) cleanup with primary secondary amine
(PSA) sorbent. The application of this procedure allows
obtaining good results for the extraction of polar as well as
nonpolar pesticides. The use of acetonitrile instead of acetone
or ethyl acetate has several advantages, such as to extract less
lipophilic compounds, facilitate the removal of residual water
with drying agents, and form well-differentiated partitioning
phases with nonpolar solvents, which can provide convenient
cleanup if necessary. The main disadvantage is that 1 g/mL of
final extract concentration is lower than the 2-5 g/mL obtained
in the most traditional methods, requiring a highly sensitive and
selective analytical instrument.
The QuEChERS method has been accepted as a standard
sample preparation method for fruits anci,vegetables by AOAC
INTERNATIONAL due to its simplicity, inexpensiveness,
amenability to high throughput, and high efficiency (68).
In addition, great interest has been recently shown on the
application of this procedure to other matrixes, such as soil.
Modifications of the original QuEChERS procedure by using
acidic-buffered extractions, adding water in order to obtain
adequate moisture, or using different adsorbents in the d-SPE to
remove matrix components, as described below in the cleanup
section, have been used for the extraction of pesticides from soil
with good results. The different QuEChERS procedures used in
pesticide extraction from soil are shown in Table 5.
Usually, the sample size is the same as that of the original
method (10 g), whereas in many cases a higher amount of
acetonitrile is used because a sufficient volume of supernatant
has to be obtained, after cleanup, to allow its transfer into a
vial and injection into the GC or LC system (58, 72, 73, 77).
On the other hand, many authors added water to soil samples
to facilitate the access of the extraction solvent to soil pores
for extracting the bound target analytes. Some authors
added water to soil samples 30 min before extraction with
acetonitrile (74, 77), whereas others added water and acetonitrile
at the same time (69, 70, 76). Several authors used buffered
extraction (pH = 5), by citrate or acetate buffering, with the aim
of enhancing recoveries for pesticides showing pH dependence.
In order to stabilize the possible losses of pesticides due to the
increase of temperature reached during the initial acetonitrile
extraction by the addition of anhydrous MgSO 4 , acidification is
carried out in some cases (69, 74, 76).
Although QuEChERS is a fast and easy method,
extraction conditions stronger than shaking may be needed
to overcome the strong binding characteristics of soil. In
this way, Asensio-Ramos et al. (73) and Santalad et al. (75)
used sonication in the QuEChERS step to enhance pesticide
extraction from soil.
SPME. SPME was developed in the 1990s by Arthur and
Pawliszyn (78). This technique is based on the use of a fiber
coated with a stationary phase that sorbs analytes from the
matrix, followed by desorption of retained compounds into
an analytical instrument. This procedure integrates extraction
and concentration into one step without using solvent. The
efficiency of analyte extraction by SPME is dependent upon the
nature of the matrix, time period of absorption and desorption,
and temperature. Moreover, the efficiency also depends on the
kind of fiber used. The main SPME methods for pesticides in
soil are carried out by the preparation of a mixture of soil with
water, or by dilution with water of the organic extract obtained
with other techniques, such as UAE or MAE, and subsequent
adsorption in the SPME fiber of the analytes from the mixture.
The soil/water suspension can be sampled by direct immersion
of the fiber or by headspace extraction (HS-SPME). Generally,
HS-SPME reduces the matrix effect because the fiber is not in
direct contact with the matrix, and only volatile or semivolatile
compounds are released into the headspace.
Several pesticides in soil samples have been determined by
—
TADEO ET AL.: JOURNAL
Table 6.
OF AOAC INTERNATIONAL VOL. 95, No. 5, 2012 1265
SPME of pesticides from soil
Matrix preparation'
Fiberb
Technique
Adsorption
Desorption
Ref.
Slurry: 2 g soil + 2 mL water
CACF
HS-SPME
Agitation with magnetic stirring
in a water bath at 500 rpm and
constant temperature
GC injection port
79
Analyte (number)
HCH
at 300°C (2 min)
Triazines (5)
DCM MAE extract, concentrated and
dissolved in benzene
MIP
DI-SPME
Agitation with a magnetron at
100 rpm (30 min)
LC desorption chamber
with Me0H (10 min)
80
Triazines (2)
Slurry: 1 g soil + 0.5 mL NaCI saturated
water
PPy-DS
HS-SPME
Agitation with magnetic stirring
in a water bath at 70°C (80 min)
Injection port of IMS'
at 220°C (60 s)
81
Pyrethroids (5)
ACN extract, diluted 1/10 with water
Sst
Agitation with stirring at 600 rpm
(30 min)
GC injection port
at 300°C (4 min)
82
Multiclass (36)
Slurry: 0.5 g soil + 0.5 mL water
PA
HS-SPME
Agitation in a water bath at 100°C
(30 min)
GC injection port
at 290°C (5 min)
83, 84
Multiclass (20)
Me0H extract evaporated to dryness,
dissolved in acetone and diluted 1:50
with 5% NaCI water
PDMS
DI-SPME
Immersion (30 min)
GC injection port
at 270°C (7 min)
85
Me0H—acetone (1 + 1, v/v) extract evaporated to dryness, dissolved in acetone
and diluted 1:50 with 25% NaCI water
PDMS
DI-SPME
Immersion at 75°C (30 min)
GC injection port
86
Multiclass (5)
a For solvent abbreviations,
at 270°C (7 min)
see Table 1.
PDMS = Polydimethylsiloxane, PA = polyacrylate, CAGE = cold activated carbon fiber, MIP = sylilated silica fiber + prometryn + metacrylic acid +
azo(bis)-isobutyronitrile + trimethylolpropane trimethacrylate, PPy-DS dodecylsulfate-doped polypyrrole, Sst = stainless steel wire etched by hydrofluoric acid.
IMS = Ion mobility spectrometer.
SPME coupled with LC or GC (Table 6). Various commercially
available SPME fiber coatings, mainly polydimethylsiloxane
(PDMS) and polyacrylate, among others, have been used in
these analyses. When HS-SPME is used, high temperatures
normally release analytes from the matrix; however, this
heat can reduce the fiber's ability to adsorb analytes because
adsorption is an exothermic process (87). On the contrary, if
high temperatures are not used, a long equilibrium time will be
required, otherwise the sensitivity will be reduced. Therefore, to
overcome this problem or enhance selectivity, other fibers, such
as cold-activated carbon (79) or molecularly imprinted polymer
(MIP; 80), have been developed. Table 6 shows the different
pesticides that have been determined in soil samples by SPME
coupled with LC or GC during the review period. Some authors
used water with NaCl to enhance retention of pesticides in the
fiber (81, 85, 86), and the use of simultaneous or sequential
application of MAE with SPME is sometimes used to overcome
strong interactions between the analyte and matrix (80).
The main advantage of SPME is to allow the simultaneous
solventless extraction and concentration of analytes, but a
disadvantage of this technique is the high RSD values obtained
when analyses are carried out with different fibers of the same
coating.
Comparison of Extraction Techniques
Conventional techniques, such as UAE, Soxhlet, and shaking,
may have had a certain decline in favor of the new extraction
techniques, but today, UAE is still a popular technique
considering the number of articles in the scientific literature
where this extraction procedure is applied for the determination
of organic compounds in soil samples (88).
UAE is faster (15-30 min) than Soxhlet extraction, several
extractions can be performed simultaneously, and, as no
specialized laboratory equipment is required, this technique
is relatively low-cost compared to most modern extraction
methods. Extraction by shaking, or in lower proportion Soxhlet
extraction, is still an attractive option in routine analysis for its
robustness and low cost.
Various studies have compared different extraction techniques
for the analysis pesticides in soil samples. Thus, Druart et
al. (89) reported the extraction of glyphosate, glufosinate, and
its major metabolite, aminomethylphosphonic acid, in soil
using three different extraction procedures: shaking, UAE,
and PLE. Among the three tested methods, extraction by PLE
showed lower efficiency, whereas shaking provided the best
results. Nevertheless, in this comparative study no optimization
of the main parameters involved in PLE extractions was
carried out. Extraction of polycyclic aromatic hydrocarbons
and organochlorine pesticides from soil was evaluated by
Wang et al. (40) in a comparative study with three different
extraction procedures (Soxhlet, MAE, and PLE). In that work,
PLE was the method for which the best extraction efficiency
was reported, followed by MAE.
Lesueur et al. (58) applied a new ultrasonic system, based
on a cylindrical probe, for the extraction of 24 pesticides from
soil samples, and the results were compared with those obtained
with different extraction methods, such as PLE, QuEChERS,
and the European Norm DIN method. The results pointed
out that this new UAE method was successful in recovering
the selected pesticides with good repeatability, whereas some
of the studied pesticides were not recovered with the other
methods. Pateiro-Moure et al. (46) compared new analytical
procedures, such as digestion-based methods, shaking, and
MAE, for determining quaternary ammonium herbicides in soil.
Recoveries of the three compounds evaluated ranged from 98
to 100% by digestion, 102 to 109% by MAE, and up to 61%
by shaking.
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TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 95, No. 5, 2012
Table 7.
Cleanup
technique'
SPE
Cleanup using SPE techniques
Sorbenta
GCB
Solvent (volume) a
Conditioning solvent (volume) a
Analytes (number)
Ref.
DCM—n-hexane (3 + 7, v/v) (10 mL)
n-Hexane (6 mL)
Pyrethroids (12)
61
OC pesticides (9)
53
Herbicides (9)
15
DCM (10 mL)
0.75% NH 4 OH in ACN
Chlorothalonil and degradates
23
n-Hexane—acetone (1 + 1, v/v) (2 mL)
Hexaconazole, myclobutanil, and
tebuconazole
31
n-Hexane—EtAc (8 + 2, v/v) (10 mL)
OASIS HLB
PSA
Me0H pH3 (2 x 1 mL)
ACN (2 x 1 mL)
Me0H (pH 3)
Me0H—(NH 4 ) 2 CO 3 0.1 M (1 + 9, v/v)
Petroleum ether—acetic ether
(95 + 5, v/v) (3 mL)
Petroleum ether (2 mL)
Acetochlor and propisochlor
7
Pesticarb/NH2
ACN—toluene (12 + 1, v/v) (2 mL)
ACN—toluene (12 + 1, v/v) (5 mL)
Enestrobwin
32
Florisil
n-Hexane—acetone (9 + 1, v/v) (10 mL)
OC pesticides
20
n-Hexane: ethyl ether (85 + 15, v/v)
(12 mL) DCM (6 mL)
Cis-trans permethrin
30
OC pesticides
25
Multiclass (62)
14
n-Hexane: diethyl ether (4 + 1, v/v)
(60 mL)
Washed before loading with
n-hexane—cliethyl ether (4 + 1, v/v)
(40 mL)
Different mixtures of DCM, hexane,
and ACN
Silica
n-Hexane (30 mL) + DCM (35 mL)
OC pesticides (10)
40
Me0H—HCI 6.5 M (7 + 3, v/v) (10 mL)
Diquat, paraquat, and difenzoquat
46
DCM—n-hexane (2 + 3, v/v) (30 mL)
OC pesticides
18
Acidified silica (8 g)
n-Hexane (15 mL) + DCM (10 mL)
OC pesticides
52
Alumina
(deactivated 5-6%)
n-Hexane—EtAc (7 + 3, v/v) (100 mL)
OC pesticides
27
Silica (deactivated
3%) + alumina
(deactivated 3%)
DCM—n-hexane (2 + 8, v/v) (35 mL)
Prewash with DCM—n-hexane (2 +
8, v/v) (10 mL)
OC pesticides
19
EtAc (1 mL) + n-hexane (3 mL)
Metabolites—MeOH (6 mL)
Me0H (1 mL) + EtAc (3 mL) +
Me0H (2 mL) + water (2 mL)
Alachlor, metolachlor, and
metabolites
47
Me0H—water (10 + 1, v/v) (3 x 5 mL)
Water (5 mL)
Pyroxsulam, flumetsulam,
metosulam, diclosulam
34
Strata XCW (0.5 g)
2% Formic acid in Me0H—water
(7 + 3, v/v) (3 mL)
Me0H (10 mL) + 10 mM
NH 4 Ac (10 mL)
Sample adjusted to pH 7-7.5
Washed with water (10 mL)
Chlormequat
64
MWCNT
DCM 1(20 mL)
ACN (10 mL) + water (10 mL)
C18
OP (7) and buprofezin
28
Atrazine and metabolites
9
Shaking + 1 min sonication +
centrifugation 4400 rpm (10 min)
OP (10) and buprofezin
73
Vortex (1 min)
Centrifugation: 2077 x g (5 min)
Metaflumizone
70
Shaking (30 s) + centrifugation
1500 rpm (3 min)
Multiclass (38)
77
Vortex (1 min), centrifugation
(4500 rpm, 2.5 min)
Azadyrachtin, spinosad, and
rotenone
69
Multiclass (24)
58
OC pesticides (51)
24
EtAc (4 mL)
d-SPE
PSA
PSA (150 mg) + C18
(150 mg)
SBSE
a
PDMS
Water (85 mL)
900 rpm (14 h)
SPE = Solid-phase extraction; d-SPE = dispersive SPE; SBSE = stir bar sorptive extraction; Me0H = methanol; DCM = dichloromethane;
HAc = acetic acid; EtAc = ethyl acetate; ACN = acetonitrile; PDMS = polydimethylsiloxane; OC = organochlorine; OP = organophosphorus;
PSA = primary secondary amine; MWCNT = multiwalled carbon nanotubes; NH 4 AC = ammonium acetate.
TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VOL. 95, NO. 5, 2012 1267
Morozova et al. (43) developed two procedures, based on
MAE and UAE, for the extraction of chlorophenoxy acids in
soil. The results showed that MAE was more efficient and rapid
than UAE for the determination of these pesticides in soil.
The comparisons of the extraction techniques used for
the analysis of pesticides in soil show that the choice of the
extraction method will depend on the extraction techniques
available in the laboratory and on the characteristics of the
particular matrix-analyte combination.
Cleanup
The extraction of pesticides from soil is usually performed
by nonselective procedures aimed to enhance the extraction
yield, but numerous matrix components that may hinder the
determination of pesticides are also coextracted. Therefore,
it is necessary to have a cleanup step after the extraction to
allow the determination of pesticides at the trace levels found
in soil. The different approaches in the cleanup of extracts can
be classified in two categories: liquid—solid and liquid—liquid
extraction. Among the liquid—solid extraction techniques, the
most common procedure is SPE with cartridges or columns
using sorbents of different characteristics depending on the
selected pesticides (Table 7). Different sorbents, such as
Florisil, silica, alumina, and C18, have been used in methods
developed for the determination of organochlorine pesticides.
The use of graphitized carbon black (GCB) is adequate for
the removal of nonpolar and oxygen-containing compounds
due to hydrophobic, electronic, and ion-exchange interactions.
GCB allowed the determination of pyrethroids—in particular,
prallethrin, resmethrin, and deltamethrin—that could not be
determined without a previous purification due to important
matrix interferences (61). The combination of GCB and
aminopropyl adsorbent was effective to remove pigments and
organic acids from wheat and soil samples (32). For strongly
basic pesticides, such as chlormequat, SPE cleanup based
on weak cation exchange is a good choice because it can be
directly analyzed by LC, avoiding the need of evaporating
and reconstituting the extract in the corresponding mobile
phase (64).
Other materials, such as multiwalled carbon nanotubes, have
been used, but the amount of sorbent used due to the strong
retention of the pesticides in this stationary phase must be
taken into account (28). Furthermore, Min et al. (9) used this
sorbent for the determination of atrazine and its metabolites,
observing that the flow rate affected the recovery as a fast flow
rate (>5 mL/min) and did not allow the analytes to be correctly
adsorbed. Different solvents were assayed, and hexane, which
was the most hydrophobic solvent tested, had the lowest elution
efficiency.
d-SPE is a variation of SPE in which a bulk amount of SPE
sorbent is added to the extract and the mixture is shaken and
centrifuged, rather than loading the extract in a column or
cartridge packed with the sorbent. This cleanup technique is
part of the QuEChERS procedure developed by Anastassiades
et al. in 2003 for the determination of pesticides in fruits and
vegetables (67). Modifications of the original protocol have
been carried out in order to analyze other compounds in
different matrixes, including soil, but in general, most of the
works use PSA sorbent as in the original procedure, and in
some cases C18 sorbent was added to improve the cleanup of
extracts (69). Anhydrous magnesium sulfate is usually present
in d-SPE in order to eliminate residual water, but in some cases,
it is used as both cleanup and drying agent without applying any
SPE sorbent.
SBSE is another cleanup procedure that became very popular
for the extraction of liquid samples, and is starting to be used
for the determination of organic contaminants in soil. In the
optimization of SBSE procedures, the effect of such parameters
as pH, the addition of salt, stirring speed, extraction time, and
temperature are usually evaluated. The commercial stir bars
used in this procedure are coated with PDMS, but new materials
are being developed to provide a more selective extraction of
the target pesticides, and this will be discussed below. In the
determination of 51 persistent organic pollutants that included
organochlorine pesticides, SBSE with PDMS was carried out
for 14 h after the dilution with 85 mL of water of the methanolic
extract obtained with UAE (24). The addition of water was
necessary to increase the polarity of the mixture and shift
the equilibrium in favor of the PDMS coating, improving the
recovery of the analytes.
New sorbents, such as MIPs, have also been used in the
cleanup of soil extracts by applying different techniques
(Table 8). Molecularly imprinted solid-phase extraction
was applied for the determination of several triazines and
their metabolites (62, 63). Terbuthylazine was selected as a
template because this MIP extracted selectively many triazines
including their metabolites. In both works, dichloromethane, an
aprotic and weakly polar solvent, was used before the elution
to favor the interaction between the analytes and the binding
sites. Methanol was used as the elution solvent because it is
a very polar and protic solvent that disrupts the hydrogen
bonds between the polymer and the analytes. As indicated
above, d-SPE is a variation of SPE that has been mainly
applied using PSA sorbent when QuEChERS is selected as the
extraction technique. However, Peng et al. (29) prepared silica
nanoparticles coated with a metsulfuron-methyl imprinted
polymer layer for d-SPE of sulfonylurea herbicides in crop and
soil samples. SBSE, as already mentioned, is usually performed
with stir bars coated with PDMS, although new coatings based
on MIP technology have been used for the selective sorption of
triazines (38) and nicosulfuron (90). Hu et al. (38) developed
a stir bar coated with terbuthylazine-MIP that exhibited a
concentration factor that was 54-fold higher than that of an
MIP-coated SPME fiber. In this work, the durability of the stir
bar was tested, and no obvious differences were observed in the
extraction efficiencies after the same stir bar was used 50 times,
which reduces the costs in routine laboratories. The effects of
pH and ionic strength on the efficiency of the stir bar coated
with nicosulfuron-MIP were evaluated (90). Increasing ionic
strength, due to the addition of NaCl, decreased the extraction
because NaC1 molecules entered the inner surface of the MIP
layer; hence, NaCl was not added. The process had to be done
at a pH range of 4-5, so that nicosulfuron binds to the MIP by
hydrogen bonds, because at pH >5 the carboxyl group is ionized
and at pH <4 nicosulfuron is protonized.
Another approach applied in the cleanup of soil extracts
is the use of classic and modern procedures based on liquid—
liquid extraction that are summarized in Table 9. When large
volumes of extraction solvent are used, the cleanup is usually
carried out by liquid—liquid partitioning in which NaC1 is
generally added to enhance the migration of the analytes to
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TADEO ET AL.: JOURNAL OF AOAC INTERNATIONAL VoL. 95, No. 5, 2012
Table 8.
Cleanup using MIPs
Cleanup technique
MISPE
Template
Solvent (volume)8
Conditioning solvent (volume) e
Analytes (number)
Ref.
Terbuthylazine
Me0H (3 mL)
DCM (10 mL)
Atrazine and
metabolites (2)
63
Me0H (3 mL)
Me0H (1 mL) + water (1 mL) + 50 mM
NH 4 H 2 PO 4 (pH 3) (1 mL)
Washing: 0.1 M HCI (1 mL) + water (1 mL)
Triazines (9) and
metabolites (3)
62
d-SPE
Metsulfuron-methyl
on silica nanoparticle
Chloroform (1 mL)
Incubated (30 min)
Filtered and cleaned with CHCI 3 (2 mL) and
dispersed in Me0H—HAc (9 + 1, v/v; 1 mL)
Sulfonylurea
herbicides (4)
29
SBSE
Terbuthylazine
Toluene
500 rpm (60 min)
Triazines (9)
38
Nicosulfuron
Aqueous solution (100 mL)
600 rpm (180 min)
Nicosulfuron
90
a
MISPE = Molecularly imprinted; d-SPE = dispersive SPE; SBSE = stir bar sorptive extraction; Me0H = methanol; DCM = dichloromethane;
HAc = acetic acid.
the organic phase. Although d-SPE is the cleanup technique
used in the original QuEChERS technique, Rashid et al. (74)
executed a liquid-liquid partitioning and obtained clean
extracts. Dispersive liquid—liquid microextraction (DLLME) is
a modern liquid—liquid extraction procedure used in the cleanup
and concentration of extracts. The extraction solvent has to
meet four requirements: higher density than water, low water
solubility, high extraction capability, and good chromatographic
behavior. In summary, 5 mL of aqueous extract containing
the target pesticides are rapidly injected with 0.8-1 mL of
methanol acting as the disperser solvent, and 10-50 At of
tetrachloroethylene or carbon tetrachloride as the extraction
solvent. With a little gentle shaking, a cloudy solution swiftly
forms, consisting of droplets of the extraction solvent dispersed
within the aqueous solution. The mixture is then centrifuged,
and the extraction solvent is collected for subsequent analysis.
Wu et al. (91) performed a modified DLLME cleanup, applying
ultrasound radiation to accelerate the mass transfer between the
two immiscible phases that leads to an increase in the extraction
efficiency. The addition of salt to promote the transfer of analytes
was considered, but it was discarded because it increased the
viscosity and, as a consequence, the organic phase could not
be dispersed in fine droplets and therefore the efficiency of the
emulsion formation was reduced drastically. A very similar
procedure, named homogeneous liquid—liquid extraction
(HLLE), was used for the determination• of three pesticides
(malathion, lambda-cyhalothrin, and cypermethrin; 17). The
main difference between HLLE and DLLME is that in HLLE,
the mixture of solvents produces a homogeneous solution prior
to the separation of the two phases by the addition of salt or an
auxiliary solvent. In the case of DLLME, the phase separation
is accomplished instantly by rapidly adding the mixture of
disperser and extractio,n solvents into the aqueous solution. The
use of two different liquid—liquid extraction cleanup techniques,
DLLME and hollow fiber-liquid phase microextraction
(HF-LPME), was evaluated for the determination of six
organosulfur pesticides (92). Although DLLME is more sensible
than HF-LPME, filtration and dilution of the extract prior to
DLLME were required when analyzing complex matrixes such
as soil. Hence, HF-LPME was considered more suitable for the
analysis of soil samples because it could be carried out without
filtration and dilution.
Today, the trend is to develop analytical methods where the
extraction and purification are carried out simultaneously. In this
way, PLE with in-cell cleanup has been successfully applied in
the determination of pesticide residues in soil. The addition of a
layer of Florisil inside the PLE cell at the flow out end provided
clean extracts ready for their analysis (54, 57).
Application to Environmental Studies
The contamination of agricultural soils with pesticides
represents a major environmental concern. Thus, the analytical
methodologies described in this review may be applied
to different studies related to the presence of pesticides in
the environment, such as monitoring, fate and transport,
modeling, ecotoxicology, risk assessment, and management
strategies (93, 94).
The persistence and mobility in soil are important processes
for the efficacy of pesticides over the growing season, although
this may increase concerns about environmental contamination.
Various sample preparation techniques have been used in the
assays of persistence and mobility of pesticides in soil; PLE is
the technique most often used lately (59), with UAE and shaking
also applied as alternative procedures in some studies (35, 95).
Sorption and degradation are key processes affecting the
behavior of pesticides in soil, thereby determining their
persistence and distribution in the field. Sorption coefficients
of pesticides are normally obtained by analyzing their
concentration in the aqueous phase after partition, but in order
to know the mass balance their levels in soil are also determined.
Pesticide extraction from soil samples by UAE or shaking with
an adequate solvent has been frequently used (35, 96), although
MAE may also be used to increase extraction efficacy for
hydrophobic pesticides more strongly adsorbed (48).
The distribution of pesticides in the environment through
their transport to water and air compartments has been studied
by means of the application of SPE and SPME to those
matrixes (97-99). SBSE has also been used lately to increase
the sensitivity obtained (100).
Degradation and metabolism of pesticides are important
processes controlling their persistence in soil, and they are
influenced by a number of factors including environmental
conditions and pesticide and soil properties. For the study of
these processes in soil, miniaturized procedures have being
proposed (96), and sample preparation methods able to extract
pesticides together with their metabolites or degradation
products have been developed in recent years (101, 102).
TADEO ET AL.: JOURNAL
Table 9.
OF AOAC INTERNATIONAL VOL. 95, NO. 5, 2012
1269
Cleanup using liquid—liquid extraction
Cleanup technique'
LL partitioning
DLLME
HLLE
HF-LPME
Extract solvent (volume) s
Solvent (volume) a
Conditioning
Analytes (number)
Ref.
ACN (1 mL)
Hexane (5 mL) + water (1 mL)
Vortex (5 min)
Organochlorine pesticides
(19)
74
Acetone extract (25 mL)
Water (25 mL) + saturated NaCI solution (25 mL) + n-hexane (3 x 50 mL)
Bromoxynil octanoate
4
NaOH 0.1 M extract (10 mL)
EtAc (20 mL) + NaCI (2 g)
pH adjusted <2
Clopyralid and picloram
3
Me0H (1 mL)
Water (5 mL) + TCE (50 pL)
3500 rpm (3 min)
Carbaryl and triazophos
6
Aqueous extract (5 mL)
Chlorobenzene (100 pL)
Sonication 3 min at 25°C
+ 3500 rpm (5 min)
Triazines (5)
91
Aqueous extract (5 mL)
Me0H (800 pL) + CCI4 (10 pL)
3000 rpm (15 min)
Organosulfur pesticides (6)
92
Acetone (1 mL)
Water (5 mL) + CCI4 (40 pL)
+ 0.3 g NaCI
3000 rpm (4 min)
Malathion, cypermetrin, and
lamda cyhalothrin
17
Aqueous extract (5 mL)
o-Xylene (5 pL)
Organosulfur pesticides (6)
92
'LL = Liquid—liquid; DLLME = dispersive liquid-liquid microextraction; HLLE = homogeneous liquid—liquid extraction; HF-LPME = hollow fiber liquid
phase microextraction; ACN = acetonitrile; Me0H = methanol; EtAc = ethyl acetate; TCE = tetrachloroethane.
Conclusions and Future Trends
X. (2009) Chromatographia 70, 1697-1701. http://
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Hu, J-Y., Zhen, Z-H., & Deng, Z-B. (2011) Bull. Environ.
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Guermouche, M.H., & Bensalah, K. (2008) Chromatographia
67, 63-68. http://dx.doi.org/10.1365/s10337-007-0465-6
Chaves, A., Shea, D., & Danehowe, D. (2008)
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90, 1659-1669
Park, J-H., Mamun, M.I.R., Choi, J-H., El-Ati, A.M.A.,
Assayed, M.E., Choi, W.J., Ion, KS., Han, S.-S., Kim, H.K.,
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Nielli, S., Pareja, L., Asteggiante, L.G., Roehrs, R., Pizzutti,
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Mt. 93, 425-431
Nishina, T., Kien, C.N., Noi, N.V., Ngoc, H.M., Kim, C.S.,
Tanaka, S., & Iwasaki, K. (2010) Environ. Monit. Assess. 169,
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Wang, X., Zhao, X., Liu, X., Li, Y., Fu, L., Hu, J., & Huang,
C. (2008) Anal. Chim. Acta 620, 162-169. http://dx.doi.
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Xin-Li, X., Shi-Hua, Q.I., Yuan, Z., Dan, Y., & Odhiambo, J.O.
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Wang,
In recent years, techniques that reduce or eliminate solvent
consumption in the sample preparation step have been
developed and are gaining in popularity. Implementing these
new developments, especially those that can be automated, into
routine laboratories continues to be an important step in the
analysis of pesticides.
Classical techniques like shaking and UAE will probably
continue being widely used due to their robustness, simplicity,
and low cost. In the use of classical techniques, the trend is
to miniaturize the method with the aim of reducing the use of
solvents and glassware.
The development of very selective techniques, such as
those hyphenated to MS/MS, will reduce the requirements for
a cleanup of sample extracts, although it still will be needed
in the analysis of dirty samples due to the presence of matrix
effects and the negative impact of coextracted compounds on
the analytical equipment.
These methods have been applied to study the presence and
behavior of pesticides in the environment. Public concern about
pesticide use will probably increase the need for rapid and
robust methods for pesticide analysis in soil, in which sample
preparation techniques will continue to play a central role.
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