plants, the original non-buffered salts (MgSO 4 + NaCl), the EN-buffered salts
consisting of the original salts and the addition of citrate buffers
(MgSO4 + C 6 H 5 Na 3 O 7 •2H 2 O + Na 2 C 6 H 6 O 7 •1.5H 2 O), and the AOAC method
with acetate buffer (MgSO 4 + CH 3 COONa). The pH is extremely important in this
extraction procedure. The EN and AOAC extraction salt kits consist of buffered salts
which result in a pH between 5 and 5.5 for the EN method and a pH < 5 for the
AOAC. In contrast, the final pH of the original method mainly depends on the nature
of the initial matrix [142]. However, current studies suggest that the original salt kit
is the most used salt mix to extract pharmaceutical residues from plant tissues, with
excellent results [45, 140, 143]. On the other hand, AOAC salts and EN salts were
widely used providing more stability to the target compounds [7, 121]. Aside from
the extraction phase, Anastassiades and colleagues also developed a new clean-up
methodology to remove any interferents based on a solid-phase extraction in the
dispersive phase (d-SPE) [19]. Further considerations are shown in the clean-up
section. The matrices studied so far are mainly vegetables such as lettuce, radish, and
cabbage (Table 6). In particular, plants consisting of a branched root are the most
studied samples, but fruits such as cucumber, tomato, and strawberry are also being
studied. QuEChERS has become the most used extraction method with exponential
growth in recent years, and everything seems to indicate that thanks to its versatility
and its possible future automation, this growth seems even more pronounced in the
near future.
5.4 Other Extraction Methods (MAE, Soxhlet, Solid-Liquid
Extraction)
The microwave-assisted extraction method (MAE) is a technique widely used in
soils but has not had the same diffusion with plants, at least for now. The MAE
consists in the use of a correctly selected organic solvent for the extraction of target
drugs which is heated by microwaves, combining sonication and temperature properties [144]. This technique is widely used for the analysis of metals and heavy
metals in plant material. However, it is also used in the field of PhACs, although the
quantity of solvent is reduced and the obligation to operate at high temperatures
could adversely affect the extraction of the target compounds. The organic solvents
used are the same as previously described for other methodologies. Basically, ACN,
MeOH, and water, including their mixtures, are the solvents selected for vegetables.
Only two studies with plants have been reported, one with Phragmites australis and
the other with cereals [33, 145].
Recent works have demonstrated the ability of the classic Soxhlet extraction
method to detect pharmaceutical products in plants and to detect 12 pharmaceutical
products in different vegetables [34]. However, Soxhlet extraction is not usually
used because, due to the high working temperatures required by Soxhlet and also the
long exposure times, this method could alter the properties of the target compounds,
Development of Methods for the Determination of PhACs in Soil/Earthworm/Crop. . .
465
consisting of the original salts and the addition of citrate buffers
(MgSO4 + C 6 H 5 Na 3 O 7 •2H 2 O + Na 2 C 6 H 6 O 7 •1.5H 2 O), and the AOAC method
with acetate buffer (MgSO 4 + CH 3 COONa). The pH is extremely important in this
extraction procedure. The EN and AOAC extraction salt kits consist of buffered salts
which result in a pH between 5 and 5.5 for the EN method and a pH < 5 for the
AOAC. In contrast, the final pH of the original method mainly depends on the nature
of the initial matrix [142]. However, current studies suggest that the original salt kit
is the most used salt mix to extract pharmaceutical residues from plant tissues, with
excellent results [45, 140, 143]. On the other hand, AOAC salts and EN salts were
widely used providing more stability to the target compounds [7, 121]. Aside from
the extraction phase, Anastassiades and colleagues also developed a new clean-up
methodology to remove any interferents based on a solid-phase extraction in the
dispersive phase (d-SPE) [19]. Further considerations are shown in the clean-up
section. The matrices studied so far are mainly vegetables such as lettuce, radish, and
cabbage (Table 6). In particular, plants consisting of a branched root are the most
studied samples, but fruits such as cucumber, tomato, and strawberry are also being
studied. QuEChERS has become the most used extraction method with exponential
growth in recent years, and everything seems to indicate that thanks to its versatility
and its possible future automation, this growth seems even more pronounced in the
near future.
5.4 Other Extraction Methods (MAE, Soxhlet, Solid-Liquid
Extraction)
The microwave-assisted extraction method (MAE) is a technique widely used in
soils but has not had the same diffusion with plants, at least for now. The MAE
consists in the use of a correctly selected organic solvent for the extraction of target
drugs which is heated by microwaves, combining sonication and temperature properties [144]. This technique is widely used for the analysis of metals and heavy
metals in plant material. However, it is also used in the field of PhACs, although the
quantity of solvent is reduced and the obligation to operate at high temperatures
could adversely affect the extraction of the target compounds. The organic solvents
used are the same as previously described for other methodologies. Basically, ACN,
MeOH, and water, including their mixtures, are the solvents selected for vegetables.
Only two studies with plants have been reported, one with Phragmites australis and
the other with cereals [33, 145].
Recent works have demonstrated the ability of the classic Soxhlet extraction
method to detect pharmaceutical products in plants and to detect 12 pharmaceutical
products in different vegetables [34]. However, Soxhlet extraction is not usually
used because, due to the high working temperatures required by Soxhlet and also the
long exposure times, this method could alter the properties of the target compounds,
Development of Methods for the Determination of PhACs in Soil/Earthworm/Crop. . .
465