selectivity, degradation, and toxic components. The temperature range used for ASE
soil extraction ranges from 40 to 200
C, with 100
C being the most used temperature. A good temperature choice is 100
C as it exceeds the boiling point of most of
the organic solvent and is low enough to avoid the degradation of the analytes, the
excess of co-extracts, and the formation of toxic compounds. Since tetracyclines can
undergo unwanted transformations at high temperature (they can be converted into
their epi or anhydro form), some studies have performed their extraction at room
temperature [58–60]. Tetracyclines and fluoroquinolones are extracted with lower
temperatures from ambient to 100
C [58], whereas sulfonamides showed the necessity of higher temperature ranging from 50 to 200
C [61]. Many studies [62–64]
have demonstrated that the extraction pressure has no influence on the recoveries and
extraction efficiency. However, an elevated pressure is important to maintain the
solvent in its liquid state. Additionally, it helps to wet the sample, resulting in an
improved extraction efficiency [65]. Pressures from 500 to 1,500 psi were usually
employed, with 1,500 psi the most common pressure.
The extraction time is the time in which the solvent and the matrix are in contact
with the desired temperature and pressure and depends on the matrix and the target
analyte characteristics [57]. It should be as short as possible but also adequate for
mass transfer. Moreover, extraction time depends on the extraction mode (static or
dynamic). Static mode was always chosen for pharmaceutical extraction from soil
rather than the dynamic mode. It is worthy to mention that different instruments are
designed for each mode. In the static mode, the solvent and the sample are
maintained for a specific time at constant temperature and pressure, and the solvent
is only replaced partly or completely when another cycle is employed [66]. If this
time is longer than necessary, thermal degradation may occur for the extracted
analytes resulting in a slower and less efficient extraction procedure. In addition,
an equilibrium point of the analytes is established in the matrix and solvent in static
mode, and beyond this point, the extraction efficiency will not increase. Hence, static
extraction time should be optimized carefully. Most often, a static extraction time of
5 min (2 Â 5 min cycles) or 10 min (1 Â 10 min cycle) is employed.
The volume of the solvent can be introduced in several cycles or in one cycle.
Fractioning the solvent in several cycles can improve extraction efficiency by
avoiding the analyte equilibrium point to take place. However, it could also provide
lower recoveries due to high co-extraction of matrix interferences [67]. Hence, a
moderate number of cycles should be employed. Mainly 1–3 cycles are used.
The soil samples are generally mixed with a cellular matrix (dispersant) to prevent
clogging of the cell and improve soil contact with the solvent. Among the different
cellular matrices available, Ottawa sand [64], quartz sand [68], sea sand [55],
diatomaceous earth [69], and Hydromatrix diatomaceous earth [59] are usually the
dispersants commonly used with the soil. Lastly, flush volume is the least mentioned
in studies, even though it is important to know the final volume of the extract. It is
expressed as the percentage of the cell volume, and it ranges from 50 to 150% in all
studies dealing with soil as matrix. This interval is sufficient to extract most of the
analytes suitable for quantitative analysis, in a minimum volume of solvent, reducing
the solvent consumption and concentrating the analytes as much as possible. Finally,
446
R. Manasfi et al.
soil extraction ranges from 40 to 200
C, with 100
C being the most used temperature. A good temperature choice is 100
C as it exceeds the boiling point of most of
the organic solvent and is low enough to avoid the degradation of the analytes, the
excess of co-extracts, and the formation of toxic compounds. Since tetracyclines can
undergo unwanted transformations at high temperature (they can be converted into
their epi or anhydro form), some studies have performed their extraction at room
temperature [58–60]. Tetracyclines and fluoroquinolones are extracted with lower
temperatures from ambient to 100
C [58], whereas sulfonamides showed the necessity of higher temperature ranging from 50 to 200
C [61]. Many studies [62–64]
have demonstrated that the extraction pressure has no influence on the recoveries and
extraction efficiency. However, an elevated pressure is important to maintain the
solvent in its liquid state. Additionally, it helps to wet the sample, resulting in an
improved extraction efficiency [65]. Pressures from 500 to 1,500 psi were usually
employed, with 1,500 psi the most common pressure.
The extraction time is the time in which the solvent and the matrix are in contact
with the desired temperature and pressure and depends on the matrix and the target
analyte characteristics [57]. It should be as short as possible but also adequate for
mass transfer. Moreover, extraction time depends on the extraction mode (static or
dynamic). Static mode was always chosen for pharmaceutical extraction from soil
rather than the dynamic mode. It is worthy to mention that different instruments are
designed for each mode. In the static mode, the solvent and the sample are
maintained for a specific time at constant temperature and pressure, and the solvent
is only replaced partly or completely when another cycle is employed [66]. If this
time is longer than necessary, thermal degradation may occur for the extracted
analytes resulting in a slower and less efficient extraction procedure. In addition,
an equilibrium point of the analytes is established in the matrix and solvent in static
mode, and beyond this point, the extraction efficiency will not increase. Hence, static
extraction time should be optimized carefully. Most often, a static extraction time of
5 min (2 Â 5 min cycles) or 10 min (1 Â 10 min cycle) is employed.
The volume of the solvent can be introduced in several cycles or in one cycle.
Fractioning the solvent in several cycles can improve extraction efficiency by
avoiding the analyte equilibrium point to take place. However, it could also provide
lower recoveries due to high co-extraction of matrix interferences [67]. Hence, a
moderate number of cycles should be employed. Mainly 1–3 cycles are used.
The soil samples are generally mixed with a cellular matrix (dispersant) to prevent
clogging of the cell and improve soil contact with the solvent. Among the different
cellular matrices available, Ottawa sand [64], quartz sand [68], sea sand [55],
diatomaceous earth [69], and Hydromatrix diatomaceous earth [59] are usually the
dispersants commonly used with the soil. Lastly, flush volume is the least mentioned
in studies, even though it is important to know the final volume of the extract. It is
expressed as the percentage of the cell volume, and it ranges from 50 to 150% in all
studies dealing with soil as matrix. This interval is sufficient to extract most of the
analytes suitable for quantitative analysis, in a minimum volume of solvent, reducing
the solvent consumption and concentrating the analytes as much as possible. Finally,
446
R. Manasfi et al.