the analytes covering the entire polarity range of the compounds, reducing the
presence of other components of the matrix in the final extracts. An instance for
this, Durán-Alvarez and coworkers [54] obtained improved recoveries when the mix
of solvents acetone/hexane (1:1, modified with 2% acetic acid) was employed
instead of single solvent, with which acetone is the polar solvent and hexane is the
non-polar solvent. The addition of some buffers such as ammonia, citric acid,
phosphoric acid, formic acid, sodium acetate, and ethylenediaminetetraacetic acid
(EDTA) has also shown an improvement in recoveries. The commonly used mixtures are generally methanol/water or acetonitrile/water with some of the previously
mentioned buffers. In some cases, those mixtures were also used unbuffered
[54, 55].
The use of EDTA in the ASE method has not been recommended in most cases
for several reasons. An example is the greater co-extraction of the matrix with
consequent lower sensitivity and clogging of the ASE apparatus (cells and device
tube) [56]. However, in some cases, its addition did not significantly improve analyte
recoveries [52]. In contrast, EDTA has been successfully employed in the techniques
discussed below (USE, MAE, and QuEChERS).
In addition to the selection of the solvent, several other parameters must be
carefully considered in the development of the ASE method, such as temperature,
pressure, extraction time and extraction mode, number of cycles, cell matrix, and
flush volume. Those parameters are related and should be balanced to obtain
acceptable recoveries for all target analytes. By far, temperature has proven to be
the most important parameter. Theoretically, the high temperature helps to stop the
strong interactions between pharmaceutical products and soil components; therefore,
the higher the temperature, the higher the extraction yield [51, 57]. Furthermore, the
temperature influences the physico-chemical properties of the solvents, therefore
viscosity, density, and polarity. For example, the dielectric constant of pure water
drops from 79 to 35 when the conditions are changed from room temperature and
pressure to 200
C and 1.5 MPa, obtaining a water solvent similar to methanol at
room temperature, with low density and polarity [57]. In other words, under high
temperature conditions, the properties of the solvents are modified so as to reduce the
viscosity and surface tension and greater diffusivity, which improves the wetting of
the matrix and improves the mass transfer from the matrix to the solvent. All in all,
the reduced interactions between analytes and matrix and the greater diffusion of the
solvent in the high temperature matrix, in addition to the increasing solubility of the
analytes, allow a faster mass transfer and a complete extraction process. On the other
hand, in addition to all these strengths of the high temperature used in the ASE, some
weaknesses are also presented, and it is worth mentioning them: (1) the co-extraction
of other unwanted compounds from the matrix due to the bonds which break under
high temperature conditions, thus converting the ASE into a less selective method;
(2) the degradation of the thermolabile analytes at high temperature; (3) and, finally,
the formation of toxic compounds due to chemical reactions that occur at high
temperature. However, these weaknesses can be avoided by minimizing the extraction time and the number of cycles. Overall, the extraction temperature should be
optimized to extract the analytes of interest with less matrix interference, loss of
Development of Methods for the Determination of PhACs in Soil/Earthworm/Crop. . .
445
presence of other components of the matrix in the final extracts. An instance for
this, Durán-Alvarez and coworkers [54] obtained improved recoveries when the mix
of solvents acetone/hexane (1:1, modified with 2% acetic acid) was employed
instead of single solvent, with which acetone is the polar solvent and hexane is the
non-polar solvent. The addition of some buffers such as ammonia, citric acid,
phosphoric acid, formic acid, sodium acetate, and ethylenediaminetetraacetic acid
(EDTA) has also shown an improvement in recoveries. The commonly used mixtures are generally methanol/water or acetonitrile/water with some of the previously
mentioned buffers. In some cases, those mixtures were also used unbuffered
[54, 55].
The use of EDTA in the ASE method has not been recommended in most cases
for several reasons. An example is the greater co-extraction of the matrix with
consequent lower sensitivity and clogging of the ASE apparatus (cells and device
tube) [56]. However, in some cases, its addition did not significantly improve analyte
recoveries [52]. In contrast, EDTA has been successfully employed in the techniques
discussed below (USE, MAE, and QuEChERS).
In addition to the selection of the solvent, several other parameters must be
carefully considered in the development of the ASE method, such as temperature,
pressure, extraction time and extraction mode, number of cycles, cell matrix, and
flush volume. Those parameters are related and should be balanced to obtain
acceptable recoveries for all target analytes. By far, temperature has proven to be
the most important parameter. Theoretically, the high temperature helps to stop the
strong interactions between pharmaceutical products and soil components; therefore,
the higher the temperature, the higher the extraction yield [51, 57]. Furthermore, the
temperature influences the physico-chemical properties of the solvents, therefore
viscosity, density, and polarity. For example, the dielectric constant of pure water
drops from 79 to 35 when the conditions are changed from room temperature and
pressure to 200
C and 1.5 MPa, obtaining a water solvent similar to methanol at
room temperature, with low density and polarity [57]. In other words, under high
temperature conditions, the properties of the solvents are modified so as to reduce the
viscosity and surface tension and greater diffusivity, which improves the wetting of
the matrix and improves the mass transfer from the matrix to the solvent. All in all,
the reduced interactions between analytes and matrix and the greater diffusion of the
solvent in the high temperature matrix, in addition to the increasing solubility of the
analytes, allow a faster mass transfer and a complete extraction process. On the other
hand, in addition to all these strengths of the high temperature used in the ASE, some
weaknesses are also presented, and it is worth mentioning them: (1) the co-extraction
of other unwanted compounds from the matrix due to the bonds which break under
high temperature conditions, thus converting the ASE into a less selective method;
(2) the degradation of the thermolabile analytes at high temperature; (3) and, finally,
the formation of toxic compounds due to chemical reactions that occur at high
temperature. However, these weaknesses can be avoided by minimizing the extraction time and the number of cycles. Overall, the extraction temperature should be
optimized to extract the analytes of interest with less matrix interference, loss of
Development of Methods for the Determination of PhACs in Soil/Earthworm/Crop. . .
445