without dipole has extended the MAE application to a wide variety of analytes (polar
and non-polar) (Table 4). Methanol, acetonitrile, and water are the most commonly
used as a solvent for green extraction. The presence of water in the extraction solvent
has been shown to increase extraction rates. This can be explained by the absorbance
of the microwave energy by the water, resulting in efficient heating of the samples.
High pressure and temperature include the risk of degradation of pharmaceutical
products and should be carefully optimized. The temperature is generally set
between 60 and 120
C, while the extraction time is generally very short when
applying MAE, since the solvent is heated directly with microwave energy and
can last from 3 to 20 min.
4.4 QuEChERS (Quick, Easy, Cheap, Effective, Rugged,
Safe)
The selection of the extraction solvent is a crucial parameter. Acetonitrile (ACN) has
been shown to provide the best extraction efficiency of a wide range of compounds,
including pharmaceutical ones, with the lowest matrix interference. Furthermore, it
was used by the QuEChERS fathers when the method was first introduced
[19]. ACN was used as extraction solvent in all the studies dealing with soil
extraction by QuEChERS technique, besides water or McIlvaine buffer as hydration
solvent of the sample [101–106]. Hydration of the soil with water is important to
weaken the interactions of the target analytes with the adsorption sites of the soil
humic substance, promoting desorption and allowing the ACN to obtain better
access to the soil pores. Likewise, the McIlvaine buffer is able to prevent the
complexation of analytes with cations such as Mg
2+ or Ca
2+ . To the greatest extent,
the hydration step allows the reconstitution of the dry samples with a high water
percentage, for which the QuEChERS method was originally designed [107]. However, satisfactory results were obtained with water in almost all the abovementioned
studies. Only Meng and coworkers employed McIlvaine buffer in their extraction
procedure [105]. Besides, the acidification of the hydration solution and/or ACN
with hydrochloric acid or acetic acid has shown to improve analyte recoveries [101–
104].
Three QuEChERS salts are known which are used intensively: (1) the original
non-buffered method (MgSO 4 + NaCl), (2) the AOAC method with acetate buffer
(MgSO 4 + CH 3 COONa), and (3) the EN method with citrate buffer
(MgSO 4 + C 6 H 5 Na 3 O 7 • 2H 2 O + Na 2 C 6 H 6 O7 • 1.5H 2 O). The AOAC and EN
buffers have the ability to acidify the medium. The extraction pH is a very important
parameter, and a small variation can affect extraction efficiency, especially for acidic
and basic compounds [19]. In the EN method, the citrate buffer provides a pH of
5–5.5, while in the AOAC method, the acetate buffer provides a more acidic pH of
4.8 [108]. Higher recoveries and bigger number of compounds were extracted using
AOAC method [103, 104, 106]. This is referred to the acidic conditions obtained by
448
R. Manasfi et al.
and non-polar) (Table 4). Methanol, acetonitrile, and water are the most commonly
used as a solvent for green extraction. The presence of water in the extraction solvent
has been shown to increase extraction rates. This can be explained by the absorbance
of the microwave energy by the water, resulting in efficient heating of the samples.
High pressure and temperature include the risk of degradation of pharmaceutical
products and should be carefully optimized. The temperature is generally set
between 60 and 120
C, while the extraction time is generally very short when
applying MAE, since the solvent is heated directly with microwave energy and
can last from 3 to 20 min.
4.4 QuEChERS (Quick, Easy, Cheap, Effective, Rugged,
Safe)
The selection of the extraction solvent is a crucial parameter. Acetonitrile (ACN) has
been shown to provide the best extraction efficiency of a wide range of compounds,
including pharmaceutical ones, with the lowest matrix interference. Furthermore, it
was used by the QuEChERS fathers when the method was first introduced
[19]. ACN was used as extraction solvent in all the studies dealing with soil
extraction by QuEChERS technique, besides water or McIlvaine buffer as hydration
solvent of the sample [101–106]. Hydration of the soil with water is important to
weaken the interactions of the target analytes with the adsorption sites of the soil
humic substance, promoting desorption and allowing the ACN to obtain better
access to the soil pores. Likewise, the McIlvaine buffer is able to prevent the
complexation of analytes with cations such as Mg
2+ or Ca
2+ . To the greatest extent,
the hydration step allows the reconstitution of the dry samples with a high water
percentage, for which the QuEChERS method was originally designed [107]. However, satisfactory results were obtained with water in almost all the abovementioned
studies. Only Meng and coworkers employed McIlvaine buffer in their extraction
procedure [105]. Besides, the acidification of the hydration solution and/or ACN
with hydrochloric acid or acetic acid has shown to improve analyte recoveries [101–
104].
Three QuEChERS salts are known which are used intensively: (1) the original
non-buffered method (MgSO 4 + NaCl), (2) the AOAC method with acetate buffer
(MgSO 4 + CH 3 COONa), and (3) the EN method with citrate buffer
(MgSO 4 + C 6 H 5 Na 3 O 7 • 2H 2 O + Na 2 C 6 H 6 O7 • 1.5H 2 O). The AOAC and EN
buffers have the ability to acidify the medium. The extraction pH is a very important
parameter, and a small variation can affect extraction efficiency, especially for acidic
and basic compounds [19]. In the EN method, the citrate buffer provides a pH of
5–5.5, while in the AOAC method, the acetate buffer provides a more acidic pH of
4.8 [108]. Higher recoveries and bigger number of compounds were extracted using
AOAC method [103, 104, 106]. This is referred to the acidic conditions obtained by
448
R. Manasfi et al.