phase. It is usually performed by filtration. For this purpose, different filters are used.
The most commonly used filters are membranes with a pore diameter of 0.22 μm
[8, 57, 86] or 0.45 μm [3, 50, 58, 63, 64, 103, 104, 109] and 2.7 μm glass microfiber
filters [5, 107], 1.6 μm [16], 1.2 μm [57, 60, 66], 1 μm [71], 0.7 μm [7, 62, 74, 83, 87,
88, 112], and 0.45 μm [5, 16, 24, 52, 69, 71, 73, 89] that have been previously baked
at 450
C for at least 2 h [118]. Sometimes, ultracentrifugation is used [113].
Refrigeration minimizes chemical changes caused by biological activity
[121]. For this reason, once the sample is collected, it is transferred to the laboratory
at 4
C [5, 8, 16, 17, 24, 32, 57, 71, 105, 108], packed on ice [76, 82, 110], or
frozen [45].
Acidification prevents biological degradation [30] and prevents cation precipitation [118]. In order to acidify the sample, hydrochloric (HCl) [3, 24, 28, 33, 49, 57,
58, 63, 69–71, 73, 103, 104], phosphoric (H 3 PO 4 ) [16], formic (CH 2 O 2 ) [51], or
sulfuric (H 2 SO 4 ) acid is used [52, 66, 76, 82, 90, 110]. The volume that is added
depends on its concentration. The pH is generally adjusted to between 2 and 3 [3, 16,
24, 28, 30, 32, 33, 45, 49, 51, 52, 57, 58, 63, 64, 67, 69–71, 73, 76, 79, 90, 104]
although some authors adjust the pH to <2 [110, 113] and others adjust it to pH > 4
[66, 103]. Sodium azide has also been used to prevent biological degradation [112].
Once filtered and acidified, the sample is stored until the next stage of the analysis
at 4
C [44, 50, 53, 62–66, 74–77, 80, 82–84, 86, 89, 91, 92, 104, 109] or frozen at
temperatures between À20 and À4
C [17, 57, 60, 67, 70, 84, 103, 112].
3.2.2 Stage 2: Extraction and Concentration of the Analytes of Interest
The low concentrations of NSAIDs in water samples and the complexity of the
matrices require that the sample undergo extraction and concentration stages before
carrying out the final analytical determination. The most common route of extraction
is to put the matrix in which the analytes are in contact with another phase, so they
are transferred to the extraction phase. Ideally, the extraction should be exhaustive,
but this depends on different factors, such as the nature of the extraction phase, the
sample and extraction phase volumes, temperature, pH, salinity, among other
parameters. The techniques used for this purpose are ion exchange (IE) [117],
liquid-liquid extraction (LLE) [13, 60, 106], and solid phase extraction (SPE)
[5, 7, 8, 16, 17, 24, 28, 30, 32, 33, 43, 48, 49, 51, 56, 62, 66, 67, 70–72, 74, 79–
81, 84, 86, 88, 93, 105, 107, 108, 110, 114], which is the most used technique to
analyze NSAIDs in water due to the high availability of sorbents, lower amounts of
organic solvents, shorter extraction times, and the possibility of coupling it in line
with the chromatographic methods used in the next stage. Combined with ultrasound
(US), the SPE method is called SPE assisted by US [74, 121].
For SPE, reverse phase cartridges of different materials and formats are used. The
Oasis HLB stand out [5, 7, 8, 16, 17, 24, 32, 48, 50, 62, 66, 70, 72, 79, 80, 82, 88,
108, 110, 112] and is composed of a copolymer of N-vinyl pyrrolidone and
divinylbenzene [13] with different hydrophilic-lipophilic balance that serve to
extract both polar and non-polar compounds [121, 123]. The MCX Oasis [28, 30,
Quantification of Non-steroidal Anti-inflammatory Drug in Water
91
The most commonly used filters are membranes with a pore diameter of 0.22 μm
[8, 57, 86] or 0.45 μm [3, 50, 58, 63, 64, 103, 104, 109] and 2.7 μm glass microfiber
filters [5, 107], 1.6 μm [16], 1.2 μm [57, 60, 66], 1 μm [71], 0.7 μm [7, 62, 74, 83, 87,
88, 112], and 0.45 μm [5, 16, 24, 52, 69, 71, 73, 89] that have been previously baked
at 450
C for at least 2 h [118]. Sometimes, ultracentrifugation is used [113].
Refrigeration minimizes chemical changes caused by biological activity
[121]. For this reason, once the sample is collected, it is transferred to the laboratory
at 4
C [5, 8, 16, 17, 24, 32, 57, 71, 105, 108], packed on ice [76, 82, 110], or
frozen [45].
Acidification prevents biological degradation [30] and prevents cation precipitation [118]. In order to acidify the sample, hydrochloric (HCl) [3, 24, 28, 33, 49, 57,
58, 63, 69–71, 73, 103, 104], phosphoric (H 3 PO 4 ) [16], formic (CH 2 O 2 ) [51], or
sulfuric (H 2 SO 4 ) acid is used [52, 66, 76, 82, 90, 110]. The volume that is added
depends on its concentration. The pH is generally adjusted to between 2 and 3 [3, 16,
24, 28, 30, 32, 33, 45, 49, 51, 52, 57, 58, 63, 64, 67, 69–71, 73, 76, 79, 90, 104]
although some authors adjust the pH to <2 [110, 113] and others adjust it to pH > 4
[66, 103]. Sodium azide has also been used to prevent biological degradation [112].
Once filtered and acidified, the sample is stored until the next stage of the analysis
at 4
C [44, 50, 53, 62–66, 74–77, 80, 82–84, 86, 89, 91, 92, 104, 109] or frozen at
temperatures between À20 and À4
C [17, 57, 60, 67, 70, 84, 103, 112].
3.2.2 Stage 2: Extraction and Concentration of the Analytes of Interest
The low concentrations of NSAIDs in water samples and the complexity of the
matrices require that the sample undergo extraction and concentration stages before
carrying out the final analytical determination. The most common route of extraction
is to put the matrix in which the analytes are in contact with another phase, so they
are transferred to the extraction phase. Ideally, the extraction should be exhaustive,
but this depends on different factors, such as the nature of the extraction phase, the
sample and extraction phase volumes, temperature, pH, salinity, among other
parameters. The techniques used for this purpose are ion exchange (IE) [117],
liquid-liquid extraction (LLE) [13, 60, 106], and solid phase extraction (SPE)
[5, 7, 8, 16, 17, 24, 28, 30, 32, 33, 43, 48, 49, 51, 56, 62, 66, 67, 70–72, 74, 79–
81, 84, 86, 88, 93, 105, 107, 108, 110, 114], which is the most used technique to
analyze NSAIDs in water due to the high availability of sorbents, lower amounts of
organic solvents, shorter extraction times, and the possibility of coupling it in line
with the chromatographic methods used in the next stage. Combined with ultrasound
(US), the SPE method is called SPE assisted by US [74, 121].
For SPE, reverse phase cartridges of different materials and formats are used. The
Oasis HLB stand out [5, 7, 8, 16, 17, 24, 32, 48, 50, 62, 66, 70, 72, 79, 80, 82, 88,
108, 110, 112] and is composed of a copolymer of N-vinyl pyrrolidone and
divinylbenzene [13] with different hydrophilic-lipophilic balance that serve to
extract both polar and non-polar compounds [121, 123]. The MCX Oasis [28, 30,
Quantification of Non-steroidal Anti-inflammatory Drug in Water
91