Biomedical Engineering Reference
In-Depth Information
2.2
Solid Phase Extraction (SPE)
Different SPE procedures have been described using sorbents based on reversed-phase
mechanism (C8, C18, hydrophilic-lipophilic balance), where the sample is loaded
in basic conditions to guarantee analyte retention. After washing the cartridge,
elution is performed with different organic solvents, sometimes in acid conditions
[
33,
66-
71
]. Due to the basic properties of antidepressants, mixed-mode sorbents
simultaneously acting by reversed-phase and cation-exchange mechanisms, allow
for a more selective extraction by removing nonbasic endogenous material [
32,
34,
35,
43,
72,
73
]. To allow analyte retention by cation-exchange mechanism, the sam-
ple should be initially conditioned with acidic solutions; after the washing step(s),
analytes are eluted using alkalinized organic solvents. Common elution solvents
are MeOH, ACN, dichloromethane, 2-propanol, or mixtures of them. The extrac-
tion process can be facilitated using semiautomated SPE robots [
32,
66
] . Moreover,
online SPE-LC-MS instruments have also been employed for antidepressant analy-
sis [
72
]. In these instruments, SPE extraction of the sample is performed in parallel
to the chromatographic separation of the previous one, allowing complete
automation of the whole analytical method, and the consequent sample high
throughput.
2.3
Online Extraction
Online extraction techniques by means of supports coupled to the chromatographic
system have also been applied to the determination of antidepressants [
74-
78
] .
These systems allow for direct sample injection, since large molecules from the
biological matrix can readily pass through the column, while the analytes of interest
can be retained under aqueous conditions, and subsequently eluted using high
organic solvents composition. There are different supports, such as the so-called
restricted access media (RAM), large particle size (LPZ) and monolithic supports,
which can be used in the single column configuration or, more frequently, in the
column-switching approach.
2.4
Solid-Phase Microextraction (SPME)
Another alternative technique, solid-phase microextraction (SPME), was used for
the determination of fluoxetine [
79
] and several TCAs [
80
]. SPME is a miniaturized
and solvent-free technique, where analytes are extracted from the sample by adsorp-
tion on a thin polymer coating fixed to the solid surface of a fiber, located inside an
injection needle or a capillary. Its main disadvantage is that special strategies are
needed to couple SPME to the LC-MS analysis.
