Biomedical Engineering Reference
In-Depth Information
real samples. Authors analyzed real samples with this method and good correlation
was shown between plasma and OF results. Matrix effects were compensated by the
use of ISTDs.
10.1.2
Liquid-Liquid Extraction
Many solvents are used, none of which meet the ideal criteria, and most of them
require specialized storage, handling and disposal procedures. The most widely
used organic solvents are ethyl acetate, diethyl acetate, hexane, chloroform, butyl
acetate, dichloromethane. Liquid-liquid extraction (LLE) is simple, robust, and
transferable; it shows good reductions in matrix effects for plasma and it may be
more suited to urgent analyses than SPE [
97
]. In most cases, though, there is a need
to evaporate the solvent extract before redissolving the residue in mobile phase.
This extra step costs time and increases error. Gottardo et al. used LLE for the anal-
yses of MA, MDA, MDEA, MDMA, methadone, cocaine, morphine, codeine,
6-MAM, BEG in blood. The sample was treated with ammonium sulfate and then
extracted with chloroform-isopropanol (9:1) [
98
] .
Couchman et al. developed a method for simultaneous analysis of amisulpride,
metamphetamine, and amphetamine in plasma by HPLC-MS/MS. To the sample
(200 ml), ISTDs solution and NaOH solution were added to the extraction solvent
(butyl acetate-butanol, 9:1). HPLC separation was performed by strong cation-
exchange. The use of methanol as eluent improved sensitivity for analytes due to
more efficient in-source desolvation when compared with aqueous eluents [
99
] .
10.1.3
Solid Phase Extraction
Many solid phase extraction (SPE) materials, when the sorbent becomes dry, risk
inactivation of the bonded phase, leading to the poor recovery of analytes. However,
some newer, polymeric sorbents are more resistant to this problem, can tolerate rela-
tively
dirty
samples and are stable over a wider pH range compared to silica-based
materials [
100
]. Mixed-mode materials, with a combination of interactions, allow
efficient cleanup by using relatively harsh wash steps with minimal loss of analyte(s),
and they are increasingly used for sample preparation [
101
] . Such methods typically
involve the sequential elution of acidic, neutral, and basic compounds using sol-
vents at appropriate pH, and this versatility has led to their increasing popularity.
For comprehensive analyses, the elutes from different fractions are usually com-
bined and evaporated before reconstitution in an LC-compatible solvent. Several
SPE materials were evaluated for extraction of a range of drugs from serum/plasma,
noting that a divinylbenzene (DVB) material offered the best combination of extrac-
tion capacity and desorption efficiency amongst those tested.
A series of SPE sorbents ranging from nonpolar, to mixed-mode, to polymeric,
were tested for their performance in the STA of a diverse range of drugs (17 ana-
lytes) in whole blood [
102
]. In this experiment a C
8
-modi fi ed silica material was
